Underlying nature of specific language impairment

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0021-9630/92 $500 + 0.00

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@ 1992 Association for Child Psychology and Psychiatry

J Child Psychol Psychial Vol. 33, No. I, pp. 3-66, 1992Prioted in Great Britain

V. M. BishopD

Introduction

Specific language impairment is diagnosed where there is a failure of normal languagedevelopment that cannot be explained in terms of mental or physical handicap, hearingloss, emotional disorder or environmental deprivation. In the past, "developmentaldysphasia" or "developmental aphasia" were widely used to refer to this condition,but these terms have fallen into disfavour in the U.K. and U.S.A., largely becausethey misleadingly imply that we are dealing with a single condition with a knownneurological basis. These me<;iical terms are, however, still popular in continentalEurope, and among some paediatric neurologists inEnglish-sp~aking countries (e.g.Rapin, 1987). Most specialists in the U.K. and U.S.A. prefer the more neutral terms"developmental language disorder", or "specific language impairment" (SLI), andit is the latter that will be adopted here.

Much of the extensive literature on SLI is concerned with documenting thedifficulties experienced by these children with different aspects of language. A smallerbody of work has concentrated on formulating psychological models that explain SLIin terms of impairment in a particular aspect of cognitive processing. This paper aimsto bring together these approaches and to assess how far specific linguistic deficitscan be explained in terms of particular cognitive impairments. Each of the followinghypotheses will be reviewed and evaluated in turn.

Hypothesis 1. Underlying linguistic competence is intact, but there is an impairmentin the processes that are involved in converting this underlying knowledge into a speechsignal, i.e. the problem is an output disorder.

Hypothesis 2. SLI results from impairment of auditory perception, which influencesthe course of language acquisition.

Keywords: Language impairment, child cognition, linguistics

Accepted manuscript received 5 July 1991

Requestsjor reprints to: Dr Dorothy Bishop, MRC Applied Psychology Unit, 15 Chaucer Road, CambridgeCB2 2EF, U.K.

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4 D. V. M. Bishop

Hypothesis 3. There is an isolated impairment of the specialized linguistic mechanismsthat have evolved to handle language processing.

Hypothes,s 4. There is a generalized deficit in conceptual development that affects, butis not restricted to, language processing.

Hypothesis 5. Learning strategies are abnormal, with a failure to apply appropriatehypothesis-testing procedures.

Hypothesis 6. The problem is not with handling particular types of mental operation,but rather arises because of limitations in the speed and capaciry oj the information-processingsystem.

The paper will conclude by considering some general principles for future researchthat are suggested by this review.

1. Language Impairment as Output Disorder

Information-processing accounts for cognitive functions draw a distinction betweenmental representations of information, and the cognitive operations that are involved intransforming information from one representation to another.

Associated with any language stimulus will be a range of different representations.A spoken utterance impinging on the ear is initially encoded in the auditory systemin terms of the acoustic characteristics of the stimulus. This representation must thenbe converted into a more abstract representation in terms of a seq~ence of speechsounds. This will not contain information about superficial acoustic details, such asthose characteristics specific to a particular speaker: the sound /b/ will be treated thesame irrespective of incidental features such as pitch or presence of background noise.Familiar sequences of sounds are then matched against existing lexical representationswhich are maintained in a long-term store. If we hear an unfamiliar word then nolexical representation will be activated. A representation of the abstract grammaticalstructure of the utterance will be derived from an analysis of the pattern of contentwords and grammatical morphemes, and this information will be combined withknowledge of meaning of lexical items to derive a representation of the propositionalcontent of the utterance.

A similar hierachy of levels of representation is implicated in the process of speechproduction which starts with a representation of the meaning of the intended message,and ends with a specification of motor commands to the articulators. The processof producing meaningful speech involves generating representations of the grammaticalstructure needed to convey the relations expressed in the message, accessing appropriatelexical representations that specify the sequence of speech sounds corresponding towords with relevant meanings, and computing the articulatory correlates of a particularsequence of sounds and the motor movements necessary to produce these.

If we adopt this kind of model, we are led naturally to the question of where inthe language processing chain the deficit of SLI children is located. For instance,when a child produces a grammatically simplified utterance, is this because theunderlying representation of grammatical structure is inadequate, or is the problemsomewhere in the chain of processes that are involved in converting an abstractgrammatical representation into speech output? Note that an output disorder is distinct

5Specific language impairment

from a peripheral impairment of motor control. The distinction may be clarified bydrawing an analogy with the situation where a person is asked to draw a bicycle andends up producing a woefully inadequate picture (as many adults do). Logically, onecan see that the poor drawing could indicate that the person had only a vague ideaof what a bicycle looked like (a disorder of a central representation), or it could bethat they had a perfect mental image of a bicycle, but lacked the ability to transformthis image into appropriate motor movements. This would correspond to an outputdisorder. Contrast this with the person who is unable to execute a drawing becauseof some peripheral problem such as paralysis or tremor.

Early accounts of SLI did not use this conceptual framework, but they impliedthat many expressive language problems could be regarded as output disorders. Mostchildren were thought to have an "expressive" form of "developmental aphasia",where the difficulties were not explicable in terms of peripheral motor problems (e.g.dysarthria), nor in terms of defective understanding. This view ofSLI may be evaluatedwith regard to the different components of language that are affected.

Output explanations of abnormal speech

Virtually all children with SLI have some abnormality of speech production (Haynes& Naidoo, 1991) and any theory of the disorder should be able to account for this.

In some children, problems with speech sound production are the only obvioussigns of disorder. Traditionally, such cases were diagnosed as "functional articulationdisorders", and treated as quite distinct from more pervasive language disorders.However, there is some evidence that this might be a false dichotomy, and thatdisorders restricted to speech output are on a continuum with more severe conditionswhere a wider range of language functions is affected. Evidence for continuity is oftwo kinds. First, many children whose problems appear confined to speech soundproduction do prove to have more widespread impairments when standardizedlanguage tests are administered (Shriner, Holloway & Daniloff, 1969; Whitacre,Luper & Pollio, 1970; Marquardt & Saxman, 1972; Saxman & Miller, 1973). Second,if children are followed over time, the picture often changes from a child having generaldifficulties with grammar and speech production, to one with isolated speech problems(Bishop & Edmundson, 1987a; Scarborough & Dobrich, 1990).

The term "functional articulation disorder" implies that control of the articulatoryapparatus is defective either because of poor motor skills or inadequate oral sensoryfeedback. However, in recent years, the emphasis has moved from articulatoryprocesses to consider other explanations of speech problems.

In terms of the framework introduced here, an output explanation of' 'functionalarticulation disorders" maintains that the child has an adequate representation ofthe phonological system of the language, and normal control of the articulatoryapparatus. What is lacking is the ability to convert an abstract phonologicalrepresentation into a set of motor commands to the articulators. Before we can evaluatethis hypothesis, we must consider alternative explanations.

One view, that will be considered in greater detail below, maintains that theunderlying problem affects input rather than output processes: abnormal speech isseen as a secondary consequence of inadequate auditory perception. If children do

6 D. V. M. Bishop

not distinguish between two sounds perceptually, then it would be expected that theycould not learn to produce them distinctively.

Another possibility is that input processes are unimpaired, but the child fails todevelop a normal representation of the phonological system of the native language.The problem is in abstracting the underlying structure from speech input. Thosewithout training in linguistics tend to assume that language contains a finite numberof speech sounds and the child's task is to learn the articulatory configurationcorresponding to each sound. This is not so. Languages vary in the ways in whichthey partition the domain of possible speech sounds into sets that contrast differencesin meaning. The task confronting the child is not just leaming how to articulate sounds,but also discovering which sounds signal a change in meaning and which do not.For instance, French distinguishes the vowel sounds in the words "doux" and 'du",whereas English does not (both sound like renderings of English "do"). Conversely,English contrasts the initial consonant in "then" and "zen", whereas French doesnot. English speakers of French frequently make errors on the doux/du contrast,whereas French speakers of English confuse then/zen; not because of any inherentperceptual or articulatory limitations, but because they do not have an adequateknowledge of the phonological system of the language. It is possible that childrenwith SLI have a similar difficulty; they fail to learn which sounds are contrastivein their language. This will be termed the learning hypothesis.

We can see, then, that output explanations for speech disorders in SLI are notthe only possible account: the problem could arise from auditory perceptual deficits,or it might reflect difficulty in learning how to partition the universe of possible speechsounds into a set of phonemes that can signal contrasts in meaning. Let us considerwhat types of evidence may be used to choose between these competing hypotheses.

Circumstantial evidence from associated motor deficits. One line of evidence comesfrom studies investigating other areas of motor function in children with SLI. Fewchildren with SLI have hard neurological signs, but several studies have found thatthey are often immature on tests of motor skill (Stark & Tallal, 1981; Bishop &Edmundson, 1987b; Robinson, 1987). While this evidence is only correlational, itdoes give credence to the notion that speech difficulties may be but one indicationof general difficulties in programming coordinated sequences of motor movements.

Analysis of speech errors. A second approach to distinguishing explanations of speechdifficulties involves analysing the errors themselves. The traditional approach wasto classify" articulation errors" in terms of substitutions and omissions of particularsounds. The involvement of linguists in the study of language impairments led toradical changes in how abnormalities of speech production were assessed andconceptualized. Grunwell (1982) noted that traditional error analysis failed todistinguish between phonetic deviations and phonological disorders. A speech disorderis appropriately described as a phonetic deviation when the child marks all the contraststhat are used to signal differences in meaning in the adult system, but productionof particular sounds is distorted. A common example is where /r/ is produced as a


labiodental approximant lpJ (so that it sounds intermediate between a normal Irl andIw/). Although one perceives such speech as abnormal, the production of Irl and Iwlis clearly contrasted, so that "ride" and "wide" are produced differently. In suchcases, the underlying phonological system is intact: the abnormality is in howarticulatory correlates of specific sounds are represented. One might wonder whetherthe distorted production of particular sounds could be a consequence of distortedperception: however, this seems implausible because any perceptual impairment shouldapply when monitoring one's own speech output as well as when listening to the speechof others, so should not result in a mismatch between perception and production.(One is reminded of the claim that Modigliani's elongated figures may be attributedto astigmatism. The fallacy is apparent when one considers how Modigliani's figureswould appear to the artist: if normal figures look elongated to him then his ownelongated drawings should appear even more abnormal).

Although there is a lack of systematic data on this point, most children with SLIappear to have difficulties that go beyond phonetic distortions, in that they do notmake all the contrasts between phonemes that are required by the adult system andso, for instance, many not distinguish Idl and Ig/. In such cases all three of thehypotheses considered above are plausible: the difficulty could be in perceiving thedistinction between the speech sounds, in learning which sounds are distinctive inthe language, or in motor output process. Leonard (1982) found close similaritiesbetween phon910gical errors of SLI children and those seen in normal development.Analyses of the phonological production of normal young children indicates that theirerrors are systematic in two respects. First, they involve entire classes of speech sounds.To take a common example, if the child tends to produce "cat" as "tat", "goat"as "doat" and "sing" as "sin" we may account for all these changes by specifyingthat velar sounds are produced at the alveolar place of articulation. Second, manychanges operate in one direction only: e.g. "k" is produced as "t", but not viceversa. Stampe (1969), who was interested in explaining patterns of phonologicaldevelopment across different languages, termed these systematic patterns "phonologicalprocesses" and argued that they operated to simplify speech output by mergingcontrastive pairs of phonemes to the more easily articulated form. This, then, is anoutput explanation of phonological errors which assumes that the child's lexicalrepr~sentations of the phonological forms of words are accurate, but simplificationoccurs at the articulatory stage. Stampe's account is attractive because it can explaincertain of the characteristics of common processes that are not easily accounted forin terms of perceptual or learning hypotheses. One of these is the fact that substitutionstend to be undirectional. If the child treated Ik/ and It I as different exemplars of thesame phoneme one might expect either form to be produced interchangeably, butthis is not observed; Ikl is often produced as It/, but one seldom finds It I producedas Ik/. Also, many processes operate at the suprasegmentallevel; thus children tendto delete final consonants (e.g. "bag" becomes "ba"), reduced consonant clusters(e.g. "string" becomes "ting") and omit weak syllables in polysyllabic words (e.g."banana" becomes "nana"). Furthermore, there may be consonant harmony,whereby the production of a sound is influenced by other sounds earlier or later inthe word (e.g. "daddy" is pronounced correctly but "doggie" becomes "goggie").

8 D. V. M. Bishop

Such patterns are not easily explicable in terms of difficulty in perceiving certainconsonant distinctions, or as the consequence of a phonological system that collapsescertain categories. However, they are readily explained in terms of simplification ofarticulation.

Grunwell (1981) has argued against applying this type of output explanation tophonological disorders on the grounds that children with such difficulties are usuallyable to imitate speech sounds in isolation, but this is not incompatible with an outputexplanation. If motor movements were slow, clumsy or poorly coordinated, one cansee that a child might succeed on the simple task of imitating one sound, but revealproblems on more taxing tasks which involve integrating a sequence of sounds intoa fluent word form.

There is a third pattern of speech error that has been described in SLI children,which is not readily categorized as either phonetic or phonological. This is wherethe child's speech production is unpredictable, both in terms of whether a particularword will be pronounced correctly, and in terms of which sound will be substitutedif an error is made. Accuracy of speech production is more a function of length ofthe utterance than of its phonological constitution. A common way of identifying thistype of speech problem in children is to contrast ability to repeat isolated speech sounds(p - p - p, t - t - t - , or k - k - k -), with ability to repeat a sequence of differentsounds (e.g. p - t - k). Many children who are able to repeat isolated sounds havedifficulty with the p - t - k sequence. By analogy with acquired neurological disorders,this has been termed "developmental verbal dyspraxia" (Edwards, 1973), but thediagnosis remains controversial, and defining criteria are inconsistently applied.

Evidence from perceptual tasks. The evidence reviewed so far indicates that many ofthe phonological problems observed in SLI children are compatible with an explanationin terms of output processes and are hard to explain in terms of a simple perceptualor learning theory that maintained that the child simply collapsed certain phonemiccontrasts into a single category. However, as we shall see in the next section, thereis one set of evidence that is difficult to reconcile with an output explanation, namelythe finding that most SLI children, including those regarded as having dyspraxicdisorders of motor programming, are impaired on a range of phonological processingtasks ~hat do not involve any speech output.

One way of explaining such findings is to propose a new version of a perceptualor learning theory, in! which the problem is not viewed as failure to distinguish betweencontrastive phonemes, but as a more basic inability to segment the speech streaminto phonemes. This hypothesis will be considered more fully below. An alternativeapproach is to retain an explanation in terms of defective speech output and to explainperceptual problems as a consequence of the output difficulties. Winitz (1969), forexample, suggested that repeated mispronunciation of words affected the child'sperceptual system so that contrasts which were misproduced would eventually alsobe misperceived. One way of testing this idea is to consider speech perception inchildren whose speech production is impaired for physical reasons, i.e. dysarthricchildren, where one would expect a similar effect to be observed. Bishop, Byers Brown


and Robson (1990) carried out such a study, comparing two groups of people withcerebral palsy, matched on age and non-verbal ability: a group with speech difficulties(dysarthria or anarthria) and a control group with normal speech. Speech-impairedindividuals were impaired relative to controls on speech discrimination when assessedusing a task that involved judging whether minimal pairs of non-words were the sameor different, but they were not impaired when the same contrasts were tested usingLocke's (1980) procedure (see below), which does not require the child to retainunfamiliar strings of sounds. This study indicated that speech difficulties can influenceperformance on speech discrimination tests, depending on which assessment methodis used.

Grammatical impairment as a secondary consequence of phonological disorderPhonological and grammatical errors do tend to co-occur in the expressive language

of SLI children and Leonard, Sabbadini, Leonard and Volterra (1987) consideredwhether failure to produce certain grammatical morphemes might reflect the operationof common phonological processes in the speech of children with SLI. It could beargued that their tendency to omit grammatical morphemes that form phonologicallycomplex clusters (e.g. Itsl in "cats", Iptl in "helped") reflects a process of clusterreduction, and that omission of unstressed sylables (e. g. "is" in "he is big") arisesthrough a process of weak syllable deletion. In support of this view, Panagos andPrelock (1982) found that syntactic errors increased with syllabic complexity in asentence repetition task, and Paul and Shriberg (1982) noted that in some SLI childrenomissions of grammatical morphemes could be totally accounted for in terms ofphonological processes such as final consonant deletion.

However, data collected by Leonard et al. (1987) showed very clearly that expressivephonological limitations alone cannot explain the range of morphological deficits inEnglish-speaking children with SLI. The children were much more likely to producethe regular noun plural (e. g. dog - dogs, cat - cats) in obligatory contexts than the third

person singular verb (e.g. go -goes, kick -kz'cks) or the contracted copula ('s) althoughall these grammatical forms involve adding Isl or Izl to the stem.

Another re~son for rejecting an explanation of grammatical deficits in terms purelyof output processes is that this could not account for the comprehension problemsseen in these children. Bishop (1979) found that most children with expressivegrammatical disorders were impaired on a test of grammatical comprehension, eventhough their deficits might not be obvious in casual conversation when they couldrely on context and redundancy to decode language.

Output explanations of vocabulary deficitsSLI children do poorly on tasks of naming vocabulary, raising the question of

whether the child's long-term store of words (the "mental lexicon") is impoverishedor whether the child has adequate lexical representations but cannot retrieve these.Problems of lexical access are common in acquired aphasia, where the patient knowsthe word, and may be able to generate it if provided with a cue such as the first sound,but suffers from a severe form of the "tip of the tongue" phenomenon that may affecteven the commonest words (see Lesser, 1978, for a review). Inability to access an

10 D. V. M. Bishop

intact lexical representation corresponds to an output deficit in the conceptualframework adopted here.

Dollaghan (1987) argued that vocabulary deficits in SLI children reflect problemsin retrieval rather than perception or storage of words. However, the evidence forthis conclusion was ambiguous. She studied a group of 4 - 5-year old SLI children,none of whom had major phonological problems that might interfere with the abilityto pronounce novel words, but all of whom had significant limitations of expressivesyntax. Subjects were exposed to a novel word in a game. They were asked to hidea pen and fork, before being asked to hide the "koob", which was the term usedto refer to an oddly shaped white plastic ring. The novel word was presented onceonly. Comprehension was then assessed by placing these three objects on the tabletogether with two unfamiliar objects, to discover whether the child could select thecorrect item when asked for fork, pen and koob. They were then asked to name allthree items. SLI and control children did not differ on the comprehension task, butthere was a striking difference on the production task. Only one of the 11 SLI childrenproduced the word "koob" compared with seven out of 11 control children. Also,the control children were more likely to produce some of the phonemes of the novelword correctly, even if they did not produce the whole word. The mismatch betweenthe successful performance by SLI children on the comprehension task and their poorperformance on production led Dollaghan to conclude that the problem was one ofretrieval rather than storage. However, children could have succeeded on thecomprehension task without retaining any phonological information about the novelword, simply by selecting the funny object that they had seen before when presentedwith a novel word. A proper test between explanations in terms of perception, storageand retrieval would require that children be taught more than one novel word and/orthat they be given "catch trials" on a comprehension test in which they were askedto select an object to correspond to a novel word that they had not previouslyencountered.

If the main difficulty for SLI children is in retrieval of adequately stored lexicalrepresentations, then it should be possible to improve performance dramatically byproviding retrieval cues. Kail, Hale, Leonard and Nippold (1984) studied theeffectiveness of retrieval cues on the free recall of word names by SLI children. Theyfound that SLI children were poor at free recall overall, but they did not improveany more than control children when retrieval cues were provided. They concludedthat the problem for SLI children is in the initial storage of lexical representations,rather than in lexical access.

Output explanations of memory deficitsOne striking deficit seen in most SLI children is limitation of immediate memory

span (see, e.g. Haynes & Naidoo, 1991). Contemporary models of working memory(Baddeley, 1986) stress the importance of an "articulatory loop" that is used to maintainitems in memory by a process of rehearsal and to translate material from visual toverbal form. According to this model, people who speak slowly rehearse fewer words,so speech rate will be a limiting factor in memory span. It is therefore predicted thatindividuals with expressive phonological problems would have reduced spans. Kamhi,


Catts, Mauer, Apel and Gentry (1988) found that SLI children were poor at short-tenn memory tasks and favoured this form of explanation for their findings, suggestingthat the problem lay in planning an articulatory program for a complex phonologicalsequence. Gathercole and Baddeley (1990), however, in a similar study found thatSLI children's performance was affected by the number of syllables in the non-words,but not by articulatory complexity. They included non-words that incorporatedconsonant clusters as well as others that used only single consonants, and they foundthat this had no effect on performance by SLI children.

If slow speech rate causes memory limitations, then we should find memory deficitsin children whose speech is impaired for purely physical reasons, i.e. cerebral palsiedchildren with dysarthria or anarthria. There is some disagreement on whether thisis the case. Bishop and Robson (1989) found no differences in memory functionbetween cerebral palsied individuals with nonnal speech and those who were dysarthricor anarthric, and they concluded that rehearsal does not depend on covert articulation.However, in a study by Raine, Hulme, Chadderton and Bailey (1991) speech-disordered children were found to be impaired on tests of short-term memory,regardless of whether there was a physical basis for the speech difficulties.

Evaluation: output explanations of SLIA major problem for output explanations of language difficulties is that most SLI

children have deficits in phonology, vocabulary and grammar even when these abilitiesare tested receptively without requiring any speech production. The only way suchresults could be accounted for in terms of theory of output disorder would be bymaintaining that limited ability to speak leads to restricted receptive languagedevelopment. Studies of children who are unable to speak because of physical handicapsallow us to assess the validity of such an explanation. There is some evidence thatsuggests that expressive speech difficulties can restrict short-term memory for verbalmaterials, and this could lead to receptive deficits on tasks where there is a heavymemory load. However, many children with total anarthria develop normal receptivelanguage skills well in excess of those seen in children with SLI (Bishop et al., 1990).Although output difficulties may lead to secondary impairments in how children handlelanguage processing tasks, it does not seem feasible to treat all receptive difficultiesas secondary consequences of expressive problems. We need to consider alternative

explanations.

2. Language Impairment as an Auditory Disorder

Eisenson (1972) was one of the first to popularize the notion that auditoryimpairments were the cause ofSLI. He maintained: "The use of the term developmentalaphasia, or one of its synonyms, implies that the child's perceptual abilities for auditory(speech) events underlies his impairment for the acquisition of auditory symbols. Hisexpressive disturbances are a manifestation of his intake or decoding impairment"(p. 69). He regarded SLI as a unitary disorder, ranging in severity from profoundreceptive aphasia at one end, to mild syntactic and phonological deficits at the other.He proposed that all disorders on this spectrum had the same underlying cause:

12 D.V. M. Bishop

auditory perceptual impairment. The evidence for auditory deficits in SLI will firstbe reviewed before going on to consider how well this theory can account for therange of different language impairments seen in this condition.

Evidence for impairment in processing rapid transient stimuliBy far the most comprehensive investigations of auditory processing in SLI have

been carried out by T~l~ and her colleagues in a series of studies conducted overthe past two decades. On the basis of initi~ studies of a group of 12 SLI childrenrecruited from a British residenti~ school, T~l~ and Piercy (1973a,b) concludedthat language-impaired children had a selective deficit in processing rapid or briefsign~s in the auditory mod~ity. Later work, however, using larger samples ofAmerican children suggested that auditory perceptu~ problems are but one indicationof a more gener~ized impairment in perception and production of rapid sequences.

In ~l these studies care was taken to devise experiment~ tasks that could beperformed with no verb~ instructions and which involved contrasting only two stimuli.The child is presented with a box which has two response panels. First he or she istrained to press panel A in response to stimulus 1. Then stimulus 2 is presented onsever~ occasions, and the child trained to press panel B. Once this has been mastered,random sequences of stimuli are given, and the child presses the appropriate panelfor each one. The aim of the test procedure up to this point is to establish that thechild can discriminate the stimuli adequately. Various manipulations may then beintroduced, either by reducing the duration of each stimulus, curtailing the interv~between stimuli, or increasing the length of the sequence of stimuli. Using thisprocedure, Tall~ and Piercy (1973a,b) demonstrated that performance ofSLI childrenwith non-verb~ auditory stimuli was cruci~ly dependent on timing. When stimuliwere of brief duration or when the interv~ between them was very short, performancedeclined dramatic~ly. In contrast, no deficit was found when visu~ stimuli werepresented in an~ogous tasks. This study provided experiment~ support for Eisensons's(1972) claim that "the aphasic child's basic perceptu~ impairment (is) one for auditoryperception for speech at the rate at which speed is normally presented" (p. 66, my it~ics).Tall~ (1976) argued that the observed deficits were not simply a consequence of immaturelanguage skills, because the pattern of performance seen in SLI children did notresemble that ofnorm~ children at any age. When the interv~ between stimuli waslong, SLI children performed better than younger control children, but they didsubstanti~ly worse than this group when the interv~ was less than 305 msec (see

Fig. 1).T~l~ and Stark (1981). conducted a larger investigation of 35 SLI children aged

from 5 to 9 years, selected by stringent and objective criteria. In this studydiscrimination of auditory and visu~ stimuli was explicitly compared. The auditorystimuli included complex tones, as used by T~l~ and Piercy (1973a,b), as wellas synthesized speech sounds (ba and da). The visu~ stimuli were letter-likeforms.

The results agreed in many respects with those from the British sample, but therewere ~so points of difference. As in the earlier studies, there were striking differencesbetween SLI and control children in their ability to discriminate tone pairs whena variable inter-stimulus interv~ was used (T~l~, Stark, K~lman & Mellits, 1981).


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Fig. 1. Relationship between inter-stimulus interval and perception of brief tone pairs by SLI childrencompared to younger normal controls (based on Tallal, 1976).

The SLI group was also significantly impaired in ability to process sequences of morethan two stimuli, even when the interval between stimuli was relatively long.

There were, however, some interesting differences from previous work. Some SLIchildren performed poorly with auditory stimuli that were neither transient nor rapid.Two children failed to reach criterion on a task that sim~y required them to learnto associate a different response with each tone. A further seven children performedat chance level with auditory sequences of two tones even when there was a long intervalbetween stimuli.

Another striking difference from earlier work was that the SLI group was deficientrelative to controls with visual as well as auditory stimuli on tests of sequencing, rateprocessing and serial memory. Post hoc analysis indicated that this finding was a functionof the age of the subjects: the younger SLI children were impaired in visual andauditory modalities, whereas the older ones were impaired with the auditory stimulionly.

~his age effect could reflect sampling factors, but it could also indicate that theprofile of impairment changes as the child grows older. To investigate this possibility,Bernstein and Stark (1985) traced 29 children from the original Stark and Tallal (1981)study and retested then on perception of synthetic ba - da contrasts 4 years after theinitial testing, at which time 23 children still met criteria for SLI. Overall, there wasa substantial improvement in children's performance. When first tested, 19 of theSLI children had failed to discriminate reliably between these synthetic speech sounds,whereas on retest 24 of them reached criterion, and there was no overall differencebetween the SLI group and controls. When presented with sequences of stimuli ata fixed inter-stimulus interval, performance of SLI children was variable. Many of

D. V. M. Bishop

them did very well, but a few made many errors. All those who did pass this subtestwere able to reach criterion on a rate processing subtest that included trials with briefinter-stimulus intervals. Furthermore, most of the SLI and control children couldrepeat sequences of four or five items on a serial memory subtest.

Bernstein and Stark (1985) noted: "Considering time 2 results alone, we couldnot conclude that specific language impairment in older children is caused by perceptualdeficits in rapid rate processing of phonemes" (p. 28). However, there are two pointsto note when interpreting these results. First, performance was near ceiling levelson retest, so it could be the case that SLI children still had an auditory perceptualdeficit, but the tests were too insensitive to detect this. The second point is that itwould be erroneous to conclude that the language disorder was not caused by perceptualimpairments just because no perceptual impairment was evident at time 2. As Bernsteinand Stark concluded: "Language disabilities may result when inadequate processingof sensory information occurs during early childhood although the original processingdeficit may no longer exist" (p. 28).

Auditory deficit as the explanation for phonological problems in SLIA crucial question is whether the perceptual deficit described by Tallal and colleagues

can account for the types of phonological problems observed in SLI children. Oneapproach to studying the relationship between auditory perception and phonologicalimpairment is to consider whether phonological errors are made on the sounds thechild has difficulty discriminating. On the basis offmdings with non-verbal tone stimuli,Tallal and colleagues made predictions about the types of speech sounds that SLIchildren should find difficult to discriminate and produce. These were broadlyconfirmed (Tallal & Piercy, 1974; Tallal, Stark & Curtiss, 1976; Tallal & Stark, 1981),with poor discrimination found for stop consonants, such as ba and ga and relativelygood performance with vowels, which are characterized by steady-state rather thantransient acoustic features. Tallal and Piercy (1975) went on to show that the advantagefor vowels over consonants could be reversed by altering temporal characteristics ofsynthetic speech stimuli, i.e. stretching formant transitions of consonants or truncatingvowels. However, it was found that the perceptual deficit of SLI children was notlimited to discrimination of temporal cues. They also did poorly when asked todiscriminate sa and sha, which differ on spectral cues.

Another way of investigating the relationship between auditory perception andphonological production is to consider whether individual differences in phonologicalstatus are related to auditory perceptual processing. Frumkin and Rapin (1980)subdivided a group of SLI children according to whether they had any phonologicalimpairment at the time of testing. Children with phonological disorders showed thecharacteristic deficit described by Tallal and Piercy. They had difficulty indiscriminating between synthetic ba and da, but their performance improved whenthe duration of the formant transition was increased from 40 to 80 msec. In contrast,SLI children with normal phonology were unimpaired on this task, but they werepoor at discriminating between brief vowel sounds, and had difficulty in reportingthe order of pairs of consonants presented in quick succession. This intriguing studysuggested that auditory perceptual problems may underlie several distinct varieties~


of SLI, but impairment in processing rapid transitional information is the deficit thatis most clearly linked to phonological competence.

Auditory deficit as the explanation for phonological impairments in milder disordersMuch of the research relating phonological impairment to auditory perception has

been conducted on children diagnosed as having" phonological disorders", who donot have the broader range of linguistic impairments that characterize SLI. As notedabove, however,. a case can be made for treating such disorders as on a continuumwith SLI, representing a mild form of the disorder. If this characterization is correct,we should expect to find similar underlying causes, and it is interesting therefore toconsider how far an auditory deficit theory can account for phonological difficultiesin children with no other obvious language problems.

A substantial body of work has been concerned with the question of whetherexpressive phonological problems can be explained in terms of deficits in auditorydiscrimination of speech sounds, but results from such studies have been contradictory,with some researchers reporting auditory discrimination deficits and others failingto confirm these. To some extent these discrepancies may reflect population differences.Some studies concentrated on children whose problems appear to have been restrictedto phonetic distortions, whereas others included children with limited or abnormalphonological systems. Some investigations excluded children with more widespreadlanguage difficulties whilst others did not. Stark and Tallal (1988) did include asubgroup of children with isolated articulation disorders in their study, but theyreported that these children were unimpaired on tests of rapid auditory processing,unlike the SI..I subgroup.

Locke (1980) and Seymour, Baran and Peaper (1981) pointed out that anotherreason for discrepant results lay in the unsatisfactory procedures used to test auditorydiscrimination. Most tests used in research settings adopted one of two paradigms.The first involves presenting the child with words that constitute" minimal pairs",i.e. they differ by one phoneme, e.g. goat - coat. The child is shown a set of picturesand has to select the one whose name is spoken. A major disadvantage of this approachis that it is difficult to find a vocabulary of minimal pairs of words that are pictureableand familiar to young children. This limits the phoneme contrasts that can be assessed,and many tests adopting this format include many contrasts that are seldom confusedin children's speech.

An alternative approach is to use non-words, in a same - different paradigm. Thechild hears a minimal pair, such as gub - guv and must say if they are the same ordifferent. However, this type of task is often impracticable with young children, whomay quickly tire attending to pairs of meaningless verbal stimuli. Given that thereis a 50% chance of giving a correct answer by guessing, it is necessary to use a longsequence of items to get a sensitive index of performance.

Locke (1980) proposed a novel approach in which the child is shown a picture witha familiar name (e.g. dog) and has merely to judge whether or not the tester saysthe name correctly (e.g. "is this a gog?"). This procedure overcomes the majorproblems of other test methods, and also has the advantage that one can design atest individually for a child so as to assess whether contrasts that are not distinguishedin the child's speech are also misperceived. Bird and Bishop (1991) included a

D. V. M. Bishop

test based on Locke's procedure in a study of 14 children with phonological disordersbut normal receptive vocabulary. They found that this group performed significantlyless well than control children, individually matched on age and non-verbal ability.However, there was wide individual variation, and several children had no difficultyin discriminating phonological contrasts that they did not produce distinctively intheir speech. There are two possible ways of viewing this result. One is to concludethat children with phonological impairments are heterogenous, with some havingperceptual deficits and others not. If this is so, one would expect to find othercharacteristics correlated with presence of perceptual deficit, e.g. the type ofphonological problem or its prognosis. An alternative interpretation is suggested bythe longitudinal study by Bernstein and Stark (1985) described above. They foundthat many SLI children who demonstrated perceptual impairments when young nolonger did so when seen a few years later. It may be that a perceptual impairmentearly in life is sufficient to cause a phonological disorder, which persists even afterthe perceptual impairment has resolved. Thus to demonstrate a link between expressivephonological problems and perceptual impairment it may be necessary to study childrenat an early age.

Bird and Bishop (1991) found that, whereas some of their subjects did havedifficulty discriminating between phonemes, many more of them had a different typeof problem, namely a difficulty in perceiving phoneme constancy across different wordcontexts. Consider the sound Isl when saying "soup" or "see". In "soup" the lipsare rounded, whereas in "see" they are spread, in each case anticipating the followingvowel. There are both acoustic and articulatory differences between these sounds,yet in English they are classified as exemplars of the same phoneme. To test abilityto perceive such constancy a task was devised in which children were shown a puppetwho liked things that began with the first sound of his name, "Sam". They werethen required to judge, e.g., whether the puppet liked the "sock" or the "ball".The contrasting sounds (in this case, Ibl and Is/) were selected to be perceptuallydistinctive, avoiding sounds that children had difficulty in producing contrastively.In another task, children were given training in rhyme generation and then askedto produce rhyming words (e.g. "tell me something that rhymes with cat"). Thesetasks revealed substantial deficits in children with phonological problems. They couldnot judge that the initial sound of Sam was the same as the initial sound in sock, andhence match Sam with sock rather than ball, despite the substantial differences betweenthe sounds Isl and Ib/. Also, they were very poor at rhyme generation. Such resultsare hard to explain in terms of poor discrimination. If the child had difficulty indistinguishing two phonemes, then certain classes of sounds would be collapsedtogether. For instance, if the child treated It I and Ik/ as instances of the same sound,we might then expect that when asked for a rhyme for cat, the response "back" wouldbe given. However, this was not the type of error that was observed. When askedto generate rhymes, the commonest type of response seen in phonologically impairedchildren was for the child to give a semantic associate (e.g. replying "dog" or"hamster" to "cat"). These children seemed to have no idea of what was requiredof them, despite repeated demonstration. Bird and Bishop (1991) explained thesefindings by arguing that children with phonological problems failed to segment wordsinto phonemes. jusczyk (1986) proposed that young children in the early stages of


language acquisition do not operate at the level of the phoneme. Rather they makedistinctions between larger, un-analysed chunks of sound. Only gradually do theylearn to recognize common elements in the words that they have mastered, and thishelps them to organize their growing vocabulary and to imitate new words that theyhear on the basis of production rules for particular phonemes. Bird and Bishop (1991)proposed that children with phonological problems continue to analyse speechin this immature manner, learning each new word as an entire unsegmented pattern.Consequently, they do not appreciate that words are composed of a small numberof building blocks, and their language learning is inefficient and protracted.

It remains open to question whether this failure to learn phonological principlesis a consequence of auditory perceptual limitations. This could arise, perhaps, if slowand inefficient processing of auditory signals led to masking of later phonemes byearlier ones. What is clear is that the problems go beyond failing to discriminatebetween similar sounds: rather there is a failure to identify the basic units necessaryfor efficient perception and storage of the sounds of words.

18 D. V. M. Bishop

and Piercy (1974, 1975), with adequate discrimination of steady-state vowels but verypoor performance with consonants. When the formant transitions of consonants wereextended, performance of these two children improved. The study was of interestin demonstrating that children with severe comprehension problems do not appearto be qualitatively different from those with milder forms of SLI: rather they havethe same types of auditory perceptual impairment, but in more pronounced form.

If, as Rapin et at.'s (1977) diagnostic label implies, the impairment in these childrenis restricted to the processing of speech sounds, one should find normal perceptionof non-verbal auditory stimuli. The tone stimuli used by Tallal and colleagues havenot been systematically applied to children diagnosed as having verbal auditory agnosia.In most cases the only evidence against a more general auditory deficit is that perceptionof environmental sounds is unimpaired. This, however, is not very satisfactory. Oftenthe child is offered choices between environmental sounds that differ substantiallyin acoustic characteristics and performance is near ceiling level. It may prove moreaccurate to speak of a general" auditory agnosia" in these children, rather than aspecifically verbal impairment.

What if one bypasses the postulated auditory perceptual deficit and presents childrenwith visual forms of language? If an auditory processing problem were responsiblefor comprehension failure, then it seems reasonable to predict that one should observemuch better understanding for written or signed language, provided the child hadhad adequate opportunity to learn these. Denes, Balliello, Volterra and Pellegrini(1986) presented a detailed case study of a child who had little speech and virtuallyno understanding of spoken language, but could communicate adequately throughreading and writing. However, he did have a tendency to make grammatical errorsin written language. Grammatical deficits in comprehending visual language werealso found by Bishop (1982), who compared children with severe receptive languagedisorders withYQunger normal children. Several of the children studied by Bishophad been taught using an artificial sign system that mapped directly on to English,so that one could sign any English sentence, complete with inflectional endings, andso directly compare comprehension across modalities for similar materials. The resultswere clear-cut: modality of sentence presentation had no effect on performance ofa group of children with receptive language disorders, whereas grammatical complexitydid. Thus, most children misinterpreted reversible passive sentences, regardless ofwhether these were spoken, written or signed. At first glance, this looks like strongevidence against an explanation in terms of auditory processing deficit. However, ina study which will be described in greater detail below, Bishop (1982) demonstratedthat congenitally deaf children performed just like the SLI children, demonstratingmajor problems in comprehending complex sentences, regardless of whether thesewere spoken (and hence perceived by lip-reading), written or signed. This studydemonstrated that it is quite wrong to assume that an auditory deficit will affectcomprehension only in the auditory modality.

Most children with SLI do not have the severe comprehension problems describedin children with' 'verbal auditory agnosia", but nevertheless their understanding istypically below age level. Several studies have investigated the nature of their difficultiesand found that SLI children do have distinctive problems in comprehendinggrammatically complex sentences. Bishop (1979) found that SLI children had major


difficulties with reversible passive sentences, although they performed better withreversible active sentences, whereas Van der Lely and Harris (1990), studying slightlyyounger children selected on the basis of poor understanding, found poor performanceby SLI subjects on all items where word order was used to express thematic relations.Leonard (1989) suggested that such impairments are consistent with an interpretationin terms of perceptual deficit. To perceive the contrasts between' 'the man chasesthe dog" and' 'the man is chased by the dog", the child must detect the -ed endingon the verb and the preposition by, neither of which is strongly stressed. If the childperceives both sentences as "man chase dog", then there will be problems in learninghow grammatical variants in word order alter meaning. * Note that one does not need

to maintain that the child never perceives the weakly stressed morphemes; even if thechild sometimes processes them, learning will be impeded because the relationshipbetween syntax and meaning becomes opaque.

Auditory deficit as the explanation for expressive grammatical problemsIt is relatively straightforward to predict that an auditory perceptual impairment

will result in problems in comprehension, but it is less clear how the structure ofexpressive language might be affected. Leonard et al. (1987) noted that the grammaticalerrors made by SLI children tended to be concentrated on verb inflections, auxiliariesand the copula (i.e. that part of the verb to be that connects subjects to predicate,as in "he is rich"). Several explanations may be offered for this finding.

(i) The grammatical problems might be a consequence of expressive phonologicallimitations. Evidence against this explanation has been reviewed above whenconsidering the "output disorder" hypothesis.

(ii) SLI children might be regarded as normal language learners confronted witha systematically altered input, i.e. an auditory perceptual deficit.

(iii) There might be impairment of an innate module specialized for learning

grammar.Leonard et al. (1987) and Leonard, Sabbadini, Volterra and Leonard (1988)

attempted to dissociate predictions made from these different hypotheses by comparingexpressive difficulties in two different languages, English and Italian. Eight English-speaking children were contrasted with eight Italian-speaking children, all diagnosedas cases of SLI. The English children all used word final Isl, Izl, It I and Id/ in singularnouns such as bus and bed, so any failure to produce plural or past tense morphemescould not be attributed to difficulty in producing these phonemes. The two groupsof children were matched in terms of mean length of utterance in words. Samplesof their language were collected, using pictures to elicit examples of grammatical formsof interest. As can be seen from Table 1, Italian children did not show strikingdifferences between regular noun plurals and third person singular verb inflections,both were produced in obligatory contexts at a much higher rate than was observedin English children. While there was no overall difference between Italian and Englishchildren in the production of articles, the Italian children used the feminine forms

-However, recent unpublished data by Vander Lely (1990) suggest that SLI children can discriminatedifferent grammatical forms. Her subjects performed consistently correctly on truncated passives, suchas "the man is chased".

20 D. V.M. Bishop

Table 1. Use of grammatical morphemes in obligatory contexts by English and Italian SLI children

book/booksball/balls

dog/dogskey/keys

libro/libri

palla/pallecane/canichiave/chiavi

mean = 94.4%S.D. = 9.8range 71 - 100

mean = 79.3%S.D = 27.1range 18 - 100

Third personsingular

to buy/buysto sell/sellsto open/opens

comprare/compravendere/vende

aprire/apre

mean = 7.9%S.D. = 17.6range 0 - 50

mean = 92.0%S.D. = 11.3range 68 - 100

Uncontracted

copula

the book is red il libro e' rosso mean = 51.9%S.D. =27.0range 0 - 78

mean = 76.5%S.D. = 16.5range 51 - 100

Contracted

copula

the book's red mean = 26.0%S.D. = 20range 0 - 53

mean = 6.8%S.D. = 10.3Range 0 - 26

mean 64.5%S.D. = 36.8range 16 - 100

provare/provatovestire/vestito

to try/triedto dress/dressed

Regular pasttense, not

involvingnumber/genderagreement

he returnedshe returned

mean = 61.8%S.D. = 30.5range 20 - 100

Regular pasttense involving

number/genderagreement

lui eo tornatolei eo tornata

Third personplural

to see/they see vedere/vedono mean = 31.8%S.D. = 34.3range 0 - 75

Articles endingin vowels

the, a la, una, i, Ie median = 55%range 12 - 64

median = 74%range 15 - 100

Articles endingin consonants

median =7%range 0 - 50

iI, un

of the definite and indefinite articles, la and una, significantly more often than thecorresponding masculine forms, il and un. Given that the number of obligatory contextsfor il and un was as high for la and una, it is difficult to explain this difference exceptin terms of phonological structure: i.e. whether or not the article ends in a vowel.Most errors with articles involved omission of the article rather than substitution ofan alternative. Finally, Leonard et al. (1987) also noted that Italian SLI childrencorrectly marked gender agreement of possessive pronouns and adjectives in nearlyall instances where this was required.

Specific language impairment

Leonard (1989) argued that many of the errors made by SLI children could beaccounted for by assuming that perceptual limitations impair the learning of specificfeatures. These include those represented by non-syllabic consonant segments (e.g.in English, plural, possessive or third person singular -s, past -ed and contracted formsof to be) and those represented by unstressed syllables (e.g. a, the, infinitive particleto, complementizer that). On this interpretation, Italian SLI children find it easierto learn verb inflections because most of these consist of stressed syllabic affiXes. Theirability to handle gender agreement is attributed to the fact that the relevant affixeshave a clear relationship to one another; in most cases the final vowels of adjectivesmatch those of the nouns they modify.

One might ask why a perceptual problem should have such a selective effect onmorphological development. If children have problems learning that -ed marks thepast tense of a verb (e. g. played), why don't they have similar difficulty in perceivingthe final consonant in other contexts, e.g. in single words such as raid? Leonard arguedthat, first, there is some evidence that SLI children do indeed have difficulty inperceiving unstressed final consonants in single words, and second that when learningthe significance of the ed morpheme children must not only perceive /d/, they mustalso relate played to play, hypothesize that /d/ is a morpheme, and work out itsgrammatical function and semantic correlates.

Although a perceptual deficit explanation can account for many of the results, somepuzzles remain. English children were much more likely to produce the regular nounplural -s than they were to produce third person singular -so This indicates that factorsother than perceptual salience are important. One possibility is that the semanticcorrelate of the morpheme is easier to deduce for the plural than the third person.However, if this were the explanation, one would expect a similar difference betweenthese two types of morpheme in Italian children, and this was not found. Parametersetting theory, considered in more detail below, provides some clues as to how suchdifferences may be explained.

Perceptual impairment or memory impairment?It was noted above that most children with SLI do poorly on tests of auditory-verbal

short-term memory, such as the digit span subtest of the Wechsler Intelligence Scalefor Children (Wechsler, 1974). Tallal and Piercy (1937b) included tests of memoryusing non-verbal tone stimuli in their assessment of auditory processing by SLIchildren. They found that, even where auditory stimuli were adequately discriminated,SLI children frequently had difficulty in retaining sequences of more than three orfour items. One way of accounting for this result in terms of their theory is to supposethat while perceptual processing is adequate to discriminate between pairs of speechsounds, it is slow and inefficient. If the child were still processing stimulus 1 whenstimulus 2 arrived then stimulus 2 would be inadequately processed, and hence poorlyremembered. The critical question that remains to be answered is whether SLI childrenhave memory deficits over and above those that would be expected on the basis of

their perceptual impairment.One way in which a perceptual deficit might affect memory processing would be

by influencing the way in which memorized material was encoded. Kirchner andKlatzky (1985) carried out a study of free recall by SLI children that supported the

22 D. V. M. Bishop

idea that they encode material in terms of meaning rather than phonologicalcharacteristics. The children were presented with pictorial stimuli to remember andwere explicitly instructed to rehearse the picture names aloud. Their rehearsal andoverall recall was less efficient than that of age-matched control children. Moststrikingly, and unlike control children, the SLI children made many intrusion errorsthat involved giving the name of a semantic associate of an item from the list. Thiswould be consistent with the view that the pictures were remembered in terms ofmeaning rather than sound. More recently, however, Gathercole and Baddeley (1990)conducted a study that indicated that SLI children can and do encode words inphonological form. Their subjects showed the normal sensitivity to phonologicalcharacteristics of memorized materials, with poorer recall of similar word sets suchas bat, cap, cat than of distinctive sets such as bus, clock, hand. They were also poorerat remembering pictures with long names (e.g. banana, elephant, ladybird) than thosewith short names (e.g. boat, cat, egg), just like control children. This observation providesevidence that children did rehearse the names of pictures rather than just forminga semantic or pictorial representation of materials. However, the authors noted thateffects of phonological similarity were not found for lists of more than four items.They suggested SLI children might switch to an alternative encoding strategy whenlists exceed their storage capacity. This would be consistent with the findings ofKirchner and Klatzky (1985), who used a free recall procedure with lists of 12items.

Gathercole and Baddeley (1990) challenged the view that memory limitations inSLI children are a secondary consequence of auditory perceptual deficits. They foundthat SLI children were extremely poor at repeating non-words. If this reflected aperceptual problem, one might expect poor performance even on monosyllables, butGathercole and Baddeley showed that performance depended on the number ofsyllables in the non-word, with much greater impairment on polysyllabic items. Therepetition scores of SLI children fell below even those of younger children matchedon receptive vocabulary. Similar results were reported by Kamhi and Catts (1986)and Kamhi et al. (1988), who noted that SLI children were impaired at repeatingpolysyllabic non-words when compared to reading retarded children as well as controlchildren. (On several other tasks that they used, reading retarded and SLI childrenshowed similar levels of impairment.) Another piece of evidence against an explanationin terms of perceptual deficit was that the SLI children studied by Gathercole andBaddeley (1990) were unimpaired on a phoneme discrimination task that involvedmaking same - different judgements about pairs of words and non-words. However,

this evidence is not conclusive, because performance of all groups was near ceilingand the sound contrasts they tested were not chosen to reflect the types of phonologicalerrors made in the memory task. Gathercole and Baddeley suggested that the memorydifficulties they observed might arise because of limitations in storage capacity, leadingto fewer items being stored, or because phonological representations in memory areless richly specified, leading to a less adequate memory trace. Although they did notthink that problems in distinguishing individual phonemes were responsible, theydid suggest that impairment of phonemic segmentation might be a factor that ledto degraded phonological representations in memory. This interpretation would meshwell with the findings of Bird and Bishop (1991), discussed above (p. 14).


Regardless of whether or not the non ..word repetition deficit is explicable in termsof a more basic perceptual deficit, it does have implications for vocabulary acquisitionin SLI children. The processes involved in repeating a non-word resemble thoseimplicated in forming a lexical representation of a new vocabulary item, and Gathercoleand Baddeley (1989) confirmed that in normal children, ability to repeat non-wordsis strongly predictive of vocabulary growth. Limited vocabulary is common in SLI(Haynes & Naidoo, 1991), and Gathercole and Baddeley's analysis provides a simpleexplanation for this in terms of difficulty in retaining unfamiliar phonological strings.A stringent test of this hypothesis would involve teaching SLI children novel wordsvarying in length. The prediction is that they should be disproportionately poor atlearning polysyllabic words.

Evaluation: the auditory deficit theory of SLIThe view of SLI as the consequence of auditory perceptual impairments has stood

the test of time remarkably well. The work ofTallal and colleagues has demonstratedthat these children do have problems in discriminating non-verbal auditory stimuliwhen these are brief or rapid. It has sometimes been argued that the auditory deficithypothesis cannot account for the specific types of grammatical impairment seen inmany SLI children. For instance, Leonard (1979) concluded a review of the areaby stating that' 'The very nature of the restricted speech used by language impairedchildren seems to suggest that auditory processing deficits may be a corollary to, ratherthan a cause of language difficulties" (p. 227). However, more recent work by Leonardand colleagues has revised that opinion and studies of receptive language functionby Bishop (1982) illustrate the dangers of assuming that one can specify preciselyhow auditory impairment will affect grammatical development. The problems forthe future are not so much to test whether auditory limitations can hinder languageacquisition - they undoubtedly can and do - but rather to consider how wide arange of the linguistic and non-verbal deficits seen in SLI children can be attributedto this cause. One crucial question is whether the observed memory deficits aresecondary consequences of perceptual abnormalities or whether there is a primaryimpairment in the memory system itself.

3. Linguistic Interpretations of SLI

One way of studying normal language acquisition is to attempt to simulate theprocess by computer, but to date all attempts to do so have foundered. No-one hassucceeded in formulating a learning algorithm that will derive the grammar of anylanguage when provided with input from that language. This has been termed the"learnability" problem and has led to the conclusion that humans must come to thelanguage learning task with some innate system specialized for grammatical processing.The problem is then to discover what type of innate knowledge would constrainlearning to make it possible to master a grammar, while at the same time being flexibleenough to allow one to learn anyone of the diverse languages that might beencountered. Given that grammatical difficulties are a hallmark of SLI, it may beasked whether these children lack the postulated language-learning module and are

24 D. V. M. Bishop

therefore confronted with the same problem as the computers that are programmedto extract grammatical regularities from language input.

In a review of the literature, Cromer (1978) noted that most theories of SLIattributed the children's language problems to some underlying deficit in non-linguisticprocessing, such as defective auditory perception, poor short-term memory, orconfusion in sequencing. Cromer argued that we had ignored the most obviousexplanation of SLI, which was that these children were unable to master thosegrammatical relations that Chomsky (1965) had proposed were processed by an innate"language acquisition device". Clearly, there is a danger of circular reasoning inarguing that the child fails to learn language because of lack of a language acquisitiondevice, unless one can be more specific about what this device does, and hence predictthe nature of language errors that should occur. Recently, several accounts have beenproposed that make specific predictions about the types of language difficulty thatshould be observed.

Failure to appreciate underlying hierarchical structureCromer (1978) noted that language learning involves extracting underlying

hierarchical structure from a temporal sequence. Consider the sentence: "The cheesein the refrigerator is green". If language were interpreted by relating each word tothose nearest to it in the sentence, we would conclude that the refrigerator ratherthan the cheese was green. Because we can utter only one word at a time, whenexpressing complex relationships we need a grammar that allows us to keep trackof underlying relationships between sentence elements that are separated in the surfacestructure. This involves appreciation of hierarchical structure, recognizing that somecomponents of the sentence are subordinate modifiers of other elements. If one lackedthe ability to extract hierarchical structure, this would have profound implicationsfor language learning. Some progress might be made by learning sentence framesand sequential dependencies between word classes, but there would be severe difficultiesin understanding constructions where processes such as subordination were involved.

Cromer carried out two studies with a small group of SLI children with severecomprehension problems ("receptive aphasics"). In both these studies he comparedchildren with receptive language disorders and deaf children. This is a potentiallypowerful method, as the deaf children can indicate how far language limitations canreasonably be attributed to auditory perceptual problems. However, these studieswere flawed by lack of comparability of deaf and SLI children on crucial variables.

In the first study, Cromer (1978) gathered samples of written language from childrenwith receptive language disorders and contrasted these with samples produced bydeaf children. He concluded that the written language of SLI children was characterizedby use of simple sentence patterns that could be interpreted in terms of sequentialdependencies. The deaf children attempted many more complex constructions,although they frequently made errors with these. This exploratory study, was, however,ambiguous. The differences between SLI and deaf children could have reflectededucational method rather than genuine differences in underlying disorder. The SLIchildren came from a school where language was taught using a structured approachthrough written sentences and they were explicitly trained to produce certain simplesentence forms.


Setting aside for the moment such objections, it may be asked whether, if ahierarchical planning deficit is implicated in SLI, this is a language-specific impairmentor a more general inability. In a subsequent study, Cromer (1983) investigated thisquestion using a task that involved extraction of hierarchical structure, but whichdid not involve any language. Children were required to copy either two-dimensionalor three-dimensional patterns of the kind shown in Fig. 2. Previous work showed

Fig. 2. Two-dimensional stimulus for copying used in Cromer's (1983) study of hierarchical processing.

that normal young children use a chain strategy, starting their construction at oneside and working sequentially up the figure, across the middle and down the otherside (Greenfield & Schneider, 1977). Older children, however, usually start theirconstruction with the superordinate connecting level, and work down through thehierarchy of levels. This involves interrupting the chain. Cromer scored children'sreproductions in terms of how far they followed the underlying hierarchical structureof the pattern, and demonstrated that SLI children obtained low scores on a measureof hierarchical organization compared to a control group of deaf children.Unfortunately, in this study too the matching of SLI and deaf children was inadequate.The deaf children came from a selective grammar school, entry to which wasdetermined by written examination, and their mean non-verbal IQ was 114. Theaverage non-verbal IQ of the SLI children was 99, and it seems probable that theirverbal skills were also lower than those of deaf children. The possibility cannot beexcluded that the differences in the model-copying task were simply a function oflanguage level and non-verbal ability.

Bishop (1982) noted that Cromer's theory predicted specific types of comprehensionproblem in SLI children. Given a sentence such as "the book on the table is brown"children should not make random sequencing errors, such as selecting a table on abook rather than a book on a table, because they can appreciate sequential order.They should, however, wrongly attribute the colour adjective to the nearest noun,i.e. the table. Bishop devised a set of items designed to test this hypothesis, an exampleof which is shown in Fig. 3.

26 D. V. M. Bishop

1 2

~

\

3

Fig. 3. Test item used by Bishop (1982) to assess comprehension of hierarchical relationships in complexsentences. The child must select the item corresponding to "the circle on the star is black."

All had the structure "The X inion/under/behind the Y is Z", where X and Ywere nouns and Z a colour term. Understanding of all nouns and colours was firstestablished in a pretest. Test items were presented in three modalities: spoken, writtenand signed using the Paget Gorman Sign System (a version of signed English). Resultswith nine children with severe receptive language problems gave support to Cromer'shypothesis that these children interpreted sentences sequentially without extractingdeeper hierarchical structure. Performance was very poor but errors were not random.Instead, irrespective of modality of presentation, there was a tendency to attach theadjective, Z, to the nearest noun, Y. Thus for the item in Fig. 3, they wo\;lld selectpicture 4 rather than the correct picture 2. This tendency was most marked whenthe test was given using written presentation, where the child did not have to rememberthe sentences.

However, when these same materials were given to profoundly deaf children, thesame pattern of performance was observed. Thus these results supported Cromer'shypothesis as a descriptive account of the nature of the grammatical problemsexperienced by children with receptive language disorders, but they challenged theview that these problems arose because of a primary impairment affecting the languageacquisition device. In ;ddition, Bishop noted that studies of written language ofhearing-impaired children carried out in the U.S.A. yielded findings that were


strikingly similar to those obtained by Cromer (1983) with SLI children. Quigley,Wilbur, Power, Montanelli and Steinkamp (1976) noted that deaf children tendedto impose a subject - verb - object pattern on sentences, and would connect the nearestnoun phrase and verb phrase, apparently treating English as if it had linear ratherthan hierarchical structure. No-one would maintain that deaf children are born withouta language acquisition device: their language problems arise because the main inputto the language processor is visual rather than auditory. Bishop concluded that thevisual system is not well suited to processing temporal information, so that merelypresenting an auditory language in visual form will not overcome the language-learningproblems of those who cannot perceive aural input adequately.

In sum, Cromer's account of defective hierarchical processing in SLI makes accuratepredictions about the types of expressive and receptive problems these childrenexperience with grammatical structure. However, it is unlikely that inability to analysehierarchical structure is the primary problem. Rather, it seems that this difficultyarises as a secondary consequence of auditory processing deficits that force childrento rely on the visual modality for language learning.

Semantic relations and the acquisition of argument structureRecent theories of language acquisition have focused on the ways in which meaning

relationships are encoded in grammatical structure. When we produce a sentence,we are describing relationships between properties, things, places and actions. Theverb is vital for expressing such relationships. Verb definitions may be regarded ascomposed of a small set of basic semantic categories (thing, event, state, place, path,property and manner) that form a scaffolding of grammatically relevant meaningto which are added specific pieces of conceptual information (Pinker, 1989).

Note that a verb does not simply specify a particular type of action. It also entails theexistence of other sentence elements. Thus, the verb to fall entails that there is a noUnacting in the role of theme, which is the grammatical subject of the verb. Other verbshave more complex entailments. Consider the following examples.

AcceptableJohn fell.John smashed the egg.John put the egg on the table.

UnacceptableJohn smashed.John put.John fell the egg.John put the egg.John put on the table

These examples illustrate the different argument structures of the verbs. Fall (anintransitive verb) has only a theme expressed as a subject, smash (a transitive verb)requires an agent and a theme, expressed grammatically as subject and object. Putentails an agent, theme and location, expressed grammatically as subject, object andoblique object.

Knowledge of verb argument structure is crucial for grammatical languageproduction. Suppose the child relied solely on abstract rules, such as one stating thatsubject-verb-object is a grammatical string. Such a rule would generate sentences

28 D. V. M. Bishop

such asJohnfell the egg orJohnput the egg. To be grammatical, a sentence must containthe obligatory arguments specified by the verb's argument structure.

It follows that if the child has not learned adequate verb definitions, thenungrammatical utterances will result. According to Pinker (1989), manyovergeneralization errors produced by normal young children arise for precisely thisreason. Utterances such as he get died, don't say me that are not the consequence of failureto learn grammatical rules, but errors in learning verb semantics. Of course, knowledgeof verb semantics is not sufficient for grammatical language, it is necessary also tomap argument structure on to grammatical functions. For instance, there is a rulemapping agents on to subjects and patients on to objects. According to Pinker (1989),normal children do not have difficulty learning these linking rules, which he suggestsare innate.

Pinker's theory has barely been applied to the study of normal language acquisition,let alone to children with disordered development. Nevertheless, it generates someinteresting predictions. Suppose that SLI children did indeed lack the innate componentof grammar: they might then have no knowledge of linking rules, but in other respectsbe cognitively normal. On this view the child with SLI might have perfectly adequatesemantic representations of verbs, but be unable to work out how to express theunderlying argument structure grammatically.

This hypothesis is of interest because it predicts that the grammatical errors observedin SLI children should differ from those seen in normal development. According toPinker, underspecified representations of verb meaning are the main cause ofovergeneralization errors in normal young children, and the progression to adultsentence constructions develops as verb meanings become more finely tuned.

One way of evaluating this hypothesis is to compare different ways that childrenlearn word meanings. Pinker proposed that the child first deduces information aboutargument structure from perceptual and conceptual analysis of the context in whichthe word is used. Suppose the adult produces a bear who is punching the air andmoving forward, and tells the child "this iskaboozling". A probable deduction wouldbe that kaboozle was an intransitive verb describing the action the bear performed.Contrast this with the situation where the bear performs an identical action, but thistime in doing so hits a giraffe and makes it move forward. Here the natural deductionwould be that kaboozle was a transitive verb describing the action of an agent (bear)on a patient (giraffe). Pinker argues that by using contextual information to deducethe meaning of words, the child can take the first steps to acquiring grammar. Thisis termed semantic bootstrapping. * If SLI children are unimpaired in their ability

to learn verb meanings and argument structure from context, then we would predictthat they should perform normally on tasks designed to test semantic bootstrapping.

There is, however, another route to language learning, Landau and Glietman (1985)noted that if the visual context was an important cue to meaning and structure, thencongenitally blind children should have major difficulties in language acquisition.They reported a case study demonstrating this need not be so. To explain how a

.The tenD "bootstrapping" is borrowed from computer science, which likened the process of startinga system from scratch to the operation of pulling oneself up by one's bootstraps.


blind child can learn to use verbs like "look" and "see" they proposed that, oncesome grammatical knowledge is available, a child may perform a syntactic analysison an input sentence containing an unfamiliar word, and deduce the meaning of theword from its syntactic characteristics. For instance, if an adult describing a cartoonto a blind child says: "Tom really walloped Jerry", the blind child can deduce thatthe word wallop refers to an action in which Tom was agent and Jerry was patient.In effect, the child uses linking rules in reverse, to work out argument structure onthe basis of syntactic structure, i.e. syntactic bootstrapping. If SLI children havedifficulty in using linking rules, then they should find it hard to learn language this way.

An early study relevant to this issue was carried out by Leonard et al. (1982) whocontrasted vocabulary learning in 14 SLI children and 14 normal control children.These children were exposed to novel words depicting objects or actions in a playsession. For instance, the adult would say "Here's the gourd" (object word) or "Watchthe baby kneel" (action word). Each word was produced five times in each of 10sessions, and the child's comprehension was tested at the end of each session withthe command "give me the. . . " or "make the baby. . . ". Comprehension and

production of the words was then tested in the final session. Leonard et al. (1982)were surprised to find that, overall, there were close similarities between SLI childrenand control children in comprehension and production of the experimental words.We know that SLI children do have weak vocabularies, and it had been anticipatedthat this study would reveal that they were slow at learning new words. Leonard (1989),however, pointed out that an analysis in terms of bootstrapping processes could clarifythese findings. He argued that the apparent ease of learning new words could bea consequence of the fact that the new words were presented in an inflectionally barecontext, e.g. "Here's the gourd"; "Watch the baby kneel", and thus the main factordetermining learning was the child's ability to perform a conceptual analysis of theobject or action from which a semantic representation could be formed, i.e. semanticbootstrapping. However, he suggested that in other situations where novel wordsare presented in a range of grammatical contexts, syntactic bootstrapping assumesimportance and slower lexical acquisition would be anticipated.

Van der Lely (1990) carried out a study designed to compare semantic and syntacticbootstrapping processes. Six children with SLI were compared with 17 youngerchildren who were matched on "language age". In the semantic bootstrapping task,the child was shown toys performing novel actions, accompanied by novel words,e.g. toy A jumps up and down on the back of toy B and the experimenter says "thisis voozing". The child's ability to infer grammatical relations was then tested byasking him or her to (i) describe the behaviour of new toys carrying out the sameactions; and (ii) act out sentences such as "the horse voozes the lion" or "the lionis voozed by the horse". In contrast, in the syntactic bootstrapping task no semanticinformation or contextual cues were provided. The child was simply asked to makeup a meaning for a new word, and to show this to the experimenter by acting outwhat he or she thought was meant by sentences such as "the lorry yols the car".Responses were scored in terms of the semantic relationships between the toys. Inthe example given, a child who appreciated the way in which syntactic structure usuallyencodes thematic roles should make the lorry perform some action on the car, i.e.lorry is agent and car is patient. Van der Lely (1990) found that SLI children did

30 D. V. M. Bishop

not differ from language-matched controls on the semantic bootstrapping task, butthey were significantly impaired on the syntactic bootstrapping task.

These results indicate that SLI children could form a semantic representation ofverb argument structure on the basis of contextual information. However, they seemedunable to deduce thematic roles from syntactic functions. It is possible that they lackedlinking rules altogether, and succeeded on the semantic bootstrapping task simplyby using response strategies based on their knowledge of thematic roles. Van derLely (1990) favoured an alternative explanation, which was that SLI children coulduse linking rules, but only in one direction: linking from a cognitive semanticrepresentation to a grammatical structure, but not vice versa. On either view, thefundamental deficit in SLI is grammatical rather than semantic. However, this primarygrammatical deficit will lead to semantic deficits, because the child is unable to usesyntactic bootstrapping to deduce word meanings.

As with the work conducted by Cromer (1978), it cannot be assumed that, becauseSLI children behave as if they are impaired in using linking rules, there is an innatedeficiency in a central language module. It is possible that inadequate auditoryperception could lead them to misperceive crucial grammatical features. An explanationin terms of primary auditory deficit would be strengthened if similar deficiencies onbootstrapping tasks were found in children with hearing loss.

"Feature blindness"Current linguistic theories have proposed that marking of grammatical features

on lexical items is an independent component of grammar. Syntactico-semanticfeatures, which mark information such as number, gender and animacy, have twomain effects on surface structure: they influence the morphological form of the feature-marked word, and they constrain the form of other items in the sentence. Thus inEnglish a plural noun is marked by a plural morpheme (/s/, Izl or hz/, dependingof the phonological form of the word). Plural marking on a noun constrains the formof the verb of which it is subject, and the form of preceding determiners (some, ortwo would be permissible preceding dogs but not dog, whereas a, one or this would beallowable before dog but not dogs).

Clahsen (1989) studied grammatical errors produced by German children with SLIand concluded that the grammatical problems of his subjects were confined tomorphology. Gender and number agreement in the noun phrase were often in error,and subject - verb agreement caused great difficulty. Verbs were restricted lar,gelyto uninflected stem forms, infinitive forms and those suffiXed with -to However, somekinds of verb morphology (e.g. rules for participles) were unimpaired. The childrenwere able to use word order to express thematic relations, but they were impairedat using morphological case markers for accusative, genitive and dative. Clahsenconcluded that the German children he studied had some ability to use grammaticalmorphology, but were selectively impaired in marking grammatical agreement.

Gopnik (1990a, b) and Gopnik & Crago (1991) also argued that the grammaticalimpairment in SLI is confined to grammatical morphology, but they proposed amore pervasive problem, which they termed "feature blindness", indicating a totalfailure to master syntactico-semantic features. Gopnik and Crago (1991) reported strikingdeficits shown by English-speaking individuals with SLI in using features such as


number, gender, animacy, mass/count distinction, proper names, tense and aspect.Extracts from notebooks kept by some of the subjects illustrate their problems withfeatures such as number ("All the children got present"), aspect ("Carol is cry inthe church' '), and proper names ("A Patrick is naughty' '). It was noted that whilemany utterances produced by SLI individuals may use such features in apparentlygrammatical fashion, they do not apply the features systematically to mark contrastsin meaning, and so may say "a boys", "a boy", "two boy" or "two boys". Ratherthan concluding that they have a shaky knowledge of the adult grammatical system,it was argued that it is more reasonable to say that their grammar lacks features. Aform such as final /s/ is regarded as a phonological variant with no associated meaning.Thus grammatically correct utterances are generated by the same defective grammaticalprocessor as the incor;rect ones.

To investigate mastery of the plural feature, Gopnik and Crago (1991)administered an experimental task in which subjects were shown a nonsense creatureand told, for example, "this is a zoop". They were then shown several such creaturesand asked: "These are. . . ?" It was found that subjects with SLI were poor at thistask and did not appear to have an internalized, unconscious set of rules for formingplurals.

Another test assessed ability to use tense features. A sentence was presented inone tense, and the subject then prompted to produce an analogous sentence in adifferent tense. For example, the tester would say, "Every day the man walks eightmiles. Yesterday he. . . (walked eight miles)". The SLI individuals gave semanticallyrelevant answers but seldom made appropriate changes to verb tense. They also haddifficulty with a related test in which sentences were given to elicit similar forms withdifferent morphological endings, e.g. "There is a lot of sun. It is very. . . (sunny)".

In another test, subjects were asked to judge whether a sentence was grammatical,and, if not, to correct it. The SLI subjects could judge which sentences were correct.Gopnik and Crago pointed out that this is to be expected as their grammar doesgenerate such sentences. However, they were poor at detecting ungrammaticalityin sentences that had errors in feature-marking, and if they did detect the error theyhad difficulty in correcting it.

Gopnik and Crago (1991) did not find any deficit in SLI subjects on comprehensiontests requiring understanding of such distinctions as reflexive/non-reflexive (" shewashes herself" vs "she washes her"), pronoun gender ("he holds him" vs "he holdsher"), passives and negatives ("the truck does not pull the car" vs "the truck is notpulled by the car") and reversible possessives ("the mother's baby" vs "the baby'smother"). They found that individuals whom they studied were able to produce somecomplex sentences such as "I know how to play basketball", could detect theungrammaticality of sentences that did not represent obligatory verb argumentstructure (e.g. "he puts"), and did not make such errors in their spontaneous speech.It was therefore concluded that their grammatical impairment was specific to syntactico-semantic features and did not affect mastery of thematic relations.

The feature-blindness hypothesis provides an interesting perspective on grammaticalproblems in SLI, but the current evidence produced in its support is sparse, and maybe open to alternative interpretation. Gopnik and Crago (1991) themselves found

32 D. V. M. Bishop

several aspects of feature-marking to which SLI subjects did appear to be sensitive.For instance, they performed correctly on a comprehension task that involvedresponding to commands such as "Point to the book" vs "Point to the books". Ifthey regarded the plural inflection as a meaningless phonological variant, performanceshould be at chance. This discrepant result was explained by suggesting that nounplurals such as "books" were learned as complete lexical items referring to groupsof objects, so would have a separate lexical representation, rather than being generatedby applying a rule of adding a plural marker to "book". A similar explanation wasproposed for the fmding that some past tense forms were produced correctly, by arguingthat each one is learned as a new item, but the child does not generalize to new forms.While this is an interesting possibility, it should be subjected to further testing. Thiscould be done by presenting SLI individuals with plural markers attached to novelforms, e.g. by showing a nonsense animal and saying "this a zoop", and thenpresenting a picture of a girl chasing a zoop and another of a girl chasing severalzoops, to see whether the subject could respond differentially to the instruction' 'thegirl chases the zoop" vs "the girl chases the zoops". Since the item "zoops" is novelit cannot have a pre-existing lexical representation~ and so if correct performancewere achieved, this would provide evidence that SLI individuals do have someawareness of the feature of number, even if they are poor at using it productively.

Gopnik and Crago (1991) make a bold claim that the deficit they have identifiedindicates that a separate, innate component of the grammar is absent in theseindividuals. If so, then the error pattern observed in SLI subjects should not be seenin the course of normal development. This prediction needs to be tested by comparingSLI subjects with younger normal children.

Another claim that requires more stringent testing is the assertion that the featureproblems shown by these subjects could not be explained in terms of an auditorydeficit of the kind proposed by Tallal and colleagues. Two lines of evidence havebeen put forward to support this statement. First, Gopnik (1990a) noted that the deficitsare apparent in spontaneous speech, grammaticality judgements, writing andrepetition, and she concluded' 'because the deficits are apparent in all aspects oflanguage their roots probably lie in the underlying grammar rather than in a peripheralprocessing system" (p. 715). The unsoundness of this argument is demonstrated byBishop's (1982) study, which found that peripheral hearing loss resulted in grammaticaldeficits in comprehension of written and signed language. Second Gopnik and Crago(1991) noted that SLI individuals do produce instances of phonological formsinvolving rapid acoustic transitions (e.g. past tense Id/), but sometimes omit otherforms that are acoustically salient, ~.g. subject pronouns. However, as is apparentfrom the studies of Leonard et at. (1987), a perceptual deficit theory can be adoptedwithout making the extreme prediction that children will omit all low phoneticsubstance morphemes; instead it can be argued that contrasts signalled by suchmorphemes will be especially difficult to learn, and this may have repercussions ondevelopment of the entire grammatical system. It would be most interesting to seehow children with mild to moderate hearing loss would perform on Gopnik's tasks.

There are major discrepancies between the feature-blindness theory and otherexplanations of grammatical impairments, not just at the theoretical level, but alsoin terms of the empirical data cited in support. Gopnik (1990b) described a bilingual

Specific language impairment 33

child with SLI who made similar feature errors in French and English, as would beexpected from her theory. However, this is in contrast to the findings of Leonardet at. (1987) of reliable differences in pattern of morphological errors between English-speaking and Italian children with SLI. In English, formal grammatical features suchas agreement with person are limited to the verb system, but in Italian there is genderagreement betwen nouns, possessive pronouns and adjectives. Furthermore, Italianverb morphology is more complex than that of English, with different verb inflectionsfor each of the six person-number combinations. The feature blindness hypothesiswould seem to predict that these features should pose far greater problems for childrenlearning Italian than for those learning English. However, the results (see Table 1)go against this prediction, with Italian children making fewer morphological errorsthan those learning English. Furthermore, in contrast to Clahsen's (1989) findingswith German children, Leonard et at. (1987) found that adjective gender agreementposed few problems for Italian SLI children. As noted above, Leonard (1989) favouredan explanation in terms of impaired morphological learning as a consequence ofperceptual limitations.

Another difference between data described by Leonard et at. (1988) and Gopnikand Crago (1991) concerns omission of obligatory elements of argument structure.Gopnik and Crago maintained that their subjects did not make such errors. However,Leonard et at. (1988) reported that English-speaking SLI children frequently omittedobligatory subjects, whereas in Italian (where the subject is not obligatory, so itsomission does not constitute an error) children were prone to omit obligatory objects.Such findings support the view that the syntactic impairments of SLI children extendbeyond grammatical morphology to encompass difficulties in mapping argu;~ents onto syntactic forms.

Yet another discrepancy with Gopnik's findings was noted in a recent study byLeonard, Bortolini, Caselli, McGregor and Sabbadini (1991) which extended thecorpus of comparative data on English and Italian-speaking SLI children. In thisstudy, attention was focused on incorrect use of grammatical morphemes, as whena child says "one cats" or "a Bernard was naughty". According to Gopnik and Crago(1991), the tendency of SLI children to produce such utterances indicates thatthey treat the morphemes as meaningless phonological variants. However, Leonardet at. (1991) found that SLI children were no more likely than normal children ofsimilar mean length of utterance (MLU) to produce grammatical morphemes ininappropriate contexts. Furthermore, SLI children sometimes produced over-regularizations, such as "drawed". Such examples are problematic for the feature-blindness hypothesis, because these forms could not have been acquired by rote learningand so provide direct evidence for the child's ability to identify morphological endingsand use them creatively.

Discrepancies in empirical findings are not just encountered when one considersdifferent languages: even within the same language quite different patterns ofimpairment have been described. In contrast to Clahsen (1989), Grimm and Weinert(1990) reported that the use of word order in German-speaking SLI childrenwas strikingly deviant, with many children producing utterances with the verb orsubject at the end of the sentence. In English, several researchers have reported resultsvery different from those ofGopnik. For instance, Bishop (1979, 1982) and Van der

34 D. V. M. Bishop

Lely and Harris (1990) found major deficits in comprehension of reversible passivesentences.

Such discrepancies in the data are bewildering for anyone attempting to proposea general theory for SLI. To some extent the differences may be a function of testprocedure. There are many pitfalls in language test design, and poor choice of itemscan lead to subjects making errors for the wrong reasons or performing correctlywithout real competence. Berndt and Caramazza (1980) provided a critique of testsof "grammatical comprehension", arguing that for many of them performance couldbe influenced by misperception of pictures, inadequate distractors, response biasesdetermined by factors intrinsic to test materials, and use of unfamiliar vocabulary.Furthermore, grammatical comprehension changes dramatically with age in normalindividuals, making it difficult to interpret the significance of errors unless there isadequate standardization across a wide age range. It is possible that Gopnik's failureto find deficits in comprehension of thematic relations in her SLI subjects arose becauseshe relied on a comprehension test devised for adult aphasics and its validity withchildren had not been established. In the absence of norms for children, she comparedgroups of affected and unaffected individuals covering a wide age range. It is likelythat the variance within either group would be large, making this an insensitiveprocedure for demonstrating group differences.

However, it seems probable that this is not the whole explanation and that someof the discrepancies reflect genuine differences between the individuals who werestudied. Gopnik's (1990a) subjects were members of a single extended family in whichthere was a dominant pattern of inheritance of SLI. Although there is mountingevidence for a genetic contribution to SLI (Bishop, 1987, 1991) such a clear-cutpedigree, where 50 % of offspring of affected individuals have the disorder, is unusual.The severity of the language impairment was probably greater in Gopnik's subjectsthan in the children participating in the cross-linguistic studies of Leonard and hiscolleagues, many of whom were below 6 years of age. As further children are studied,we may find that the population of SLI fractionates into different subgroups, somewith selective problems with syntac~ico-semantic feaures, and others with differentgrammatical impairments, such as inability to link argument structure to grammaticalfunction. It seems unlikely that the grammatical difficulties of this family are entirelyidiosyncratic, given that other cases have been described with similar problems (e.g.

Gopnik, 1990b).

Parameter settingIt was noted above that if children had a fundamental incapacity to process

grammatical features, then similar patterns of morphological error would be expectedacross different languages, whereas in fact there are cross-linguistic differences, asdocumented by Leonard et at. (1987, 1988, 1991). Loeb and Leonard (1988) consideredwhether parameter setting theory might illuminate the problems experienced by SLIchildren. This theory aims to account for the fact that a normal child can learn thegrammar of any language it is exposed to: therefore any language-learning mechanismpossessed by humans must be very general indeed (Chomsky, 1988). It is assumedthat the innate knowledge that the child brings to the language-learning situationis awareness of the types of information that can be encoded in grammar. The child


has knowledge that certain components of the grammar ("parameters") exist, butnothing is known about the way in which they are encoded in any given language(i.e. the "settings" of parameters). Thus the language-learning task involvesdetermining the correct setting of each parameter for the specific language that isbeing learned. Settings are not fixed for once and for all: the child will change settingsas language is acquired on the basis of evidence from the input language. One problemfor anyone attempting to apply parameter setting theory to SLI is that the theoryis still being developed and new parameters are being postulated. Loeb and Leonard(1988) used a version of the theory in which learning of verb morphology dependson the Null Subject parameter. All children start out with this parameter set to [+uniform], indicating (i) that the language permits sentences lacking an overt subjectand (ii) that verb morphology is uniform. For some languages, such as Italian, thissetting is appropriate. However, for other languages, such as English, this settingis not appropriate because overt subjects are required and verb morphology is notuniform, i.e. some verb forms require inflectional endings (e.g.jumping, jumped, jumps),whereas others do not (e.g. jump). While the initial setting is retained, the child willassume that sentence subjects are not obligatory and that English verb morphologyis uniform, lacking any complex forms (such as jumping, jumped, or jumps). Failureto reset this parameter has effects not just on those grammatical features specifiedby the parameter, but also on other features. Because the child is slow to master verbinflections, the learning of tense is delayed, and this in turn affects the learning ofauxiliary verbs.

Loeb and Leonard (1988) tested the hypothesis that SLI children might be unusuallyslow in setting the Null Subject parameter. They examined language samples fromeight English-speaking SLI children to see whether their grammatical deficits couldbe explained in terms of this hypothesis, but the data did not support this explanation.The SLI children they studied did, like others described in the literature, have limiteduse of verb inflections and auxiliaries and often failed to use grammatical subjects.However, these errors did not covary in the specific way predicted by the hypothesis.For instance, several children had begun to use modal auxiliaries, yet did not usethe tense inflections that are postulated as necessary to set the occasion for thesefeatures.

A particularly interesting point about parameter setting theory is that it makesdifferent predictions about the relative difficulty of learning grammatical features indifferent languages. For instance, learning of verb morphology should be easier inItalian than in English because the Null Subject parameter does not need changingfrom the unmarked setting in Italian. Leonard (1989) considered whether parametersetting theory could explain his cross-linguistic data on Italian and English SLIchildren. Once again, the pattern of performance did not conform to the theory.Although Italian SLI children had better mastery of verb inflections than childrenlearning English, they did nevertheless have difficulty with the third person pluralinflection.

Although parameter setting theory in its current form is inadequate to explain errorpatterns in children with SLI, it does suggest a new way of thinking about some ofthe counter-intuitive cross-linguistic differences observed in Leonard's study. Ratherthan hindering language learning, it may be that the complex morphological system

36 D. V. M. Bishop

of Italian facilitates the process of acquisition. Parameter setting theory proposes thatverb inflections are hard to learn in English, because only the third person singularverb is inflected. Since no other person - number combination carries an inflection,the child will tend to assume that verb forms are grammatically unmarked.Furthermore, in English the same inflection (-s) is used to represent different features(plural, possessive and third person singular), making learning more difficult. InItalian, however, the unmarked form of the verb does not occur, and the widevariations in verb endings could lead the child to pay more attention to grammaticalmorphemes.

Rom and Leonard (1990) went on to consider grammatical development in childrenlearning Hebrew, a richly inflected language that does not permit verbs with barestems. Finite verbs in present tense are always inflected for number and gender, andin past tense they are inflected for number, gender and person. These features aremarked by a combination of vocalic infixes and word-final affixes, as is illustratedby the example in Table 2. Noun plurals are formed by adding the stressed syllabic

Table 2. Past and present tense inflections of the Hebrew verb "go"

Singular Plural

Present

Masculine

Feminine

holxim

holxot

holex

holexet

Past

hal

hal

hal

hal,

hal

haIaxr

haIaxt

haIaxt

haIKu

haIKu

1st person

2nd person masculine

2nd person feminine

3rd person masculine

3rd person feminine

suffix -im for masculine and -at for feminine. Rom and Leonard (1990) found thatHebrew SLI children were more likely to produce present and past verb inflectionsand noun plural inflections than English SLI children of comparable language level.They were also more likely to use the definite article ha than English children wereto use the or a. Although these results could to some extent be accounted for by theperceptual salience of the morphemes, they are not entirely consistent with thatexplanation, as the Hebrew definite article ha is usually unstressed yet was frequentlyproduced by SLI children. Rom and Leonard (1990) concluded that the higherpercentage of morphological use by Hebrew SLI children was consistent with an account

axti

axta

axt

ax

xa

IU

em

en


in terms of morphological richness, as proposed to explain the data from Italianchildren. Thus the high frequency and obligatory nature of morphological informationin Hebrew tunes the child in to morphology in general.

Although this view is highly speculative, it has important implications for thoseinvolved in teaching children with language difficulties. People tend to assume thatlanguage learning will be facilitated if the child is exposed to grammatically simplifiedlanguage similar to the language that he or she is producing. Some educationalprogrammes (e.g. Waryas & Stremel-Campbell, 1978) explicitly encourage teachersto say "Doug eat cookie", rather than "Doug's eating the cookie". The work onEnglish/Italian differences by Leonard and colleagues suggests that this approach maybe counterproductive. The more syntax is made explicit by function words andinflections, the easier it seems to be for the child to learn, even if the input is morecomplex. The notion of syntactic bootstrapping is also relevant here. By performinga syntactic analysis the child can obtain clues as to word meaning. Inclusion ofdeterminers, function words and inflections signals which words are nouns and verbs,and make the structure of the input sentence clearer.

Evaluation: linguistic interpretations of SLILinguistic explanations of SLI have the undeniable advantage of plausibility.

Learnability theory has concluded on logical grounds that language learning wouldnot be possible unless humans possessed innate neurological modules specialized forthis process. Studies of adults with acquired aphasia have confirmed that focal brainlesions can impair specific components of the language function, leading to suchselective impairments as word-deafness, agrammatism or anomia. It is easy, thereforeto accept the idea that some children may have a congenital deficiency in this specialisedsystem which will result in selective language impairment. The main obstacles toaccepting this explanation concern the evidence advanced in its support, which isboth logically inadequate to make the point and inconsistent from one study to another.Few of those advancing linguistic explanations for SLI have considered alternativehypotheses. A minimal requirement would be to compare SLI children with youngernormal control children to demonstrate that language seen in SLI is deviant ratherthan merely delayed. It would then be necessary to show that SLI children differalso from children with language deficits arising from other causes (e.g. hearingimpairment, mental retardation). It may be that as more sophisticated analyses oflinguistic characteristics are adopted, we will obtain better evidence that SLI childrendo have quite distinctive problems with particular components, but to date the evidenceis lacking.

4. Explanation of SLI in Terms of Piagetian Theory

A very different type of explanation maintains that SLI is not a perceptual deficit,nor a primary language disorder, but a manifestation of a higher level conceptualdeficit not restricted to anyone modality.

It is usually assumed that because, by definition, SLI children perform much betteron non-verbal intelligence tests than on language tests, their underlying conceptual

38 D. V. M. Bishop

skills are intact. On this view, any problems they have with linguistic developmentarise because of difficulty in learning the verbal forms used to refer to concepts, ratherthan because there is any fundamental conceptual limitation. However, this viewhas been questioned by researchers working in the Piagetian tradition who havesuggested that the limitations of children with SLI can be explained in terms of failureto master a particular stage of representational thought.

Defective symbolic processingPiaget viewed language acquisition within the broader context of symbolic

development (see Piaget & Inhelder, 1969). Towards the end of the period ofsensorimotor development, the child begins to show the ability to use objects insymbolic fashion, to represent something else. For example, in pretend play, the childmight use a stick as if it were a spoon, or make a doll behave like a person. Theseearly symbols usually bear some physical similarity to their referents. As languagedevelops, the child extends this symbolic capacity to the use of signs, which havesocially shared meanings but are arbitrary, in that they bear no systematic resemblanceto the thing they signify.

Morehead and Ingram (1976) suggested that the language deficits ofSLI childrenreflected an underlying difficulty in using symbolic representations. One predictionfrom this hypothesis is that we should find deficits of symbolic behaviour in domainsother than language. Supporting evidence for this position comes from studiesdemonstrating that SLI children do engage in less symbolic play than normal childrenof the same age (Udwin & Yule, 1983; Terrell, Schwartz, Prelock & Messick, 1984).

This approach can be taken a little further by asking whether the problem is inthe formation of an abstract mental representation, or whether the difficulty is oneof attaching meaning to such a signifier when it does not r~semble what it signifies.Johnston and Ellis Weismer (1983) suggested that SLI children are impaired at usingimagery even when the task involves manipulating meaningless images. They studiedanticipatory imagery in SLI children using the mental rotation paradigm. The childwas presented with pairs of stimulus arrays composed of shapes in the same or differentsequence (see Fig 4). The task was to judge as quickly as possible whether the two

Fig. 4. Experimental stimuli used by Johnston and Ellis Weismer (1983). The child must judge whetherthe two arrays of shaDes are in the same linear order.


arrays were the same. The left-hand array was always vertical, but the right-handarray was either vertical or rotated about its centre at 45°,90° or 135°. Most peoplewho perform such a task have a subjective sensation of mentally rotating an imagein the head, and the usual finding is that the latency of response is dependent onthe degree of rotation. This task requires the child to generate and act upon a mentalrepresentation, but the representation that is formed is not only non-verbal but alsomeaningless, in that it does not stand for any object in the real world other than thepicture that the child observes. Johnston and Ellis Weismer (1983) found that SLIchildren were as accurate as control children on this task, and showed the normalrelationship between degree of rotation and reaction time. However, they respondedmuch slower overall (see Fig 5).

(/)"CC0UQ)(/)

0 45 90 135

degree of rotation

Fig. 5. Mean reaction time on mental rotation task for SLI and control children, showing relationshipin all groups between reaction time and angle of rotation. (Based on Johnston & Ellis Weismer, 1983).

Johnston and Ellis Weismer (1983) concluded that SLI children can invoke visualimages and transform these at the normal rate. If image transformation had beenslowed, steeper functions relating degree of rotation and reaction time would beexpected. However, they argued that the overall lag in responding by SLI childrenindicated that image generation, maintenance or interpretation were slowed. Thisconclusion, however, is questionable. An alternative explanation would be in termsof generally slow rate of responding in speeded tasks. Evidence for a slow rate ofprocessing has been found by Tallal, Stark and Mellits (1985) and Bishop andEdmundson (1987b) on tasks that involved no mental imagery. Johnston (1988)conducted a further analysis on the data from this study, using speed of responseduring training as a covariate, and found that the group differences persisted. She

40 D. V. M. Bishop

argued therefore that the slow responding by SLI children could not be interpretedin terms of a general lag in performance on all tasks. However, although theexperimental task did give SLI children disproportionate difficulty, it did not seemto be the imagery requirements of the task that were critical, because performancewas slow even in the baseline condition in which it was necessary only to comparetwo stimuli that were side by side in the same orientation. It may be that SLI childrenare not only generally slow, but also benefit less from practice than normal children,and so their speeds improved less between training and experimental trials. Johnston(personal communication) has noted one other difference between training andexperimental tasks that might prove crucial in explaining why SLI children weredisproportionately slow on the latter: in the experimental task, children needed toswitch from one condition to the next at random.

There are, however, other studies reporting symbolic deficits in SLI children, usingtasks where accuracy rather than speed was the dependent variable. Kamhi, Catts,Koenig and Lewis (1984) used a cross-modal matching task in which the child wasrequired to blindly feel a geometic shape and then point to a visual drawing of thecorresponding shape from an array of 15 stimuli. In agreement with a previous studyby Kamhi (1981), it was found that SLI children performed more poorly than controlchildren matched on age and non-verbal ability, and performance was strongly relatedto receptive vocabulary, as assessed by the Peabody Picture Vocabulary Test (Dunn,1965), while correlations with other language measures were non-significant. It wasproposed that this task required the child to generate and interpret a symbolicrepresentation of an unseen object, and the association with vocabulary arose becauseboth tests placed heavy demands on SLI chidren' s defective symbolic abilities. Inlater studies, Savich (1984) and Kamhi et at. (1988) went on to show impairmentsin SLI children on other untimed tasks involving visual symbolization, including theMinnesota Paper Form Board and a paper folding task based on work by Piaget andInhelder (1971). These results are supported by informal observations by Inhelder(1976) and de Ajuriaguerra et ai. (1976), who also remarked on impairment ofanticipatory imagery in SLI children.

Deficiency in concrete operationsAccording to Piaget, the young child, before the age of 7 years or so, is able to

form symbolic representations and manipulate these, but is still dominated by thesuperficial appearance of external reality, and so cannot, for instance, judge that twolines may contain an unequal number of counters, although they are the same length.It is only when the stage of concrete operations has been reached that the child cancarry out operations such as conservation, transitive inference, classification andseriation without being misled by immediate perceptual impressions. It has beensuggested that appreciation of certain late-developing grammatical notions dependson mastery of concrete operations; for example Beilin (1975) contended thatcomprehension of word order depends on appreciation of reversibility of cognitiveoperations. Could a delay in mastering concrete operations be a factor hindering fullgrammatical competence in SLI children? This notion seems attractive when weconsider that comprehension of word order is usually disproportionately poor in SLIchildren (Bishop, 1979; Van der Lely & Harris, 1990). However, testing of this


hypothesis is hampered by the fact that the classic Piagetian tests of concrete operationsinvolve fairly complex language, so SLI children might do poorly simply becausethey do not fully comprehend the questions.

Inhelder (1976) took care to avoid this problem by adapting tasks to be suitablefor children with limited language competence. She described the case of a 5-year-old child with SLI who was well able to solve a range of problems involving concreteoperations, provided these were presented to him using simple language that heunderstood. Interestingly, she found that to solve the problems the SLI child wouldresort to schemes of physical action which seemed to substitute for verbalization ofthe task, Inhelder (1976) used these findings to support the Piagetian view that concreteoperational thought is not dependent on language level, but her data also served tomake the point that underlying conceptual deficiencies do not appear to be theexplanation for language problems, at least in this child.

In 1981, two large scale studies were published, both aiming to evaluate concreteoperational thought in SLI children using non-verbal methods. Siegel, Lees, Allanand Bolton (1981) compared 26 SLI and 26 age-matched control children aged from3 to 5 years on understanding of concrete operations. Examples of the test items areshown in Fig. 6. A match-to-sample paradigm was used to assess one-to-one

~

. . .0000

ONE-TO-ONE CORRESPONDENCE

CONSERVATION

SERIATION

Fig. 6. Sample test items used by Siegel et at. (1981) to test understanding of Piagetian concepts inSLI children.

42 D. V. M. Bishop

correspondence and number conservation, with the child being asked to select fromtwo choices the card that contained the same number of dots as a sample card. Inthe seriation subtest, the task was to select the second largest line from any array.Kamhi's (1981) study included a number conservation task and a classificationtask in which the child was asked to sort geometric shapes according to size, shapeor colour. He compared 10 SLI children with 10 controls matched on mental age,and 10 younger children matched on MLU. Both studies used the Leiter InternationalPerformance Scale (Leiter, 1969) as the basis for matching control children on mentalage.

Siegel et at. (1981) reported that their SLI group had significantly more difficultythan age-matched controls with all three tests of concrete operations. Only two childrenpassed one-to-one correspondence, one passed conservation, and none passed seriation.In contrast, the percentages of control children passing these tasks ranged from 23 %to 46%. Kamhi (1981), however did not find any significant difference between SLIchildren and mental age-matched controls on classification or number conservation,although there were non-significant trends on all subtests for SLI children to do morepoorly than age-matched controls, but better than MLU-matched controls.

These discordant results are difficult to interpret. It could be argued that Kamhi's(1981) study lacked statistical power because of the small sample size, and thatdifferences between SLI and age-matched controls might have achieved significancehad a larger group been used. However, on several subtests, one or two SLI childrendid achieve the maximum score, suggesting that even if there is an overall delay inmastery of concrete operations, there is not a developmental barrier in SLI thatprevents certain stages being attained. Siegel et at. (1981) suggested that even wheredeficits were found in SLI children, these were more probably consequences ratherthan causes of language disorder, because on the more complex tasks language maybe important for holding information or place-marking in a sequence. It can be seen,for instance, that the tests of one-to-one correspondence and number conservationwould be made easier if the child adopted a strategy of counting.

It is interesting to compare these findings with results obtained by Furth andcolleagues in a series of studies with congenitally deaf people. Furth (1966) foundthat while deaf children performed below control levels on certain tests of concreteoperations, their performance was heavily dependent on how the tasks were presented,and some children showed dramatic improvements when given minimal training.Furth and Youniss (1975), summarizing this work, concluded that as far as concreteoperations were concerned, the intellectual development of deaf children wasremarkably normal, despite their limited oral language skills. Occasional failuresrelative to hearing controls were explicable in terms of limited experience.

Evaluation: Piagetian explanations of SLIOverall, there is little support for the view that SLI can be explained in the Piagetian

framework as a delay in mastering concrete operations. SLI children are usually ableto succeed on Piagetian tasks and any deficit they do show seems to be more a functionof specific task requirements, rather than the particular types of mental operationinvolved. Nevertheless, research in the Piagetian tradition has revealed one quiteunexpected source of difficulty for SLI children: they appear to be poor at usin~


anticipatory imagery, even where they are merely required to manipulate mentalimages of perceived objects, without having to associate them with specific meanings.The results from these studies are intriguing, but interpretation is complicated bythe finding of similar deficits in children with reading difficulties but no measurableimpairment on tests of spoken language (Kamhi et at. 1988). If symbolic deficits arethe cause of language problems, then we would expect to find language problemsin all children with such deficits. Furthermore, it is difficult to see why Kamhi et at.(1984) found that a non-verbal symbolic deficit was related only to receptive vocabularylevel without having more widespread ramifications for other aspects of language.Overall, there seems to be good evidence that impairments on tests of non-verbalimagery are a common correlate of SLI, but it remains to be demonstrated that suchdeficits reflect the underlying cause of language difficulties. Methodological approachesto the problem of disentangling causal relationships will be considered in more detailbelow.

5. Learning Strategies in SLI

Most of the theories of SLI considered so far suppose that there is an underlyingdeficit in the use of particular types of mental representation or cognitive operation.A different type of explanation suggests that the problem is more to do with the learningprocess than with what is learned.

Hypothesis-testing abilitiesWord meanings and grammatical constructions are usually learned indirectly rather

than by explicit instruction. Consider the problem confronting the child learning themeaning of the word' 'furniture": there is no unique or obvious set of physicalattributes that defines items in this category, yet most children learn to use the termappropriately to refer to chairs, tables, cupboards, etc., but not to other large inanimateobjects such as cars, nor to functional household appliances such as vacuum cleaners.Similarly, the child is not told that the morpheme -s indicates plurality, but mustrather deduce this from the language input. As noted above, acquisition theories suchas parameter setting theory regard the learning process as involving testing of successivehypotheses about grammatical structure. Kamhi et al. (1984) considered whetherlanguage difficulties in SLI children might be a consequence of general problemsin hypothesis-testing and concept formation. They used a concept formation task whichmay be regarded as a high artifical model of the problem confronting the child learningnew vocabulary. The child is shown exemplars of nonsense animals varying on fourdimensions, e.g. spotted or plain, with short or long legs, two or three ears, and curlyor pointed tail. The experimenter defines a concept in terms of specific features. Ina simple task, a single feature is crucial, e.g. all animals with curly tails are zonks.In a more complex task, a conjunction of features is critical, e.g. only those withtwo ears and a curly tail are wugs. The child's task is to decide whether each animalis a member of the category: thus in the simple task, each animal would be presentedand the child would be asked "is this a zonk?". Feedback was given after each trial,and the dependent measure was the number of trials to reach a criterion of eight

44 D. V. M. Bishop

consecutive correct responses. After the categorization of items from the training sethad been learned, children were tested on novel items varying on the same dimensionsto measure transfer of the concept to new materials. Kamhi et al. (1984) compared10 SLI children with 10 control children matched on age and non-verbal intelligence.The SLI children obtained significantly poorer scores on a test of receptive vocabulary .Kamhi et al. (1984) were surprised to find no difference between SLI and controlchildren on this task, with both groups performing at or near ceiling levels in thetransfer task

Kamhi et al. (1984) also used a discrimination learning task in which children wereshown pairs of jars, each with two coloured semicircles on the lid. They were toldthat one of the four colours used in the study was a special colour, and the jar withthe special colour always had a ball in it. On each trial the child was asked to findthe jar with the ball in it, and was then allowed to open both jars to check the accuracyof the choice. After each block of eight trials, the child was asked what the specialcolour was. SLI and control children did not differ in the number of trials to reachcriterion on this task.

In a further study of hypothesis testing, Kamhi, Nelson, Lee and Gholson (1985)used a discrimination learning paradigm, in which the child would be presented, forinstance, with a choice between a red circle on the left and a blue square on the right.The child selects the red circle and is told "Yes, that is correct". This feedback isconsistent with several hypotheses: the crucial feature may be colour (red), shape(circle), or position (left), or the rule may be a more complex one (e.g. alternate betweenred and blue). By examining the child's responses on subsequent trials where differentcolour/shape pairs are presented without feedback one can deduce which hypothesesthe child has formulated. Additional feedback is then given to see how capable the childis of modifying a hypothesis in the light of further information. Kamhi et al. (1985)found that SLI children performed as well as age-matched control children on this task.

Despite these negative findings, Nelson, Kamhi and Apel (1987) argued that theresults were inconclusive, because the tests included items that were too demandingfor the normal children in the study. They therefore conducted a further study usingan easier test in which the same materials and procedures were used as in the 1985study, except that the series of test trials was preceded by a set of four exemplar trials,in which the experimenter selected one of the pair of items, according to apredetermined rule. The variation on the procedure resulted in a marked improvementin the performance of control children relative to the previous experiment from 19 %correct to 81 % correct, but SLI children made smaller gains from 28% to ca 50%per cent correct. On this occasion the difference between groups was statisticallysignificant, even though they were carefully matched on mental age, using theColumbia Mental Maturity Scale (Burgemeister, Blum & Lorge, 1972). Nelson etal. (1987) favoured an explanation of these findings in terms of an encoding deficit,but alternative explanations (e.g. memory impairment, failure to learn throughmodelling vs personal experience) have not been ruled out.

Evaluation: learning strategies in SLIPerformance on hypothesis-testing tasks is heavily dependent on task demands.

As Nelson et al. (1987) noted. SL1 children are competent on performance 10 tests

4:5Specific language impairment

such as the Columbia Mental Maturity Scale, where the child must deduce rules suchas "odd one out", and recognize a range of non-verbal concepts. Hypothesis-testingper se does not seem to be deficient in SLI children, but they appear to have difficultyin learning underlying rules if the task requires them to encode and rememberinformation over time.

6. Limited Information Processsing Capacity as Explanation for SLI

Kirchner and Klatzky (1985) carried out an investigation of verbal memory inchildren with SLI, with the aim of pinpointing a specific component of the workingmemory system that was impaired. They were unsuccessful, however, findingimpairment in a whole range of different processes. They concluded that a deficitthat involved so many processes could be viewed in terms of a limited capacity modelof information processing (Shiffrin & Schneider, 1977). This model assumes that thehuman information processing system has limited cognitive resources that can beallocated to various activities. Overlearned tasks may become automatized so thatthey can be performed with little or no demand on resources. Non-automatic activitiesdemand resources and their performance will suffer if the system is overstretched.Kirchner and Klatzky (1985) suggested that SLI children have a reduced capacityfor verbal processing.

Other work, however, suggests that Kirchner and Klazky's (1985) explanatioQmaybe too narrow in restricting consideration to verbal processes. Wyke and Asso (1979)looked at memory for visuospatial relations in children with SLI. Children were showndisplays, such as that in Fig. 7, and allowed to study them for 3 seconds. The display

Fig. 7. Sample item used by Wyke and Asso (1979) to test memory for spatial relations. The childhas to place three targets in position on the grid to match a sample seen 8 seconds previously.

was then removed and after an 8-second interval the child was asked to reconstructit. SLI children were significantly impaired on this task relative to control childrenmatched on age and non-verbal ability. It could be shown that the problem was notwith the initial perception of the stimuli, because all children performed at ceilinglevel when asked to copy the display while it remained in view. Nor did it seem plausibleto argue that verbal encoding of stimulus configurations was used in the task, becausecontrol children were unable to describe the configurations in words, and theirperformance did not suffer when verbal encoding of arrays was prevented by requirinf?;

46 D. V. M. BishoD

children to count during the retention interval. This unexpected result suggests thatSLI children may suffer from some general limitation of information-processingcapacity not restricted to language.

Non-verbal reasoning and processing limitationsThis view of a deficit in a general purpose limited capacity system agrees with

suggestions by Kamhi et al. (1985) and Johnston (1991). Kamhi et al. (1985) arguedthat there was a pattern in the cognitive strengths and weaknesses shown by SLIchildren, and that the key variable was the information-processing demands of thetask: the greater the memory and representational demands of a task, the more childrenwith SLI will show a deficit relative to age-matched controls. Johnston and Smith(1989) conducted an experiment that lent support to this view. Children aged from3 to 5 years were taught to playa follow-the-leader game with two experimenters.On each trial a set of three toys was distributed to each participant. The firstexperimenter selected a toy or toys saying "I take this one" or "I take these two".The second experimenter then did the same, and the child was invited to "Do whatwe did. Take one/two of your toys and put them here". SLI children, like controlchildren, performed at near ceiling levels when required to select an item identicalto that picked by the adults (e.g. a green aeroplane). They also had no difficulty on"identity analogy" trials, where the task was to select the two identical items froman array of three (e.g. first adult selects two red cars, second adult selects two whitedaisies, child given a choice of two green cups and a yellow banana). However, theywere impaired on tasks involving dimensional thinking, such as those illustrated inFigs 8 and 9. Figure 8 illustrates the situation where matching was on the basis ofcolour. Analogous items were given to test ability to match on the basis of size (e.g.first adult selects small blue car, and second adult selects small red car). Figure 9illustrates a more abstract task that involved identifying the crucial dimension onwhich two items matched. In this example, colour is the critical dimension, andthe correct response is to select the two blue houses. Similar items were given to testability to extract size as the crucial dimension (e.g. first adult selects two big houses,second adult selects two small houses, correct response is for the child to select twomedium-sized houses).

Johnston and Smith (1989) compared performance on this non-verbal task witha similar verbal task, where the child was asked, for example, to "take the red one"or "take the two that are the same colour". On the basis of previous research onacquisition of conceptual skills, it might have been anticipated that children wouldshow understanding of size and colour concepts in the non-verbal task before theycould adequately comprehend the language used to refer to these concepts (Siegel,1978). However, the results went directly against this prediction. For control children,performances on the non-verbal dimensional tasks was identical to performanceon the verbal tasks. However, SLI children of similar mental age, performance onthe verbal tasks was superior to performance on the non-verbal tasks. The SLI childrencould respond appropriately when explicitly asked to pick' 'the big one" or "the twothat are the same size", but they showed a striking deficit with non-verbal tasksinvolving the dimension of size. This study, then, provided a nice demonstrationth~t thp ST.T rhilnrpn nin nn~~p~~ thp rnnrpnt11~1 -:rhpm~t~ nprp~~~rv tn ~nlvp thp


~ first adult

~second adult

child

Dyellow redblue

Fig. 8. Item illustrating follow-the leader task used by Johnston and Smith (1989) to assess dimensionalthinking. Each player has three items varying in colour and size. The child must deduce the rule for

selecting an item on the basis of the choices made by two adults.

48 D. V. M. Bishop

first adult

~~

second adult

~6ODD!DnD

child

~~~~I

Fig. 9. Item illustrating procedure used by Johnston and Smith (1989) to test ability to identify a crucialdimension on which two toys match.

problem. Why, then, did they not apply these in the non-verbal task? Johnston andSmith (1989) suggested that some kind of capacity limitation impeded their effectiveutilization of available knowledge. They suggested that size is more demanding ofcognitive capacity than colour because whereas colour is inherently nominal, sizeis an ordered dimension, and to categorize items by this attribute involves makingjudgements about relationships (A may be bigger than B but smaller than C) ratherthan about class membership.


Explaining language impairments in terms of information-processing limitationsImplicit in this account is the notion that the cognitive system can process individual

pieces of information in isolation, but has difficulty in performing operations involvingseveral pieces of information simultaneously. It can be seen that this would inevitablylead to widespread language deficits. We would expect impairment not only in learningof relational terms, but also in acquisition of syntax, which involves forming abstractconcepts such as subject and object. In addition, we might anticipate that childrenwould have difficulty in integrating different types of information in their utterances.Johnston, Smith and Box (1988) used a referential communication task in which thechild was shown a set of three toys and was asked to describe two of them to ablindfolded puppet. They found that SLI children aged from 3 to 5 years and controlchildren of similar mental age were largely successful on this task, giving messagesthat were communicatively adequate. However, the two groups did differ in thetypes of message they formulated. Normal children usually referred to thetwo target items using a single phrase (e.g. "two green ones") whereas the SLIchildren commonly used two phrases, one for each object (e.g. "a green big one anda green little one' ').. An intriguing point about these results is that, from a linguisticperspective, the utterances produced by the SLI children (two conjoined phrases)are more complex than those produced by the control children (a single phrase).Johnston et al. (1988) argued that this demonstrates underlying limitations in non-verbal cognitive processes. They suggested that the children were overloaded by theneed to notice similarities between items, make an explicit judgement about thecommon attribute, and consider the dimensional space for this attribute, judge thefoil object on this dimension, compare the targets with the foil, treat the targ<:'t objectsas members of a class, and specify the size of this set. Thus the number of reasoningsteps, their hierarchical structure and simultaneity are seen as influencing the languagethat is produced.

A similar explanation could be put forward to explain results obtained by Bishopand Adams (1991) in a study of 54 SLI children, aged from 8 to 12 years, and60 control children aged from 4 to 12 years. Children were presented with a set ofeight cards, representing all possible combinations of three binary variables (e.g. boyvs girl; red vs yellow bicycle; big vs little wheels). A star was affixed to the undersideof one card, and the child's task was to locate the card with the star when the adultwas not looking, and then describe this card so that the adult could find it. A crucialpart of the procedure was a post-test, in which the child was given pairs of itemsdiffering on just one of the critical dimensions and asked to describe how the two picturesdiffered. This made it possible to establish whether the child could percieve the crucialdifference and describe it (e.g. "that one's big and that one's little"). In fact, fewSLI children had any difficulty in describing the dimensions in this explicit situation.Nevertheless, on the main task where one card was described from an array of eight,SLI children did more poorly than age-matched controls. The processing limitationsproposed by Johnstonet al.(1988) provide a plausible explanation for this result. Tosucceed in this task, it is not enough for the child to know the critical words (e. g.red, boy and bz'g). First the array must be scanned to work out which three featurescritically differentiate the eight objects. All three features must then be kept in mindwhile the child formulates a message that describes the properties of the target item

50 D. V. M. Bishop

on those variables. These aspects of the task appear to be the crucial factor limitingperformance of SLI children.

These studies demonstrate that SLI children have difficulty in integrating differenttypes of information when formulating messages, even though they have the relevantvocabulary. Other studies suggest a similar integrative deficit in comprehension, wherechildren fail to construct a composite representation of different pieces of information,even though they may understand each individual piece. Tallal (1975) showed thatSLI children might have difficulty in comprehending a sentence, although thevocabulary and grammatical structure were kept very simple. She used the TokenTest (de Renzi & Vignolo, 1962), in which the child is confronted with an array oftokens varying in terms of colour (red, green, blue, yellow and white) and shape (circleor square). For some trials, size is also varied. The test proceeds through five subtests.In the first four subtests, complexity increases by making the instructions longer,moving from subtest 1 (e.g. "point to the red circle") up to subtest 4 (e.g. "pointto the small white square and the large white circle"). In subtest 5, grammaticalcomplexity is introduced with instructions such as "if there is a black circle, pickup the red square". Tallal (1975) found that subtest 4 gave SLI children greaterdifficulty than subtest 5, and that where errors occured on subtest 5, they involvedselecting the wrong token, rather than failing to comprehend the syntax.

Bishop (1979) obtained results that at first glance seem quite discrepant with Tallal's(1975) study. She found that sentence complexity was a more crucial factor thansentence length in determining children's errors on an early version of the Test forReception of Grammar. This version of the test included a subset of items termed"Word Combinations", in which the child had to select the picture depicting thecorrect combination of words, but word order and grammatical structure wereirrelevant. For example, for the sentence "the spotty cat sits under the tree", thechoice was between the correct picture, a plain cat under the tree, a spotty cat standingunder a tree, and a spotty cat sitting under a table. It had been established in a pretesth h h.ld d d h .. al d " """" ." d " t "t at t e c I un erstoo t e crItic wor s spotty, cat, SIt an ree, so

all that was required for correct performance was for the child to select the pictureshowing the conjuction of these four words. SLI children had little difficulty withitems of this kind. In contrast the same children made many errors on other sentencesof similar length such as "the boy is chased by the dog", where word order was crucial,the choice being between a dog chasing a boy and a boy chasing a dog. It was concludedthat grammatical complexity was more important than sentence length as a determinantof comprehension in SLI children.

These results appear to be in conflict, but when the information-processing demandsof the different tests are considered, they are easy to reconcile. There is now ampleevidence that one does not wait until the end of a sentence to start decoding themeaning. Rather, comprehension occurs "on-line", with a representation of meaningbeing constructed as words are heard (Marslen-Wilson & Tyler, 1980). On this view,processing load will depend not just on the number of words in a sentence, but alsoon the complexity of the parsing involved, and the rate of presentation. If the parsingprocess is inefficient, then the parser will not keep up with the incoming message.Although both tests used long, grammatically simple sentences, viewed in terms ofon-line parsing, the test items are of quite different levels of complexity. Bishop's


(1979) long sentences required the child to retain at most three or four crucial words,and word order was irrelevant. Furthermore, the sentence could be repeated if thechild requested it. In the Token Test sub test 4, however, the input must be parsedinto noun phrases each involving two adjectives and a noun, so six critical wordshave to be processed, with serial order assuming crucial importance as any confusionabout which noun phrase the adjectives were assigned to would lead to error. Theresults of both Tallal (1975) and Bishop (1979) are compatible with an interpretationthat maintains that for SLI children information-processing capacity is exceeded bynormal rates of speech and so interpretive processes cannot keep up with verbal input.

A similar type of explanatiqn can be proposed for higher level comprehensiondeficits. For many years, comprehension tests were largely restricted to instrumentssuch as those we have just considered in which the child is either asked to act outcommands or to select a picture to match a sentence. Real life comprehension is moredemanding in that is usually involves keeping track of a series of pieces of informationand engaging in constructive processing to integrate these. We understand far morethan is literally stated, making bridging inferences based on general knowledge tofill in details that are not mentioned. For instance, if we hear: "John was at the beach.He stepped on some broken glass. He had to go to hospital.", we infer that Johncut his foot, although we have not been told this.

Ellis Weismer (1985) investigated story comprehension in SLI children. Subjectswere presented with three-item verbal or pictorial sequences and then asked questionsabout these, some of which required them to make an inference. Ellis Weismer (1985)found that SLI children were poor on this task relative to age-matched controls, butthe deficit was apparent for pictorially as well as for verbally presented stories. Bishopand Adams (in press) conducted a similar study, using longer and more complexstory sequences. Results agreed broadly with those obtained by Ellis Weismer (1985).SLI children were impaired on story comprehension, even when stories were presentedvisually as a sequence of pictures. These studies, then, provide evidence that SLIchildren have difficulty in integrating sequentially presented information, and theseprocessing limitations extend beyond situations where language is overtly implicated.

We have considered so far how the information-processing load of a specific taskcan affect performance. A limited capacity model makes a further prediction thatthere should be interference between different tasks if they both consume attentionalresources. Some evidence supporting this prediction comes from studies of relationshipsbetween syntax and phonology in SLI children. Panagos and Prelock.( 1982) notedthat phonological errors increased with syntactic complexity of utterances, and theyshowed that syntactic errors were more numerous when polysyllabic words were usedin a sentence repetition task. This kind of trade-off would be expected if neitherphonology nor syntax had become automatized and there were limitations on centralprocessing capacity. In a paper that was well ahead of its time, Manning, Keappockand Stick (1976) demonstrated that some children whose phonological problemsappeared to have resolved started to make phonological errors when exposed to auditorymasking. They argued that these children had not automatized correct articulatoryproduction, and they found that this simple test of automatization was effective inpredicting which children would do well without further speech therapy. The theoreticalapproach of Shiffrin and Schneider (1977) provides a framework for understanding

52 D. V. M. Bishop

results of this kind, where the same child may demonstrate impaired or normal ability,depending on task demands.

Evaluation: rate and capacity limitations in processing by children with SLIA theory in terms of limited information-processing capacity is attractive because

it predicts that SLI children's difficulties will depend not on the type of representationor mental operation implicated in a task, so much as on the amount of material tobe integrated and the time available to perform such integration. However, the veryfactor that makes this type of model attractive, namely its wide scope of application,is also potentially problematic because unless our models of cognitive processes areclearly specified, it becomes all too easy to explain almost any pattern of results interms of capacity limitations.

What is now needed is a theoretical specification that is tight enough to yield newpredictions about performances of SLI children, rather than just acting as a post hocexplanation for observed phenomena. A limited capacity explanation of SLI can beinterpreted in the light of two contemporary models of information processing. Onthe one hand we have Baddeley's (1986) working memory model, which includesa central executive that is implicated in tasks that involve simultaneous retention andmanipulation of representations, such as logical reasoning or comprehension tasks.On the other hand, we have the related ideas derived from Shiffrin and Schneider's(1977) account of cognitive resources being divided between competing demands.These models may prove fruitful in suggesting novel approaches to tackling theinvestigation of capacity limitations in SLI. In particular, it would be of interest toadopt dual task methodology (see below).

Perhaps the greatest challenge for a model in terms of capacity limitations is thatit must be able to account for the cognitive strengths of SLI children as well as theirweaknesses. Why are SLI children able to perform normally in certain non-verbaldomains, such as on performance IQ tests and on some of Kamhi's hypothesis-testingtasks, which also make demands on information processing capacity? A speculativeanswer is that SLI children will be impaired on tasks that require them to operateon mental representations that are processed sequentially rather than simultaneously.Proficiency on tasks such as the block design subtest of the WISC might be posssiblebecause all the necessary information for solving the task is present simultaneouslyand the individual blocks can be mentally manipulated into a single spatialrepresentation. Where the crucial information is transient, or where a transientrepresentation must be held in mind while another representation is formed, then thechild will experience difficulties. Note that some tasks require formation of transientrepresentations, even though the test materials may be continuously present: forinstance in the non-verbal reasoning task used by Johnston and Smith (1989) (seeFigs 8 and 9) the test materials are continuously present, but in order to performcorrectly, the child must first form a representation of the critical relationship betweentwo stimuli and then keep this is mind while analysing another array.

Future Directions for Research

The initial impression striking anyone reviewing the literature on underlyingprocesses in SLI is one of total confusion. Every investigator has a different explanation,


and each advances evidence that seems to support their viewpoint. How are we toprogress beyond this chaos? To some extent, different approaches may be reconciledby adopting new theoretical perspectives, such as limited capacity models that canexplain why a child may pass one test and fail on another, even though both appearto involve the same cognitive operations. In addition, there are two methodologicalproblems that need to be addressed: (i) dealing with heterogeneity of SLI; and (ii) adoptingmethods that make it possible to distinguish cause and effect in correlational studies.

The need to consider heterogeneity of SLIThe first problem concerns the definition of SLI itself. Traditionally this has been

a disorder defined by exclusion: the child had significant language difficulties thatare not explicable in terms of hearing loss, low intelligence, abnormal environmentor physical handicap. A superficial review of the research literature might lead tothe conclusion that SLI children will be impaired relative to age-matched control groupon virtually any task with a verbal component. Acquisition of phonology is delayed,utterances are short and simple and include grammatical errors, vocabulary is limited,and the ability to adapt language use to specific contexts is poor. However, the pictureof a pervasive impairment affecting allaspec~s of language development looks lessplausible when one moves away from group data to consider performance in individualchildren, who may show quite uneven profIles of language ability and disability. Thedefinition of SLI encompasses on the one hand the child who understands virtuallyno spoken language, who produces only short and largely unintelligible utterances,and on the other hand the child who chatters profusely in long and clearly articulatedutterances, but who seldom responds appropriately to adult attempts to initiateconversation. It also includes children who seem to understand most of what is saidto them, but are limited in their ability to form any but the simplest utterances, aswell as those who are able to talk clearly in single words, but become increasinglyunintelligible as the length of their utterances increases. Given the diverse range oflinguistic manifestations that are encompassed under this diagnostic label, it maybe unrealistic to look for a single underlying factor that can explain all cases of SLI.

Attempts have been made to use statistical methods to identify subtypes of SLI.These have been unsatisfactory for two reasons. First, the multivariate analyticaltechniques used for this purpose require that the sample size greatly exceeds the numberof variables entered into the analysis if reliable results are to be obtained. Everitt(1975) recommended that ideally there should be 10 times as many subjects as variables,and he argued that any study where the ratio of subject to variables is less than 5to 1 should be viewed with scepticism. Evaluated with regard to this criterion, studiessuch as those of Aram and Nation (1975) (47 subjects with scores on 14 variables)or Wolfus, Moscovitch and Kinsbourne (1980) (19 subjects with scores on 16 variables)are woefully inadequate. A second point is that a statistically derived classificationis only as good as the data entered into it. For instance, there is no possibility ofidentifying a subgroup of children with predominantly pragmatic difficulties unlesssome measure of pragmatic function is used. If crucial measurements are excluded,one cannot expect to be any more successful in identifying subtypes than one wouldbe if an attempt was made to derive a classification of mammals on the basis ofmeasurements of weight, girth and speed.

54 D. V. M. Bishop

Wilson and Risucci (1986) suggested that it is preferable to derive an initialclassifactory framework from the basis of clinical observations and then attempt tovalidate the derived classification using more objective means. Clinical inference isused to generate a theoretically coherent typology which can be regarded as a hypothesisthat then needs to be tested using a suitably large sample. Some preliminary evidenceof validity may be provided where similar classifications are proposed by differentresearchers working independently (e.g. Rapin & Allen, 1983; Bishop & Rosenbloom,1987). However, it is then necessary to go further to demonstrate that the proposedsubgroups are distinct both from one another and from other disorders. In principle,this approach has the advantage that one can devise dependent measures that arespecifically designed to capture clinically important differences between postulatedsubtypes, and then enter these into a statistical classificatory analysis. However, todate, classifications such as Rapin and Allen's (1983) have not yet been validatedstatistically, and attempts to devise measures that will clearly discriminate betweensubtypes of language impairment that have been described clinically have met withlimited success (Adams & Bishop, 1989; Bishop & Adams, 1989, 1991, in press).

Heterogeneity of SLI should not discourage us from conducting studies using groupsof children, but it should influence the way we analyse our data. It is not adequatemerely to compare group means and to assume that the mean is representative ofall individuals in the group. We need also to examine the extent of variability withinthe group, and, where appropriate, to attempt to relate this to explanatory factors.In some cases we may find that variability within the SLI group can be accountedfor in terms of variables such as age, personality and intelligence of the child, oreducational experience. However, in other cases we may find clear associations betweenlanguage profile and experimental measures that suggest we are dealing withqualitatively different subgroups; the study by Frumkin and Rapin (1980), describedabove, is a good example of this type of approach.

One source of heterogeneity that is often overlooked is variation associated withdevelopmental change. The clinical picture of SLI can change quite markedly aschildren grow older. Furthermore, many young children who present with SLI inthe preschool years do appear to grow out of their difficulties (Bishop & Edmundson,1987a). The population of SLI children from which research samples is drawn is acontracting one, and the older the sample, the more children with severe and persistentdisorders will be included. For too long, research on SLI has ignored the importanceof age as a factor determining results, yet it is probably an important variable inaccounting for some of the conflicting results in this field (see e.g. the discussion ofthe "feature-blindness" hypothesis, above).

Distinguishing cause and effect in correlational studiesSuppose it is postulated that the underlying basis of SLI is a specific deficit in

performing a particular type of mental operation, that may be designated as X. Xmight be auditory discrimination, short-term memory, sequencing, symbolic thoughtetc. The most obvious way of testing this hypothesis is to compare SLI children witha normal control group of similar age and non-verbal ability on a task that providesa measure of the postulated deficit, mx. Studies of this kind are frequently used toprovide supporting evidence: a significant association is shown between presence of


SLI and a deficit in mx, and it is concluded that impairment in X has caused thelanguage difficulties. However, although this conclusion is compatible with the data,it is not the only possible explanation, as can be seen from Fig. 10. Model A illustrates

model A

model B

model C

Fig. 10. Alternative models of causal relationships to explain associations between language impairtnentand measures of specific cognitive functions (mx). Latent variables representing unobservable constructsare shown as circles, and measurable variables as rectangles. A primary deficit is represented by shading.

Any variable linked to a shaded variable by an arrow from that variable will show impairment.

the hypothesis under test, where a deficit in X leads to impaired language and a poorscore on mx. Model B shows the situation where X is not implicated in causing SLI,which is determined by a deficit in Y, but language level affects performance on mx.Thus the deficit in mx reflects a consequence rather than a cause of SLI. Model Cpostulates deficits in two underlying processes, X and Y, which independently leadto deficits in mx and language functions. Although such a model is less parsimoniousthan Models A or B. it is not inherentlv implausible when we consider that the

56 D. V. M. Bishop

neurological abnormality responsible for SLI might extend across several brain regionsmediating different cognitive processes. On this view, there is no direct causalrelationship between mx and language functioning.

A practical illustration of these models is provided by the study by Kamhi et at. J(1984) demonstrating a significant correlation between cross-modal tactile - visualshape-matching and receptive vocabulary in SLI children. Model A (Fig. 11)corresponds to the explanation favoured by Kamhi et at. (1984). On this view theprimary deficit X is in symbolic representation, and this leads to impaired performanceon both cross-modal matching mx and receptive vocabulary, a measure of languagefunctioning. Alternative explanations, however, are possible. Model B postulates aprimary deficit in some other domain as the cause of receptive vocabulary deficit.

model A

model B

model C

Fig. 11. Alternative explanatory models of association between language impairment and cross-modalmatching performance found by Kamhi et at. (1984).

57Spcci/ic languagc impairmcnt

Weak vocabulary may lead to poor cross-modal performance because the child is lesslikely to adopt a strategy of verbal labelling to encode the shapes. No primary symbolicimpairment is postulated. The nature of the primary deficit in model B remainsunspecified, but there is no shortage of candidates. It could be proposed, for instance,'that an auditory processing deficit was "responsible for retarding vocabularydevelopment. Model C, on the other hand, assumes that there are two underlyingdeficits, one impairing performance on sensorimotor tasks and the other leading tolanguage difficult.ies. On this view it is the proprioceptive aspect of the task, ratherthan its representational demands, that leads to poor scores on cross-modal matching(mx). This model is consistent with other studies demonstrating sensorimotor deficitsin SLI children in tasks that can be regarded as neither symbolic not linguistic, e.g.speed of moving pegs on a pegboard (Bishop & Edmundson, 1987b) or fingerlocalization (Stark & Tallal, 1981).

Correlational studies are clearly inadequate to distinguish between differentexplanations. Does this mean that they should therefore be abandoned? The answeris no, for two reasons. First, they provide a simple means for rejecting theories whenthe anticipated association between cognitive deficit and language impairment is notfound (as for example, in the studies by Kamhi et ai., 1984, 1985, demonstratingnonnai hypotheses-testing skills in SLI children). Second, where impairments are found,this can provide useful practical information for those concerned with remediation,even if the underlying casual relationships are ambiguous. Consider for exampleGopnik's (1990a) demonstration that individuals with SLI. were unable to processcertain grammatical features. Gopnik maintained that this deficit indicated lack ofan innate language~processing module. Presumably she would explain any associatednon-verbal deficits in these individuals as either secondary"'consequences of languageimpairment (model B), or as associated deficits no.t c;:aSually rela~ed to language disorder(model C). However, her data do not allow us to. rule out the possibility that thegrammatical deficits might themselves be secondary consequences of an auditoryprocessing impairment (i.e. model B, where X corresponds to the innate language-processing module, mx is a measure of use of grammatical features, and Y is auditoryperception). If our interest is in arriving at a theoretical understanding of SLI, wemust adopt other research designs to disentangle the complex web of causalrelationships. However, those working with the child might argue that the underlyingcausal relationships are unimportant and that the crucial thing is to know what thechild can and cannot do so that appropriate remediation can be implemented. Thecorrelational method is quite adequate for this purpose.

Language-matched control group. This is the commonest method used to distinguishdifferent casual relationships. Essentially, it aims to rule out explanations of the kindshown in Model B (Fig. 10), where a deficit on a specific task is. a secondaryconsequence of language impairment rather than an indication on the underlyingcause. The logic is as follows: if poor performance on mx is a consequence of thelow level of language skill, then any child with a comparable language level shouldshow an equivalent performance on mx. Thus the same relationship between mx andlanguage level is predicted for SLI children, and for younger children with equivalent

58 D. V. M. Bishop

language skill who are developing normally. If it is found that the language-matchedcontrol group performs better on mx thaI;} the SLI group, than language level isshown to be dissociated from mx, demonstrating that limited language skills cannotbe the explanation for the poor performance on mx in the SLI group.

The language-matched control group has become a popular method in studies ofSLI children, but it has drawbacks that are seldom appreciated. It is an inefficientmethod in so far as a finding of no difference on mx between the SLI group andthe language-matched control group is relatively uninformative. The problem is clearlystated by Goswami and Bryant (1990) in their discussion of the use of reading-agematch designs in the study of reading problems, where the same methodological issuesarise. "In these comparisons the poor readers are more advanced intellectually -have a higher mental age - than normal readers. This may make it possible forthe poor readers to conceal a genuine deficit. They may have real difficulties withthe ski11~that the task is designed to test, but end up doing it as well as the other children,because they understand the instructions better or because they know how to attendbetter" (p. 83). A finding of no difference between groups does not allow us to ruleout model A, but it is also compatible with B, C or more complex models.

Another difficulty inherent in this design is that implicit in the idea of a language.matched control group is the notion that the language ability of 'an SLI child canbe equated with that of a younger normally developing child. Matching is most oftendone of the basis ofM~U. Howevcr, the profile of language abilities in SLI childrenis frequently very uneven, so a "different matched control group would be obtainedif another language meas'.1re .wire used (see Bishop & Adams, 1991). One approachthat avoids this difficulty involves measuring a range of language functions in a youngercontrol group. One can s~e how each language function relates to mx and use analysisof covariance to compare SLI and control groups after adjusting for language levelon one or more measures.

Comparison with other disorders. If a deficit in X is postulated as the cause of SLI,then it follows that any child who has this deficiency should have poor language skills.It is therefore interesting to consider whether there is any group of children who areknown to be deficient in X, who may act as a contrast group for SLI children. Totake one example, it might be argued that limited vocabulary in SLI children is theconsequence of their expressive phonological difficulti~s, which limit the amount ofspeech they produce. If restricted expressive speech does indeed cause vocabularylimitations, then similar limitations should be found in any child with restricted speech,whatever the cause. To test this hypothesis receptive vocabulary development couldbe studied in children who found it physically difficult"to speak (see Bishop el a/., 1990).

A second example is provided by case studies of mentally handicapped personswho have remarkably good language abilities despite performing at the preoperationallevel on Piagetian tests (Yamada, 1988; Bellugi, Marks, Bihrle & Sabo, 1988). Itfailure to master concrete operations limits language development, then we shouldnot expect to find normal language development in children with limited intellectualattainments. Such cases have been used to argue that language d~velopment ismediated by a separate module, independent of other cognitive processes. While

59Specific language imp.airmcnt

intriguing, such evidence must be treated with caution. In virtually all these cases,the evidence for good language skills is based mainly on analysis of spontaneous speech.On formal language tests performance is severely impaired. Two interpretations ean'be offered. One possibility is that the impression of "good" language is misleadingand these individuals are merely parroting language heard from others rather thangenerating it creatively. If so, it might reasopably be argued that the evidence doesnot invalidate the Piagetian position that language depends on underlying cognitivedevelopment because true language skill is lacking. The alternative view is that theseindividuals do have genuinely good language skills but language tests do not accuratelyindicate their level of proficiency because other factors (e.g. poor attention, lack ofmotivation) intervene to impair performance. However, on the basis of what is saucefor the goose is sauce for the gander, it could be argued that the same is true forthe Piagetian tests: these too do not accurately reflect the true level of cognitivecompetence of the individual. Although the interpretation of the evidence might bequestioned, these studies are of great potential interest for those working on SLI becausethey provide a way of assessing how far a given cognitive deficit can be a plausibleexplanation for poor language development.

Of particular interest for evaluation of theories that attribute SLI to auditoryperceptual impairments are comparisons with hearing-impaired children. The valueof this approach was demonstrated many years ago in work by Furth (1966), whowas able to show that congenital deafness can influence performance on certain non-verbal tasks that use visual presentation. More recently, Bishop (1982) showed thatpatterns of abnormal comprehension that were thought to be specific to children withSLI could also be observed in hearing-impaired childreq, eyen when written or signedmeans of presentation were used. Preliminary stu~ies of Dutch-speaking childrensuggest that some of the expressive syntactiG, deficits. that were thought to becharacteristic of SLI can also be seen in children with moderate levels of hearing loss(Bol & Kuiken, 1990). It would be wrong to assume that the auditory deficit hypothesispredicts exact parallels between hearing-impaired and SLI children. The effectivelanguage input received by a child with a sensory loss will be different from that ofa child who has some kind of higher level perceptual disturbance. Nevertheless, bystudying the sequelae of hearing loss we can obtain useful information about the rangeof consequences that can arise as a result of inability to hear language adequately.

Longitudinal study. A longitudinal study provides the opportunity to see whichcognitive processes at one point in time predict language status later on. In principle,such a study can suggest very different answers to theoretical questions than a cross-sectional study. This is nicely illustrated by the follow-~p study of SLI childrenconducted by Bernstein and Stark (1985). Children who previously had shown clearevidence of auditory perceptual impairments no longer did so whe~ followed up4 years later. Had the children been studied only at time 2, without the earlier databeing available, it might well have been concluded th~t perceptual factors play norole in the causation of SLI. The longitudinal data tell a different story, however,and suggest that auditory perceptual impairment early in development may disruptlanguage acquisition permanently, even if the perceptual problem resolves over time.

60 D. V. M. Bishop

i~

Dual task methodology. The notion of a limitation on processing capacity is not capturedby any of the models shown in Fig. 10. This hypothesis would not represent theunderlying deficit as specific to one kind of mental operation: rather it proposes thatthe resources available for performing mental operations are limited, so problemswill arise if the system is overloaded. This type of theoretical approach has alreadybeen applied to good effect in the study of developmental dyslexia by Nicholson andFawcett (1990), who used a dual task paradigm in which children were required tobalance on a beam while counting backwards. They found that children with specificreading problems could perform either task in isolation but were dramatically impairedwhen they attempted to do both tasks together. Their conclusion was that these childrenhad a generalized failure to automatize new skills.

If SLI children do have limited information-processing capacity then is should bepossible to show that their performance suffered disproportionately when they wereasked to perform a secondary task while carrying out an activity that demandedattentional resources.

Conclusions

Perhaps the most notable theoretical development in this field over the past decadehas been the formulation of hypotheses that attempt to explain SLI as a primarylinguistic deficit. On this view, ,.the '-language difficulties that are observed are notsecondary to some other: cogp.itj;tfe limitation, but rather reflect maldevelopment ofa module specialized forJariguage processing. This line of theorizing has ledinvestigators to focus on the language impairment itself, rather than concentratingon related deficits. Cross-linguistic studies have been particularly fruitful in throwingnew light on the nature of language disturbances. However, those favouring linguisticexplanations of SLI still have a long way to go to make a convincing case. Whereappropriate controls are adopted it is generally found that children with SLI do notdiffer markedly from children with hearing impairments, indicating that quitedistinctive patterns of linguistic impairment can arise because of auditory perceptuallimitations. What is now needed is research that combines the linguistic sophisticationof studies conducted on SLI children with the more powerful methodology offeredby a design that contrasts children with different type~ of disorder.

The:auditory perceptual deficit hypothesis has been rigorously tested over the pasttwo decades and has withstood this scrutiny remarkably well. There is compellingevidence that most SLI children do have subtle auditory perceptual problems andthese provide a plausible explanation for many of the problems seen with syntax andphonology. The major difficulty confronting the auditory deficit hypothesis is notthat of explaining language problems but its inability to account for the non-linguisticimpairments associated with SLI.

Most children with SLI do show evidence of broader cognitive impairments. Therather vague concept of "limited processing capacity" has proved useful in providinga common framework in which to interpret the wide range of deficits seen in SLI.

It is possible to integrate the auditory deficit hypothesis with the processing limitationhypothesis by ar~in~ that the fundamental deficit in SLI is a slowerl r~te of

61Specific language impairmcnt

..

information-processing, that leads to impairment in any task requiring integrationof. rapidly presented information. On this view, processing of auditory infqrmationwill normally be much more impaired than processsing of visual information becauseauditory stimuli are typically brief and sequential, whereas: visual stimuli usual persist.Language development will suffer disproportionately relative to other aspects of mentaldevelopment be,cause language entails rapid"processing of auditory information. Onthis view, the underlying deficit is not restricted to auditory perception, but it hasdisproportionately severe consequences for the processing of auditory stimuli comparedto those in other modalities.

This speculative attempt to integrate the available evidence is unlikely to explainadequately all types of language difficulty seen in children, but it is presented asthe most promising general account that is compatible with current researchfindings.

Acknowledgement-l would like to thank Heather Van der Lely for comments on an earlier draft of thismanuscript.

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Underlying nature of specific language impairment

Education

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