A Statistical Basis for Speech Sound...

A Statistical Basis for Speech SoundDiscrimination

Jennifer L Anderson James L Morganand Katherine S WhiteBrown University

J L Anderson J L Morgan and K S White 155

Abstract

Infants under six months are able to discriminate native and non-native con-sonant contrasts equally well but as they learn the phonological systemsof their native language this ability declines Current explanations of thisphenomenon agree that the decline in discrimination ability is linked to theformation of native-language phonemic categories The goal of this studywas to evaluate the role of input statistics in learning these categories ourhypothesis was that relative frequency is a determinant of the relative orderin which categories are acquired English-learning infants of two age groups(65 months and 85 months) were tested on their ability to discriminatenon-native consonant contrasts using the Conditioned Head Turn ProcedureAs predicted older infants were worse in their performance on the morefrequent coronal stop contrast than on the less frequent dorsal stop contrastIn contrast 65-month-olds discriminated both contrasts equally well Anadult control group tested with an AX task also discriminated both con-trasts equally These results provide preliminary confirmation of the

hypothesis that frequency plays an important role in tuning of phonological systems to prop-erties of the native language A simple attractor model suffices to account for these and previousresults on loss of discrimination of non-native-language contrasts and suggests that the techniqueof measuring graded loss of multiple contrasts in combination with observation of input fre-quencies can offer a powerful method of assessing infantsrsquo phonological representations

Key words

infant speechperception

non-native contrasts

phoneme discrimination

phonological categories

statistical learning

Acknowledgements This research was supported by a grant from the National Instituteof Health (5 R01 HD32005) to JLM JLA was supported by the National ScienceFoundation IGERT Program (9870676) We thank Janet Werker and Catherine Best forcontributing their stimuli for use in the studies Karen Rathbun for her assistance indata collection and Catherine Best Heather Bortfeld Katherine Demuth Leher SinghMelanie Soderstrom and Janet Werker for comments on drafts and development ofstudies Requests for reprints should be directed to James Morgan Department ofCognitive and Linguistic Sciences Brown University Box 1978 Providence RI 02912

Address for correspondence Jennifer L Anderson Department of Cognitive amp LinguisticSciences Brown University Box 1978 Providence RI 02912 phone 401-863-1160fax 401-863-2255 e-mail ltjandersomicrosoftcomgt James Morgan Department ofCognitive amp Linguistic Sciences Brown University Box 1978 Providence RI 02912phone 401-863-2462 fax 401-863-2255 e-mail ltjames_morganbrownedugtKatherine S White Department of Cognitive amp Linguistic Sciences Brown UniversityBox 1978 Providence RI 02912 phone 401-863-1167 fax 401-863-2255 e-mailltkatherine_s_whitebrownedugt

LANGUAGE AND SPEECH 2003 46 (2 ndash3) 155 ndash 182 155

Language and SpeechlsquoLanguage and Speechrsquo is copyKingston Press Ltd 2003

1 Introduction

Infants begin life with the ability to learn any of the worldrsquos languages While adultsare often hampered in their ability to identify or discriminate phones that are not usedcontrastively in the phonological system of their language(s) young infants do notshow such language-specific effects In the early months of life infants discriminateboth native and non-native contrasts with equal ease As experience with a nativelanguage accumulates evidence of increased attunement to properties of the ambientlanguage emerges By the end of the first year infants like adults fail to discriminatemany non-native contrasts In this article we begin to explore whether there may bea simple statistical basis for the particular order in which non-native contrasts arelost

Adults show difficulty discriminating non-native single-feature contrasts if thespeech sounds are not used to distinguish meaning in their native language A classicexample is the difficulty that native Japanese-speaking adults have in discriminatingbetween English ra versus la (Sheldon amp Strange 1982) The same pattern isevident in English-speaking adultsrsquo difficulties in discriminating Hindi retroflex anddental stops (Werker Gilbert Humphrey amp Tees 1981) Nthalamkampx ThompsonSalish velar and uvular ejectives (Werker amp Tees 1984) and Czech retroflex andpalatal fricatives (Trehub 1976)

Infants younger than six to eight months however can categorically discriminateboth contrasts that are phonemic (eg for English ba-pa Eimas SiquelandJusczyk amp Vigorito 1971) and contrasts that are not phonemic in their nativelanguage (for a review see Werker amp Tees 1999) By 10 to 12 months infants performlike adults in their discrimination of non-native contrasts This change in discrimi-native abilities within the first year of life has been reported for English-learninginfants for several different consonant contrasts including the Hindi stops the Salishejectives and the Czech fricatives cited above as well as several different Zulu con-trasts mdash bilabial plosive implosive lateral voiced voiceless fricative and velarvoiceless ejective distinctions (Best 1995) Similar loss of discriminatory ability hasbeen reported for Japanese-learning infantsrsquo discrimination of English r and l (Kuhl1998) The same developmental pattern emerges in the perception of vowel con-trasts although the discriminative decline appears to occur somewhat earlier KuhlWilliams Lacerda Stevens and Lindblom (1992) found evidence of distinctive pro-totype effects for vowels in English- and Swedish-learning infants by six monthsand Polka and Werker (1994) found effects of native language experience on perceptionof vowels between four and six months

Initially this developmental pattern was explained as an absolute loss of abilitydue to lack of exposure (see Werker 1994 for a detailed historic overview) Such aMaintenance Theory assumes that experience is required to maintain perceptual sen-sitivities that are already present in the infant (Aslin amp Pisoni 1980) Evidence fromBest McRoberts and Sithole (1988) however contradicts such a view Infants andadults were tested on their ability to discriminate a Zulu apical versus lateral clickcontrast in which neither sound was like any English sound Unlike other non-nativecontrasts previously tested both older infants and adults showed high levels of

Language and Speech

156 Statistical basis

discrimination Moreover adultsrsquo ability to discriminate certain non-native contrastscan improve with training (see Logan Lively amp Pisoni 1991 Werker amp Polka 1993)These findings suggest that the loss of ability is not absolute but is instead due toperceptual reorganization (Werker 1994) Two dominant theories used to explainthese developmental changes are the Perceptual Magnet Effect (or Native LanguageMagnet Theory NLM Kuhl 1991 1995 2000) and the Perceptual AssimilationModel (PAM Best 1993 1995 Best amp McRoberts this volume Best McRobertsamp Sithole 1988) Though differing in many of their particulars these theories assumea restructuring of perceptual space or a redistribution of attention rather than anabsolute loss of ability

Kuhlrsquos NLM originally formulated to account for the changes in vowel dis-crimination noted earlier but intended to apply to consonant discrimination as wellproposes that experientially derived prototypes function as perceptual magnets forother sounds in speech perception These prototypes act to warp perceptual spaceserving as attractors in speech sound discrimination by pulling other members of thecategory towards themselves Thus nonprototypical members of categories are per-ceived as more similar to the category prototype than to each other even thoughthe physical distance between the stimuli may be equivalent The fashion in whichperceptual space is warped is a function of learning about the phonological organizationof the native language

Bestrsquos PAM devised to account primarily for the changes in consonant dis-crimination noted earlier but intended to account for vowel discrimination as wellhypothesizes that incoming speech sounds are perceptually assimilated to the phonemiccategories of the native language whenever possible Non-native phones are perceivedwith respect to their similarities and differences to native phones If a non-nativephone is reasonably similar to a native-category phone then listeners are likely toassimilate it to that native category In the case of discrimination of non-native con-trasts if both phones are assimilated to a single native category listeners will showdifficulty in discrimination Therefore the ability to discriminate non-native contrastsis dependent upon how similar the sounds are to those found in the native languageBest (1993 1995) offered a taxonomy for classifying how non-native contrasts mayrelate to native language phonology First two non-native phones may be assimilatedto a single native phonetic category such Single Category contrasts should be dif-ficult to discriminate Second each of the non-native phones may be assimilated toa separate native phonetic category Two Category contrasts should be easy to dis-criminate Third one of the native phones may be a good exemplar of a native categorywhile the other is a poor exemplar (cf Volaitis amp Miller 1992) Category Goodness con-trasts should also be relatively easy to discriminate Fourth one or both of thenon-native phones may be unlike any native phone though both are perceived asspeech sounds Uncategorizable contrasts will be easy if one of the phones is similarto a native language phone but difficult otherwise Fifth the two non-native phonesmay be perceived as nonspeech sounds the difficulty of discriminating Nonassimilablecontrasts depends on the acoustic distance between the phones

Although these accounts predict which non-native contrasts should become dif-ficult to discriminate and which should remain easily discriminable they do not

Language and Speech


directly predict either how or when changes in perception of such contrasts shouldoccur Studies investigating infantsrsquo discrimination of non-native contrasts tradi-tionally used measures involving achievement of some criterion of performanceThis coupled with relatively wide separation of the age groups tested may make itappear as though loss of discrimination is categorical No evidence however existsto support the notion that development in this domain is discontinuous more likelyprevious findings reflect endpoints of a continuous (albeit fairly rapid) process Themore recent adoption of graded measures of performance such as Arsquo (eg Pegg ampWerker 1997) are in keeping with this view

With respect to the question of when the logic of predicting the relative orderin which non-native contrasts are lost is straightforward at least for Single Categorycontrasts If non-native contrasts that are discriminable early in development laterbecome indiscriminable due to their relation to emerging native-language phoneticcategories then the order in which non-native contrasts become indiscriminableshould correspond to the order in which corresponding native-language categoriesemerge

NLM and PAM may be embedded within general models of phonologicaldevelopment that seek to account for development of phonetic categories (Behnke1998 Plaut amp Kello 1999) Jusczykrsquos (1993 1997) WRAPSA model provides a broadframework in which development of speech perception may be viewed as one com-ponent involved in acquiring a receptive lexicon and spoken word recognition skillsmodels such as PAUSEmir (Werker amp Curtin in press) and DRIBBLER (MorganSingh Bortfeld Rathbun amp White 2001) have recently begun to fill in details ofthis framework All of these models devote particular attention to the statisticalproperties of the input for example Behnke (1998 p 43) writes ldquoThe developmentof [phonetic categories ] is initially mainly based on the distributional characteristicsof incoming signals and is the first step in the direction of language-specific processingof speechrdquo This concern with input statistics mirrors both recent developments inphonological theory (eg Pierrehumbert 2001 in press) and recent findings in infantspeech processing

In several domains infants have proven to be highly capable statistical learnersNewborn infants can discriminate sets of words based on distributions of correlatedacoustic and phonetic features within the sets (Shi Werker amp Morgan 1999) Aseries of studies has shown that infants are able to use statistical regularities of theinput to discover boundaries of word-like units (Aslin Saffran amp Newport 1998Goodsitt Morgan amp Kuhl 1993 Saffran Aslin amp Newport 1996 Saffran NewportAslin Tunick amp Barrueco 1997) Infants are sensitive to the relative frequency ofphonotactic sequences (Jusczyk Luce amp Charles-Luce 1994) as well to the like-lihood of particular sequences occurring within versus between words (MattysJusczyk Luce amp Morgan 1999) Recent work by Maye Werker and Gerken (2002)has shown that infantsrsquo sensitivities to input statistics also extend to learning ofphonetic categories The distributions of speech sounds with which six to eight monthold infants were presented were shown to affect the way in which they later classifiedthe sounds Infants presented with the sounds in a bimodal distribution formed twocategories whereas those exposed to a unimodal distribution formed only one

Language and Speech


According to a statistical learning account the statistics of the input languageplay a fundamental role in the warping of perceptual space and the formation ofnative language categories as indeed Kuhl (2000) has suggested Maye et al (2002)have demonstrated that the location of input exemplars in phonetic space influencescategory development but the frequencies of exemplars within regions must also playa role Unless categories are extremely well separated variation within categories ishighly restricted and infantsrsquo perceptual and representational capacities are non-noisy (a trio of unlikely assumptions) reasonably large numbers of exemplars will beneeded for learners to be able to infer whether these are drawn from one or manyunderlying distributions Moreover if phonetic category acquisition were insensitiveto exemplar frequencies then sparse sets of exemplars might suffice to trigger formationof new categories Even brief exposure to another language might suffice to alter theorganization of the learnerrsquos phonological system Available data suggest to thecontrary however that exposure to varied (and hence necessarily numerous) sets ofinstances is required for robust category formation (Logan Lively amp Pisoni 1991 seealso Bomba amp Siqueland 1983 Quinn Eimas amp Rosenkrantz 1993) Therefore wehypothesize that other things being equal the order of emergence of native-languagephonetic categories will mirror the frequencies with which exemplars of those cate-gories appear in the input This hypothesis provides a basis for predicting when changesin perception of non-native contrasts should occur Non-native contrasts relating tocategories of sounds with higher frequencies in the native language should be lost earlier

In particular for English-learning infants discriminability should begin todecline earlier for non-native coronal contrasts than for non-native dorsal or labialcontrasts As every contestant on Wheel of Fortune knows the most frequent Englishconsonantal phonemes are t d s and n mdash all coronals (French Carter ampKoenig 1930 Tobias 1959) Previous studies investigating infantsrsquo ability to dis-criminate non-native contrasts have focused on either coronal or dorsal stop consonantsStudies by Werker and colleagues have used coronal and dorsal voiceless stop con-trasts whose crucial differentiating factor is place of articulation (eg dental vsretroflex both of which are coronal or velar vs uvular both of which are dorsalWerker amp Tees 1984 Werker et al 1981) The ability to discriminate both coronaland dorsal non-native contrasts disappears between 6 and 10 months but the relativeorder in which these contrasts are lost is not known We hypothesize that due tothe higher frequency of coronals in English discrimination of non-native coronalcontrasts should decline earlier than discrimination of non-native dorsal contrasts

2Experiment 1

Infants were tested on their ability to discriminate a coronal Hindi retroflex dentalstop contrast ([OcircAElig] versus [ t9AElig] ) and a dorsal Salish velar uvular ejective contrast ([k1AElig ]versus [q1AElig ] ) In previous studies infants have been tested on one or the other ofthese two contrasts Using the Conditioned Head Turn Procedure Werker et al(1981) and Werker and Tees (1984) showed that 6 ndash 8-month-old infants were able todiscriminate the coronal Hindi contrast at native levels By 8 ndash 10 months infantsshowed a slight decline in performance and by 10 ndash 12 months they were no longerable to discriminate the contrast Testing infants on the dorsal Salish contrast yielded

Language and Speech


similar results (Werker amp Tees 1984) Because we were concerned with relative orderof loss we tested the same infants on both contrasts Although there has been no large-scale study that has systematically tested the same infants on both contrasts Werkerand Tees (1984) reported a small-scale longitudinal study in which six infants wererepeatedly tested on both the coronal Hindi and dorsal Salish contrasts Contraryto our prediction at eight months all infants reached criterion on the coronal Hindicontrast but only half reached criterion on the dorsal Salish contrast Howeverwhether these initial results reflect differences in discriminative ability or whether theymight be ascribable to repeated testing or testing order effects is not clear

21Method

Subjects Subjects were 18 full-term infants (10 males 8 females) recruited fromRhode Island birth records Subjects were between 79 and 91 months (mean = 252days range = 236 to 273 days) and were from English-speaking homes (exposure toanother language was limited to less than 10) An additional 13 infants wereexcluded from the study due to crying or fussiness that prevented completion of oneor both tests (5) experimenter error (2) and loss of interest in reinforcers (5) onefamily rescheduled their appointment several times so that their infant was too oldwhen finally tested

Stimuli Stimuli were the same as those used in previous experiments by Werker etal (1981) Werker and Tees (1984) and Best (1995) The coronal contrast was aHindi voiceless unaspirated retroflex ( [OcircAElig] ) versus dental ( [ t9AElig] ) distinction English-speaking adults tend to perceive this as a Single Category contrast (Werker 1991)Retroflex consonants are produced by curling the top of the tongue back and forminga closure posterior to the alveolar ridge Dentals are produced by placing the tip (orblade) of the tongue against the back wall of the upper front teeth (Werker amp Tees1984) The closure for English voiceless coronal stops usually occurs at the alveolarridge between the closure points for the Hindi consonants

The dorsal contrast was an Nthalamkampx (an Interior Salish language spokenin British Columbia) voiceless unaspirated glotallized velar ( [k1AElig ] ) versus uvular( [q1AElig ] ) stop distinction (Werker amp Tees 1984) English-speaking adults tend to perceivethis as either a Single Category or Nonassimilable contrast (Werker 1991 Best ampMcRoberts this volume characterize both of the contrasts used here as SingleCategory contrasts) The velar versus uvular place of articulation is the crucial dif-ference The sounds are produced by obstructing the airflow by raising the back ofthe tongue either against or behind the velum In English only velar stops carry sig-nificance Infants were tested on their ability to discriminate sets of stimuli three foreach speech sound By using multiple exemplars subjects were forced to ignorewithin-category acoustic variability and differentiate the sounds according to phoneticcategory (Werker amp Tees 1984)

The contrasts were both produced by male native speakers For the coronalcontrast from Hindi Werker and colleagues selected three exemplars from eachcategory from multiple recordings so that variations in fundamental frequencyduration and intonation were randomized both within and between categories (Werker

Language and Speech


et al 1981 Werker amp Tees 1984) All of the coronal stimuli displayed similar falling-rising pitch contours and we further edited them to ensure that duration and RMSamplitude were equal Werker and Tees note that they had difficulty in eliciting setsof Nthalamkampx stimuli with the same vowel They identified one instance of [k1AElig ]and one instance of [q1AElig ] that had similar vowels and then created sets of three stimulieach by splicing these vowels onto different bursts Consequently there was morevariability among the coronal stimuli than among the dorsal stimuli We testedwhether this affected the intrinsic discriminability of the contrasts in Experiments 2and 3 below with six-month-olds and adults respectively

In previous reports the Hindi phones were transcribed with a following vowel[ a] and the Salish phones were transcribed with a following vowel [ i] (Werker etal 1981 Werker amp Tees 1984) According to Werker (personal communicationMarch 7 2002) these transcriptions reflect the relative values of these vowels intheir respective vowel systems However these two vowels have very similar formantvalues in comparison to English the two vowels fall in a portion of F1 F2 vowel spacelabeled as [Aring] by Peterson and Barney (1952) most closely described as a midcentralvowel In listening to the vowels we could detect no trace of retroflection and thereforehave transcribed them as [AElig]

To verify that coronal stops occur more frequently than dorsal stops in typicalspeech to children we examined transcriptions of mother to child speech obtainedfrom the CHILDES database (MacWhinney 1999) The four corpora we used(Bernsteinrsquos corpora Brownrsquos Adam Eve and Sarah corpora) all involved mother-child dyads Transcriptions of the mothersrsquo speech began when the children werebetween one and two years and continued for at least four months (Bernstein 1982Brown 1973) Across these four corpora there were approximately 300000 wordsuttered Each transcribed word type was converted into a phonemic pronunciationusing the on-line Wordsmith dictionary token pronunciations were adjusted foreffects of phonological contexts in continuous speech by applying rules of Englishphonology suggested by Ladefoged (2001) We reasoned that this procedure wouldsuffice to provide rough estimates of phoneme frequencies

In the four corpora there were approximately 750000 consonantal phonemesPhoneme token frequencies were evaluated with respect to features of both place(labial coronal dorsal) and manner of articulation (nasal liquid semivowel fricativeaffricate stop) There are multitudes of ways in which frequency of these phonemescan be computed Not only could infants be representing sounds in different ways(eg features phones demisyllables) but they could also be computing frequencywith respect to different contexts (eg word-initial syllable-initial or perhaps overallfrequency) However as is evident in Table 1 (overleaf) across a wide variety ofmethods of measurement the result is always the same in English voiceless coronalstops are more frequent than voiceless dorsal stops (c2 p lt 001 for all counts)

Language and Speech


TABLE 1

Occurrence of coronal and dorsal sounds in American English child-directed speech

Voiceless stops All oral stops

coronal dorsal coronal dorsalPosition t k t d k g

Word Initial 18099 14375 40746 27157

Syllable Initial 26555 20062 56372 35315

Any 106832 38325 157297 55720

Procedure We used a modified version of the Conditioned Head Turn procedure (fora description see Werker Polka amp Pegg 1997) The experimental procedure includedtwo phases conditioning and testing Throughout the procedure the infant wasseated on a parentrsquos lap across a small table from an experimenter in a sound-treatedroom A loudspeaker (JBL Sentry 100A) and visual reinforcers were located at a 90-degree angle to the left of the infant and parent The visual reinforcers included twomechanical toys in smoked Plexiglas boxes which could be lighted and activated andfour Sony Trinitron televisions arranged in a cube on which an animated movie couldbe displayed A Comprehensive Video matrix switcher connected a video cassetteplayer to the televisions so that the movie could be displayed on all four or any com-bination of the televisions To maintain infantsrsquo interest the reinforcers that wereactivated varied across trials once activated reinforcers remained on for 4500 ms

Both the experimenter and the parent listened to music over Bose noise-can-cellation headphones to prevent them from hearing the stimuli A second experimenterlocated outside the room operated the computer and observed the testing session viaa Panasonic (BP-WV550) low light video camera and a Panasonic (WV-540) monitorThe experiment was controlled by custom software running on a PC-compatiblecomputer with a TBS Montego II Sound Blaster Pro Emulation soundboard Stimuliplayed through an Onkyo Integra stereo preamplifier (P- 304) and an Onkyo Integrastereo power amplifier (M-504) and were set to play at a conversational level (66 dB)

For each infant one stimulus set of the contrast pair being tested was des-ignated as the background set the other was the target or change set Assignmentof stimulus sets to background or target status was counterbalanced across infantsThe background stimulus was repeated continuously with 1300 ms interstimulusintervals throughout the procedure except during target trials as noted below

During the conditioning phase the infant was given the opportunity to learnthe association between a sound change and the activation of the visual reinforcersIn this phase of the experiment only one exemplar each from the background andtarget sets were used When the infant was judged to be in a state of readiness quietlyattending at midline the experimenter could initiate a trial during which the targetstimulus played three times (again with 1300ms ISIs) This allowed a response windowof approximately five seconds Afterwards the background stimulus resumedConditioning proceeded by initially activating the reinforcers as soon as the first

Language and Speech


target stimulus was presented After the first three trials a delay was inserted betweenpresentations of the target stimulus and activation of the reinforcers The additionof a delay allowed the infants to initiate head turns prior to the activation of the rein-forcers This delay was gradually lengthened to a maximum of 45 seconds Followingthe standard established by Pegg and Werker (1997) once the infant made three con-secutive anticipatory head turns (ie before the reinforcers were automaticallyactivated) or reached a maximum of 15 trials testing began

In testing all three exemplars from the background and target sets were usedPresenting infants with three exemplars required them to attend to categorical dif-ferences among the speech stimuli in order to discriminate successfully Betweentrials each background stimulus was repeated three times after which a new back-ground stimulus was chosen (with replacement) from the background stimulus setAs before when the infant was judged to be in a state of readiness the experimentercould call for a trial which was initiated at the first possible interstimulus intervalThe type of trial control or test was randomly selected by the computer so thatchange trials occurred on about 55 of the trials and control trials occurred on 45of the trials No more than two control or change trials were allowed consecutivelyIn each trial one stimulus chosen at random from the appropriate stimulus set wasrepeated three times with ISIs of 1300 ms As in conditioning this allowed a totalresponse window of approximately five seconds Trials in which a target stimuluswas presented and the infant turned were counted as hits trials in which a back-ground stimulus was presented and the infant did not turn were counted as correctrejections Correct responses comprised hits and correct rejections

Seven correct responses out of any eight consecutive trials was adopted as afloating criterion of discrimination (see Aslin amp Pisoni 1980 Jusczyk Shea amp Aslin1984 Kuhl 1985) Testing concluded when the infant reached this criterion or com-pleted 26 trials1 If on the 26th trial the infant was within two trials of reachingcriterion an additional five trials were presented Infants who failed to reach thecriterion and had fewer than three hits or fewer than five head turns total (hits plusfalse alarms) were judged to have lost interest in the reinforcers and to no longer beon task Data from these infants were discarded

During testing if an infant had three consecutive incorrect responses a retrainingstage was entered In retraining the infant was presented with only the conditioningstimuli and every trial presented was a change trial If no head turn occurred on thefirst three retraining trials the reinforcers were automatically activated on the fourthtrial After two consecutive correct responses or five retraining trials testing resumedEach infant was limited to three retraining sets which were not included in theanalyses

Infants were tested on both contrasts in two sessions on the same day counter-balanced for the order of presentation of the contrasts (coronal or dorsal)

Language and Speech


1 We had intended this to be 25 trials as is the norm but our software mistakenly begannumbering trials from 0 rather than 1

22Results amp Discussion

Conditioning proceeded in a similar fashion for both of the contrasts tested The rateat which trials were presented during conditioning did not differ across the contrastsFor the coronal contrast there was an average of 1207 background stimuli (approx-imately 18 seconds) between trials for the dorsal contrast there was an averageof 1148 background stimuli (approximately 17 seconds) between trials t (17) = 076p gt4 Also the average number of conditioning trials per infant did not differ acrosscontrasts (coronal 1277 dorsal 1133 t (17) = 157 p gt1) However whereas 14 of 18infants reached the conditioning criterion (3 consecutive anticipatory head turns infewer than 15 trials) on the dorsal contrast only 11 did so on the coronal contrastTen infants reached the conditioning criterion on both contrasts three infants didso on neither We consider later whether performance in testing was associated withperformance during conditioning

Two types of dependent measures both regularly used in previous work wereused to assess infantsrsquo performance in the testing phase First subjects were scoredcategorically on their ability to discriminate each contrast using a criterial measureSecond Arsquo scores were calculated to provide graded measures of performance Arsquo isthe more suitable measure assuming that loss of discriminability of non-native con-trasts is a continuous process we include measures of criterial performance forcompatibility with previous reports

On the coronal contrast seven of 18 subjects reached the pre-established floatingcriterion of seven correct responses on any eight consecutive trials (within 15 trialson average) On the dorsal contrast 11 of 18 subjects reached criterion (within 173trials on average) Three infants reached criterion on both contrasts three othersreached criterion on neither Eight infants reached criterion on the dorsal contrastbut not the coronal whereas four infants reached criterion on the coronal contrastbut not the dorsal This asymmetry is consistent with the hypothesis that sensitivityto the coronal contrast is lost first but the difference is not significant by McNemarrsquosQ p gt 35

The 7- of - 8 floating criterion has been used in a number of studies employingthe conditioned head turn procedure (Kuhl 1985 Polka Jusczyk amp Rvachew 1995Werker Polka amp Pegg 1997) The justification for this criterion is that the binomiallikelihood of seven correct responses in eight trials is less than 005 (p = 035) so thatclaims that individual infants can discriminate particular contrasts may be justifiedHowever application of the 7- of - 8 criterion is problematic for at least two reasonsFirst subjects may perform at a level that exceeds this criterion but nonetheless failto meet it Consider a subject who performs correctly on six consecutive trials thenmisses two trials and then gets six more correct This subject has not reached the 7-of - 8 criterion even though the likelihood of 12 correct in 14 trials is five times lessthan that of seven of eight This may be straightforwardly remedied by consideringthat any performance whose likelihood is equal to or less than the likelihood of 7-of - 8 meets criterion What matters is not any particular sequence of responses butrather whether infantsrsquo performance exceeds the standard

This raises the second problem What is the likelihood of 7- of - 8 The true

Language and Speech


likelihood of reaching this criterion is not 035 unless subjects receive only eighttrials When applied as a floating rather than blocked criterion likelihood increasesaccording to the total number of trials In nine trials the likelihood of reaching 7-of - 8 by chance is 049 in 10 trials the likelihood is 061 To estimate the likelihoodof 7- of - 8 in larger numbers of trials we conducted a series of Monte Carlo simu-lations each including 10000000 random sequences of n trials results of thesesimulations are shown in Table 2 Also shown in Table 2 are the number correctneeded in n trials to achieve likelihoods less than or equal to those of 7- of - 82

TABLE 2

Floating 7-of - 8 criterion likelihoods

Trials Criterion Likelihood- Trials Criterion Likelihood-Likelihood equivalent equivalent

number numbercorrect correct

8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12

Applying the likelihood-equivalent criteria performance on the coronal contrastremains the same Seven of 18 subjects reach criterion Given that the likelihood ofreaching the floating criterion by chance within 26 trials is 024 we cannot rule outthe possibility that seven of 18 infants might do so by chance (binomial p = 12)However under this standard 14 of 18 subjects reached criterion on the dorsalcontrast (within 166 trials on average the likelihood of this occurring by chance isp lt 0001) Five subjects reached criterion on both contrasts two reached criterion

Language and Speech


2 These likelihoods perhaps render the criterion unsuitable as a basis for claims aboutindividual subjects However the criterion may be used as a basis for binomial analysesof group performance with a substantial advantage over the default assumption ofp= q = 50 that must be used elsewhere (eg in assessing the numbers of subjects wholistened longer to one stimulus set over another in the Headturn Preference Procedure)For example under the default assumption the likelihood of 8 of 10 infants reachingcriterion is 055 whereas with the probability of reaching 7- of - 8 within 25 trials thelikelihood drops to 0002 and only 6 of 10 must reach criterion (p = 013) to exceed the 05standard

on neither Under this criterion the asymmetry between the two contrasts is morepronounced Nine subjects reached criterion on the dorsal contrast but not thecoronal whereas only two showed the opposite pattern However as before thisasymmetry falls short of significance McNemarrsquos Q p lt 07

Although we believe that the likelihood-equivalent approach to calculating acriterion of performance is logically more consistent arguably neither form of cri-terion is entirely relevant to our present enterprise Our fundamental concern is therelative discriminability of the coronal and dorsal contrasts tested we do not wishto advance claims that infants can discriminate this contrast or cannot discriminatethat one Use of continuous measures that can be analyzed with parametric statisticsare best suited for deciding questions of relative discriminability Thus as a more sen-sitive measure of discrimination subjectsrsquo Arsquo scores were calculated for each of thecontrasts Arsquo is a measure of sensitivity similar to drsquo but designed for use on a smallernumber of trials (Grier 1971) which takes into account the proportion of hits andfalse alarms to give an estimate of a subjectrsquos ability to detect the sound change giventhe overall frequency of head turns Scores range up to 1 By definition 5 correspondsto a chance level of performance (hits = false alarms) whereas Arsquo scores that are sig-nificantly below chance suggest that subjects are using a strategy different from thatrequired for the task Polka (1991) defined adult ldquonative-likerdquo performance at 95corresponding to a 10 error rate however we would expect the performance ofinfants to be somewhat lower

Arsquo scores were computed for each subject and compared across contrasts tochance levels of performance Results are shown in Figure 1 Dorsal Arsquo scores weresignificantly higher than chance (M = 73 SD = 19 t (17) = 526 p lt 0001) whereascoronal Arsquo scores were not (M = 48 SD = 40 t (17) = ndash 026 p gt75) A 2 acute 2Contrast by Order mixed ANOVA on these data showed a significant main effect ofContrast F (1 16) = 495 p lt 05 The order in which the contrasts were tested madeno significant difference F (1 16) = 288 p gt 10 nor was there a significant Contrastby Order interaction F (1 16) lt 1 NS Thirteen of 18 infants had higher Arsquo scoresfor the dorsal contrast than for the coronal contrast binomial p lt 05

We may also ask whether the two contrasts engendered different biases inresponding In sessions testing the coronal contrast infants turned on 321 of thetrials (hits plus false alarms) in sessions testing the dorsal contrast infants turnedon 394 Twelve of 18 infants showed higher rates of turning to the dorsal contrast(binomial p = 12) The difference in turning rate across contrasts was not significantt (17) = 127 p gt 2 Infants did however turn more during the first session (424)than during the second session (292) t (17) = 259 p lt 02

As noted earlier we followed Pegg and Werker (1997) in allowing infants toproceed to testing after 15 conditioning trials even if they had not reached the cri-terion of three consecutive anticipatory head turns Our motivation for doing so wastwo-fold First limiting the length of the sessions increased the likelihood that infantscould successfully complete both tests thereby helping to minimize subject mortalitySecond restricting the variance in amount of exposure to both the contrasts and thereinforcers was in keeping with our general strategy of counterbalancing Our intentwas to treat infants in the same fashion during both sessions that this was accomplished

Language and Speech


is demonstrated by the data reported at the beginning of this section Nevertheless14 of 18 infants reached the conditioning criterion on the dorsal contrast whereasonly 11 of 18 infants did so on the coronal contrast Here we consider whether dif-ferences in test performance across the contrasts might have been associated withdifferences in conditioning

On the dorsal contrast there was an association between achievement of theconditioning criterion and achievement of the (likelihood-equivalent) test criterionFisher exact p = 02 Of the 14 infants reaching the conditioning criterion 13 alsoreached the test criterion However this pattern did not hold for the coronal contrastOf the 11 infants reaching the conditioning criterion on this contrast only five alsoreached the test criterion On this contrast there was no association between theachievement of these two criteria Fisher exact p = 42 For both contrasts subjectsfailing to reach the conditioning criterion also tended to fail to reach the test criterionby a ratio of three to one We also considered whether number of conditioning trialsmight be correlated with Arsquo scores for the coronal contrast Although the statistic hadthe expected sign it fell well short of significance r = ndash 014 p gt 25 (1-tail)

Ten infants reached the conditioning criterion on both contrasts Half of thesewere tested first on the dorsal contrast and half were tested first on the coronalcontrast All 10 of these infants reached the test criterion on the dorsal contrast(binomial p lt 0001) whereas five reached the test criterion on the coronal contrast(binomial p lt 07) Within this subsample Arsquo scores were significantly higher on thedorsal contrast (M = 79 SD = 13) than on the coronal contrast (M = 48 SD =37) t (9) = 244 p lt 05 Eight of 10 had higher Arsquo scores on the dorsal contrast thanon the coronal contrast binomial p lt 06 The essential question is whether continuing

Language and Speech


Figure 1

85-month-oldsrsquodiscrimination of coronal and dorsal non-native contrasts in a ConditionedHeadturn task

conditioning unti l all infants had reached criterion would have substantively changed our results The data suggest that if anything the differences between thetwo contrasts might then have been more pronounced There is no indication thatthe differences we observed in test performance were attributable to differences inconditioning

Werker and Tees (1984) and many subsequent studies include a control conditionthat we omitted When an infant failed to reach test criterion before deciding thatthat infant could not discriminate the contrast Werker and Tees required that theinfant pass criterion on an English-language contrast ba versus da Werker andTees (p 54) note ldquoThis was done to ensure that the failure of the infant was due toan inability to readily perceive the sound difference and was not due to nonspecificfactors such as boredom dirty diapers etcrdquo

One motivation for eliminating this control was feasibility The likelihood ofgetting infants through three sessions in a single day is remote More importanthowever we believe that given our design this additional session is logically unnec-essary Counterbalancing the order of testing served to strain out extraneous factorsThe essential use of this additional session is to provide a basis for interpreting nullresults by assessing continued interest in reinforcers and ensuring that the infant hasremained on task As noted earlier we do not advance claims that infants fail to dis-criminate either or both of the contrasts tested but rather make the weaker assertionthat performance differs across the two in a fashion that is predictable from the sta-tistical properties of input speech Our argument does not rest on null results sothe interpretive difficulty with which Werker and Tees were concerned does not arise

Moreover the efficacy of the control session is unclear given its lack of simul-taneity with the behavior of interest Between sessions performance may vary for anyof a multitude of reasons For example the diaper gets changed or becomes soiledand the infantrsquos performance changes for better or worse accordingly Thus an infantwho was inattentive during a session testing a non-native contrast may becomeattentive on a following control session testing a native contrast or vice-versa Nonecessary logical relation holds between performance in the two sessions Interest inreinforcers may be gauged by observing infantrsquos overall turning rate within a sessiondata from infants failing to turn at some criterion rate may be excluded as we didhere Turning rate provides a clue to whether an infant remains on task beyond thiscounterbalancing provides the most appropriate solution Recall that half the infantswere tested first on the coronal contrast and half were tested first on the dorsalcontrast so that any extraneous nonspecific influences on performance were ran-domized across the two contrasts

The results of this experiment indicate that 85-month-old English-learninginfants are better able to discriminate a non-native dorsal contrast correspondingto a lower frequency English phonological category than they are to discriminate anon-native coronal contrast corresponding to a higher frequency category There wasa tendency for more infants to reach criterion on the dorsal contrast than on the coronalcontrast (either the traditional 7-of - 8 criterion or its probabilistic equivalent) moreimportant infants were significantly more sensitive to the dorsal contrast than thecoronal contrast as measured by Arsquo scores These results are consistent with our

Language and Speech


statistical account The more frequent category appears to be drawing in the non-native contrasts sooner

However it is possible that the 85-month-olds were better at discriminatingthe dorsal contrast for reasons other than frequency Perhaps the reason for the dis-crepancy in performance across the contrasts was simply because the dorsal contrastwas inherently easier to discriminate or because the dorsal exemplars were less variablethan were the coronal exemplars These possibilities were evaluated in the next twoexperiments

3Experiment 2

Werker and Teesrsquo (1984) results provide an initial argument against the hypothesisthat the coronal and dorsal contrasts we tested differed in their inherent discriminabilityYounger infants (6- to 8-month-olds) were tested on either the coronal or the dorsalcontrasts the proportion of subjects reaching criterion on the two contrasts wasequivalent This was however a between-subjects test To evaluate explicitly whetherthe dorsal contrast was easier to discriminate it is necessary to perform a within-subjectstest on infants who are young enough to discriminate both contrasts

31Method

Subjects Twenty-four English-learning infants between six and seven months oldwere tested in this procedure (11 males 13 females M = 199 days range = 180 ndash 212days) An additional 29 infants were tested and subsequently excluded from analysisdue to crying or illness (11) failure to turn during testing (7) Arsquo scores significantlybelow chance on both contrasts (3) failure to complete the second Session (3)parental or sibling interference (3) experimenter error (1) and foreign languageexposure in excess of 10 (1)

Stimuli The stimuli used were the same Salish (dorsal) and Hindi (coronal) con-trasts described in Experiment 1

Procedure Infants were tested using the CHT procedure outlined in Experiment 1To accommodate the younger infants two changes were made to the procedureFirst in pilot tests many of the younger infants made headturns on change trials imme-diately following the 45 second window thus not activating the reinforcer Becauseof this we increased the reinforcer window to 475 seconds This slight increase aidedin capturing the slower responses of the younger infants

Second once we began to test the infants it became evident that performanceon the second test was greatly reduced There were a much higher number of infantsdiscarded for crying or failure to turn to the reinforcer compared to the 85-month-olds For these reasons after testing 12 infants in one-day sessions we tested theremaining infants on both contrasts over two different days The second testingsession always occurred within three days of the first As in Experiment 1 testingwas counterbalanced for both the order of presentation of the contrasts (coronal ordorsal) and for the syllable that was used as the background stimulus (ie retroflexor dental velar or uvular)

Language and Speech



Fourteen of 24 infants reached the conditioning criterion (3 consecutive anticipatoryhead turns in fewer than 15 trials) on the dorsal contrast as did 12 of 24 infants onthe coronal contrast Nine infants reached the conditioning criterion on both con-trasts seven infants did so on neither As was the case for the older infants there wasno significant difference in the number of conditioning trials for the two contrasts(Coronal 1208 Dorsal 1129 t (23) = 806 p gt 4)

Nineteen of 24 infants reached the 7- of - 8 criterion on the dorsal contrast(within 1537 trials on average) whereas 17 of 24 infants reached the 7- of- 8 cri-terion on the coronal contrast (within 1382 trials on average) Fourteen infantsreached this criterion on both contrasts Five infants reached this criterion on thedorsal contrast but not on the coronal contrast whereas three infants exhibited thereverse pattern There was no difference in the proportion of subjects reaching the7- of- 8 criterion on the two contrasts by McNemarrsquos Q p gt 70

Twenty-two of 24 infants reached the likelihood-equivalent criterion on thedorsal contrast (within 1477 trials on average) as did 18 of 24 infants on the coronalcontrast (within 1278 trials on average) Seventeen infants reached this criterion onboth contrasts Five infants reached this criterion on the dorsal contrast but not onthe coronal contrast whereas only one infant exhibited the reverse pattern ByMcNemarrsquos Q this asymmetry was not significant p gt 20

Arsquo scores were calculated for the infants for both contrasts and were analyzedin a 2 acute 2 acute 2 Contrast (coronal vs dorsal) by Group (same day vs different days)by Order mixed ANOVA There were no main effects or interactions except for themain effect of Order F (1 20) = 103 NS in all instances F (1 20) lt 1 As Figure 2indicates both coronal and dorsal Arsquo scores were significantly higher than chance levelsof performance coronal t (23) = 6603 p lt 0001 dorsal t (23) = 10992 p lt 0001

Results from this experiment fail to provide evidence in support of the hypothesisthat there was an inherent difference in the difficulty of the coronal and dorsalcontrast tested 65-month-old infants discriminated both contrasts equally wellThis was evident in the proportion of subjects reaching criterion as well as in thesubjectsrsquo Arsquo scores on the two contrasts These results are consistent with previousbetween-subjects tests of these contrasts (Werker et al 1981 Werker amp Tees 1984)

Younger and older infantsrsquo Arsquo scores on the two contrasts across experimentswere compared in a 2 acute 2 Contrast by Age Group mixed ANOVA which revealeda significant Contrast by Age Group interaction F (1 40) = 453 p lt 05 Arsquo scoresfor the coronal contrast were significantly different t (40) = 302 p lt 01 whereasArsquo scores for the dorsal contrast were not t (40) = 85 p gt 40 By 10 ndash 12 monthsas shown by Werker and Tees (1984) English-learning infantsrsquo abilities to discriminateboth coronal and dorsal non-native contrasts have declined This pattern of resultsis consistent with our prediction that the coronal category is acquired earlier due toits higher frequency of occurrence in input consequently decline in discriminabilityappears earlier for the non-native coronal contrast than for the non-native dorsalcontrast

Language and Speech


4Experiment 3

To further evaluate the possibility that there might be an inherent difference in thediscriminability of the coronal and dorsal contrasts tested here or that these con-trasts might vary in their relation to corresponding English categories we testedadult English speakers on their perception of these contrasts Adult perception ofeach of the contrasts used here has been examined previously though in independentstudies (Polka 1991 Werker amp Logan 1985) Both of these studies used AX tasks

Werker and Logan tested English speakers on the Hindi dental versus retroflexvoiceless stop contrast used in Experiments 1 and 2 here including four exemplarsof each (the 3 exemplars of each used here were drawn from this larger set) Groupsof subjects were tested using ISIs of 250 500 and 1500ms in five 96-trial blocks wediscuss only results from the last group because this ISI was most similar to that usedwith the infants in Experiments 1 and 2 (1300 ms) Werker and Logan reported like-lihood of hits and false alarms from the data given (see their Fig 3) we calculatethat average Arsquo was about 070 Across blocks subjects showed considerable improve-ment from an approximate Arsquo of 060 in the first block to an approximate Arsquo of 080in the fifth block (see their Fig 4) Werker and Logan did not report on subjectsrsquo char-acterization of the stimuli though elsewhere Werker (1991) has noted thatEnglish-speaking adults tend to perceive this as a Single Category contrast

Polka examined effects of phonemic and phonetic familiarity on identificationand discrimination of non-native dorsal contrasts using both AX and ID tasks we

Language and Speech


Figure 2


note only results from the former In her first experiment English and Farsi speakerswere tested on the Salish velar versus uvular glottalized voiceless stop contrast usedin Experiments 1 and 2 here Speakers of both languages performed significantlyabove chance but significantly below native level (Arsquo = 095) for Farsi speakersthere was no benefit of familiarity with the phonemic dimension underlying thecontrast (Farsi contrasts velar [ g ] vs uvular [ G] voiced stops) English speakersshowed significant improvement in the AX task between the first and second 90-trial halves In her second experiment English speakers were tested on the Salishcontrast as well as the Farsi dorsal contrast subjects performed slightly better on theFarsi contrast suggesting that there might be some influence of the phonetic famil-iarity of [g] Subjects offered varying descriptions of the Salish stimuli some Englishspeakers characterized these as ldquofunny krsquosrdquo (consistent with perception of this as aSingle Category contrast) whereas others perceived them as unusual combinationsof speech sounds (consistent with perception of this as a non-assimilable contrast)

In this experiment we tested English-speaking adults on an AX task in a within-subjects design on the coronal and dorsal contrasts used in Experiment 1 and 2Polka instructed some subjects that one of the Salish sounds was similar to kwhereas the other was not other subjects received illustration of velar versus uvularplaces of articulation To better foster comparability with infantsrsquo performance likeWerker and Logan we instructed subjects simply that they would hear two syllablesin each trial and should judge whether these were the same or different

41Method

Subjects Participants included 16 native English-speaking adults 14 right-handedand two left-handed 14 female and two male Partial data from one additionalparticipant were discarded because the software crashed partway through theexperiment

Stimuli The stimuli used were the same Hindi and Salish contrasts described inExperiment 1 Within each contrast each of the six stimuli was paired with itself andevery other stimulus to form a block of 36 pairs Within each pair the syllables wereseparated by a 1500 ms silence

Procedure Participants were tested individually with an AX task using a PC-com-patible computer and Koss headphones Participants were instructed that they wouldhear pairs of syllables and should press one key on the keyboard if they thought thesyllables were the same or another key if they thought the syllables were differentParticipants first received a series of practice trials using endpoint stimuli from anEnglish [da]-[ ta] VOT continuum on which feedback was provided No feedbackwas provided on trials with experimental stimuli

Four blocks of 36 syllable pairs each were presented The first half included oneblock of coronal stimuli and one block of dorsal stimuli as did the second halfSubject to this constraint all possible orderings of blocks were employed (coronal-dorsal-coronal-dorsal coronal-dorsal-dorsal-coronal dorsal-coronal-dorsal-coronaland dorsal-coronal-coronal-dorsal) Within each block syllable pairs were orderedrandomly for each participant Subjects had an opportunity to pause after every 18trials

Language and Speech


Participantsrsquo responses were scored as hits if they responded ldquodifferentrdquo topairs of stimuli drawn from different non-native categories and were scored as correctrejections if they responded ldquosamerdquo to pairs of stimuli drawn from a single non-native category Arsquo scores were calculated as described earlier In addition latenciesfrom the onset of the ldquoXrdquo item to the key press were recorded Three seconds afterthe participantrsquos response the next trial began


Mean Arsquo scores for the coronal and dorsal contrasts are shown in Figure 3 Performanceon the coronal contrast (M = 072 SD = 008) was similar to that reported by Werkerand Logan (M lt 070) and was significantly better than chance t (15) = 1053 plt 0001but worse than native-level performance t (15) = ndash1090 p lt 0001 Performance onthe dorsal contrast (M = 074 SD = 016) was very similar to that reported by Polka(M = 077 SD = 013) as in Polkarsquos experiments performance was significantly betterthan chance t (15) = 606 p lt 0001 but also significantly worse than native-level performance t (15) = ndash 519 p lt 0001 A 2 acute 2 Contrast by Half repeated-measuresANOVA conducted on the Arsquo scores revealed no main effect of Contrast (coronal vsdorsal) F (1 15) = 025 p gt 50 Half (first vs second) F (1 15) = 292 p gt 10 andno Contrast by Half interaction F (1 15) = 021 p gt 50

Although we found no difference in overall accuracy for the two contrasts thisdoes not rule out the possibility of more subtle differences in performance Such dif-ferences might have arisen for example if the coronal stimuli were more similar to

Language and Speech


Figure 3

Adultsrsquo discrimination of coronal and dorsal nonnative contrasts in an AX task

the corresponding English alveolar [ t ] than the dorsal stimuli were to the corre-sponding English velar [k ] For example participants might have taken longer torespond correctly that dental (coronal) stimuli were different from retroflex stimulithan they did to respond correctly that velar (dorsal) stimuli were different fromuvular stimuli due to increased competition in the former case To test this weanalyzed participantsrsquo latencies Mean latencies and standard deviations were calculatedfor each participant and latencies more than two standard deviations from theparticipantrsquos mean were discarded Overall latencies on dorsal stimulus pairs (M =134909ms SD = 37564) were slightly longer than latencies on coronal stimuluspairs (M = 129466 SD = 36729) but this difference was not significant t (15) = 092p gt 35 Separate analyses conducted on latencies for each of the four response typesalso failed to show any differences these are shown in Table 3 Thus there is noevidence that the coronal stimuli were any more or less different from English [ t ] thanthe dorsal stimuli were from English [k ]

TABLE 3

Latencies by response type Experiment 3

Coronal Dorsal

Response type M SD M SD t p

Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50

Correct Rejection 153284 54838 151027 51803 021 gt50

Note Latencies are in ms One subject had no false alarm responses on dorsal stimuli thuson this comparison df = 14 On all other comparisons df = 15

The results of this experiment partially replicate those of Werker and Logan(1985) and Polka (1991) Overall performance was very similar across their studiesand ours However whereas Werker and Logan and Polka both found improvementacross blocks on the stimuli they tested we did not On the view that improvementover time provides information about the treatment of a particular contrast thiscould be interpreted as an important difference For example a significant pro-portion of Polkarsquos subjects may have perceived the Salish contrast as nonspeechwhereas our subjects perceived these stimuli as speech sounds Perhaps thenimprovement occurs when subjects perceive the sounds as nonspeech but not asassimilating to a single native-language category However contrary to the suggestionby Best (1993) Werker and Loganrsquos data suggest that improvement is not diagnosticof the distinction between Single Category and Nonassimilable contrasts In their studythere was improvement for a contrast subjects described as falling within a single native-language category Furthermore there were methodological differences that caneasily account for discrepant findings across the studies Differences in instructionsbetween our study and Polkarsquos were noted earlier In addition Werker and Loganrsquossubjects received 480 trials on the Hindi contrast Polkarsquos subjects received 180 trials

Language and Speech


per contrast (Salish and Farsi) and subjects tested on both contrasts were tested oneach contrast on different days In contrast our participants received only 72 trialsper contrast and were tested on both the Hindi (coronal) and Salish (dorsal) contrastsin a single session In light of these methodological differences variance in improvementacross studies does not appear to be meaningful There is no indication on the basisof our results that the two contrasts were perceived as differing qualitatively in theirassimilability to native-language categories

Like 65-month-olds English-speaking adults discriminate the dorsal andcoronal contrasts equally well (or equally poorly) Taken together results fromExperiments 2 and 3 indicate that there is no significant inherent difference in thediscriminability of the two non-native contrasts we tested Nor is there any evidencethat these two non-native contrasts were perceived as falling in different categoriesin Bestrsquos (1993 1995) taxonomy The explanation for the difference in performanceon these two contrasts in Experiment 1 by 85-month-olds thus must be soughtelsewhere

5General Discussion

The goal of this study was to examine whether frequency of occurrence in inputplays a role in the acquisition of phonological categories We investigated this byexamining infantsrsquo relative abilities to discriminate two non-native contrasts whosecorresponding English categories differ in frequency Dominant theories accountingfor loss of discrimination of non-native contrasts in infants all tie this loss of abilityto learning of native categories

If frequency does play a role in the developing phonological system we wouldexpect infants to learn more frequent categories earlier than less frequent categoriesThis would result in earlier declines in discrimination performance on non-native con-trasts relating to the more frequent categories An examination of transcriptions ofspeech to children confirmed that coronal stops are more frequent than dorsal stopsIn Experiment 1 85-month-old infants discriminated a non-native coronal stopcontrast significantly worse than they discriminated a non-native dorsal stop contrastIn Experiment 2 65-month-old infants performed equally well on the two contrastsas did adults in Experiment 3 indicating that 85-month-old infantsrsquo poor per-formance on the coronal contrast was not due to inherent differences in discriminabilityof the contrasts tested Taken together these results are consistent with the hypothesisthat frequency plays a role in determining the order in which native language phono-logical categories are acquired and hence the order in which non-native contrastsare lost

An alternative explanation of our results might appeal to the ldquospecialrdquo statusof coronals in phonology Across languages coronal consonants are more likely toappear in phonological inventories than are consonants at other places of articulation(Ferguson 1963 also see contributions to Paradis amp Prunet 1991) Furthermore withinlanguages inventories of consonants tend to be richer for coronals than for alternativeplaces of articulation (Maddieson 1984 also see contributions to Paradis amp Prunet1991) Thus it is likely that the high relative frequencies of coronals observed in

Language and Speech


English reflect a widespread if not universal pattern These observations have ledphonologists to suggest that coronals are unmarked (see contributions to Paradis ampPrunet 1991) Hence the contrasts that we tested may be distinguished by theirmarkedness as well as by differences in the frequencies of corresponding Englishcategories

A markedness explanation of our results is less adequate than a frequency-based account for several reasons If these two accounts are to be distinguishablethere must be some frequency-independent means by which learners can judge whichphonological categories are unmarked Learners have no access to phonologicalinventories of other languages so the cross-linguistic commonness of coronals isnot relevant Learners might be innately disposed to classify coronals as unmarkedAlternatively they might observe that there is a larger inventory of coronals in theirlanguage Or learners might observe that coronals behave differently than do otherphonological categories with respect to certain phonological processes Some alter-nations apply uniquely to coronals for example in English word-final coronal nasalsmay assimilate in place of articulation to a following labial or dorsal obstruent (butnot vice versa) In other instances coronals may be more likely to result from certainphonological processes such as neutralization We consider each of these accountsof the origin of markedness in turn

On the view that learners have some innate sense of markedness the unmarkedstatus of coronals may be consistent with the notion that such categories are learnedearlier than others3 However on this view it is not at all clear why there should bea relation between ldquoless markedrdquo and ldquoless discriminablerdquo To the contrary if learnersare predisposed to learn multiple coronal categories (which must be distinguished byfiner gradations than are the sparser sets of categories at other places of articu-lation) they ought to have enhanced discriminative abilities for these less markedsounds Moreover if there were an innate relation between ldquoless markedrdquo and ldquolessdiscriminablerdquo it ought to be developmentally constant An innate markednesshypothesis cannot account for the fact that 65-month-olds discriminate both coronaland noncoronal contrasts whereas 85-month-olds discriminate only the latter

Changes in discriminative abilities across age might be accommodated by theview that learners develop a sense of markedness from observations of phonologicalprocesses or phonological inventories in the native language However on this viewthe (eventual) unmarked status of coronals is irrelevant to the order in which phono-logical categories are acquired This is because in order to observe that one categorybehaves differently than others with respect to phonological alternations these cat-egories must have already been acquired Similarly noting that inventories ofconsonantal categories are not uniformly distributed with regard to place of artic-ulation requires that these categories already be learned Regardless of whethermarkedness is learned by observing phonological processes or inventories learning

Language and Speech


3 Evidence of preference for unmarked categories or phonotactic sequence should appearearly in development indeed Smolensky Davidson and Jusczyk (in press) found that 45-month-old infants prefer unmarked syllable sequences with assimilated nasals (egumpo) to marked sequences with unassimilated coronal nasals (eg unpo )

which categories are marked requires induction to be performed over already existingcategories Thus on this view the order of acquisition of native language cate-gories mdash and hence the order of loss of non-native contrasts mdash must be independentof markedness The innate markedness and the learned markedness accounts canoffer at best post hoc accounts of the present results

In contrast the frequency-based account that we advocate offers a straight-forward explanation of these results One way to conceptualize this phenomenon isin terms of an attractor model The DRIBBLER (Dimensionally Reduced Item-Based Lexical Recognition) model we have been developing (Morgan et al 2001) issimilar to the attractor models proposed by Hinton and Shallice (1991) TaborJuliano and Tanenhaus (1997) and Tabor and Tanenhaus (1999) used for syntacticprocessing DRIBBLER is an attempt to flesh out Jusczykrsquos (1993 1997) WRAPSAmodel Like WRAPSA DRIBBLER posits a front-end encoder that takes as itsinput the output of domain-general sensory analyzers This encoder seeks to reducethe dimensionality of representation (roughly from phonetic space to phonemicspace) by extracting covariation from word-like stretches of speech Phonological cat-egories emerge as nexuses of position-specific variation (see also Pierrehumbert2001) Order of emergence depends jointly on frequency and distribution of exemplars(Maye Werker amp Gerken 2002)

In this model items are projected to locations in representational space determinedby the encoder Perceptually similar items will be projected near one another in spaceclusters of exemplars emergently constitute categories The location of each item isstored for future use Categorization is competitive The movement of the current inputitem under the combined gravitational attraction of all previously stored exemplarsis simulated and the endpoint of the trajectory reached within criterial time determinesthe perceptual categorization of the item In general items will be attracted to clustersaccording to (1) the perceptual similarity of the new item to previous exemplars (2)the presence of neighboring competing clusters and (3) the density of the clusters mdashclusters containing more exemplars will more strongly attract new items

In terms of the present data when a listener hears a foreign speech sound thesound will gravitate toward a perceptually similar native category For more frequentsounds clusters will have coalesced earlier and be denser than for less frequentsounds Thus if an infant hears a non-native coronal sound that exemplar will belikely to gravitate quickly to a dense native cluster Conversely if an infant hears anon-native dorsal or labial sound that sound would be less likely to gravitate to thesparser native cluster within criterial time but rather may be perceived as an exemplarof a new category

In older speakers classification of coronals should occur more rapidly thanclassification of dorsals or labials One prediction following from this is that phonemeidentification should be less affected by higher-level (eg lexical) information forcoronals than for sounds at the other places of articulation particularly when thesesounds occur in word-initial position Indeed several studies of top-down influenceson phoneme identification in adults have found effects for dorsals and labials butnot for coronals (Burton Baum amp Blumstein 1989 Newman Sawusch amp Luce1997 Pitt amp Samuel 1993)

Language and Speech


If one were to adhere to the idea that coronals are learned earlier because theyare unmarked it would be expected that coronals would always be learned first Thefrequency-based account however does not necessarily predict this In those casesin which coronals are not the most frequent sounds the two hypotheses make divergentpredictions Determination of precisely what these cases are may provide a meansof ascertaining the nature of infantsrsquo phonological representation of input

Currently we are unsure of the fashion in which infants might compute statisticspertaining to speech sounds Intuitively there appear to be several types of repre-sentations that infants could be considering These representations can vary both inthe size of the unit and in the context with respect to which the frequency of the unitis tabulated For example infants could be considering the overall frequency ofphonetic features Alternatively at the other end of the spectrum infants could besensitive to CV demisyllables with respect to their word-relative position

For the comparison that we examined we considered the frequency of phonesFor these counts context is irrelevant in English the outcome is always the samethe voiceless coronal stop t is always more frequent than the voiceless dorsal stopk However if we consider the frequency of demisyllables we find that coronal-stop-onset demisyllables are not always more frequent than dorsal-stop-onsetdemisyllables For instance coronal stop onsets followed by I are more frequent thandorsal stop onsets followed by I (a pattern clearer for voiced than voiceless stops)However when the same onsets are followed by E the pattern reverses (again moreclearly for voiced than for voiceless stops) Coronal stop onsets followed by E areless frequent than are dorsal stop onsets followed by E (Recall that the coronal anddorsal stimuli used here had similar midcentral vowels the closest English vowelcategory is Because coronals occur with much higher frequency before thando velars the present results are neutral with respect to the size of representationalunit that infants may have used)

Thus opposing predictions may be made about the relative rate of loss dependingon whether frequency is computed by phone feature or by demisyllable By testinginfants on their abilities to discriminate non-native contrasts that relate to native-language categories differing in relative frequency depending on the unit ofrepresentation assumed we hope to learn more about the types of representationsinfants are using The technique of measuring graded loss of multiple contrasts incombination with observation of input frequencies can offer a powerful method ofassessing infantsrsquo phonological representations

The nature of these representations in turn bears on whether the statisticalapproach we advocate is sufficient to account for the order in which native languagecategories are acquired or whether it must be supplemented with additional mech-anisms Recall that loss of discrimination of non-native consonant contrasts onwhich we have focused begins at about eight months and is largely complete by theend of the first year whereas effects of native language experience on perception ofvowels begin sometime between four and six months (Polka amp Werker 1994) In ourtabulation of phones in child-directed speech however we found a ratio of con-sonants to vowels of about 151 If infants are computing statistics of speech soundsin terms of phones without regard to context (ie with respect to phonemic categories)

Language and Speech


frequency alone cannot account for why perceptual changes occur first for vowels Inthis case some supplemental mechanism would be required One might posit thatsegments with high amplitude and long duration (such as vowels) are especially salientto learners and may thus receive early preferential processing Behnkersquos (1998) MAPCATmodel assumes such a filter which is instrumental there in determining that vowel cat-egories are acquired first However if infantsrsquo tabulations are restricted to countingphones as they occur in particular syllabic positions (ie with respect to allophonicrather than phonemic categories) then the ratio of consonants to vowels reversesin the child-directed speech we examined we found a ratio of syllable-initial con-sonants to syllable-medial vowels of about 115 Thus if the representation on whichfrequency counts are based is suitably constrained the statistical account can sufficeto explain why changes in discrimination are seen earlier for vowels than for consonants

The experiments in this article provide evidence suggesting that the order inwhich infants lose the ability to discriminate non-native contrasts may be accountedfor at least in part by a simple frequency-based theory The ramifications of this con-clusion are twofold By examining the frequencies of occurrence of sounds in alanguage we can make predictions about the order in which native phonological cat-egories are acquired Moreover examining the relative order of loss of discriminationof non-native contrasts gives us a novel way to explore the granularity of phonetic infor-mation that infants are representing and exploiting in learning their native languages

References

ASLIN R N amp PISONI D (1980) Effects of early linguistic experience on speech dis-crimination by infants A critique of Eilers Gavin and Wilson (1979) Child Development51 107ndash 112

ASLIN R N SAFFRAN J R amp NEWPORT E L (1998) Computation of conditionalprobability statistics by human infants Psychological Science 9 321ndash 324

BEHNKE K (1998) The acquisition of phonetic categories in young infants A self-organizingartificial neural network approach Nijmegen MPI (MPI series in psycholinguistics 5)

BERNSTEIN N (1982) Acoustic study of motherrsquos speech to language learning children Ananalysis of vowel articulatory characteristics Unpublished Doctoral Dissertation BostonUniversity Boston

BEST C T (1993) Emergence of language-specific constraints in perception of non-nativespeech A window on early phonological development In B de Boysson-Bardies amp Sde Schonen et al (Eds) Developmental neurocognition Speech and face processing in thefirst year of life (pp 289 ndash 304) New York Kluwer

BEST C T (1995) Learning to perceive the sound pattern of English In C Rovee-Collieramp L P Lipsitt (Eds) Advances in infancy research (pp 217 ndash 304) Norwood NJ Ablex

BEST C T McROBERTS G W amp SITHOLE N M (1988) Examination of perceptualreorganization for non-native speech contrasts Zulu click discrimination by English-speaking adults and infants Journal of Experimental Psychology Human Perceptionand Performance 14 356 ndash 360

BOMBA P C amp SIQUELAND E R (1983) The nature and structure of infant form cat-egories Journal of Experimental Child Psychology 35 294 ndash 328

BROWN R (1973) A first language The early stages Cambridge MA Harvard UniversityPress

Language and Speech


BURTON M BAUM S amp BLUMSTEIN S (1989) Lexical effects on the phonetic cate-gorization of speech The role of acoustic structure Journal of Experimental PsychologyHuman Perception and Performance 15 567 ndash 575

EIMAS P D SIQUELAND E R JUSCZYK P amp VIGORITO J (1971) Speech perceptionin infants Science 171 303 ndash 306

FERGUSON C A (1963) Assumptions about nasals A sample study in phonological uni-versals In J H Greenberg (Ed) Universals of language (pp 53 ndash 60) Cambridge MAMIT Press

FRENCH N R CARTER C W amp KOENIG W (1930) The words and sounds of telephoneconversations Bell Systems Technical Journal 9 290ndash 324

GOODSITT J V MORGAN J L amp KUHL P K (1993) Perceptual strategies in prelingualspeech segmentation Journal of Child Language 20 229ndash 252

GRIER J B (1971) Nonparametric indexes for sensitivity and bias Computing formulasPsychological Bulletin 75 424 ndash 429

HINTON G E amp SHALLICE T (1991) Lesioning an attractor network Investigations ofacquired dyslexia Psychological Review 98 74 ndash 95

JUSCZYK P W (1993) From general to language specific capacities The WRAPSA modelof how speech perception develops Journal of Phonetics 21 3 ndash 28

JUSCZYK P W (1997) The discovery of spoken language Cambridge MA MIT PressJUSCZYK P W LUCE P A amp CHARLES-LUCE J (1994) Infantsrsquo sensitivity to phono-

tactic patterns in the native language Journal of Memory and Language 33 630 ndash 645JUSCZYK P W SHEA S L amp ASLIN R N (1984) Linguistic experience and infant

speech perception A re-examination of Eilers Gavin amp Oller (1982) Journal of ChildLanguage 11 453 ndash 466

KUHL P K (1985) Methods in the study of infant speech perception In G Gottleib amp NA Krasnegor (Eds) Measurement of audition and vision in the first year of postnatal lifeA methodological overview (pp 223 ndash 251) Westport CT Ablex

KUHL P K (1991) Human adults and human infants show a ldquoperceptual magnet effectrdquofor the prototypes of speech categories monkeys do not Perception and Psychophysics50 93 ndash 107

KUHL P K (1995) Mechanisms of developmental change in speech and language Proceedingsof the International Congress on Phonetic Science Stockholm 2 132ndash 139

KUHL P K (1998) Effects of language experience on speech perception Journal of theAcoustical Society of America 103 2931

KUHL P K (2000) Language mind and brain Experience alters perception In M SGazzaniga (Ed) The new cognitive neurosciences (2nd ed pp 99 ndash 115) CambridgeMA MIT Press

KUHL P K WILLIAMS K A LACERDA F STEVENS K N amp LINDBLOM B (1992)Linguistic experience alters phonetic perception in infants by six months of age Science255 606ndash 608

LADEFOGED P (2001) A course in phonetics (4th ed) Orlando FL Harcourt BraceLOGAN J S LIVELY S E amp PISONI D B (1991) Training Japanese listeners to identify

English r and l A first report Journal of the Acoustical Society of America89 874 ndash 886

MACWHINNEY B (1999) The CHILDES project Tools for analyzing talk (3rd ed) HillsideNJ Lawrence Erlbaum Associates

MADDIESON I (1984) Patterns of sounds Cambridge MA Cambridge University PressMATTYS S JUSCZYK P W LUCE P A amp MORGAN J L (1999) Phonotactic and

prosodic effects on word segmentation in infants Cognitive Psychology 38 465 ndash 494MAYE J WERKER J F amp GERKEN L (2002) Infant sensitivity to distributional infor-

mation can affect phonetic discrimination Cognition 82 B101ndashB111

Language and Speech


MORGAN J L SINGH L BORTFELD H RATHBUN K amp WHITE K (2001)Effects of speech and sentence position on infant word recognition Paper presented at theBoston University Conference on Language Development Boston MA

NEWMAN R S SAWUSCH J amp LUCE P D (1997) Lexical neighborhood effects inphonetic processing Journal of Experimental Psychology Human Perception andPerformance 23 873 ndash 889

PARADIS C amp PRUNET J-F (1991) Phonetics and phonology 2 The special status ofcoronals San Diego CA Academic Press

PEGG J E amp WERKER J F (1997) Adult and infant perception of two English phonesJournal of the Acoustical Society of America 102 3742 ndash 3753

PETERSON G E amp BARNEY H L (1952) Control methods used in a study of vowelsJournal of the Acoustical Society of America 24 175ndash 184

PIERREHUMBERT J B (2001) Exemplar dynamics Word frequency lenition and contrastIn J H Greenberg (Series Ed) amp J Bybee amp P Hopper (Vol Eds) Typological studiesin language Vol 45 Frequency and the emergence of linguistic structure (pp 137ndash 157)Philadelphia PA John Benjamins Publishing Company

PIERREHUMBERT J (in press) Probabilistic phonology Discrimination and robustnessIn R Bod J Hay amp S Jannedy (Eds) Probability theory in linguistics CambridgeMA MIT Press

PITT M amp SAMUEL A (1993) An empirical and meta-analytic evaluation of the phonemeidentification task Journal of Experimental Psychology Human Perception andPerformance 19 699 ndash 725

PLAUT D C amp KELLO C T (1999) The emergence of phonology from the interplay ofspeech comprehension and production A distributed connectionist approach In BMacWhinney (Ed) The emergence of language (pp 381ndash 415) Mahwah NJ LawrenceErlbaum

POLKA L (1991) Cross-language speech perception in adults Phonemic phonetic andacoustic contributions Journal of the Acoustical Society of America 89 2961ndash 2977

POLKA L JUSCZYK P W amp RVACHEW S (1995) Methods for studying speech per-ception in infants and children In W Strange (Ed) Speech perception and linguisticexperience Theoretical and methodological issues in cross-language speech researchTimonium MD York

POLKA L amp WERKER J F (1994) Developmental changes in perception of non-nativevowel contrasts Journal of Experimental Psychology 20 421 ndash 435

QUINN P C EIMAS P D amp ROSENKRANTZ S L (1993) Evidence for representationsof perceptually similar natural categories by three-month-old and four-month-old infantsPerception 22 463ndash 475

SAFFRAN J R ASLIN R N amp NEWPORT E L (1996) Statistical learning by eight-month-old infants Science 274 1926ndash 1928

SAFFRAN J R NEWPORT E L ASLIN R N TUNICK R A amp BARRUECO S(1997) Incidental language learning Listening (and learning) out of the corner of yourear Psychological Science 8 101 ndash 105

SHELDON A amp STRANGE W (1982) The acquisition of r and l by Japanese learnersof English Evidence that speech production can precede speech perception AppliedPsycholinguistics 3 243ndash 261

SHI R WERKER J F amp MORGAN J L (1999) Newborn infantsrsquo sensitivity to per-ceptual cues to lexical and grammatical words Cognition 72 B11ndash B21

SMOLENSKY P DAVIDSON L amp JUSCZYK P (in press) The initial and final statesTheoretical implications and experimental explorations of the richness of the base InR Kager J Pater amp W Zonnevald (Eds) Fixing priorities Constraints in phonologicalacquisition Cambridge UK Cambridge University Press

Language and Speech


TABOR W JULIANO C amp TANENHAUS M K (1997) Parsing in a dynamical systemAn attractor-based account of the interaction of lexical and structural constraints insentence processing Language and Cognitive Processes 12 211ndash 271

TABOR W amp TANENHAUS M K (1999) Dynamical models of sentence processingCognitive Science 23 491 ndash 515

TOBIAS J V (1959) Relative occurrence of phonemes in American English Journal of theAcoustical Society of America 31 631

TREHUB S E (1976) The discrimination of foreign speech contrasts by infants and adultsChild Development 47 466ndash 472

VOLAITIS L E amp MILLER J L (1992) Phonetic prototypes Influence of place of artic-ulation and speaking rate on the internal structure of voicing categories Journal of theAcoustical Society of America 92 723 ndash 735

WERKER J F (1991) The ontogeny of speech perception In I G Mattingly amp M Studdert-Kennedy (Eds) Modularity and the motor theory of speech perception (pp 91 ndash 110)Hillside NJ Lawrence Erlbaum Associates

WERKER J F (1994) Cross-language speech perception Developmental change does notinvolve loss In J Goodman amp H Nusbaum (Eds) The development of speech perceptionThe transition from speech sounds to spoken words (pp 93 ndash 120) Cambridge MA MITPress

WERKER J F amp CURTIN S (in press) The PAUSE Mir model of infant speech pro-cessing In D B Pisoni amp R Remez (Eds) The handbook of speech perception

WERKER J F GILBERT J H V HUMPHREY K amp TEES R C (1981) Developmentalaspects of cross-language speech perception Child Development 52 349 ndash 355

WERKER J F amp LOGAN J S (1985) Cross-language evidence for three factors in speechperception Perception amp Psychophysics 37 35 ndash 44

WERKER J F amp POLKA L (1993) Developmental changes in speech perception Newchallenges and new directions Journal of Phonetics 21 83 ndash 101

WERKER J F POLKA L amp PEGG J E (1997) The conditioned head turn procedureas a method for testing infant speech perception Early Development and Parenting6 171 ndash 178

WERKER J F amp TEES R C (1984) Cross-language speech perception Evidence for per-ceptual reorganization during the first year of life Infant Behavior and Development7 49 ndash 63

WERKER J F amp TEES R C (1999) Influences on infant speech processing Toward anew synthesis Annual Review of Psychology 50 509 ndash 535

Language and Speech


1 Introduction

Infants begin life with the ability to learn any of the worldrsquos languages While adultsare often hampered in their ability to identify or discriminate phones that are not usedcontrastively in the phonological system of their language(s) young infants do notshow such language-specific effects In the early months of life infants discriminateboth native and non-native contrasts with equal ease As experience with a nativelanguage accumulates evidence of increased attunement to properties of the ambientlanguage emerges By the end of the first year infants like adults fail to discriminatemany non-native contrasts In this article we begin to explore whether there may bea simple statistical basis for the particular order in which non-native contrasts arelost

Adults show difficulty discriminating non-native single-feature contrasts if thespeech sounds are not used to distinguish meaning in their native language A classicexample is the difficulty that native Japanese-speaking adults have in discriminatingbetween English ra versus la (Sheldon amp Strange 1982) The same pattern isevident in English-speaking adultsrsquo difficulties in discriminating Hindi retroflex anddental stops (Werker Gilbert Humphrey amp Tees 1981) Nthalamkampx ThompsonSalish velar and uvular ejectives (Werker amp Tees 1984) and Czech retroflex andpalatal fricatives (Trehub 1976)

Infants younger than six to eight months however can categorically discriminateboth contrasts that are phonemic (eg for English ba-pa Eimas SiquelandJusczyk amp Vigorito 1971) and contrasts that are not phonemic in their nativelanguage (for a review see Werker amp Tees 1999) By 10 to 12 months infants performlike adults in their discrimination of non-native contrasts This change in discrimi-native abilities within the first year of life has been reported for English-learninginfants for several different consonant contrasts including the Hindi stops the Salishejectives and the Czech fricatives cited above as well as several different Zulu con-trasts mdash bilabial plosive implosive lateral voiced voiceless fricative and velarvoiceless ejective distinctions (Best 1995) Similar loss of discriminatory ability hasbeen reported for Japanese-learning infantsrsquo discrimination of English r and l (Kuhl1998) The same developmental pattern emerges in the perception of vowel con-trasts although the discriminative decline appears to occur somewhat earlier KuhlWilliams Lacerda Stevens and Lindblom (1992) found evidence of distinctive pro-totype effects for vowels in English- and Swedish-learning infants by six monthsand Polka and Werker (1994) found effects of native language experience on perceptionof vowels between four and six months

Initially this developmental pattern was explained as an absolute loss of abilitydue to lack of exposure (see Werker 1994 for a detailed historic overview) Such aMaintenance Theory assumes that experience is required to maintain perceptual sen-sitivities that are already present in the infant (Aslin amp Pisoni 1980) Evidence fromBest McRoberts and Sithole (1988) however contradicts such a view Infants andadults were tested on their ability to discriminate a Zulu apical versus lateral clickcontrast in which neither sound was like any English sound Unlike other non-nativecontrasts previously tested both older infants and adults showed high levels of

Language and Speech






Language and Speech






Language and Speech




2Experiment 1


Language and Speech



21Method





Language and Speech






Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech






Language and Speech






Language and Speech




2Experiment 1


Language and Speech



21Method





Language and Speech






Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech






Language and Speech




2Experiment 1


Language and Speech



21Method





Language and Speech






Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech




2Experiment 1


Language and Speech



21Method





Language and Speech






Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech



21Method





Language and Speech






Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech






Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech


TABLE 1




Word Initial 18099 14375 40746 27157


Any 106832 38325 157297 55720





Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech







Language and Speech









Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech








Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech



TABLE 2




8 035 7 18 156 12

9 049 8 19 167 13

10 061 8 20 178 13

11 074 9 21 188 14

12 086 9 22 199 14

13 097 10 23 210 14

14 109 10 24 220 15

15 121 11 25 231 15

16 133 11 26 241 16

17 144 12


Language and Speech








Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech







Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech





Language and Speech


Figure 1







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech







Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech




3Experiment 2


31Method





Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech









Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech


4Experiment 3




Language and Speech


Figure 2




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech




41Method





Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech






Language and Speech


Figure 3



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech



TABLE 3


Coronal Dorsal


Hit 139242 48283 149194 42272 109 gt25

Miss 140965 43687 147912 50987 104 gt30

False Alarm 157992 61003 164665 67327 058 gt50




Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech




5General Discussion




Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech






Language and Speech








Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech







Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech







Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech




References










Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech

























Language and Speech




















Language and Speech
















Language and Speech




















Language and Speech
















Language and Speech
















Language and Speech


A Statistical Basis for Speech Sound...

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Transcript of A Statistical Basis for Speech Sound...