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8/13/2019 Purser 2013 (15) Poor Phonemic Discrimination Does Not Underlie Poor Verbal Short-term Memory in Down Syndro
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Poor phonemic discrimination does not underlie
poor verbal short-term memory in Down
syndrome
Harry R.M. Purser a,, Christopher Jarrold b
a School of Psychology, Criminology, and Sociology, Faculty of Arts and Social Sciences, Kingston University, Kingston KT1 2EE, UKb School of Experimental Psychology, University of Bristol, Clifton, Bristol BS8 1TU, UK
a r t i c l e i n f o
Article history:
Received 31 October 2012
Revised 24 December 2012
Available online 28 February 2013
Keywords:
Down syndrome
Memory
Short-term memory
Verbal
Language
Phonological
Phonemic discrimination
Recognition
a b s t r a c t
Individuals with Down syndrome tend to have a marked impair-
ment of verbal short-term memory. The chief aim of this study
was to investigate whether phonemic discrimination contributes
to this deficit. The secondary aimwas to investigate whether phono-
logical representations are degraded in verbal short-term memory
in people with Down syndrome relative to control participants. To
answer these questions, two tasks were used: a discrimination task,
in which memory load was as low as possible, and a short-term rec-
ognition task that used the same stimulus items. Individuals with
Down syndrome were found to perform significantly better than a
nonverbal-matched typically developing group on the discrimina-
tion task, but they performed significantly more poorly than that
group on the recognition task. The Down syndrome group was out-
performed by an additional vocabulary-matched control group on
the discrimination task but was outperformed to a markedly greater
extent on the recognition task. Taken together, the results strongly
indicate that phonemic discrimination ability is not central to theverbal short-term memory deficit associated with Down syndrome.
2013 Elsevier Inc. All rights reserved.
Introduction
Individuals with Down syndrome (DS) tend to perform poorly on tests of verbal short-term mem-
ory (Mackenzie & Hulme, 1987; Marcell & Armstrong, 1982; Marcell & Weeks, 1988). This deficit is
0022-0965/$ - see front matter 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jecp.2012.12.010
Corresponding author.
E-mail address: [email protected](H.R.M. Purser).
Journal of Experimental Child Psychology 115 (2013) 115
Contents lists available at SciVerse ScienceDirect
Journal of Experimental Child
Psychologyj o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j ec p
http://dx.doi.org/10.1016/j.jecp.2012.12.010mailto:[email protected]://dx.doi.org/10.1016/j.jecp.2012.12.010http://www.sciencedirect.com/science/journal/00220965http://www.elsevier.com/locate/jecphttp://www.elsevier.com/locate/jecphttp://www.sciencedirect.com/science/journal/00220965http://dx.doi.org/10.1016/j.jecp.2012.12.010mailto:[email protected]://dx.doi.org/10.1016/j.jecp.2012.12.010http://crossmark.dyndns.org/dialog/?doi=10.1016/j.jecp.2012.12.010&domain=pdf -
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reflected by the relatively poor performance of people with DS on the digit span task, in which partic-
ipants listen to a series of digits spoken by an experimenter and then attempt to repeat them back in
serial order. Numerous studies have shown that the digit spans of individuals with DS are poorer than
those of matched control participants (Jarrold & Baddeley, 1997; McDade & Adler, 1980; see also Nss,
Lyster, Hulme, & Melby-Lervg, 2011). Individuals with DS have also been found to perform more
poorly on verbal short-term memory tasks than control participants when matched for performance
on nonverbal short-term memory tasks (Brock & Jarrold, 2004; Jarrold & Baddeley, 1997; Jarrold,
Baddeley, & Hewes, 1999; Jarrold, Baddeley, & Phillips, 2002; Purser & Jarrold, 2005), indicating that
this deficit is specific to the verbal modality.
This deficit in verbal short-term, or phonological, memory may give rise to further cognitive defi-
cits. Baddeley and colleagues (Baddeley, Gathercole, & Papagno, 1998; Gathercole & Baddeley, 1990)
have argued that phonological memory might play a causal role in vocabulary acquisition and lan-
guage comprehension. Although direct evidence for this suggestion in the context of Down syndrome
is rare (Laws, 1998; Mosse & Jarrold, 2011), language abilities, and expressive language abilities in
particular, are poorer in individuals with DS than predicted by their general cognitive abilities (e.g.,
Chapman, 1995, 1997; Fowler, 1990; Nss et al., 2011).
However, Hulme and Roodenrys (1995)argued that, in contrast to Baddeley and colleagues sug-gestion, the verbal short-term memory deficit associated with DS might be a consequence of language
difficulties because performance on phonological memory tasks is influenced by general language
abilities. Successful performance on verbal short-term memory tasks is likely to depend on the ability
to encode phonological representations (cf.Brady, 1997) because one cannot correctly output an item
from memory that was not correct at the input to that memory system. Metsala (1999)argued that
typically developing childrens phonological representations emerge as a consequence of vocabulary
development. In this way, the general language delay shown by individuals with DS could result in
relatively poor phonological discrimination skills and, consequently, poor verbal short-term memory
performance.
Phonological awareness does appear to be an area of relative difficulty for individuals with DS.
Recently, Roch and Jarrold (2008) assessed the phonological awareness skills of a DS group, andreading-matched controls, with three paradigms. One was initial sound detection, where participants
attempted to match a target picture to one of three response pictures on the basis of sharing the same
initial sound (e.g., Which starts with the same sound as beetable, bed, or sun?). Another was
phoneme deletion, which was presented in a similar fashion; the task involved deciding which of
three pictures would match the sound of the target picture following a particular deletion (e.g., If
d is removed from deer, which would matchdoor, eye, or ear?). The remaining task was rhyme
detection, in which participants tried to decide which of three response pictures rhymed with a target
picture. Although the DS group performed more poorly than controls on each phonological awareness
task, the DS group demonstrated particular difficulties on the rhyme detection test, in line with other
studies (Cardoso-Martins, Michalick, & Pollo, 2002; Snowling, Hulme, & Mercer, 2002, see alsoNss,
Melby-Lervg, Hulme, & Lyster, 2012).Another aspect of phonological awareness is phonemic discrimination, sometimes referred to as
segmental awareness. Phonemic discrimination abilities of individuals with DS were investigated by
Brock and Jarrold (2004) with an item discrimination task similar to that developed by Bridgeman
and Snowling (1988). In this task, participants were auditorily presented with pairs of words or non-
words and then asked to respond as to whether the two items were the same or different. The task
performance of the DS group was found to be impaired relative to that of the control participants.
However, there was a possible limitation of the discrimination task used; the participants needed
to hold two items in memory and access the phonemic features of both simultaneously to be success-
ful at the task. Given that individuals with DS show impaired verbal short-term memory, relatively
poor performance on the task may simply reflect poor memory ability. Thus, it remains possible that
individuals with DS do not have a particular impairment of phonemic discrimination ability. Further-more, it is possible that any group that scores poorly on a test of phonemic discriminationor indeed
any measure of phonological awareness more generallymight do so because of poor verbal short-
term memory if the test makes demands on that system (see Ramus & Szenkovits, 2008, for a detailed
discussion of how findings of phonological discrimination can depend on noncentral task demands).
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This might be one reason why individuals with DS struggle with rhyme detection, for example. There
is evidence that children with specific language impairment have difficulties in identifying and encod-
ing phonemes, especially with consonant rather than vowel discriminations (Leonard, McGregor, & Al-
len, 1992). Because both children with specific language impairment and individuals with DS show
generally delayed language abilities (Laws & Bishop, 2003), it is possible that individuals with DS have
a similar pattern of difficulties.
In addition to possible atypicalities in the discrimination of vowels and consonants, individuals
with DS may have atypical short-term memory for vowel or consonant information given their
verbal short-term memory difficulties. Results from a study of typical development suggest that,
for verbal stimuli, vowels are more useful than consonants as placeholders in serial memory,
providing the basis for correct ordering. Using CV (consonantvowel) syllables, Drewnowski
(1980) constructed vowel-only lists, in which the order of consonants remained constant across
trials but the order of vowels was altered. In addition, consonant-only lists were constructed,
where only the order of consonants was varied across trials. Order recall of vowel-only lists was
markedly better than that for consonant-only lists, whereas item recall was roughly equivalent
for these two types of lists. Importantly, the superior order recall of vowel-only lists over conso-
nant-only lists was apparent even when the vowel-only lists were more phonologically confusable.Consistent with this notion, Service, Maury, and Luotoniemi (2005) found impaired recall of
vowel-redundant lists (where each list item had the same vowels) but found unimpaired recall
of consonant-redundant lists (see Luotoniemi, Service, & Maury, 2007, for a review of short-term
recall for vowels and consonants).
Given the above findings, the current study had two main aims: (a) to investigate systematically
whether phonemic discrimination difficulties contribute to the verbal short-term memory deficit
associated with DS and (b) to investigate whether phonological representations are degraded in
verbal short-term memory in people with DS relative to typically developing control participants.
A discrimination task was administered, in which memory load was as low as possible (similar to
that used by Jarrold, Thorn, & Stephens, 2009), using the same stimulus items as a subsequent
recognition memory task. In this way, differences across tasks could not be attributed to differencesin stimuli.
There was also a phonemic manipulation to the task, so that the names of the pictures differed
either in a single consonant or a vowel (e.g., a pig and a pin, a coat and a kite). In addition, the verbally
presented word was masked with either a high or low amount of noise. The effect of noise masking
was to make stimuli less discriminable without manipulating phonology. In this way, it was possible
to assess whether individuals with DS are poorer at phonemic discrimination than matched control
participants and, if so, whether this problem is strictly one of phonemic discrimination or a more gen-
eral auditory discrimination problem.
In addition to the discrimination task, a recognition memory task was employed, based on the test
of item memory used by Brock and Jarrold (2004). If individuals with DS were found to be particularly
susceptible to the phonemic manipulation (whether a consonant or vowel was substituted), thiswould imply that the quality of phonemic representations within phonological memory are degraded
in people with DS.
It was important to include a control group matched on nonverbal ability, rather than vocabulary
age, because there is evidence that verbal short-term memory is a causal factor in determining
vocabulary acquisition in young children (Baddeley et al., 1998; Gathercole & Baddeley, 1990). Con-
sequently, matching for performance on vocabulary tasks would bias toward finding group differ-
ences in verbal short-term memory performance. Hence, if a group of 5-year-olds had the same
average vocabulary mental age as a 20-year-old comparison group with a developmental disorder,
one would expect the older group to have a smaller verbal short-term memory span than the youn-
ger group (cf. Jarrold, Baddeley, Hewes, Leeke, & Phillips, 2004). As a result, our approachnamely
matching for nonverbal ability while allowing for a vocabulary advantage in DSwould bias againstfinding a verbal short-term memory deficit in the DS group. Nevertheless, a vocabulary age-matched
control group was also included to ensure that any group differences found between individuals
with DS and nonverbal ability matched controls could not be attributed to differences in verbal
knowledge.
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Method
Participants
There were three groups: 14 individuals with DS (DS), 19 typically developing (TD) children
matched for nonverbal mental age, and 19 TD children matched for vocabulary age. All members of
the DS group had confirmed trisomy 21 without mosaicism and were recruited via local support
groups and schools. The TD children were recruited from local schools. Selection of children was on
the basis of being judged at average ability for age by their teachers. All participants had good hearing
and demonstrated a clear understanding of what the task required. All participants were fully in-
formed about the aims of the experiment, had parental consent, and also consented on the day of test-
ing. In a preliminary test session, participants were tested on the British Picture Vocabulary Scale II
(BPVS;Dunn, Dunn, Whetton, & Burley, 1997), a measure of receptive vocabulary. Participants were
also assessed on Ravens Coloured Progressive Matrices (RCPM; Raven, Raven, & Court, 1998), a test
of nonverbal mental age. The DS group had a mean chronological age of 19 years 4 months (19;4 years,
range = 13;726;4, SD= 3;8), a mean vocabulary mental age of 7;11 years (range = 4;1010;7,
SD= 1;6), and a mean RCPM score of 18.6 (range = 1230,SD= 5.1). The nonverbal-matched grouphad a mean chronological age of 6;0 years (range = 5;26;10, SD= 0;6), a mean vocabulary age of
5;11 years (range = 4;38;0, SD= 1;0), and a mean RCPM score of 18.4 (range = 1324, SD= 3.4).
The vocabulary-matched group had a mean chronological age of 8;5 years (range = 8;08;11,
SD= 0;3), a mean vocabulary age of 8;0 years (range = 5;59;4,SD= 1;1), and a mean RCPM score
of 27.7 (range = 1234,SD = 6.2).
A series of Bonferroni-corrected ttests showed that the individuals with DS were significantly older
than the nonverbal-matched group, t(31, Welch-corrected) = 13.09, p< .001, had reliably higher
vocabulary mental ages, t(31) = 4.16, p< .001, but were closely matched for nonverbal mental age,
t(31) = 0.11, p= .912 (without Bonferroni adjustment). The DS group was reliably older than the
vocabulary-matched group, t(31, Welch-corrected) = 10.75,p< .001, with poorer average nonverbal
mental age, t(31) = 4.61, p< .001, but the groups were closely matched for vocabulary age,t(31) = 0.28,p = .781 (without Bonferroni adjustment).
Procedure
Discrimination task
A total of 32 monosyllabic words, in 16 phonologically similar pairs, were used in the experiment.
Of these 16 pairs, 8 differed by one consonant (fish/dish, sun/nun,goat/boat, hair/chair,pig/pin, lock/sock,
box/fox, and ring/king) and 8 differed by the vowel (bell/ball, cat/cot, girl/goal, boy/bee, kite/coat, man/
moon, pan/pen, and wall/well). Stimulus details are given in the Appendix. Stimulus words were re-
corded using Sound Edit 16 (all samples 16 bits, 32 kHz). Two sets of words were recorded: one with
an adult male voice and one with an adult female voice.On each trial, a pair of large cartoon pictures was displayed on a computer touch-screen, each
depicting one member of one of the above word pairs (e.g., fish anddish). After 1 s had elapsed, a
word of 500 ms duration was auditorily presented in a male voice. This word corresponded to
one of the displayed pictures (e.g., either fish or dish). The participant was required to touch the pic-
ture that corresponded to the auditorily presented word. To successfully perform the task, the par-
ticipant needed only to hold a single word in mind long enough to make a single response. In
addition, the two response pictures remained visible until a response was made, reducing memory
demands even further.
The experimental task involved two different types of phonemic distinction (consonant and vowel)
and two levels of noise masking (high and low). These factors were crossed to give four conditions:
consonanthigh, consonantlow, vowelhigh, and vowellow.There were 32 trials of each of these inter-mixed within four blocks. The dependent variable was the number of correct responses. The stimuli
were spread evenly across conditions.
Low noise masking was achieved by mixing a normalized recording of pink noise at 20%
intensity (of the speech stimulus). High noise masking was achieved by mixing each word
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recording with the pink noise at 80% intensity. Pink noise is weighted, such that each band-
width is equal in power and, thus, masks evenly across the broad frequency spectrum of natural
speech.
Prior to testing, participants were first shown each of the experimental pictures individually and
required to name them, with feedback given as to which words they represented in the forthcoming
experimental task. This was then repeated to ensure that participants were familiar with the verbal
labels ascribed to the pictures. Participants were then given 12 practice trials, balanced across the four
conditions, with feedback as to the correct responses.
The discrimination task was always administered prior to the memory task in a separate
session.
Memory task
Presentation was on a laptop, prominently featuring a cartoon gray mole character to the left of the
screen and a cartoon pink mole character to the right. On each trial, a list of one, two, or three words
were presented auditorily, spoken by the gray mole in a male voice. This was the presentation list. In
a female voice, the pink mole would then speak a sequence of the same number of words, which wasthe recognition list. The presentation of each word lasted 500 ms, with an interstimulus interval (ISI)
of 500 ms. A 1-s interval separated the presentation and recognition lists. The lists were constructed
from the word pairs used in the discrimination task.
On half of all trials, the recognition list was the same as the presentation list. Only noise-masking
level was manipulated on these trials, giving the two control conditions: samehigh and samelow. The
task involved two different types of phonemic distinction (consonant and vowel) and two levels of
noise masking (high and low). These factors were crossed to give four experimental conditions: con-
sonanthigh, consonantlow, vowelhigh, andvowellow. Only the presentation list was masked with
noise; no noise was applied to the recognition list in any condition.
The dependent measure wasA0, a nonparametric analog ofd0.A0 is a measure of discriminability like
d0, but it can be calculated when the participant has a hit or false alarm rate of 1 or 0 in addition to
intermediate values. Unliked0,A0 does not require homogeneous variance. Values ofA 0 vary from 0 to
1, with 0.5 indicating chance performance (seeMcNicol, 1972). Essentially,A 0 is a measure of perfor-
mance that takes response biases into account; therefore, it is particularly suited to yes/no recogni-
tion tasks and is calculated as follows:
A0
1
2 H F
1 H F=4H1 F; 1
whereH is the hit rate (proportion of trials on which the participant correctly detected a difference
between lists) and Fis the false alarm rate (proportion of trials on which the participant incorrectly
reported a difference between lists).
There were 16 trials of one-item list length consisting of 2 consonanthigh (e.g., box/fox, highnoise), 2 consonantlow, 2 vowelhigh (e.g., bell/ball, high noise), 2 vowellow, 4 samehigh, and 4
samelow trials. There were 32 trials of two-item list length made up of 4 consonanthigh, 4 conso-
nantlow, 4 vowelhigh, 4 vowellow, 8 samehigh, and 8 samelow trials. There were 24 trials of
three-item list length composed of 3 consonanthigh, 3 consonantlow, 3 vowelhigh, 3 vowel
low, 6 samehigh, and 6 samelow trials. Over trials where a phonemic substitution was made during
the recognition phase, the substitutions were evenly spread across serial position. The stimuli were
also spread evenly across conditions.
The participant was required to judge whether the pink mole had correctly repeated the gray mo-
les words (She got it right) or whether a word had been substituted (She got it wrong). Responses
were verbal and were recorded into the computer by the experimenters key-press. If a participant
changed his or her mind about a response on a particular trial, it was noted and the response filewas amended after testing.
Participants were given practice trials prior to testing, with 4 trials each at list lengths of one, two,
and three items. Feedback was given as to the correct responses throughout these trials. Across these
12 trials, the above conditions were evenly represented.
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Results
Discrimination task
The data were analyzed using a three-way mixed-design analysis of variance (ANOVA), with with-
in-participants factors of noise masking and phonemic distinction and with a between-participants
factor of group. Post hoc comparisons were paired-samplesttests with Bonferroni correction, with al-
pha level set atp = .016. Descriptive statistics for the groups performance under the different condi-
tions of the discrimination task are given in Table 1. There was a significant main effect of group,
F(1, 49) = 39.34, p< .001, gp2 = .616, due to the DS group performing better than the nonverbal-
matched group and the vocabulary-matched group performing better than both other groups (all
ps < .001). The analysis also revealed a significant main effect of noise masking, F(1, 49) = 341.29,
p< .001, gp2 = .874, due to superior performance with low noise masking, but no significant main ef-
fect of phonemic distinction,F(1,49) < 1,p = .54,gp2 = .008.
There was a reliable interaction of noise masking and group, F(2, 49) = 38.69,p< .001, gp2 = .612. All
groups performed better with low noise than with high noise (all Fs > 50, all ps < .001). For high
noise-masking trials, there was a reliable group effect, F(2, 49) = 44.60, p< .001, gp2 = .645; the DSgroup performed better than the nonverbal-matched group (p< .05), and the vocabulary-matched
group performed better than both other groups (both ps < .001). For low noise-masking trials, there
was also a reliable group effect, F(2,49) = 6.22, p< .01, gp2 = .203; the nonverbal-matched group
performed worse than both the vocabulary-matched and DS groups, but there was no reliable
difference between the DS and vocabulary-matched groups.
There was also an interaction of phonemic distinction and group, F(2,49) = 3.89,p< .05,gp2 = .137.
Post hoc analysis of simple effects revealed that the nonverbal-matched group was marginally better
at discriminating consonant changes over vowel changes, F(1, 18) = 4.00,p = .061, gp2 = .182, whereas
the vocabulary-matched group was marginally better at discriminating vowel changes over consonant
changes, F(1,18) = 3.27, p= .087, gp2 = .154. However, the DS group showed no such difference,
F(1,13) < 1. The interaction, then, arose from two marginal effects in opposite directions coupled witha lack of effect in the DS group.
Furthermore, there was a three-way interaction of group, phonemic distinction, and noise mask-
ing, F(2,49) = 3.89, p< .05, gp2 = .137. There was no significant interaction of phonemic distinction
and noise masking for either the DS group, F(1, 13) < 1, or the nonverbal-matched group,
F(1,18) = 1.74, p= .204, gp2 = .088; however, this interaction was reliable for the vocabulary-
matched group, F(1,18) = 9.87, p< .01,gp2 = .354. Restricting analysis to the high noise-masking tri-
als, the vocabulary-matched group was significantly better at discriminating vowel pairs than con-
sonant pairs, F(1,18) = 8.84, p< .01, gp2 = .329, but on low noise-masking trials, the vocabulary-
matched groups performance did not differ between vowels and consonants, F(1,18) < 1. Thus,
the only effect of phonemic distinction evident in the discrimination task was the vocabulary-
Table 1
Mean numbers of correct responses by group, noise masking, and phonological distinction in the discrimination task.
Group Noise masking Phonological distinction
Vowel Consonant
Mean SD Mean SD
DS High 24.1 3.76 24.1 3.52
Low 31.3 1.23 31.4 0.84
Nonverbal-matched High 21.0 2.40 22.5 2.80
Low 29.9 1.52 30.4 1.54
Vocabulary-matched High 29.5 1.78 28.2 1.98
Low 31.0 1.47 31.1 0.81
Note. Maximum possible score = 32. DS: Down syndrome group; Vocabulary-matched: control group matched on the British
Picture Vocabulary Scale; Nonverbal-matched: control group matched on the Ravens Coloured Progressive Matrices.
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matched groups superior performance with vowels over consonants in high noise. There are sev-
eral possible explanations for this result (e.g., superior phonotactic knowledge in the vocabulary-
matched group facilitating more of a vowel advantage in the absence of ceiling effects). However,
this result does not speak directly to the current research questions and, therefore, is not consid-
ered further.
Recognition task
The data were analyzed using a four-way mixed-design ANOVA, with within-participants factors of
noise masking, phonemic distinction, and list length and with a between-participants factor of group.
Descriptive statistics for the groups performance under the different conditions of the recognition
task are given in Table 2. There was a significant main effect of noise masking, F(1, 49) = 62.19,
p< .001, gp2 = .559, due to superior performance with low noise masking and also a significant main
effect of phonemic distinction, F(1, 49) = 8.58, p < .01,gp2 = .149, owing to superior recognition of vo-
wel changes over consonant changes. There was a reliable main effect of list length, F(2, 98) = 44.16,
p< .001, gp2 = .474. Performance was significantly better with a list length of one item than of two
or three items, and it was also better with a list length of two items than of three items. Furthermore,
there was a reliable main effect of group, F(2, 49) = 109.20, gp2 = .817, due to the DS group performing
more poorly than the nonverbal-matched group and the vocabulary-matched group performing better
than both other groups.
There was a reliable interaction of noise masking and group, F(2,49) = 9.18, p< .001, gp2 = .272;
although both the DS group, F(1, 13) = 14.16, p< .01, gp2 = .521, and nonverbal-matched group,
F(1, 18) = 50.72, p< .001, gp2 = .738, performed reliably better on low noise-masking trials than
on high ones, the vocabulary-matched group showed only a trend in the same direction,
F(1, 18) = 3.702,p= .070, gp2 = .171, perhaps reflecting near-ceiling effects for this group on the rec-
ognition task.
There was a significant interaction of phonemic distinction and group, F(2, 49) = 10.27,p< .001,
gp2 = .295 (seeFig. 1). Post hoc analysis of simple effects revealed that the nonverbal-matched groupwas better at recognizing vowel changes over consonant changes, F(1, 18) = 38.87,p< .001, gp
2 = .683,
but neither the DS group, F(1,13) < 1, nor the vocabulary-matched group, F(1,18) = 2.18, p= .157,
gp2 = .108, showed such a difference. Performance on the consonant contrasts was well above chance
for both the DS group, t(13) = 37.04, p< .001, and the vocabulary-matched group, t(18) = 55.28,
p< .001, which was close to ceiling.
Table 2
A0 values of correct responses by group, noise masking, phonological distinction, and list length position in the recognition task.
Group Noise masking Phonological distinction List length
1 2 3
Mean SD Mean SD Mean SD
DS High Vowel .75 .17 .59 .13 .54 .20
Consonant .59 .12 .63 .12 .53 .16
Low Vowel .81 .14 .69 .15 .59 .13
Consonant .83 .11 .74 .14 .58 .17
Nonverbal-matched High Vowel .78 .11 .68 .13 .61 .11
Consonant .63 .15 .64 .14 .60 .12
Low Vowel .85 .09 .81 .06 .79 .09
Consonant .86 .13 .81 .09 .62 .14
Vocabulary-matched High Vowel .95 .12 .96 .03 .85 .16
Consonant .91 .11 .96 .08 .94 .09
Low Vowel .92 .08 .96 .06 .97 .04
Consonant .98 .02 .96 .05 .93 .08
Note. DS: Down syndrome group; Vocabulary-matched: control group matched on the British Picture Vocabulary Scale;
Nonverbal-matched: control group matched on the Ravens Coloured Progressive Matrices.
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There was a reliable interaction of length and group, F(4,98) = 9.16,p< .001, gp2 = .272. The effect of
group was significant at each list length (all ps < .001), but the pattern of group differences changed
with list length; at a list length of one item, the vocabulary-matched group outperformed both the
DS and nonverbal-matched groups, but there was no reliable difference between the DS and
nonverbal-matched groups. At longer list lengths, the DS group scored lower than both the vocabu-
lary-matched and nonverbal-matched groups, and the vocabulary-matched group outperformed the
nonverbal-matched group. Thus, only longer list lengths distinguished the memory performance of
the DS and nonverbal-matched groups, with the vocabulary-matched group performing consistently
better than the other groups.
There was no significant interaction of noise masking and phonemic distinction, F(1,49) = 1.25,
p= .269, gp2 = .025, or of noise masking and length, F(2,98, GreenhouseGeisser) = 2.27, p= .109,
gp2 = .044. However, there was a significant interaction of phonemic distinction and length,
F(2,98) = 3.71, p< .05, gp2 = .070, which was qualified by a three-way interaction of noise masking,
phonemic distinction, and length, F(2, 98) = 25.30, p< .001, gp2 = .341. The interaction of phonemicdistinction and length was reliable on both high noise-masking trials, F(2, 102) = 12.74, p< .001,
gp2 = .200 (see Fig. 2A), and low noise-masking trials, F(2, 102) = 14.94, p< .001, gp
2 = .227 (see
Fig. 2B). Restricting analysis to the high noise-masking trials, post hoc pairwise comparisons re-
vealed that participants were reliably better at recognizing vowel changes over consonant changes
with a list length of one item, but there was no significant difference at a list length of two or three
items. Restricting the analysis to the low noise-making trials, post hoc pairwise comparisons showed
that participants were significantly better at recognizing vowel changes over consonant changes
with a list length of three items, but there was no reliable difference at a list length of one or
two items.
Comparison of performance across tasks
To directly compare performance on the discrimination task with that on the memory task, partic-
ipants mean scores for each task were converted into z-scores. Each participants performance was
Fig. 1. Phonemic distinction effects by group in the memory task, as assessed by A0 . Vocabulary-matched: control group
matched on the British Picture Vocabulary Scale; Nonverbal-matched: control group matched on the Ravens Coloured
Progressive Matrices; DS: Down syndrome group. Vertical lines depict standard errors of the means.
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standardized against the mean score of each TD group in each task. The z-scores of the DS group areplotted inFig. 3separately for standardization on the vocabulary-matched and nonverbal-matched
groups scores. A two-way mixed-design ANOVA was performed on thez-scores for each standardiza-
tion, omitting the nonrelevant TD group in each case, with a within-participants factor of task and
with a between-participants factor of group. The analysis confirmed that the interaction between
A
B
Fig. 2. (A) List length effects by phonemic distinction on high noise-masking trials, as assessed by A0 . Vertical lines depict
standard errors of the means. (B) List length effects by phonemic distinction on low noise-masking trials, as assessed by A 0 .
Vertical lines depict standard errors of the means.
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group and task was highly significant for both the nonverbal-matched group standardization,
F(1, 31) = 20.72, p< .001, gp2 = .401, and the vocabulary-matched group standardization,
F(1, 31) = 31.60,p < .001, gp2 = .505.
Discussion
There were two main aims of the experiment. One was to determine whether individuals with DS
have particular difficulties with phonemic discrimination, which could lead to poor verbal short-term
memory performance. The other was to investigate whether phonological representations are de-
graded in verbal short-term memory in people with DS relative to TD control participants. A discrim-
ination task was employed, in which participants ability to make phonemic and more general
auditory discriminations was assessed. This task had been designed to make minimal demands on
memory. There was also a recognition task that measured participants memory for lists of words
involving single phonological foils.
Individuals with DS were found to perform significantly better than the nonverbal-matched
group on the discrimination task but performed significantly more poorly than the nonverbal-
matched group on the recognition task. Because the same stimulus words were used in both tasks,
affording a close comparison of phonemic discrimination and verbal short-term memory abilities,
the results of the current study strongly indicate that phonemic discrimination ability is not central
to the verbal short-term memory deficit associated with DS. In addition, because relatively good
hearing was necessary to afford good discrimination task performance, the results also strongly
indicate that the verbal short-term memory deficit in DS is not primarily caused by hearing difficul-
ties. These findings on the recognition task are more striking given the fact that the DS group had a
higher average vocabulary age than this control group. Furthermore, higher average vocabulary age
cannot fully account for the DS groups relatively good performance on the discrimination task; anadditional control group, matched on the BPVS, outperformed the DS group on the discrimination
task. However, this vocabulary-matched group outperformed the DS group to a significantly greater
extent on the recognition task (an interaction was found between group and task; see Fig. 3),
Fig. 3. DS groupz-scores by standardization group and task. Vertical lines depict standard errors of the means. RCPM: DS task
scores standardized on the nonverbal-matched group; BPVS: DS task scores standardized on the vocabulary-matched group.
Vertical lines depict standard errors of the means.
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strengthening the finding that phonemic discrimination ability is not central to poor verbal
short-term memory in DS. Reflecting this verbal short-term memory deficit, the DS group demon-
strated poorer performance on the recognition task on longer lists (i.e., two or three items) than
both control groups.
Regardless of participant group, consonants and vowels were differentially affected by task condi-
tions in the recognition task. Participants recognition of consonants was markedly impaired by high
noise masking at a list length of one item compared with their recognition of vowels. However, in low
noise-masking conditions, this advantage for recognition of vowels over consonants was evident only
at a list length of three items. This suggests that recognition varies as an additive function of both
noise masking and list length but that the representations of consonants are more susceptible to deg-
radation. In this way, consonant recognition was markedly impaired by either high noise masking or a
longer list length, whereas both factors were necessary to see a comparable drop in performance for
recognition of vowels.
In a study comparing recall for vowels and consonants, Surprenant and Neath (1996) showed
that even when vowel-contrasting stimuli were manipulated to render them less discriminable
than consonant-contrasting stimuli, serial recall performance of those vowels was nonetheless
superior for TD adults. Similarly, in the recognition task of the current study, the nonverbal-matched group showed markedly superior recognition of vowel changes over consonant changes.
The vocabulary-matched group was very close to ceiling on the recognition task, so there was no
room for an effect of phonemic distinction in that group. However, the DS group also showed
no vowel advantage despite being comfortably between floor and ceiling. Because there was no
reliable evidence of such group differences in the discrimination taskif anything, the DS group
was better at discriminating vowels than the nonverbal-matched groupthis indicates that the
short-term storage of vowels and consonants in individuals with DS does not operate in the same
way as in TD children.
There are at least two interpretations of the group difference in storage of vowels and conso-
nants. First, individuals with DS might have a dysfunction of auditory sensory memory.Crowder
and Morton (1969)proposed a theory of auditory sensory memory (or the precategorical acousticstore), which assumes that auditorily presented items are stored in a relatively uncategorized
code for a period of approximately 2 s in a modality-specific memory system. Lexico-semantic cod-
ing may be accessed within a few hundred milliseconds of presentation (e.g., Marslen-Wilson &
Welsh, 1978), but auditory sensory memory is assumed not to be influenced by such coding. As
items enter auditory sensory memory, they will interfere with other items within it provided that
the new items are acoustically similar to the previous items. Research has shown that auditory
sensory memory tends to be superior for vowels, which are relatively simple acoustically and of
relatively long duration, than for consonants, which are acoustically more complex and of shorter
duration (Cowan, Lichty, & Grove, 1990; Crowder, 1971, 1973; Darwin & Baddeley, 1974; Pisoni,
1973). The DS groups failure to show superior recall of vowels over consonants in the current
study, then, is consistent with an auditory memory dysfunction. In addition, individuals with DSappear to have a verbal short-term memory that is limited in capacity to perhaps even a single
item (Purser & Jarrold, 2010). Therefore, it is possible that this limited capacity reflects, in part,
an impairment of auditory sensory memory that would otherwise support verbal short-term mem-
ory performance.
Second, individuals with DS might have a degradation of the representations in phonological
memory. If memory representations were degraded, one might expect their subsequent identifica-
tion in memory to be more difficult, so that item memory would be poorer. However, this degrada-
tion of items would also render them less discriminable, so that order errors may also result. Of
course, similar behavior would also result from degraded input to phonological memory as a result
of poor phonemic discrimination (although it is possible that degradation at discrimination and deg-
radation within memory itself could interact nonlinearly; e.g., degraded inputs might further de-grade more quickly within memory than undegraded inputs). Phonological similarity effects in
short-term serial recall appear to be due to an increased incidence of order errors rather than item
errors (Gruneberg & Melton, 1972; Watkins, Watkins, & Crowder, 1974; Wickelgren, 1965), and
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evidence from typical development suggests that vowels are more useful than consonants for cor-
rect ordering of items in verbal short-term memory (Drewnowski, 1980; Service et al., 2005). With
this in mind, if individuals with DS have a particular degradation of vowel information in verbal
short-term memory, one might expect to see an increased incidence of order errors rather than item
errors.
Indeed, there is evidence suggesting that individuals with DS have particular difficulty with
order memory in verbal short-term memory tasks. Purser and Jarrold (2005) carried out a mod-
ified verbal probed recall task involving DS and TD participant groups. In one experiment, where
both item memory and order memory were necessary to afford good recall, the DS group per-
formed significantly more poorly than the control group. However, in a second experiment that
required only item memory, the two groups performed at a similar level to each other. Moreover,
Brock and Jarrold (2004) showed that individuals with DS performed more poorly on a test of
short-term verbal order memory than predicted by their performance on a phonological item
memory task.
Whatever the underlying reason for the DS groups lack of an advantage for recognizing vowel
changes over consonant changes, the current study clearly shows that it does not primarily lie in
difficulties of phonemic discrimination that might be related to the hearing loss typically associatedwith the condition. Moreover, due to the strong interaction found between task and group, our results
indicate that the verbal short-term memory deficit associated with DS cannot be primarily attributed
to such difficulties. This confirms findings from previous work (e.g., Brock & Jarrold, 2004; Marcell &
Cohen, 1992) but builds on them in a number of ways. First, in contrast to previous findings, the DS
group of the current study performed better on the discrimination task than the nonverbal-matched
control group, presumably because of the successful removal of memory load in our task. This
suggests that poor performance by people with DS on other tasks, such as nonword repetition (e.g.,
Cairns & Jarrold, 2005; Laws, 1998), is unlikely to owe primarily to poor phonemic discrimination.
Furthermore, the current study demonstrates the need to minimize memory load in any future inves-
tigations of phonemic discrimination, particularly for participants with etiologies associated with poor
verbal short-term memory.It should be noted that the results of the current study do not imply that poor phonemic discrim-
ination plays no role at all in determining verbal short-term memory performance. The fact that the DS
group performed markedly worse on the discrimination task than the vocabulary-matched group
indicates that phonological processing is weak in DS and is delayed relative to lexico-semantic
knowledge. This phonological processing deficit would be expected to constrain verbal short-term
memory performance, particularly when memoranda are less phonologically distinct (e.g., in school
classrooms;McSporran, 1997).
More general, it might benefit researchers to bear in mind the verbal short-term memory
demands made by various tests used to assess phonological awareness (Gathercole, 2006). For
example, in the oddity task, developed byBradley and Bryant (1978), the participant listens to trip-
lets of words and, on oddity trials, is required to select the odd word out. Clearly, however, the childmust hold in mind at least two words to succeed at the task. Similar concerns clearly apply to
various rhyme tasks and also to synthesis tasks, in which participants must hold in mind the various
subcomponents of a word before joining them together to respond with the target word. One
possible way of addressing these concerns would be to take a concomitant measure of verbal
short-term memory to be used as a covariate in subsequent analysis. Where no attempt has been
made to reduce or account for memory demands in a task of phonological awareness, conclusions
should be viewed with caution.
Acknowledgments
This research was funded by a studentship from the Medical Research Council to Harry Purser and
was also supported by Economic and Social Research Council (ESRC) Grant RES-062-33-0005. We
thank Simon Farrell, Jon Brock, and Clive Frankish for their helpful comments and also Ioanna Angus
and Aalia Javaid for additional data collection.
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Appendix. Details, where available, of words used in the experiment
Word Rating
Rated age
of acquisition(months)
Imageability
(scale 17)
Rated
frequency(scale 15)
Ball 12.0 6.40 3.45
Bee 34.2 6.30 2.85
Bell 40.8 6.60 2.50
Boat 30.6 6.30 3.30
Box 32.4 5.60 3.65
Boy 14.4 6.25 4.10
Cat 12.0 6.40 3.40
Chair 28.8 6.45 4.00
Cot
Dish 52.0 5.94 Fish 32.4 6.75 3.05
Fox 48.0 6.55 2.15
Girl
Goat 46.8 6.30 2.00
Hair 18.0 5.75 4.25
King 37.2 6.35 2.05
Kite 49.2 6.65 1.65
Lock
Man
Moon 34.2 6.65 3.00
Nun 74.4 6.20 1.90Pan 44.4 6.70 3.90
Pen 36.0 6.35 4.45
Pig 23.4 6.75 2.50
Pin
Ring 48.0 5.95 3.45
Sock 23.4 6.20 4.05
Sun 16.2 6.70 3.95
Wall
Well 50.4 4.85 2.20
Note.All ratings are from Morrison, Chappell, and Ellis (1997)except those in bold, which are recent norms from the Universityof Bristol (http://www.pc.rhul.ac.uk/staff/c.davis/Articles/Stadthagen-Gonzalez_Davis_in_press.pdf ).
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