Short-term memory in Down syndrome: Applying the working memory model
Christopher Jarrold and Alan Baddeley
This paper is divided into three sections. The first reviews the evidence for a verbal short-term memory deficit in Down syndrome. Existing research suggests that short-term memory for verbal information tends to be impaired in Down syndrome, in contrast to short-term memory for visual and spatial material. In addition, problems of hearing or speech do not appear to be a major cause of difficulties on tests of verbal short-term memory. This suggests that Down syndrome is associated with a specific memory problem, which we link to a potential deficit in the functioning of the 'phonological loop' of Baddeley's (1986) model of working memory. The second section considers the implications of a phonological loop problem. Because a reasonable amount is known about the normal functioning of the phonological loop, and of its role in language acquisition in typical development, we can make firm predictions as to the likely nature of the short-term memory problem in Down syndrome, and its consequences for language learning. However, we note that the existing evidence from studies with individuals with Down syndrome does not fit well with these predictions. This leads to the third section of the paper, in which we consider key questions to be addressed in future research. We suggest that there are two questions to be answered, which follow directly from the contradictory results outlined in the previous section. These are 'What is the precise nature of the verbal short-term memory deficit in Down syndrome?', and 'What are the consequences of this deficit for learning?'. We discuss ways in which these questions might be addressed in future work.
Jarrold C, Baddeley AD. Short-term memory in Down syndrome: Applying the working memory model. Down Syndrome Research and Practice. 2001;7(1);17-23.
This paper has three aims. First, to review the evidence for a verbal short-term
memory deficit in Down syndrome, and in doing so to consider whether any observed
deficit really reflects an underlying memory difficulty, or is instead caused by
other factors. Second, to explore the possible implications of a verbal short-term
memory deficit for individuals with Down syndrome, and finally, to highlight key
questions that remain to be addressed by future work in this area. This paper draws
heavily on material covered in an earlier paper published in Down Syndrome Research
and Practice (Jarrold,
Baddeley & Phillips, 1999), and the reader is referred to this previous
account for a fuller treatment of many of the issues raised here.
2. A Review
Short-term memory is the ability to 'hold in mind' information. For example, one
might try to remember the string of digits in a telephone number while moving to
the phone and dialling. It differs from long-term memory, where information, although
not necessarily forgotten, is not always consciously available until recalled. Verbal
short-term memory refers to the ability to maintain verbal items such as words or
digits, and is most often assessed using 'span' paradigms in which individuals are
asked to remember increasing numbers of words or digits. The maximum number of items
that an individual can remember, in correct order, is their span. Adults typically
have a span of around seven items (Miller, 1956),
but among individuals with Down syndrome verbal short-term memory spans are usually
considerably lower than this.
In fact many studies have shown that individuals with Down syndrome have lower verbal
short-term memory spans than would be predicted by their level of verbal ability
or their general level of intellectual function. For example, Kay-Raining Bird and
compared the digit spans of a group of 47 individuals with Down syndrome and a corresponding
number of typically developing children of an equivalent level of intellectual functioning.
Among these controls digit spans ranged from 3 to 6, with a span of 5 being most
common. Although 1 of the individuals with Down syndrome achieved a digit span of
6, the average level of performance was considerably lower than that seen among
controls; the most common span for individuals with Down syndrome was 4 and a number
of individuals had a span of 2.
This study illustrates two important aspects of verbal short-term memory in Down
syndrome. The first is that, on average, verbal spans tend to be lower than one
would expect given individuals' other abilities (see also,
Bower & Hayes, 1994; Jarrold
& Baddeley, 1997;
Jarrold, Baddeley & Hewes, 1999,
2000; MacKay & McDonald,
1976; Marcell & Cohen, 1992;
& Cothran, 1988;
Marcell, Ridgeway, Sewell & Whelan, 1995;
Marcell & Weeks, 1988; McDade
& Adler, 1980). However, it is also important to note that
not all individuals with Down syndrome have low verbal short-term memory spans.
Vallar and Papagno (1993) report
the case of a young woman with Down syndrome, who, in addition to being fluent in
3 languages, had a perfectly normal verbal short-term memory span.
It is also worth noting that the problems experienced by the majority of individuals
with Down syndrome on tests of verbal short-term memory are relatively specific,
in that they do not appear to extend to all aspects of short-term memory. Short-term
memory can be assessed for visuo-spatial information, as well as for verbal material;
typically using a 'Corsi span' test (after Corsi, cited in
Milner, 1971). In this task individuals watch while the tester taps out
a series of different spatial locations on a board, and then have to repeat the
sequence in the correct order themselves. As with digit span, the number of items
to be remembered can be gradually increased to determine the maximum level of an
individual's memory ability. The majority of studies which have contrasted digit
and Corsi span among individuals with Down syndrome, in some cases relative to appropriate
controls, have shown that individuals with Down syndrome are impaired only on digit
et al., 1994; Jarrold
& Baddeley, 1997;
Jarrold, Baddeley & Hewes, 1999;
Wang & Bellugi, 1994; though see Haxby, 1989;
& Caltagirone, 1995). In other words, the individuals' visuo-spatial
short-term memory is relatively unimpaired, in contrast to their verbal short-term
However, this, in itself, does not necessarily prove that individuals with Down
syndrome have a specific verbal short-term memory problem. Their relatively poor
performance on digit and word span tasks might arise for reasons which have comparatively
little to do with memory. One obvious explanation for poor verbal short-term memory
performance is that hearing difficulties, which are known to be relatively common
in Down syndrome (Davies, 1996), affect individuals'
ability to properly encode the items being presented. Verbal short-term memory tasks
typically involve auditory presentation of the to-be-remembered items, and clearly
if one has difficulty in hearing and identifying these items then one's memory for
them is likely to be severely reduced.
This issue has been addressed in a number of elegant studies carried out by Marcell
and colleagues. Marcell and Armstrong (1992)
found that visual presentation of digits in a span task did not improve performance
among individuals with Down syndrome, which contrasts with what would be expected
if hearing difficulties were affecting the encoding of stimuli. Marcell, Harvey
and Cothran (1988)
found that presenting auditory stimuli through headphones also had no beneficial
effect on the short-term memory performance of individuals with Down syndrome. Although
this manipulation was designed to assess individuals' susceptibility to 'distractibility'
in verbal short-term memory tasks, one might expect headphones to produce a higher
quality auditory signal than normal speech. Finally, Marcell and Cohen (1992)
found that direct measures of individuals' hearing ability did not predict their
performance on a digit span task (see also
Jarrold & Baddeley, 1997). Again this is at odds with what one would
expect if hearing difficulties were a major constraint on verbal short-term memory
in Down syndrome. Having said this, Marcell and Cohen (1992)
did find that the extent of hearing loss was related to the time taken by individuals
with Down syndrome to identify digits, which may have some indirect effect on verbal
short-term memory performance (Rabbitt, 1968).
A second possible explanation of verbal short-term memory difficulties in Down syndrome
concerns problems of spoken articulation, which again are known to be associated
with the condition (Dodd, 1975;
Gibson, 1978). In word and digit span tasks, individuals typically give
their response by repeating the presented list in full. It seems intuitively likely
that an individual with difficulty in planning and/or executing a sequenced spoken
response would show reduced verbal short-term memory spans as a result of these
problems. Indeed work by Cowan and his colleagues (e.g.,
Cowan, Day, Saults, Keller, Johnson & Flores, 1992) has shown that the
time taken by typically developing individuals to produce their response in a verbal
short-term memory task has a direct effect on their memory performance.
Once again Marcell and colleagues have done much of the work aimed at addressing
this particular issue. In two studies Marcell and Weeks (1988)
adapted digit and word span tasks so as to remove the need for individuals with
Down syndrome to give a full spoken response at recall. In their first study, individuals
recreated the to-be-remembered list in a digit span task by ordering a series of
numbered blocks. In their second study, individuals gave their response to a word
span task by pointing to pictures of the presented words in the correct serial order.
In neither case did these manipulations improve the performance of individuals with
Down syndrome relative to the standard recall procedure. If removing the need for
a spoken response does not remove the difficulty with the task, then clearly this
suggests that problems of speech are not a major factor in determining verbal short-term
memory performance. A similar conclusion follows from the results of a recent study
from our own research group. Jarrold et al. (2000)
found that individuals with Down syndrome had lower verbal short-term memory spans
than a comparison group of individuals with moderate learning difficulties, despite
being able to repeat pairs of words in an articulation test at a similar rate to
these controls. Again this implies that poor verbal short-term memory is not primarily
due to problems of spoken articulation in Down syndrome.
In sum, individuals with Down syndrome tend to perform relatively poorly on tests
of verbal-short term memory; a problem which appears to be specific to verbal tasks
and does not extend to all short-term memory tests. Although one might well expect
that problems of hearing and speech, which are associated with Down syndrome, are
likely to be the cause of much of this difficulty, there is actually relatively
little evidence to support this suggestion. Of course, this raises the question
of what exactly is causing the relatively poor verbal short-term memory performance
seen among individuals with Down syndrome? One possible answer is that there really
is a fundamental short-term memory problem associated with Down syndrome. A number
of authors have advanced this view (e.g.,
Broadley, MacDonald & Buckley, 1995; Comblain,
1996; Das & Mishra 1995;
Hulme & Mackenzie, 1992;
Jarrold & Baddeley, 1997;
Kay-Raining Bird &
Chapman, 1994; Mackenzie & Hulme,
Das & Varnhagen, 1987; Wang
& Bellugi 1994), and have tended to do so with reference
to Baddeley and Hitch's 'working memory model'.
The working memory model, outlined initially by Baddeley and Hitch (1974)
and refined by Baddeley (1986), represents
an attempt to explain and account for a range of phenomena associated with short-term
memory performance in both typically and atypically functioning individuals. A key
feature of the model is that it suggests that separate systems are dedicated to
the short-term storage of verbal and visuo-spatial information. These are termed
the 'phonological loop' and the 'visuo-spatial sketchpad' respectively. Broadly
speaking there are three lines of evidence to support this account. First, the type
of experimental manipulations which affect verbal short-term memory performance
have little or no effect on visuo-spatial short-term memory performance, and vice
Brandimonte, Hitch & Bishop, 1992). Second, studies that have examined
the pattern of brain activity in individuals performing verbal and visuo-spatial
short-term memory tests have suggested that different locations in the brain are
associated with these two sets of tasks (see Baddeley,
1993). Finally, and in line with the previous point, there are cases of
adult neuropsychological patients who have suffered brain damage which has had a
selective effect on verbal rather than visuo-spatial short-term memory, and vice
versa. All three lines of evidence point to the separateness of the short-term memory
systems responsible for the storage of verbal as opposed to visuo-spatial information;
a suggestion which itself is consistent with the possibility of a selective verbal
short-term memory problem in Down syndrome. In other words, according to the working
memory model, Down syndrome may be associated with some form of 'phonological loop'
The claim of a phonological loop deficit in Down syndrome amounts to more than a
restatement of the data reviewed above, because it has, at least, two important
implications. The first follows from our knowledge of the likely consequences of
a phonological loop problem and the second from our understanding of the normal
functioning of this system.
There is considerable evidence to suggest that the phonological loop plays an important
role in the development of aspects of language, and in vocabulary acquisition in
particular (see Baddeley,
Gathercole & Papagno, 1998 for a review). The first evidence for the
role of the loop in vocabulary acquisition came from work with a neuropsychological
patient - PV - known to have impaired phonological loop functioning. Baddeley, Papagno
and Vallar (1988)
studied PV's ability to associate known target words with two types of material:
other known words or novel 'nonsense' word stimuli. They showed that while PV was
perfectly able to learn associations between pairs of known words, her ability to
pair words with nonsense words was drastically impaired. These data were seen as
being consistent with the view that the phonological loop is involved in maintaining
the novel phonological form of a newly experienced word during vocabulary learning.
Subsequent work by Gathercole and her colleagues, among others, has shown that children's
verbal short-term memory functioning relates to their ability to learn the phonological
form of new word stimuli in vocabulary learning tasks (e.g.,
Gathercole, Hitch, Service & Martin, 1997). These results imply that
individuals with Down syndrome should have problems in learning the phonological
form of words during vocabulary acquisition, given their apparent problems in verbal
In addition to knowing something about the probable consequences of a phonological
loop impairment, our knowledge of the workings of the loop has implications for
the precise type of problem experienced by individuals with Down syndrome, which
in turn has potential implications for intervention strategies. Evidence from work
with typically functioning adults has led Baddeley (1986)
to suggest that the loop can be divided into two components. One is a passive store,
which maintains information in a phonological form, but which is subject to forgetting
due to the decay of verbal memory traces. A second process - rehearsal, offsets
this decay. By subvocally repeating to-be-remembered information an individual can
prolong the maintenance of material within the phonological loop.
The importance of rehearsal is shown by the effects of blocking this process. If
adults are asked to continually repeat a word out loud during a verbal short-term
memory task, a process known as 'articulatory suppression' which blocks rehearsal,
then spans are markedly impaired (Baddeley,
Thomson & Buchanan, 1975). In addition, an insight into the nature of
rehearsal is shown by the fact that the faster an individual is able to rehearse,
as indexed by the speed with which they can articulate words out loud, the greater
their verbal short-term memory span (e.g.,
Baddeley, et al. 1975). This is thought to reflect the time-limited
nature of the store within the phonological loop - the faster one rehearses the
more words one can maintain in the store without any being subjected to forgetting.
This is further reflected in the 'word length effect'; individuals show higher spans
for words with a short spoken duration, which take relatively less time to rehearse,
than for words with a longer spoken duration (Baddeley
et al., 1975). Clearly, given the importance of rehearsal in determining
phonological loop efficiency, it is possible that the problems experienced by individuals
with Down syndrome in verbal short-term memory tasks are due to a deficit in this
process. If this is the case, then one might be able to target interventions towards
this problem, in the hope of improving rehearsal inefficiency. Indeed a number of
studies have attempted to improve or train rehearsal in individuals with Down syndrome
(Broadley & MacDonald, 1993;
& Buckley, 1994; Comblain, 1994;
Laws, MacDonald & Buckley,
Laws, MacDonald, Buckley & Broadley, 1995; for more details on rehearsal
training see Conners, Rosenquist and Taylor (2001).
The claim of a phonological loop deficit in Down syndrome therefore has implications
for our understanding of Down syndrome, and for possible intervention strategies.
However, it is important to note that the accounts outlined above, although theoretically
plausible, are not fully supported by empirical evidence. First, it is far from
clear that vocabulary abilities in Down syndrome are as poor as one would predict
given the evidence of a phonological loop deficit and the known importance of the
loop in vocabulary acquisition. For example, Rondal (1995)
reports the case of FranÃ§oise, an individual with Down syndrome with poor verbal
short-term memory, but relatively superior vocabulary and other language skills.
More generally, although researchers are not fully agreed about exactly how strong
vocabulary abilities are in Down syndrome, it appears that vocabulary tends to be
relatively advanced in comparison to other aspects of language (see
Chapman, 1995). This should not be the case if a phonological loop deficit
were affecting vocabulary acquisition directly, leading Laws (1998)
to suggest that "The link between vocabulary and phonological memory in Down syndrome
is not as well established as it is in normal development" (p. 1121).
Second, although rehearsal is certainly an important component of typical phonological
loop functioning, there is growing evidence to suggest that a problem in rehearsal
may not be the prime cause of poor verbal short-term memory in Down syndrome. This
is not to say that rehearsal is entirely unimpaired, simply that problems in rehearsal
may not be the main reason why individuals with Down syndrome show lower verbal
short-term memory spans than other individuals. In our recent work (Jarrold
et al., 2000) we compared the verbal short-term memory spans of
a group of individuals with Down syndrome and a comparison group of individuals
with learning disability but without Down syndrome. In addition, individuals' speech
rates were measured by asking them to repeat sequences made up of the words used
in the memory task as rapidly as possible. This provides a measure, albeit an indirect
one, of the speed with which individuals should be rehearsing these words in the
memory task. The individuals with Down syndrome had lower memory spans than the
comparison group, in line with the body of research outlined above, but both groups
had comparable speech rates. This implies that the problems in memory seen in these
individuals with Down syndrome are not due to slower, more inefficient rehearsal.
Of course this assumes that individuals with Down syndrome actually employ rehearsal
to aid their verbal short-term memory. In fact there are reasons for thinking that
individuals with Down syndrome might be unlikely to use rehearsal at all. These
follow from evidence that typically developing individuals do not begin to rehearse
until around the age of 7 years (see Gathercole,
1998). Although somewhat controversial, according to this view one might
not expect to see rehearsal emerge among individuals with Down syndrome until they
reach the equivalent developmental level. To assess this possibility we carried
out a further study, in which the size of the word length effect was assessed in
a 'probed recall' task. As discussed above, poorer memory for words of a longer
spoken duration - the word length effect - is potential evidence that rehearsal
is occurring. However, a word length effect might also arise because a sequence
of long words takes longer to say when it comes to providing the response to a short-term
memory test (Cowan,
et al., 1992). To counter this latter issue, memory was assessed
in a task where three words were presented and individuals were prompted to remember
either the first, the second, or the third word in the list. Under these conditions
individuals with Down syndrome did not show a word length effect, suggesting that
they were not rehearsing. Their overall level of memory was poorer than a comparison
group, but importantly these control individuals also showed no word length effect
in recall. This implies that a problem of rehearsal is not the main cause of the
verbal short-term memory deficit in this instance, because both groups showed no
evidence of rehearsal, and yet the individuals with Down syndrome still had lower
verbal short-term memory spans.
4. Key Questions for Future Work
The previous section highlighted two implications of the claim that Down syndrome
is associated with problems of phonological loop function, but also noted that both
of these implications are not clearly supported by the existing evidence from studies
of Down syndrome. A phonological loop deficit would predict impaired vocabulary
acquisition, but it is not clear that vocabulary learning is particularly problematic
for individuals with Down syndrome. Rehearsal is a key process in phonological loop
functioning, but it does not appear that problems in rehearsal are the main reason
why verbal short-term memory is poor in Down syndrome. The two questions outlined
here for future research follow from these two unresolved issues.
The first question is whether a phonological loop problem has consequences for learning
in Down syndrome. Although the existing evidence suggests that vocabulary is a relatively
strong aspect of language functioning, there are few, if any, direct studies of
vocabulary learning in Down syndrome. Chapman, Kay-Raining Bird and Schwartz (1990)
have examined 'fast mapping' abilities. This is the ability to learn or realise
that an unfamiliar word must be paired with an unfamiliar object. Fast mapping is
tested, for example, by showing individuals two familiar and one novel object, and
asking them to point to the 'koob' (or some other unfamiliar term). Chapman et al.
found that individuals with Down syndrome had no difficulty with this task. However,
this kind of task does not assess the precise aspects of vocabulary acquisition
that are thought to be supported by the phonological loop, as it does not really
test individuals' ability to learn and remember the phonological form of the novel
word. Studies of this latter form of word learning have been conducted with typically
developing children (e.g.,
Gathercole et al., 1990;
Gathercole, et al., 1997), and could be adapted for use with
individuals with Down syndrome.
A second key question concerns the precise nature of the apparent phonological loop
problem in Down syndrome. If it does not reflect a rehearsal deficit, or problems
of hearing and speech, then what is its cause? As outlined above, the phonological
loop consists of two processes or components - rehearsal and the phonological store.
If the deficit is not primarily one of rehearsal, then one is left to conclude that
it must reflect some form of impairment with the phonological store. There are two
problems with this suggestion. The first is that it is a rather 'post-hoc' account,
and the second is that, as yet, we have no clear and accepted method of measuring
phonological store functioning or capacity. One possible way in which this might
be done is to look at the rate at which individuals forget verbal information over
time. If individuals with Down syndrome do not rehearse, then the rate at which
they forget information should provide a reasonably direct index of the efficiency
of their phonological store.
This paper began with a review of verbal short-term memory in Down syndrome. Taken
together, the evidence suggests that individuals with Down syndrome do have relatively
poor verbal short-term memory abilities (although there are certainly individual
exceptions to this rule), and that these appear not to be caused by problems of
hearing or speech. This suggests that there really is a fundamental short-term memory
problem associated with the condition, which in terms of Baddeley and Hitch's working
memory model would be viewed as a phonological loop deficit. Two implications of
this claim were outlined. First, that this should predict problems of vocabulary
acquisition in Down syndrome, and second, that the problem might well be one of
inefficient or absent rehearsal. However, it appears that neither of these suggestions
is fully supported by the existing evidence, and future work is clearly needed to
determine the precise causes and consequences of a phonological loop deficit in
The authors are supported by a grant from the Medical Research Council to Alan Baddeley
and Susan Gathercole. We are grateful to Down Syndrome Education International for
their invite to present this paper at the 3rd International Conference on Language
and Cognition in Down syndrome.
Chris Jarrold â€¢ Centre for the study of Memory and Learning, Department of Psychology,
University of Bristol, 8 Woodland Road, Bristol, BS8 1TN, UK â€¢ E-mail:
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DSE's Reading and Language Intervention for Children with Down Syndrome (RLI) is an evidence-based programme designed to teach reading and language skills to children with Down syndrome.
RLI incorporates best practice in structured activities delivered in fast-paced daily teaching sessions. It was evaluated in a randomised controlled trial and found to improve rates of progress compared to ordinary teaching.
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