Memory and neuropsychology in Down syndrome
Christopher Jarrold, Lynn Nadel and Stefano Vicari
This paper outlines the strengths and weaknesses in both short-term and long-term memory in Down syndrome, and the implications of these patterns for both other aspects of cognitive development and underlying neural pathology. There is clear evidence that Down syndrome is associated with particularly poor verbal short-term memory performance, and a deficit in verbal short-term memory would be expected to negatively affect aspects of language acquisition, particularly vocabulary development. Individuals with Down syndrome also show impaired explicit long-term memory for verbal information, and may also have particular problems in explicit long-term memory for visual-object associations. However, implicit memory appears to be less affected in Down syndrome, and may therefore provide an important basis for intervention approaches. These findings are consistent with the suggestion of dysfunction within the hippocampal system in Down syndrome, and problems in verbal memory may be linked to impaired functioning of pre-frontal brain regions.
The human memory system can be divided into a number of
sub-components. Evidence from experimental studies of memory, and from adult
patients with acquired brain damage, suggests that one key distinction in memory
function is between the active, ongoing maintenance of information in a
'short-term' or 'working' memory system, and the storage of material that is not
kept active, but which can be retrieved from, long-term memory[1,2] (see
ref 4). In addition, the
short-term and long-term memory systems can also themselves be fractionated.
Again, both experimental and neuropsychological evidence suggests that there may
be separate verbal and visuo-spatial short-term memory systems, with potentially
domain-general control of these storage systems in working memory.
Similarly, long-term memory can be sub-divided along a number of lines. One
common distinction is between 'explicit' (conscious) memory for facts and events
and 'implicit' (non-conscious) knowledge and learning
ref 50). A related distinction
can be drawn between the learning of 'declarative' knowledge about facts and
'procedural' knowledge about how to perform particular tasks.
The former represents an aspect of explicit memory, while the latter is often
(though not necessarily) implicit. This review of strengths and weaknesses in
memory in Down syndrome follows these distinctions in the human memory system,
before turning to the neuropsychological implications of memory impairments
observed in the condition.
Short-term memory in Down syndrome
A long-standing finding in the Down syndrome literature is
that verbal short-term memory performance is impaired relative to visuo-spatial
short-term memory performance
ref 7). Verbal short-term memory
is typically assessed by asking individuals to repeat, in correct serial order,
a list of words that they have just heard. In contrast visuo-spatial short-term
memory tasks typically require participants to recreate a visually presented
sequence of spatial locations by manually selecting the appropriate locations in
serial order. In the clear majority of studies that have given these two types
of task to individuals with Down syndrome and appropriate comparison samples
(e.g., control groups matched for mental age), individuals with Down syndrome
have shown impaired verbal, but not visuo-spatial short-term memory performance
One obvious explanation for such a finding is that
individuals with Down syndrome struggle on verbal short-term memory tasks
because these tasks require them to perceive auditorily presented information,
and then respond verbally. Given that speech perception and production problems
are relatively common in Down syndrome, these difficulties might affect
performance on such tasks, regardless of the quality of the underlying
short-term memory system. Studies that have examined this issue, either by
measuring speech and hearing problems or by trying to remove their influence on
performance, have suggested that these 'peripheral' effects are not the
fundamental cause of poor verbal short-term memory in Down syndrome
Two studies illustrate this point particularly well. Laws
presented individuals with Down syndrome and typically developing children of a
comparable vocabulary level with two versions of a test of short-term memory for
In one condition 'focal' colours, such as red, blue, and green, were presented
visually, and participants had to recall the presented sequence by touching the
appropriate colours in the correct order on a response board. In a second
condition, hard to name 'non-focal' colours were presented for recall in the
same manner. Precisely because these colours are very difficult to generate a
verbal label for, participants are forced to maintain them in visuo-spatial
short-term memory, and indeed the two groups showed comparable performance on
the non-focal condition. However, in the focal condition individuals with Down
syndrome were impaired because they failed to recode the visual image of each
colour into a verbal label for maintenance in verbal short-term memory. In other
words, this study shows a selective deficit in verbal short-term memory ability,
even though no words were presented auditorily and responses were made manually
rather than verbally.
In a similar vein, Brock and Jarrold presented individuals
with and without Down syndrome with two short-term memory tasks, one verbal, one
visuo-spatial, which each required participants to remember the presented
sequence of items by manually selecting positions on a touchscreen.
Individuals with Down syndrome were selectively impaired when their re-ordering
of the presented sequence had to be done on the basis of verbal short-term
memory. In addition, Brock and Jarrold showed that these individuals with Down
syndrome were unimpaired in their ability to identify single verbal items,
suggesting that speech perception problems were not the cause of poor
performance on the verbal memory task.
It therefore appears that people with Down syndrome have a
real problem in representing verbal, or 'phonological' information in short-term
memory. This has potentially important implications for the condition, as other
evidence suggests that children's verbal short-term memory capabilities are
closely linked to aspects of their language development
refs 15,16). This could be
because individuals with generally poor language skills are less familiar with
the verbal material employed in typical tests of verbal short-term memory, or
have generally poorer phonological skills as a result of their language problems[17,18].
However, the above studies have shown that, in Down syndrome at least, verbal
short-term memory deficits can be observed even relative to comparison
individuals of the same level of language knowledge
An alternative hypothesis put forward by Baddeley, Gathercole, and colleagues is
that verbal short-term memory plays a causal role in aspects of language
development, particularly vocabulary, because individuals have to maintain in
short-term memory an accurate phonological representation of any new word that
they hear in order to create a more stable, long-term representation of it. If
this account were correct, then one would expect the poor verbal short-term
memory skills of individuals with Down syndrome to compromise their subsequent
The problem for this suggestion is that, while language
abilities are often particularly delayed in Down syndrome, vocabulary is by no
means the weakest of the various aspects of language function in Down syndrome;
indeed, vocabulary is typically in advance of syntactic skill for example[20,21].
Furthermore, studies of 'fast mapping' - the ability to learn that a novel sound
must apply to a novel object - have shown generally good word learning skills
among individuals with Down syndrome.
Reconciling the apparent conflict between impaired verbal short-term memory
performance and relatively good vocabulary knowledge in Down syndrome is a key
area for future research. Recent work by the first author and colleagues has
suggested that individuals with Down syndrome are unimpaired on word learning
tasks in which the experimenter provides the novel word and the participant
selects the object that it has been paired with.
This task is arguably analogous to tests of receptive vocabulary in which
individuals select the object named by the tester, and on which individuals with
Down syndrome do relatively well. In contrast, participants with Down syndrome
were impaired on a word-learning task that was more analogous to a measure of
expressive vocabulary, in which they were required to select the appropriate
non-word name that had been previously paired with a particular object. It may
well be that the latter task requires the participant to have a 'finer-grained'
and more accurate phonological representation of the non word in question, and
therefore is more closely related to verbal short-term memory abilities.
Long-term memory in Down syndrome
People with intellectual disabilities learn and successively
retain new information less efficiently than age-matched typically developing
individuals. Although this statement may seem trivial, awareness of the central
role of long-term memory impairment in the emergence of learning difficulties
and adaptation problems of people with intellectual disability has prompted a
large body of experimental literature aimed at clarifying the qualitative
characteristics and basic mechanisms of this deficit.
Down syndrome is the aetiologic group whose long-term memory
impairment has been investigated most extensively. However, memory effects vary
as a function of the type of memory being assessed. Explicit memory involves
things like facts and events that participants consciously recollect, whereas
implicit memory can be demonstrated indirectly, without conscious recollection.
For example, one common kind of implicit memory test looks at skills or
procedures, such as mirror-tracing; another common implicit memory test involves
"priming", where prior exposure to a word or picture can influence subsequent
performance on word-stem or partial-picture completion tasks even though the
participants might not recall having seen the relevant items before. This
distinction has been shown to be important in understanding organic amnesia,
since most individuals with amnesia are profoundly impaired on explicit memory
tasks but show relatively intact performance on implicit tasks. This distinction
turns out to also be important in Down syndrome.
Evidence suggests that the explicit long-term memory
abilities of individuals with Down syndrome are impaired. In a study comparing
individuals with Down syndrome, individuals with intellectual disability of
unspecified aetiology and a group of mental age-matched typically-developing
children, the Down syndrome group scored significantly lower than both of the
other groups on tests of free recall of word lists and a short story, and in the
reproduction of Rey's figure from memory [24-26].
Explicit long-term memory for visual-spatial information has
been much less investigated than memory for verbal material. In a recent study,
Vicari, Bellucci, and Carlesimo investigated the performance of a group of 15
participants with Down syndrome compared to a control group of mental
age-matched typically developing children.
A further group of people with intellectual disability was also included in that
study, namely 15 adolescents with Williams syndrome.
A visual-object and a visual-spatial learning test were
developed on the assumption that visual-object long-term memory (i.e., for the
physical characteristics of objects) and visual-spatial long-term memory (i.e.,
for position or motion in space) are mediated by different neural systems and,
therefore, constitute two distinct aspects of the organisation of explicit
During the study phase of the visual-object test, fifteen figures of common
objects (e.g., a tree) were shown to the participants. During the test phase
immediately following the study phase, four different versions of the same
object (e.g., 4 trees) were depicted on each page; only one of the four was the
same as the target object in the study phase and the other three were physically
different distracters. Study and test phases were presented three consecutive
times. In the visual-spatial learning test, the pages were divided into four
quadrants and each figure was positioned in one of the quadrants. During the
test phase, the target stimuli were presented and the participant was asked to
indicate the position of the figure on an empty page divided into four
quadrants. The entire test was administered three times. The results of this
study showed analogous learning of visual-spatial sequences and poorer learning
of visual-object patterns in the individuals with Down syndrome compared to
their own control group of mental age matched typically developing children.
Interestingly, individuals with Williams syndrome showed the opposite profile,
with poorer visual-spatial learning but analogous visual-object learning.
The performance profile observed in people with Down syndrome
highlights a dissociation between more preserved visual-spatial memory and
greater impairment of visual-object learning ability. There is general agreement
in the literature that the neuropsychological profile of people with Down
syndrome is characterised by strength in nonverbal abilities, as revealed by
their performance on graphic, constructive and spatial tests, which are
generally less impaired than linguistic abilities.
However, few studies have made a detailed analysis of the visual-spatial domain.
In the only study that compared visual-object and visual-spatial learning
abilities in people with Down syndrome, a relative sparing of visual-spatial
memory was found with respect to visual-object memory.
These authors argued that memory for the spatial position of objects is
characterised by greater automaticity than memory for visual and/or verbal
content of information and, for this reason, is less impaired in persons with
In the Vicari et al. study, the comparison of the two groups of subjects with
known genetic syndromes suggests a different type of interpretation.
In fact, individuals with Down syndrome have greater difficulty in learning
visual-object material with a substantial saving of visual-spatial learning, and
participants with Williams syndrome show the opposite pattern. This suggests
that not all persons with intellectual disability present a preserved
visual-spatial memory (as Ellis seems to suggest (see ref 30)).
Accordingly, this finding also gives greater validity to the performance pattern
exhibited by the persons with Down syndrome and Williams syndrome. It indicates
that the impairment exhibited by the two groups cannot be attributed simply to
the presence of intellectual disabilities but that it is a peculiar
characteristic of each syndrome.
In the last few years, some experimental data have been
reported regarding the possible extension to individuals with intellectual
disability of the dissociation noted above between explicit and implicit memory
processes so frequently described in brain damaged adults with memory disorders.
Regarding repetition priming,
studies investigating facilitation in identifying perceptually degraded
pictures, induced by previous exposure to the same pictures, have consistently
reported a comparable priming effect in individuals with intellectual disability
and in typically-developing subjects matched for chronological age[31,32] or mental age.
Similar findings were found using verbal material. Most of these studies were
based on the Stem Completion procedure in which subjects are requested to
complete a list of stems (i.e., the first three letters) with the first word
that comes to mind. In this test, the priming effect is revealed by a bias in
completing the stems with previously studied rather than unstudied words.
Carlesimo et al.
and Vicari, Bellucci, and Carlesimo[34,35]
reported a priming effect with this procedure in various groups of individuals
with intellectual disability (aetiologically unspecified, Down syndrome and
Williams syndrome) comparable to that of mental age matched typically developing
Less experimental work has been devoted to investigating the
ability of individuals with intellectual disability to learn
cognitive skills. In a first
study, Vakil, Shelef-Reshef, and Levy-Shiff compared the improvement in accuracy
displayed by groups of individuals with intellectual disability and mental
age-matched children on successive trials of the Tower of Hanoi and the Proteus
On both tests, the individuals with intellectual disability performed
significantly less accurately than the controls. However, on the first test
(which requires completing a spatial pattern according to a series of
predetermined rules) the rate of trial-to-trial improvement was higher in the
typically-developing than in the intellectual disability group, and on the
Proteus Maze test (which requires solving a series of mazes with the least
number of errors possible) the two groups improved at the same rate. Recently,
Vicari and co-workers pointed out an intriguing difference in the skill learning
abilities of two genetically distinct groups of individuals with intellectual
disability. In the first study,
a group of individuals with Down syndrome showed the same rate of improvement as
a group of mental-age matched typically-developing children across successive
trials of the Tower of London test (analogous to the Tower of Hanoi) and in the
comparison of the repeated versus random blocks of a facilitated version of the
Serial Reaction Time test,
which requires implicit learning of the sequential order of a series of visual
events. In a second study, a group of children with Williams syndrome showed
significantly less procedural learning than typically developing children on
both of these tests.
Certainly, the relative sparing of implicit memory function may help explain why
infants with Down syndrome show unimpaired performance on a memory task that
requires acquisition of a motor response (i.e., procedural learning)
and why 20-43-month old children with Down syndrome are able to succeed at a
deferred imitation task.
The neuropsychology of memory in Down syndrome
The different cognitive profiles exhibited by the various
aetiological groups of people with intellectual disability presumably result
from some specific characteristics of their anomalous brain development. In a
recent study, Pennington, Moon, Edgin, Stedron and Nadel tested adolescents with
Down syndrome on a range of tasks designed to directly assess the function of
specific brain systems. This 'cognitive neuropsychological' approach often uses
tasks first developed in animal models, where the critical underlying brain
circuits can be identified and carefully studied in invasive experiments. The
team started with a focus on three brain systems identified by the
neuropathological data: the hippocampal system, the prefrontal cortex, and the
cerebellum. They developed a set of tasks that could, collectively, tell us
something about how these brain systems are faring. In the first set of studies,
they found evidence of specific hippocampal dysfunction in a sample of 28
adolescents, using mental age matched controls.
This impairment in hippocampal function could in principle reflect problems in
any of the structures of the hippocampal region; a recent study of two
neuropsychological paradigms dependent on parahippocampal and perirhinal regions
(delayed nonmatching to sample and visual paired comparison), however, suggests
that these areas are functioning appropriately, and that the impairment is more
likely to reflect improper development of the hippocampus itself
Little evidence of prefrontal dysfunction was observed in a
battery of nonverbal tasks in the Pennington et al. study.
Subsequent pilot work from that group, however, suggested that verbal tasks
might yield a different result, and indeed that is what is being observed.
Using verbal tasks to explore the prefrontal cortex, these researchers found in
the young (aged 5-11) and old (aged 30-41) groups strong signs of dysfunction in
both the hippocampal and prefrontal systems. Deficits were observed in a range
of tasks although verbal mediation was necessary to bring out the prefrontal
Taken as a whole, these neuropsychological studies show that
particular problems emerge in the memory domains served by the hippocampal
system and the prefrontal system. The latter impairment appears to be linked to
the use of verbal test materials. There is not yet sufficient evidence to
evaluate the role of presumed cerebellar impairments. There is at present little
consensus on the role of the cerebellum in learning and memory. Latash discussed
some abnormalities in motor coordination in children with Down syndrome, but
future research will need to look more carefully at any cognitive sequellae of
improper development of the cerebellum.
Given the neuropsychological evidence of a developmental
trend toward hippocampal neuropathology and the fact that older individuals with
Down syndrome show many features consistent with Alzheimer's neuropathology,
there has been particular focus on hippocampal functions in neuophysiological
studies of Down syndrome. Evidence for hippocampal deficits has been observed in
mouse models. Hyde and Crnic reported that Ts65Dn mice show
hippocampal-dependent learning deficits as a function of age, and they propose
that these deficits may be related to reduced cholinergic innervation of the
A very recent study demonstrates decreased neurogenesis in the dentate gyrus of
the hippocampus in both foetuses with Down syndrome and Ts65Dn mice, suggesting
a possible mechanism for at least some of the memory related difficulties in
The dentate gyrus plays a particularly important role in contextual aspects of
long-term memory, and it has recently been shown that the neurons formed by
post-natal neurogenesis are incorporated into learning circuits and may even be
critical in fostering normal learning capacity. This is clearly an area ripe for
Recent neuro-imaging studies have also attempted to document
the presence of particular morphological cerebral characteristics to explain the
distinct cognitive and behavioural profiles observed in persons with
intellectual disability, especially of known genetic syndromes[46,47].
Magnetic resonance imaging studies have shown reductions in hippocampal volume
in Down syndrome prior to the onset of dementia, and these reductions were found
to relate to memory deficits.
Based on a morpho-volumetric analysis of the brains of
individuals with Down syndrome and Williams syndrome, Jernigan, Bellugi, Sowell,
Doherty, and Hesselink described important differences that explain, at least in
part, the peculiarities of the neuropsychological processes associated with
Specifically, people with Down syndrome have a hypofrontality associated with
cerebellar hypoplasia and with substantial saving of the trunk and the posterior
cortical structures. Therefore, Down syndrome may be associated with relatively
preserved maturation of the dorsal compared to the ventral component of the
visual system, leading to relatively better performance on visual-object than on
visual-spatial memory tests. The particularly impaired visual-object learning
found in individuals with Down syndrome is also at variance with the hypothesis
that, irrespective of the aetiology of the cerebral insult, during development
the dorsal stream is more vulnerable to brain damage than the ventral stream[50,51].
Further studies are obviously needed to investigate in more detail the presently
postulated dissociation of the long-term memory abilities underlying the
functions of the dorsal and ventral visual systems in individuals with Down
Concerning implicit memory, both neuropsychological
and functional neuroimaging
data assign a critical role to basal ganglia and cerebellum in the implicit
learning of visuo-motor skills. The brains of individuals with Down syndrome,
instead, exhibit severe cerebellar hypoplasia with normal morphology of basal
In the light of these data, we can tentatively conclude that the deficient
maturation of visuo-motor skill learning in people with Williams syndrome is
related to the deficient maturation of striatal circuits known to be critical
for this ability.
Directions for future research
The above review has highlighted a number of potential
directions for future research. While it seems clear that individuals with Down
syndrome perform particularly poorly on tests of verbal short-term memory, the
consequences of this impairment for more general language development in the
condition need to be determined. In addition, the role of impaired language
knowledge, and phonological awareness in particular
ref 54), in mediating potential
links between verbal short-term memory and vocabulary development needs to be
more precisely specified.
Similar issues are relevant for work on long-term memory in
Down syndrome, where evidence suggests that the modality of information to be
recalled moderates the degree of any deficit seen on certain tasks; particularly
in the case of verbal tests of frontally-mediated memory systems. The relation
between the degree of any impairment on such tasks, and individuals' generally
delayed language abilities, needs to be clarified in future work. At the same
time, while visual memory skills may be less impaired, there is evidence of a
particular difficulty in visual-object memory, and the causes and consequences
of such an impairment need to be outlined.
A final point to note is that the learning and memory problems that begin to
emerge in late infancy in Down syndrome become considerably more noticeable as
the infant grows to childhood and adolescence. While much of our knowledge for
this period comes from the learning of language, there is information available
about other kinds of learning and memory. One major point to be stressed from
these language learning data has less to do with the inability of children with
Down syndrome to acquire words, or linguistic constructions, or other non-verbal
material, and more to do with their inability to 'stabilise' the information
that they do manage to acquire. Wishart and Fowler
stress this point, which might reflect, among other factors, impairments in
memory consolidation, another function of the hippocampal system.
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The preparation of this paper was partly supported by a grant to the first
author from the Economic and Social Research Council of the United Kingdom
Chris Jarrold is at the University of Bristol, UK.
Lynn Nadel is at The University of Arizona, USA.
Stefano Vicari is at the Children's Hospital Bambino GesÃ¹,
I.R.C.C.S., Rome, Italy.
Chris Jarrold â€¢ e-mail: C.Jarrold@bristol.ac.uk ; Lynn Nadel â€¢
e-mail:firstname.lastname@example.org ; Stefano Vicari â€¢ e-mail:
Paper prepared from presentations and discussions at the Down Syndrome
Research Directions Symposium 2007, Portsmouth, UK. The symposium was hosted by
Down Syndrome Education International in association with the Anna and John J
Sie Foundation, Denver. Major sponsors also included the Down Syndrome
Foundation of Orange County, California and the National Down Syndrome Society
of the USA. Information about the symposium can be found at
Received: 4 October 2007; Accepted: 11 October 2007; Published online: 2 July 2008