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BACKGROUND: The Hebb effect is a form of repetition-driven long-term learning that is thought to provide an analogue for the processes involved in new word learning. Other evidence suggests that verbal short-term memory also constrains now vocabulary acquisition, but if the Hebb effect is independent of short-term memory, then it may be possible to demonstrate its preservation in a sample of individuals with Down syndrome, who typically show a verbal short-term memory deficit alongside surprising relative strengths in vocabulary. METHODS: In two experiments, individuals both with and without Down syndrome (matched for receptive vocabulary) completed immediate serial recall tasks incorporating a Hebb repetition paradigm in either verbal or visuospatial conditions. RESULTS: Both groups demonstrated equivalent benefit from Hebb repetition, despite individuals with Down syndrome showing significantly lower verbal short-term memory spans. The resultant Hebb effect was equivalent across verbal and visuospatial domains. CONCLUSIONS: These studies suggest that the Hebb effect is essentially preserved within Down syndrome, implying that explicit verbal short-term memory is dissociable from potentially more implicit Hebb learning. The relative strength in receptive vocabulary observed in Down syndrome may therefore be supported by largely intact long-term as opposed to short-term serial order learning. This in turn may have implications for teaching methods and interventions that present new phonological material to individuals with Down syndrome.

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A brief account is given of the evolution of the concept of working memory from a unitary store into a multicomponent system. Four components are distinguished, the phonological loop which is responsible for maintaining speech-based information, the visuospatial sketchpad performing a similar function for visual information, the central executive which acts as an attentional control system, and finally a new component, the episodic buffer. The buffer comprises a temporary multidimensional store which is assumed to form an interface between the various subsystems of working memory, long-term memory, and perception. The operation of the model is then illustrated through an account of a research programme concerned with the analysis of working memory in Down syndrome.

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Working memory can be defined as the ability to hold in mind information in the face of potentially interfering distraction in order to guide behavior. The experimental manipulation of working memory tasks has shed considerable light on the probable structure of the human working memory system, and, to a lesser extent, the specific processes captured by working memory paradigms. However, individual differences research has also had a crucial role to play in the development of theories of working memory. In particular, correlational approaches have been particularly informative in three areas of working memory research, each of which is reviewed here. These are, first, the importance of working memory measures as correlates of high-level cognitive skills such as reading, mathematics, reasoning, and fluid intelligence; second, the extent to which human working memory relies on domain-general or domain-specific component subsystems, and third, the precise reasons why working memory measures do relate to other important indices of human cognitive functioning. The findings from each of these areas suggest that working memory depends on a combination of domain-specific representational systems and domain-general processing and control systems, and that working memory measures capture individuals' ability to combine maintenance and processing demands in a manner that limits information loss from forgetting or distraction.

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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.

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Williams syndrome (WS) is a rare genetic disorder which, among other characteristics, has a distinctive cognitive profile. Nonverbal abilities are generally poor in relation to verbal abilities, but also show varying levels of ability in relation to each other. Performance on block construction tasks represents arguably the weakest nonverbal ability in WS. In this study we examined two requirements of block construction tasks in 21 individuals with WS and 21 typically developing (TD) control individuals. The Squares tasks, a novel two-dimensional block construction task, manipulated patterns by segmentation and perceptual cohesiveness to investigate the first factor, processing preference (local or global), and by obliqueness to examine the second factor, the ability to use mental imagery. These two factors were investigated directly by the Children's Embeded Figures Test (CEFT; Witkin, Oltman, Raskin, & Karp, 1971) and a mental rotation task respectively. Results showed that individuals with WS did not differ from the TD group in their processing style. However, the ability to use mental imagery was significantly poorer in the WS group than the TD group. This suggests that weak performance on the block construction tasks in WS may relate to an inability to use mental imagery.

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Jarrold et al. (1998) presented evidence to suggest that verbal and non-verbal abilities develop at different rates in individuals with the Williams syndrome phenotype. However, this evidence was derived from cross-sectional rather than longitudinal data. The current report presents data from a series of follow up assessments which examine the development of vocabulary and pattern construction abilities in 15 of the original sample of 16 individuals, over a 40 month period. The results confirm the original predictions, as mental age equivalent scores for vocabulary increase more rapidly than scores for the pattern construction test; a finding, which appears unlikely to be due to practice effects.

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Individuals with Down syndrome suffer from relatively poor verbal short-term memory. Previous explanations of this deficit have been framed in terms of inefficient or absent rehearsal of verbal material in Down syndrome within the phonological loop component of Baddeley and Hitch's (1974) working memory model. Two experiments are presented which test this explanation by looking for the markers of rehearsal in children with Down syndrome and verbal mental age matched controls. Both experiments confirm that individuals with Down syndrome show poorer verbal short-term memory performance than controls. However, they rule out rehearsal as an explanation of these deficits because the evidence suggests that neither individuals with Down syndrome nor matched controls are engaging in spontaneous subvocal rehearsal. Other explanations of poor verbal short-term memory performance in Down syndrome, in terms of impairments both within and outside of the phonological loop system, are discussed. Practical implications for intervention strategies aimed at improving verbal short-term memory skills in Down syndrome are also outlined.

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Three experiments investigated whether 2 characteristic aspects of the psychological profile of autism, theory-of-mind deficits and weak central coherence, might be functionally related. Experiment 1 showed that in the general population, performance on a proposed test of theory of mind was inversely related to speed on the Embedded Figures Test, a measure of central coherence bias. Experiments 2 and 3 confirmed that poor theory-of-mind performance was linked to weak central coherence among typically developing children and among children with autism; however, the correlations between these measures were reliable only after accounting for differences in individuals' verbal mental ages. This pattern of results is interpreted in terms of a relationship between individual differences in theory of mind and central coherence bias, a relationship that is separate from any developmental differences in these domains.

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Previous research has indicated that 2 processing rates may constrain verbal short-term memory performance. These have been linked to individual differences in (a) the time taken to articulate spoken words and (b) the duration of pauses that occur between words in the output responses to memory tasks. Two experiments examined whether evidence for these effects on memory can be obtained for measures taken from a single speech sample. Children articulated pairs of words as rapidly as possible. In both experiments, the spoken duration of words and the length of the pauses between them predicted significant variance in verbal short-term memory performance. It is argued that the duration of words is linked to memory performance through the processes underlying time-based forgetting in short-term memory. In contrast, the duration of pauses in speeded articulation may index individual differences in speech planning processes.

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Wang and Bellugi [J clin exp Neuropsychol 1994;16:317 22] have suggested that Down's and Williams syndrome might be associated with specific and contrasting working memory deficits; with impaired verbal short-term memory in Down's syndrome, and a visuo-spatial short-term memory deficit in Williams syndrome. In two studies we examine whether these apparent deficits might simply be a consequence of the general pattern of learning difficulties associated with these disorders. Experiment 1 compared verbal and visuo-spatial short-term memory abilities in these groups, using analysis of covariance to control for mental age differences. In Experiment 2 individuals with Williams syndrome were matched to control groups for non-verbal mental age, and the short-term memory abilities of these matched groups were compared. The results of both experiments are broadly consistent with those reported by Wang and Bellugi, and support the view that working memory can be dissociated into separate subsystems.

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Many studies have shown that children with autism perform at a much lower level than control subjects on tests of executive functioning, defined as tasks requiring subjects to hold information in mind while suppressing a prepotent response. These tasks have invariably required subjects to (a) follow arbitrary and novel rules and (b) make a nonverbal response. We report that when one of these features is absent, children with autism are not impaired relative to controls. They perform at a similar level to normally developing children on the "tubes" task (containing no arbitrary and novel rules) and on the day/night task (in which the output is verbal). Results are consistent, at least, with the hypothesis that children with autism are challenged by executive tasks because they are unlikely to encode rules in a verbal form.

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Individuals with Down syndrome are thought to perform poorly on tests of verbal short-term memory, such as measures of word span or digit span. This review critically examines the evidence for a specific deficit in verbal short-term memory in Down syndrome, and outlines a range of possible explanations for such a deficit. The potential implications of a verbal short-term memory impairment for broader aspects of development are outlined, in particular with respect to vocabulary development. Possible intervention strategies, which might improve verbal short-term memory performance in Down syndrome are also considered. However, we argue that further research is needed to fully clarify the nature of a verbal short-term memory deficit in Down syndrome, before the merits of these various intervention approaches can be properly evaluated.

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In order to ask whether autism is associated with difficulty in self-monitoring we gave a group of children and adolescents with autism a task in which they themselves had to remember whether they or another person had performed certain actions. In our first experiment, three groups of participants had to recall whether placements of picture cards had been made by themselves or by the experimenter. The participants with autism performed at a much lower level than the two comparison groups and, unlike the comparison groups, were not better at recalling their own placements. A second experiment tested the prediction arising from the monitoring-deficit account that the children with autism would be unimpaired when the placement of the items did not alternate between self and other. This prediction was confirmed moderately well. Additionally, in contrast to that of the comparison groups, the performance of the participants with autism was unaffected by whether self or other displaced the items. This is consistent with the existence of a monitoring deficit.

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With a task involving the launching of missiles at targets, Malenka et al. (1982) and C. Frith and Done (1989) showed that schizophrenic patients with delusions of alien control and auditory hallucinations were likely to leave erroneous responses uncorrected whose outcomes were not visible until the missile hit or failed to hit the target, while being able to correct visible errors adequately. This is consistent with an impairment in the central monitoring of action. Using a similar task, we found that children with autism were more likely than controls to fail to correct both kinds of error. Data are interpreted in terms of difficulties with constructing visual schemata for actions.

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One commonly cited feature of Williams syndrome is a characteristic dissociation between relatively spared language skills and severely impaired nonverbal abilities. However, the actual evidence for a dissociation between verbal and nonverbal abilities in Williams syndrome is equivocal. In two separate studies we examined these abilities in 16 individuals showing the Williams syndrome phenotype. When considered as a whole, the group did have significantly superior verbal abilities, but this difference was caused by a large discrepancy in abilities in only a small number of individuals. In both studies there was a clear, linear relation between individuals' verbal ability, and the magnitude of their verbal-nonverbal discrepancy. We suggest that these results are best explained in terms of verbal ability developing at a faster rate than nonverbal ability in this disorder. We discuss how this model of differential rates of development has the potential to reconcile the apparently inconsistent findings in this area.

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OBJECTIVE: To evaluate a 2% w/w minocycline gel (Dentomycin) in the treatment of older adults with chronic periodontitis. DESIGN: Vehicle-controlled, split-mouth SETTING: Dental Institute, St Bartholomew's and the Royal London School of Medicine and Dentistry. SUBJECTS: 22 adults (mean age 60 years). INTERVENTIONS: At baseline, patients received scaling and root planing. Test and placebo were applied at contralateral disease sites surrounding 22 test and 22 control teeth at baseline, and at weeks 2 and 4. MAIN OUTCOME MEASURE: Probing pocket depth, bleeding on probing and supragingival plaque measurements, and microbiological sampling, at one pocket site per tooth were undertaken at baseline, and at weeks 2, 4, 6 and 16. RESULTS: Microbial counts of Porphyromonas gingivalis, Prevotella intermedia and Eikenella corrodens did not change significantly over 16 weeks. No significant reductions in bleeding on probing and supragingival plaque score occurred with either gel. A larger reduction in mean pocket depth, from baseline to week 16, was found at test (6.50 to 4.95 mm; 24% reduction; P < 0.01) than at control sites (6.41 to 5.53 mm; 14% reduction; P < 0.05). CONCLUSIONS: Minocycline gel administration is a useful adjunct to scaling and root planing in the treatment of older adults with periodontitis.

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We examined the claim that children with autism have a "weak drive for central coherence" which biases them towards processing information at an analytic rather than global level. This was done by investigating whether children with autism would rapidly and automatically enumerate a number of dots presented in a canonical form, or count each dot individually to obtain the total. The time taken to count stimuli was compared across three participant groups: children with autism, children with moderate learning difficulties, and normally developing children. There were 22 children in each group, and individuals were matched across groups on the basis of verbal mental age. Results implied that children with autism did show a tendency towards an analytic level of processing. However, though the groups differed on measures of counting speeds, the number or children showing patterns of global or analytic processing did not differ significantly across the groups. Whether these results implicate a weak drive for central coherence in autism, which is both specific to, and pervasive in the disorder, is discussed.

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Short-term memory for verbal and visuospatial information was examined in a group of children and teenagers with Down's syndrome. Performance on the verbal task was impaired relative to matched control groups, but there were no group differences on the visuospatial task. Relatedly, the Down's syndrome group showed inferior short-term memory for verbal as opposed to visuospatial information, whereas controls showed the opposite pattern. These findings did not appear to result from a general superiority of nonverbal abilities in the Down's syndrome group, or from hearing difficulties that might have impacted on the verbal short-term memory task, in which material was presented auditorily. The results are consistent with the suggestion that Down's syndrome is associated with a selective impairment of the phonological loop component of Baddeley and Hitch's (1974) working memory model.

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We asked whether children with autism are specifically impaired on tests of working memory. Experiment 1 showed that children with autism were at least as likely as normal children to employ articulatory rehearsal (criterion: evincing the "word length effect") and that they had superior spans to that of children with moderate learning difficulties. In Experiment 2, participants were given "capacity tasks" in order to examine group differences in the capacity of the central executive of working memory. The performance of the children with autism was inferior to that of the normally developing group and similar to that of the children with moderate learning difficulties.

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The failure to produce pretend play seen in autism may arise from executive deficits associated with the syndrome. This experiment investigated the ability of children with autism to use various objects in pretence, the prediction being that they would have particular difficulty using props with a clear function (e.g. a pencil) to perform a different pretend function (e.g. to act as a toothbrush). However, children with autism were as likely as controls to select a prop with an inappropriate function from amongst other, nonfunctional props, suggesting that executive deficits of this particular kind cannot readily explain an absence of pretend play in autism.

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