No abstract

This study challenges the use of adult neuropsychological models for explaining developmental disorders of genetic origin. When uneven cognitive profiles are found in childhood or adulthood, it is assumed that such phenotypic outcomes characterize infant starting states, and it has been claimed that modules subserving these abilities start out either intact or impaired. Findings from two experiments with infants with Williams syndrome (a phenotype selected to bolster innate modularity claims) indicate a within-syndrome double dissociation: For numerosity judgments, they do well in infancy but poorly in adulthood, whereas for language, they perform poorly in infancy but well in adulthood. The theoretical and clinical implications of these results could lead to a shift in focus for studies of genetic disorders.

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Few investigations into the executive function (EF) skills of these groups have examined the effect of verbal/visuospatial task type on performance. Analogous verbal and visuospatial measures were administered to these populations within four EF domains: executive-loaded working memory (ELWM), inhibition, fluency and set-shifting. Performance in both groups was compared to that of typically-developing (TD) children using regression techniques controlling for potentially influential cognitive/developmental factors. Individuals with WS showed the expected relative visuospatial difficulties, as indicated by poorer performance than TD individuals, on tests of ELWM and fluency. Individuals with DS displayed the expected relative verbal difficulty in the domain of set-shifting. In addition, each population showed pervasive deficits across modality in one domain; ELWM for individuals with DS, and inhibition for individuals with WS. Individuals with WS and DS showed EF difficulties in comparison to a TD group, but, their executive performance was affected by EF task type (verbal/visuospatial) and EF domain in different ways. While the findings indicated that EF in these populations is characterised by a range of specific strengths and weaknesses, it was also suggested that the relative verbal/visuospatial strengths associated with each population do not consistently manifest across EF domains. Lastly, syndrome specificity was indicated by the differences in groups' performance patterns. Permanent repository linkKeywords: Williams syndrome, Down syndrome, executive function. INTRODUCTION Cognitive profiles associated with Williams and Down syndromesThe genetic conditions Williams syndrome (WS) and Down syndrome (DS) are characterised by roughly opposing ability profiles. Individuals with WS display relative verbal strengths alongside impairments on visuospatial tasks (Bellugi, Korenberg, & Klima, 2001;Pani, Mervis, & Robinson, 1999), particularly those involving a constructive element (Hoffman, Landau, & Pagani, 2003). Saccadic abnormalities (Brown et al., 2003;van der Geest, et al., 2004), and problems with location encoding (Farran & Jarrold, 2005), as well as the perceptual grouping of elements (Farran, 2005), have all been suggested as contributory factors with regard to these spatial difficulties, while vulnerability of the dorsal stream -a brain region thought to mediate the processing of spatial location and movement (Milner & Goodale, 1995) -has been implicated at the neurological level (e.g. Atkinson et al., 2003;Galaburda & Bellugi, 2000 Alberti, & Vianello, 2010;Porter, Coltheart, & Langdon, 2007;Rhodes, Riby, Park, Fraser, & Campbell, 2010). However, studies have not generally acknowledged verbal/visuospatial task modality as a potential contributing influence. This is surprising when the ability profiles associated with each population are considered. Furthermore, EF performance has been...

In contrast to skilled adults, both groups of children preferred to foveate the target prior to initiating a hand movement if time allowed. The TD children, however, were more able to reduce this foveation period and shift towards a feedforward mode of control for hand movements. The children with DCD persevered with a look-then-move strategy, which led to an increase in error. For the group of DCD children in this study, there was no evidence of a problem in speed or accuracy of simple movements, but there was a difficulty in concatenating the sequential shifts of gaze and hand required for the completion of everyday tasks or typical assessment items.

Visual information is vital for fast and accurate hand movements. It has been demonstrated that allowing free eye movements results in greater accuracy than when the eyes maintain centrally fixed. Three explanations as to why free gaze improves accuracy are: shifting gaze to a target allows visual feedback in guiding the hand to the target (feedback loop), shifting gaze generates ocular-proprioception which can be used to update a movement (feedback-feedforward), or efference copy could be used to direct hand movements (feedforward). In this experiment we used a double-step task and manipulated the utility of ocular-proprioceptive feedback from eye to head position by removing the second target during the saccade. We confirm the advantage of free gaze for sequential movements with a double-step pointing task and document eye-hand lead times of approximately 200 ms for both initial movements and secondary movements. The observation that participants move gaze well ahead of the current hand target dismisses foveal feedback as a major contribution. We argue for a feedforward model based on eye movement efference as the major factor in enabling accurate hand movements. The results with the double-step target task also suggest the need for some buffering of efference and ocular-proprioceptive signals to cope with the situation where the eye has moved to a location ahead of the current target for the hand movement. We estimate that this buffer period may range between 120 and 200 ms without significant impact on hand movement accuracy.