Spatial representation and attention in toddlers with Williams syndrome and Down syndrome

Abstract

The nature of the spatial representations that underlie simple visually guided actions early in life was investigated in toddlers with Williams syndrome (WS), Down syndrome (DS), and healthy chronological age- and mental age-matched controls, through the use of a “double-step” saccade paradigm. The experiment tested the hypothesis that, compared to typically developing infants and toddlers, and toddlers with DS, those with WS display a deficit in using spatial representations to guide actions. Levels of sustained attention were also measured within these groups, to establish whether differences in levels of engagement influenced performance on the double-step saccade task. The results showed that toddlers with WS were unable to combine extra-retinal information with retinal information to the same extent as the other groups, and displayed evidence of other deficits in saccade planning, suggesting a greater reliance on sub-cortical mechanisms than the other populations. Results also indicated that their exploration of the visual environment is less developed. The sustained attention task revealed shorter and fewer periods of sustained attention in toddlers with DS, but not those with WS, suggesting that WS performance on the double-step saccade task is not explained by poorer engagement. The findings are also discussed in relation to a possible attention disengagement deficit in WS toddlers. Our study highlights the importance of studying genetic disorders early in development.

Introduction

The development of mental representations used to plan eye movements (saccades), in order to select appropriate aspects of the environment to attend to, is of major importance during development. Before motor control has developed sufficiently to allow infants to explore their environment by touching or grasping, visual exploration allows them to interact with their world, and to begin to exert control over their responses to it. But the planning of saccades is no simple task. Successful spatial orientation relies on accurate perception of the physical self in relation to the environment, and adaptation to the changes both within that environment and of the position of the body. Such spatial knowledge is represented by frames of reference, a coordinate system used to code positions in space that can then be used to monitor stimuli and plan actions within the environment. The particular type of mental representation used for spatial orientation will dictate how efficiently different aspects of visual stimuli can be processed and integrated.

Attention also plays a role in the development of visual cognition, and thus in the infants ability to plan eye movements. The infant must attend to objects in the real world, and shift attention appropriately, either when an object has been fully processed, or when a new object appears in the environment. Thus, individual differences in attention levels will have an impact on infants ability to process visual stimuli.

But what happens if visual exploration or attention is impaired from early infancy onwards? In this paper, we examine this question with respect to two genetic disorders—Williams syndrome (WS) and Down syndrome (DS)—and compare these to two groups of typically developing children.

Williams syndrome is caused by a sub-microscopic deletion on chromosome 7q.11.23, and occurs in approximately 1 in 20,000 live births. Clinical features include several physical abnormalities that are accompanied by mild to moderate mental retardation and a specific personality profile. The interest of WS to cognitive neuroscientists stems from the very uneven profile of cognitive abilities, with spatial cognition seriously impaired and language and face processing relatively proficient (for full details, see [9], [14], [45]). Our knowledge of the spatial problems in the WS adult end state is relatively advanced. In contrast, we know relatively little about the development of spatial cognition in infants with WS. Furthermore, while there is some anecdotal evidence that attention is poor in children and adults, little is known about the development of attention in infants with WS.

Down syndrome is the most common chromosomal abnormality, and one of the leading causes of mental retardation, with a prevalence of 1 in 600–800 live births [13]. It is caused by extra material on chromosome 21. Trisomy 21, in which all cells have an extra chromosome 21, is the most common form of DS and accounts for 90–94% of cases. Unlike the uneven cognitive profile found in WS, the pattern of cognitive abilities in DS is usually somewhat more uniform. Both spatial cognition and attention are typically reported as problem areas in adults and children with DS (e.g. [8], [19], [43]). Prior to examining spatial representation and attention in these clinical groups, we provide details of the normal course of early development in these domains.