Spatial representation and attention in toddlers with Williams syndrome and Down syndrome
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.
Section snippets
Spatial representation in normal development
In typically developing infants, increasingly complex representations of spatial information are used throughout infancy and then childhood, before adult representations are ultimately formed. At around the age of 6 months, children tend to use body-centred representations, i.e. they use body- or head-centred coordinates (e.g. [10], [11]), while younger infants tend to rely on retinocentric representations. Gilmore and Johnson [20], [21] investigated 3- and 7-month-old infants frames of
Attention in normal development
Spatial attention, as defined by Posner and Peterson (e.g. [37]) consists of three phases: engage, disengage, and shift. Similar components are also found in attention in infancy. Lansink and Richards [34] describe three phases of infant attention: stimulus orienting, sustained attention, and attention disengagement.
The first phase of attention, stimulus orienting, involves the direction of attention toward a spatial location or object of interest. This usually (but not always) involves eye and
Spatial representation and attention in atypically developing children
The aim of the present study is to examine both spatial representation and sustained attention in toddlers with WS and DS, as well as mental age-matched (MA) and chronological age-matched (CA) healthy controls. Spatial representation will be tested in Experiment 1, using the double-step saccade paradigm employed by Gilmore and Johnson [20], [21], to determine whether the visuo-spatial impairments found in adults with WS [9], [14], [48] have precursors in early childhood, in the form of deficits
Experiment 1: saccade planning
It is proposed that toddlers with WS will be impaired on the double-step saccade task, and will make more errors of saccade planning that require body-centred frames of reference, relative to typically developing controls. A secondary prediction of Experiment 1 is that toddlers with WS will differ from typically developing controls in terms of vector summation, which would appear as looks to the central position on the first saccade, and which would indicate delayed maturation of the cortical
Experiment 2: sustained attention
It is hypothesised that toddlers from the two clinical groups, WS and DS, will be impaired relative to control groups, both on measures of duration and number of periods of sustained attention, which would indicate that the attention problems that are reported to be present in adulthood (e.g. [19], [43], [52]) already exist from an early age.
General discussion
The results of our experiments suggest that the visuo-spatial problems found in adults with WS do have precursors in early childhood. Likewise, poor sustained attention seems to be a stable deficit over developmental time in DS.
Experiment 1 revealed that toddlers with WS perform very differently from all the other groups on each of the measures from the double-step saccade paradigm. Thus, the ability to orient accurately to target locations, and the use of body-centred spatial frames of
Acknowledgements
This research was made possible by a studentship to JH Brown, Programme Grant No. G9715642 and Project Grant No. G9809880 to A Karmiloff-Smith, and Programme Grant No. G9715587 to MH Johnson, all made available by the Medical Research Council, as well as a studentship from the Downs Syndrome Association and a grant from the PPP Healthcare Foundation to A Karmiloff-Smith, and EU Biomed Grant No. BMH4-CT97-2032 to MH Johnson and A Karmiloff-Smith et al. We thank all the children and parents who
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