Ansari, Daniel; (2003) Atypical trajectories of number development: The case of Williams Syndrome. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Adult neuropsychological models predict that exact representations of number are language-dependent while representations of approximate number relate to visuo-spatial processing. Interestingly, individuals with Williams syndrome (WS) present with a relative proficiency in language coupled with weak visuo-spatial abilities, which might predict good exact and weak approximate number abilities. However, while the cognitive profile of individuals with WS is well studied, little is known about their numerical abilities. This thesis begins to fill this gap by exploring the numerical abilities in this clinical group and by considering whether WS exhibit dissociation between exact and approximate numerical abilities. Chapters 1 and 2 report results from standardized measures and from a parental number abilities questionnaire. The results reveal that number abilities are severely delayed in WS and fall between their verbal and visuo-spatial abilities. Chapters 4-6 examine the underlying developmental trajectories of basic exact and approximate representations of number. The data from these chapters demonstrate that the early development of exact number representation (the understanding of the meaning of counting) is not only delayed but follows an atypical developmental trajectory. The investigation of non-verbal, approximate number abilities (magnitude comparison and numerical estimation) indicate that participants with WS are severely delayed and that underlying developmental changes diverge in subtle ways from typical developmental trajectories. The results demonstrate that individuals with WS do not present with an exact-approximate dissociation of numerical representations. Instead, impairments and subtle differences in developmental trajectories exist for both, highlighting the importance of adopting a developmental perspective. The thesis also evaluates the wider implications of the methodological and theoretical approach adopted here for the study of atypical trajectories of number development in general. It is contended that the investigation of how 'low-level' number representations and their developmental trajectories impact on the development of 'high-level' numerical cognition is a vital way in which to study both typical and atypical number development.

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