Embracing Complexity in the Characterisation and Tracking of Neurodevelopment

Abstract

Neurodiverse diagnoses (NDs) are currently conceptualised as distinct categories with clear-cut diagnostic boundaries. However, converging research evidence is showing that this conventional approach inadequately captures the individual variation and overlaps that are commonly observed across NDs. This thesis aimed to build upon the emerging quantitative research frameworks, through adopting a transdiagnostic dimensional approach to characterising the complexity of the cognitive and mental health dimensions associated with NDs. First, the findings of the systematic review suggested that the co-occurrence of Attention Deficit Hyperactivity Disorder and Reading Difficulty remains consistently high across the lifespan, and that both children and adults experience comparable levels of difficulty across several cognitive domains. The review indicated that complex profiles of co-occurring NDs persist into adulthood, and that studies of adults are substantially underrepresented in the literature. Second, the results of an empirical study revealed that a transdiagnostic sample of neurodivergent adults was best characterised by three data-driven clusters, none of which mapped directly onto traditional diagnostic categories. These transdiagnostic groups were best characterised by a combination of strengths and weaknesses along two multivariate dimensions. Dimension 1 was associated with cognitive flexibility and processing speed abilities, and Dimension 2 was related to language, memory, emotional difficulties and hyperactive behaviour. Further, cognitive flexibility, inhibitory control and vocabulary skills were highlighted as central processes in facilitating communication between the dimensions characterising the transdiagnostic dataset, thus acting as potential mechanisms underpinning heterogeneity, overlaps and/or the co-occurrence of conditions in NDs. This thesis has provided data-driven evidence to support a dimensional structure of NDs, and highlighted several processes which may contribute to the complexity associated with NDs in adults. Further research yielding data-driven accounts of neurodiverse conditions is required to build on the present findings, and to further extend current transdiagnostic dimensional frameworks beyond strictly categorical approaches.