With the recent sequencing of the human genome, the following question has attracted much interest: can the function of single genes be linked to specific neural and cognitive processes? Within this context, developmental disorders of known genetic origins have been used as naturally-occurring models to link the function (and dysfunction) of genes with cognition. Fragile X syndrome (FXS) is a genetically inherited disorder associated with the silencing of a single gene involved in experience-dependent changes at glutamatergic synapses. In adulthood, it is associated with core attentional difficulties accompanied by seemingly proficient visuo-perception, but the profile of infants and toddlers has not been investigated. In this thesis, fragile X syndrome is used as a tool to investigate how initial changes in a generalised property of all cortical neurones can nonetheless result, in the adult, in core difficulties in the control of attention. I argue that, even in disorders associated with the silencing of a single gene like FXS, the answer requires a developmental approach. Chapter 1 delineates a theoretical distinction between endogenous and exogenous influences on attentional control, whereas Chapter 2 defines methodological issues in assessing atypical attention, such as tools for the assessment of general developmental level and choices of control groups. Part II focuses on tasks tapping endogenous attention control. In particular, Chapters 3 and 4 examine the control of eye-movements and manual response conflict in infants and toddlers with FXS and in typically developing controls. In contrast, Part III concentrates on the exogenous effects of sudden peripheral onsets on visual orienting (Chapter 5) and of the perceptual salience of targets during visual search (Chapter 6). Finally, Part IV traces longitudinal changes in visual search performance. The findings suggest that, like adults with the syndrome, infant and toddlers with FXS display striking deficits in endogenous attention. However, unlike adults, infants are also characterised by atypical exogenous influences on attention and longitudinal changes in performance point to complex developmental relationships between early and later measures of attention. The findings are discussed in terms of their theoretical implications for fragile X syndrome and other developmental disorders affecting attention. They challenge the notion of direct genotype-phenotype mappings that fail to take development into account
