Visual Abilities in Profound Deafness: A Window into the Mechanisms of Multisensory Perception

The present thesis addresses the cross-modal plasticity occurring in the visual modality due to profound deafness. In Chapter 1, we review the current perspective on the general mechanisms of cross-modal plasticity and, in particular, the changes occurring in visual modality in the case of profound deafness. Enhanced visual abilities in the deaf have typically been reported when in tasks that involve visual attention resources and processing of peripheral portions of the visual field. In this thesis, we present a series of four experimental studies aimed at specifying which visual attentional components display modulations in case of deafness. In Chapter 2 and 3, we contrasted the role of endogenous and exogenous capture of visual attention. The results showed that endogenous attention, when tested in a transient-free context, does not reveal compensations effects due to deafness (i.e., enhancement). By contrast, enhanced performance for peripheral portion of the visual field emerged when exogenous capture of selective attention was involved. We suggest that enhanced visual performance in the deaf is transients selective. In Chapter 4, we contrasted the ability to simply react to a visual event (simple detection task) and the ability to orient visual attention (shape discrimination task). The results showed that deaf posses marked enhanced reactivity compared to hearing controls, when detecting targets presented at both central and peripheral locations. Reactivity is thus an enhanced visual skill in the deaf that is not spatially selective, in the sense that it does not emerge solely at peripheral locations. In addition, RTs results support the hypothesis that deaf may display a different neural representation of the peripheral portion of the visual field. These effects were not paired by enhanced ability to discriminate between different shapes regardless of their relative position, suggesting that enhanced reactivity was not due to a better mechanism of 6 attention orienting. Finally, in Chapter 5, we present the electrophysiological data recorded during a simple visual detection task. The ERPs analysis revealed that deafness determines quantitative and qualitative modulations of visual processing already at the early stages (C1, P1). Finally, Chapter 6 resumes and discusses the implications of the overall set of results.