Investigating Neural Substrates of Visual Motion Sensitivity in Deaf Individuals

The aim of this thesis has been to explore neural substrates of enhanced far-peripheral visual motion processing in congenitally deaf adults. To do this, psychophysical measures were used as well as novel fMRI stimulus delivery methods to record responses to stimu- lation in the far-peripheral visual field. For the first time, far-peripheral visual field mapping measured an extended representa- tion of the visual field (72 ◦) in early visual cortex in deaf and hearing individuals. Using this method, unique evidence of plasticity within the cortical surface area distribution of visual field representations in the primary visual cortex was found in congenitally deaf adults, biased towards the far-peripheral visual field. Furthermore, neural responses to far-peripheral stimuli were measured in visual motion processing areas V5/MT+ and V6, and in auditory regions. Results show novel and dis- tinctive differences in response profiles in auditory, but not visual regions between deaf and hearing participants, indicating crossmodal plasticity in deaf participants, specific to coherent but not incoherent global optic flow field motion stimuli. Most importantly, the aim of the thesis was to relate neural measures to behavioural per- formance of motion perception. The results show evidence that unimodal plasticity in V1 and activation in visual motion areas V5/MT+ and V6 are not related to performance in two visual motion tasks (local motion detection and global motion direction discrimina- tion), but that response inhibition and excitation levels in auditory regions are related to motion processing performance in deaf and hearing individuals. In summary, the findings described in this thesis show for the first time that congenital deafness leads to plastic changes within primary visual cortex. In addition, auditory but not visual motion regions are recruited differentially between deaf and hearing individu- als, depending on the motion type, and this activation shows a trending relationship with visual motion performance in both groups.