The neural basis of visual performance advantages in the deaf

Individuals who are congenitally deaf display greater sensitivity to visual stimuli presented to their peripheral visual field than those with normal hearing. Interestingly, this performance advantage emerges around adolescence, an age that would be considered beyond the classical critical period for visual cortical plasticity. The purpose of this study is to investigate the neural correlates of the visual performance advantages achieved by deaf individuals. While much of the work on visual function and its neural underpinning in the deaf has appealed to differences in high level neural representations, recent work has indicated that visual performance is correlated with structural differences much earlier in the visual pathway in the retina. Investigations of the function of early visual processing areas and their relationship to visual
performance advantages is now required. My aim is to use electrophysiological and neuroimaging techniques to determine the functional and anatomical properties of early components of the visual system and relate those properties to the visual performance advantages in the deaf. There are three potential mechanisms that could act to facilitate enhanced visual performance in early visual structures: 1. Neural resources allocated to peripheral vision could be enlarged such that a greater
number of retinal and cortical neurons represent the peripheral visual field; 2. The gain of neurons representing the visual periphery could be higher; 3. The speed of processing peripheral visual targets may be faster. These three mechanisms are not mutually exclusive, but can be disambiguated to a great extent by the anatomical, electrophysiological and neuroimaging procedures I will use as follows. Visual field maps will be generated in the cortex using functional MRI (fMRI) and retinotopic mapping procedures. Neural tissue thickness will be compared for central and peripheral visual field representations in the retina using optical coherence tomography (OCT), and in cortex by comparing cortical volume derived from structural MRIs. Neural response amplitudes will be compared over different visual field representations in early visual areas in the brain using multifocal magnetoencephalography (mfMEG) and fMRI. The high temporal resolution of mfERG and mfMEG techniques will also allow the processing speed of early visual structures to be compared over the visual field.