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Abstract
The dorsal and ventral visual pathways mediating spatial processing and object recognition, respectively, have been extensively studied in hearing populations, yet little is known about the effects of lifelong deafness and visuospatial language experience on the functional anatomy of these processes. This dissertation presents two complementary studies examining these questions. The first examined the neural basis of spatial cognition using the activation likelihood estimate (ALE) technique to assess concordance across published brain imaging studies of mental rotation. Activation of the left and right superior parietal lobules was equally likely despite variation across studies in stimulus type and tasks. However, activation likelihood of the inferior parietal lobule was limited to the left hemisphere, and exhibited concordance only across studies requiring mental rotation of hand and tool stimuli. In addition, likely activation foci were identified in frontal regions, although with less concordance across studies. The second study used functional magnetic resonance imaging (fMRI) to investigate plasticity in the dorsal and ventral visual pathways consequent to congenital deafness and life-long experience with American Sign Language (ASL). During scanning, deaf native ASL users, hearing native ASL users, and hearing subjects without ASL experience attended to the spatial location or object identity of face and house photographs. Within-groups, the dorsal/ventral dichotomy for spatial versus object processing, and category-selectivity in the ventral pathway for face and house stimuli, were observed in all three groups. However, deaf subjects exhibited reduced reliance on right superior parietal cortex and increased recruitment of the left inferior parietal lobule for spatial processing compared to both hearing groups, revealing plasticity attributable to deafness. Furthermore, group differences occurred in several face-selective, but not in house-selective regions, all located in the right hemisphere, including ventral temporal cortex. Specifically, deaf signers demonstrated reduced activity in the lateral fusiform gyrus, and increased activity in the anterior superior temporal gyrus, inferior to auditory cortex, relative to hearing non-signers. Comparison of hearing signers with these groups revealed no differences, indicating that the combined effects of deafness and ASL may alter the response of deaf signers to faces in these regions. Deaf signers also demonstrated increased activity in the amygdala in comparison to both hearing groups. Taken together, these findings illustrate that plasticity subsequent to congenital deafness and/or visuospatial language experience may be more widespread than previously thought.
