Adaptive crossmodal plasticity in deaf auditory cortex: areal and laminar contributions to supranormal vision in the deaf.

This chapter is a summary of three interdigitated investigations to identify the neural substrate underlying supranormal vision in the congenitally deaf. In the first study, we tested both congenitally deaf and hearing cats on a battery of visual psychophysical tasks to identify those visual functions that are enhanced in the congenitally deaf. From this investigation, we found that congenitally deaf, compared to hearing, cats have superior visual localization in the peripheral field and lower visual movement detection thresholds. In the second study, we examined the role of "deaf" auditory cortex in mediating the supranormal visual abilities by reversibly deactivating specific cortical loci with cooling. We identified that in deaf cats, reversible deactivation of a region of cortex typically identified as the posterior auditory field (PAF) in hearing cats selectively eliminated superior visual localization abilities. It was also found that deactivation of the dorsal zone (DZ) of "auditory" cortex eliminated the superior visual motion detection abilities of deaf cats. In the third study, graded cooling was applied to deaf PAF and deaf DZ to examine the laminar contributions to the superior visual abilities of the deaf. Graded cooling of deaf PAF revealed that deactivation of the superficial layers alone does not cause significant visual localization deficits. Profound deficits were identified only when cooling extended through all six layers of deaf PAF. In contrast, graded cooling of deaf DZ showed that deactivation of only the superficial layers was required to elicit increased visual motion detection thresholds. Collectively, these three studies show that the superficial layers of deaf DZ mediate the enhanced visual motion detection of the deaf, while the full thickness of deaf PAF must be deactivated in order to eliminate the superior visual localization abilities of the congenitally deaf. Taken together, this combination of experimental approaches has demonstrated a causal link between the crossmodal reorganization of auditory cortex and enhanced visual abilities of the deaf, as well as identified the cortical regions responsible for adaptive supranormal vision.