Why deaf have ‘super vision’
Cross-modal plasticity in specific auditory cortices underlies visual compensations in the deaf. When the brain is deprived of input from one sensory modality, it often compensates with supranormal performance in one or more of the intact sensory systems. In the absence of acoustic input, it has been proposed that cross-modal reorganization of deaf auditory cortex may provide the neural substrate mediating compensatory visual function. We tested this hypothesis using a battery of visual psychophysical tasks and found that congenitally deaf cats, compared with hearing cats, have superior localization in the peripheral field and lower visual movement detection thresholds. In the deaf cats, reversible deactivation of posterior auditory cortex selectively eliminated superior visual localization abilities, whereas deactivation of the dorsal auditory cortex eliminated superior visual motion detection. Our results indicate that enhanced visual performance in the deaf
is caused by cross-modal reorganization of deaf auditory cortex and it is possible to localize individual visual functions in discrete portions of reorganized auditory cortex. Studies of deaf or blind subjects often report enhanced perceptual abilities in the remaining senses. Compared with hearing subjects, psychophysical studies have revealed specific superior visual abilities in the early deaf as well as enhanced auditory functions in the early blind. The neural substrate for these superior sensory abilities is thought to reside in the deprived cerebral cortices that have been reorganized by the remaining sensory modalities through cross-modal plasticity. Thus, in early blind animals, acoustic stimuli evoke responses in what are normally visual regions of the cerebral cortex, and heteromodal activity after early deprivation has been repeatedly demonstrated in the visual and somatosensory systems, consistent with the results of imaging studies performed in deaf subjects. In this context, it has been proposed that auditory cortex of the deaf may be recruited to perform visual functions. However, a causal link between supranormal visual performance and the visual activity in the reorganized auditory cortex has never been demonstrated. Furthermore, if auditory cortex does mediate the enhanced visual abilities of the deaf, it is unknown whether these functions
are distributed uniformly across deaf auditory cortex or whether specific functions can be differentially localized to distinct portions of the affected cortices. It is also unknown whether reorganized cortex retains any relationship to functions performed in these regions in hearing subjects. These fundamental questions are clinically important now that restoration of hearing in prelingually deaf children is possible through cochlear prosthetics. To address these issues, we first examined the visual abilities of congenitally deaf and hearing cats to identify those visual functions that are enhanced in the early deaf. We then examined the role of deaf auditory cortex in mediating the superior visual abilities by reversibly deactivating specific cortical loci with cooling. This combination of experimental approaches revealed a causal link between the cross-modal reorganization of auditory cortex and enhanced visual abilities of the deaf and identified the neural regions responsible for those improvements in visual performance. Affiliations; Centre for Brain and Mind, Department of Physiology and Pharmacology, Department of Psychology, University of Western Ontario, London, Ontario, Canada. Stephen G Lomber – Department of Anatomy and Neurobiology, School of Medicine, Virginia Commonwealth University, Richmond, Virginia, USA. M Alex Meredith – Department of Experimental Otology, Institute of Audioneurotechnology, Medical University Hannover, Hannover, Germany.Andrej Kral. Corresponding author. Correspondence to: Stephen G Lomber. News from: nature.com
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