Effects of Congenital Deafness on Vision

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Studies of visual functions after early genetic deafness indicate enhanced visual processing, at least for visual motion, visual attention, and peripheral vision. For example, deaf adults are better than hearing controls at detecting the onset or the direction of motion of a peripheral stimulus. They are also faster at switching visual attention toward a near-periphery target in the presence of distractors located at the fixation point. Electrophysiological recordings while subjects monitored moving stimuli have indicated larger visually evoked responses for deaf than hearing adults over occipital and temporal sites. These group differences were especially marked for peripheral stimuli. In an fMRI study, the effects of visual attention on motion processing were compared in deaf and hearing individuals. When participants monitored the peripheral visual field, greater recruitment ofthe motion-selective area MT was observed in deaf than in hearing participants, whereas the two groups were comparable when attending to the central visual field (Fig. 3). Further analysis suggested that changes in peripheral attention in the deaf are mediated through the modulation of the connections between earlier sensory areas and the posterior parietal cortex, which is one of the main centers of attention.

The functions altered in the deaf, i.e., motion, visual attention, and peripheral processing, share the property of being mediated predominantly by the dorsal visual pathway that projects from V1 to the motion area (MT-MST) and the parietal cortex. To test the specificity of dorsal pathway enhancement after early deafness, motion processing was compared to color processing. It is commonly accepted that, whereas motion processing is primarily mediated by the dorsal pathway, color is primarily mediated by the ventral pathway projecting from V1 to IT. Electrophysiologi-cal recordings while subjects monitored either high-spatial-frequency colored gratings (color) or low-spatial-frequency, gray scale, moving gratings (motion) were compared in deaf and hearing persons. Several specific group differences occurred in the amplitude and distribution of early sensory responses recorded over anterior and temporal regions. Deaf subjects displayed significantly greater amplitudes than hearing subjects, but this effect occurred only for moving stimuli not for color stimuli (Fig. 4). Further, whereas in hearing subjects, color stimuli elicited larger responses than did motion stimuli, in deaf subjects responses to motion stimuli were as large as those to color stimuli. These data suggest that larger changes in motion than color processing occur after early deafness. Taken together, the available data suggest that there is considerable specificity in the aspects of visual processing that are altered in con-genitally deaf individuals.

Because many genetically deaf individuals learn American Sign Language (ASL) as a first language, some of the changes reported could have been due to deafness or to the acquisition of a signed language. Signing has been shown to affect performance on tasks that require visuospatial transformations and are likely to recruit the dorsal pathway. Deaf native signers are both faster and more accurate than controls on tasks of mental rotation or when identifying objects presented from a noncanonical viewpoint. This effect is not specific to auditory deprivation; hearing native

Human Auditory Field
Figure 3 Extent of activation in MT-MST in deaf and hearing individuals as they monitored moving stimuli for luminance changes in either the center or the near periphery of the visual field. Enhanced recruitment of MT is observed for the peripheral condition in the deaf.

signers also exhibit better mental rotation performance than nonsigners. Researchers have also shown that deaf and hearing signers are faster at generating mental images than hearing nonsigners. This work establishes that familiarity with ASL results in behavioral enhancement in a number of visuospatial tasks that are likely to recruit structures in the dorsal pathway. Acquisition of ASL has also been linked with brain reorganization for motion processing. The few studies on that topic have reported a left hemi sphere advantage for motion processing in native signers, whereas hearing nonsigners displayed a tendency for a right hemisphere advantage. Interestingly, the studies available indicate that the lateralization pattern for motion processing is guided by sign language acquisition, whereas the enhancement of motion processing in the visual periphery is specific to deafness. This research illustrates the specificity of the plastic changes as a function of the nature of altered experience.

Figure 4 ERPs elicited by (a) color change and (b) motion in normally hearing and congenitally deaf adults. Recordings are from temporal and posterior temporal regions of the left and right hemispheres. Reprinted from Neville and Bavelier (1999), In ''The New Cognitive Neurosciences,'' 2nd ed., M.S. Gazzaniga, (Ed.), pp. 83-98, with permission of MIT Press.

Figure 4 ERPs elicited by (a) color change and (b) motion in normally hearing and congenitally deaf adults. Recordings are from temporal and posterior temporal regions of the left and right hemispheres. Reprinted from Neville and Bavelier (1999), In ''The New Cognitive Neurosciences,'' 2nd ed., M.S. Gazzaniga, (Ed.), pp. 83-98, with permission of MIT Press.

D. Summary

The existing literature suggests that, across auditory and visual modalities, the representation of peripheral space is more altered by early sensory experience than is the representation of central space. Close examination of the behavioral data for blind cats indicates a similar effect, i.e., a larger advantage in sound localization for blind cats at peripheral locations. Importantly, the available studies indicate that not all aspects of the remaining senses are altered after early blindness or deafness. Rather, different neurocognitive systems and subsystems exhibit different sensitivities to altered experience.

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