Impact of early deafness and early exposure to sign language on the cerebral organization for motion processing.

This functional magnetic resonance imaging study investigated the impact of early auditory deprivation and/or use of a visuospatial language [American sign language (ASL)] on the organization of neural systems important in visual motion processing by comparing hearing controls with deaf and hearing native signers. Participants monitored moving flowfields under different conditions of spatial and featural attention. Recruitment of the motion-selective area MT-MST in hearing controls was observed to be greater when attention was directed centrally and when the task was to detect motion features, confirming previous reports that the motion network is selectively modulated by different aspects of attention. More importantly, we observed marked differences in the recruitment of motion-related areas as a function of early experience. First, the lateralization of MT-MST was found to shift toward the left hemisphere in early signers, suggesting that early exposure to ASL leads to a greater reliance on the left MT-MST. Second, whereas the two hearing populations displayed more MT-MST activation under central than peripheral attention, the opposite pattern was observed in deaf signers, indicating enhanced recruitment of MT-MST during peripheral attention after early deafness. Third, deaf signers, but neither of the hearing populations, displayed increased activation of the posterior parietal cortex, supporting the view that parietal functions are modified after early auditory deprivation. Finally, only in deaf signers did attention to motion result in enhanced recruitment of the posterior superior temporal sulcus, establishing for the first time in humans that this polymodal area is modified after early sensory deprivation. Together these results highlight the functional and regional specificity of neuroplasticity in humans.

Visual attention to the periphery is enhanced in congenitally deaf individuals.

We compared normally hearing individuals and congenitally deaf individuals as they monitored moving stimuli either in the periphery or in the center of the visual field. When participants monitored the peripheral visual field, greater recruitment (as measured by functional magnetic resonance imaging) of the motion-selective area MT/MST was observed in deaf than in hearing individuals, whereas the two groups were comparable when attending to the central visual field. This finding indicates an enhancement of visual attention to peripheral visual space in deaf individuals. Structural equation modeling was used to further characterize the nature of this plastic change in the deaf. The effective connectivity between MT/MST and the posterior parietal cortex was stronger in deaf than in hearing individuals during peripheral but not central attention. Thus, enhanced peripheral attention to moving stimuli in the deaf may be mediated by alterations of the connectivity between MT/MST and the parietal cortex, one of the primary centers for spatial representation and attention.