Principal Investigator: Daphne Bavelier
Reorganization of Visual Functions in Congenitally Deaf Adults
This work tests the effect of altered auditory experience on the organization of visual functions by comparing visual abilities in congenitally deaf and hearing individuals. Two major hypotheses are concerned with the perceptual effects of deafness. The perceptual deficiency hypothesis holds that impairment in the auditory modality results in deficiencies in the visual modality, as the auditory system develops before the visual system and so normally guides the development of the visual system. In contrast, the compensatory hypothesis holds that impairment in the auditory modality results in greater reliance upon the remaining senses, possibly leading to enhanced visual abilities.
The proposed studies are aimed at testing the hypothesis of enhanced visual attention capacities in deaf individuals. In particular we will test the claim that deaf individuals have a greater 'spatial resolution of visual attention' than hearing individuals as defined by He et al. (1996). While the resolution of visual attention is known to decrease abruptly from center to periphery in hearing subjects, we hypothesize that the rate of decrease will be slower in deaf individuals, allowing an easier processing of peripheral stimuli. Spatial resolution of visual attention should not be confounded with visual resolution. For example, it can be shown that upper and lower fields have similar visual resolution but different attentional resolution. Subjects asked to detect the orientation of a tilted T presented in the middle of other Ts show higher accuracy when the stimuli fall in the lower than the upper visual field. While detecting the orientation of a T require feature conjunction (a process which is attention dependent), detecting the orientation of a single bar requires little attention. Accordingly, the lower visual field advantage disappears when the Ts are replaced by single bars. Experiment I proposes to map the spatial resolution of visual attention in hearing and deaf individuals. Increased sensitivity generally co-occur with increased amount of cortex devoted to the task. Experiment 2 will test the proposal of an increased attentional resolution in deaf by testing whether areas involved in visuo-spatial attention are enlarged in deaf using the fMRI technique. Increased attentional resolution in the periphery holds that peripheral stimuli should be easier to process. Thus, peripheral distractors should be harder to ignore in deaf than hearing individuals. This claim will be tested behaviorally in Experiment 3; its neural correlates will be assessed using fMRl in Experiment 4.
Although differences between deaf and hearing may be due to auditory deprivation, we will control for the contribution of early acquisition of sign language as all of our deaf subjects are native signers of American Sign Language. Hearing subjects born to deaf parents, raised within a signing community will also be run to disentangle the relative contribution of signing and auditory deprivation.
This work carries important implications for the deaf community as deficits
in visual attention are often cited as the source of their reading difficulties.
This work also holds promises for children with visual attention disorders
as our studies will characterize the aspects of experience that enhances
visual attention during development.
