Enhanced Vision in the Deaf

Everyone's heard that losing a particular sensory modality causes the sensitivity of the other modalities to be heightened. Blind people are supposed to hear and smell really, really well, for example. While this is something that's been talked about for ages, there are actual neuroscinetific reasons for thinking that it might be true. When an area of the brain that is designed for one function or set of functions goes unused, or is underused, that area can be co-opted by other functions. But testing to see whether people with sensory deficits (blindness, deafness, etc.) actually have heightened perception in their other sensory modalities isn't all that easy, as a review article by Bavelier et al. in the November issue of Trends in Cognitive Sciences points out. The article is on heightened visual perception in deaf individuals, and the authors note that across the deaf population, there are deaf people who have better vision, and individuals who have worse vision, than nondeaf people. The reason is that there are a lot of causes of deafness, and so it can be difficult to isolate the affects of deafness on vision specifically.

In order to test the effects of deafness specifically on visual perception, the authors limit their review to studies of a small subset of the deaf population: "deaf native signers." Here's the description of this group, and the reasons for using them from the article:

These individuals are born deaf to deaf parents; they are profoundly deaf; and they have no associated central nervous system damage. In addition, they achieve their language development milestones at the same rate and time as hearing individuals by virtue of being born within a signing community. Study of this population, representing only about 5% of the total deaf population, enables the effect of auditory deprivation to be evaluated, with minimal confounds from other factors such as language deprivation, abnormal cognitive development due to communication disruption, or comorbidity associated with deafness.

Within this subpopulation of deaf individuals, they don't find that deaf individuals see better than nondeaf individuals over all, but that certain aspects of visual perception are better. Specifically, the deaf individuals seem to be better at allocating visual attention to the periphery of the visual field, even when the task they're performing requires them to attend to something at the center of their visual field as well. For example, the authors describe studies in which deaf native signers are better at identifying the direction of motion of stimuli at the edge of their visual field, and that distracting stimuli at the edge of the visual field actually hurt the performance of deaf native signers on tasks that require them to attend to the center of the visual field more than it hurts the performance of nondeaf individuals. This latter finding indicates that the deaf native signers are processing the peripheral distractors more than nondeaf individuals.

They go on to describe several possible neural reasons for enhanced attention to the visual periphery in deaf individuals, discussing the evidence for each. They find that several brain areas associated with areas that process information from both the visual and auditory systems show changes in deaf native signers. These areas include the superior temporal sulcus, the posterior parietal cortex, and the medial temporal cortex, as well as parts of the primary auditory cortext that border on multisensory areas. These areas are shown in their figure 3 (p. 516):

If you want to learn more, you can read the entire article here.

Article: Bavelier, D., Dye, M.W.G., & Hauser, P.C. (2006). Do deaf individuals see better? , 10(11), 512-518.