Visual Processing in Deaf Signers: Psychophysics and Brain Imaging Studies

University Of California-San Diego, La Jolla CA

Investigators

Abstract

With National Science Foundation support, Dr. Dobkins and colleagues will conduct a three-year study to investigate the perceptual and neural consequences of early-onset sensory deprivation. The study of deaf individuals, who have been auditorily deprived since birth and who rely upon a visual language (i.e., American Sign Language, ASL) for communication, affords a unique opportunity to investigate how and the degree to which sensory processing within the remaining senses (specifically vision) is modified as a result of altered sensory experience. It has long been thought (mostly through anecdotal evidence) that the deaf "see" better (and that the blind "hear" better). However, owing to the difficult nature of communication between hearing researchers and deaf subjects (who speak different languages and are part of different cultures), surprisingly little is known about visual processing in the deaf. To investigate this issue, we use both psychophysical techniques (i.e., asking subjects to report about stimuli presented to them on a video monitor, e.g., "did the stimulus move up or down?") and brain imaging techniques (which localize and measure neural activity elicited by visual stimuli) to characterize differences in visual processing between deaf and hearing subjects. In our prior NSF studies, we focused on visual motion processing, with the notion that motion may be particularly important to deaf subjects, since 1) ASL comprehension relies heavily on the motion of the hands and 2) in the absence of auditory cues, deaf subjects may rely on motion in their periphery to help orient them to objects entering their visual field. The results of these studies revealed a robust right visual field / left hemisphere dominance for motion processing in deaf signers, which can potentially be accounted for by proposing that perceptual processes required for the comprehension of language (motion processing in the case of ASL) get recruited by the left, language-dominant hemisphere. In the current proposal, we expand on these findings by testing deaf and hearing subjects on motion processing, as well as two other domains of vision, namely color and form processing. In addition, as an extension of our previous attentional studies, we will investigate whether attentional resources differ between deaf and hearing subjects, particularly in the peripheral visual fields. Finally, in combination with these visual studies, we will conduct linguistic studies that examine the degree to which language is lateralized to the left hemisphere within individual subjects. This will allow us to investigate further the proposed link between the deaf's visual and linguistic experience. The intellectual merits of this proposed activity include advancing our knowledge of how developing sensory areas of the brain reorganize themselves in response to altered sensory input, which should have implications for principles of brain development in general. From the clinical standpoint, the results may be useful in designing compensatory programs for auditorily-deprived persons, who could be trained to exploit those aspects of sensory processing that are most adaptive in response to altered sensory input. Moreover, with the frequency of cochlear implants on the rise, our findings may be helpful in designing special features for these prosthetic devices in deaf individuals. The broader impacts of these projects include a strong outreach component; by their very nature, they include people from an underrepresented group (i.e., deaf people) as subjects. They also attract deaf people to be trained as researchers in our laboratory. Finally, in addition to reaching a broad audience (including researchers in the fields of psychophysics, neuroscience, development and linguistics) through conferences and journal publications, the results of these projects are brought to both undergraduate and graduate students at UC San Diego through the PI's teaching of courses such as "Sensation and Perception" and "Physiological Psychology".