Visual contrast sensitivity in deaf versus hearing populations: exploring the perceptual consequences of auditory deprivation and experience with a visual language.

Early deafness in humans provides a unique opportunity to examine the perceptual consequences of altered sensory experience. In particular, visual perception in the deaf may be altered as a result of their auditory deprivation and/or because the deaf rely heavily upon a visual language (American Sign Language, or ASL, in the US). Recently, we found that deaf, but not hearing, subjects exhibit a right visual field/left hemisphere advantage on a low-level direction of motion task, a finding that has been attributed to the deaf's experience with ASL [Psychol. Sci. 10 (1999) 256; Brain Res. 405 (1987) 268]. In order to determine whether this visual field asymmetry generalizes to other low-level visual functions, in this study we measured contrast sensitivity in deaf and hearing subjects to moving stimuli over a range of speeds (0.125-64 degrees /s). We hypothesized that if ASL use drives differences between hearing and deaf subjects, such differences may occur over a restricted range of speeds most commonly found in ASL. In addition, we tested a third group, hearing native signers who learned ASL early from their deaf parents, to further assess whether potential differences between groups results from ASL use. These experiments reveal no overall differences in contrast sensitivity, nor differences in visual field asymmetries, across subject groups at any speed tested. Thus, differences previously observed between deaf and hearing subjects for discriminating the direction of moving stimuli do not generalize to contrast sensitivity for moving stimuli, a result that has implications for the neural level at which plastic changes occur in the visual system of deaf subjects.