Mental retardation and perception of global motion

We have found that mildly mentally retarded adults are impaired in their perception of global stereoscopic forms (Fox & Oross, 1988) in ways that cannot be attributed to peripheral visual deficits or failures to comprehend. To assess the generality of that result, we measured the ability of mentally retarded adults to perceive kinematographic forms. Mentally retarded and nonretarded adults were presented with a two-choice, forced-choice detection task requiring the location of a target's spatial position. The discriminability of the forms was varied by systematic reductions in both element density and temporal correlation. We found that, relative to nonretarded adults, mentally retarded adults exhibited large qualitative deficits in their ability to discriminate these kinematographic forms when either density or correlation was reduced. After considering a number of alternative interpretations of these data based on factors such as peripheral visual impairment and a failure to attend, we could find none more compelling than a perceptual interpretation, which posits a deficit within the short-range motion system.