A deficit in perceptual matching in the left hemisphere of a callosotomy patient.

Decades of research have demonstrated dramatic differences between the hemispheres of the brain. While the most obvious asymmetries are in the areas of language and motor control, the visuospatial abilities of the left hemisphere are also known to differ from those of the right hemisphere. This hemispheric difference has been demonstrated empirically but its basis is thus far unclear. In this paper, we investigate the hypothesis that the left hemisphere is capable of sophisticated visual processing, but represents spatial information relatively crudely compared to the right hemisphere. The implication of this hypothesis is that pattern recognition is a function of both hemispheres but the right hemisphere is further specialized for processing spatial information. In a series of seven experiments we examined perceptual matching of mirror-reversed stimuli by the divided cerebral hemispheres of a callosotomy patient. In each experiment the left hemisphere's performance was impaired relative to the right hemisphere. This finding was independent of stimulus type, response bias and stimulus duration. These results are consistent with the idea that visual processing in the left hemisphere is directed towards pattern recognition at the expense of spatial information.