Interhemispheric cooperation during word processing: evidence for callosal transfer dysfunction in schizophrenic patients.

Functional lateralization and interhemispheric interaction during word processing were investigated in schizophrenic patients (n=12) and matched healthy controls (n=18). Words and phonologically regular pseudowords were presented tachistoscopically either in the left or right visual field (unilateral conditions), or simultaneously in both visual hemifields (bilateral condition). Consistent with earlier findings, healthy controls showed a right visual field advantage (RVFA), indicating left-hemispheric dominance for language. The patients showed a RVFA similar to that of controls, consistent with normal left-hemispheric language dominance. Importantly, controls performed much better on words presented in the bilateral condition, when two copies of the same word appeared twice, compared to stimulation in only one of the visual hemifields. This bilateral advantage, which has been interpreted as evidence for cooperation between the hemispheres, was absent in schizophrenics. These data show that schizophrenic patients can exhibit similar lateralization patterns as healthy controls. Their specific functional deficit may be a lack of cooperation between the hemispheres.