SELECTIVE IMPAIRMENT OF EXPRESS SACCADE GENERATION IN PATIENTS WITH SCHIZOPHRENIA

When a temporal gap is introduced between the offset of a central fixa tion point and the onset of a peripheral sacccadic target, normal subj ects generate an increased number of short latency (90-150 ms) saccade s, termed express saccades, and the profile of express saccade frequen cy across different gap sizes for any individual subject, even if untr ained in the task, shows a high test-retest reliability. In patients w ith schizophrenia, the generation of express saccades was also normal for gap sizes of 200-300 ms or in an overlap task (gap = 0 ms). Howeve r, for temporal gaps of 50-150 ms, the generation of express saccades was significantly impaired in the schizophrenic subjects. This selecti ve deficit appeared to be independent of the patients' neuroleptic med ication status and did not correlate with the severity of schizophreni c symptoms. It is postulated that the successful execution of an expre ss saccade requires that the cognitive operations of disengagement of visual attention and selection of the appropriate motor command to gen erate a saccade both be commenced or completed during the temporal gap between fixation offset and peripheral target onset. Our results sugg est that, in schizophrenia, there is an impairment in the cortical/sub cortical neural network that generates express saccades and controls t hese cognitive operations. Potential sites for such dysfunction in sch izophrenia include the parietal cortex and the GABA-ergic function of the superior colliculus.