SELECTIVE IMPAIRMENT OF EXPRESS SACCADE GENERATION IN PATIENTS WITH SCHIZOPHRENIA

When a temporal gap is introduced between the offset of a central fixation point and the onset of a peripheral sacccadic target, normal subjects generate an increased number of short latency (90-150 ms) saccades, termed express saccades, and the profile of express saccade frequency across different gap sizes for any individual subject, even if untrained in the task, shows a high test-retest reliability. In patients with schizophrenia, the generation of express saccades was also normal for gap sizes of 200-300 ms or in an overlap task (gap = 0 ms). However, for temporal gaps of 50-150 ms, the generation of express saccades was significantly impaired in the schizophrenic subjects. This selective deficit appeared to be independent of the patients' neuroleptic medication status and did not correlate with the severity of schizophrenic symptoms. It is postulated that the successful execution of an express saccade requires that the cognitive operations of disengagement of visual attention and selection of the appropriate motor command to generate a saccade both be commenced or completed during the temporal gap between fixation offset and peripheral target onset. Our results suggest that, in schizophrenia, there is an impairment in the cortical/subcortical neural network that generates express saccades and controls these cognitive operations. Potential sites for such dysfunction in schizophrenia include the parietal cortex and the GABA-ergic function of the superior colliculus.