A quantitative analysis of saccades and smooth pursuit during visual pursuit tracking. A comparison of schizophrenics with normals and substance abusing controls.

The eye movements of schizophrenic patients are characterized by decreased smooth pursuit gain and an increased frequency of saccades. However, the nature of these saccades and their function during smooth pursuit has not been clearly defined. To address this issue we examined the eye movements of 22 schizophrenic patients, 20 substance abusing patients (primarily alcohol; some with concomitant cocaine and/or cannabis abuse), and 17 normal controls during a visual pursuit task using infra-red oculography. A computerized pattern recognition algorithm divided pursuit eye movements into two basic components: smooth pursuit and saccadic eye movements. The algorithm also determined eye position error and velocity error before and after each saccade. Schizophrenic patients had lower smooth pursuit gain (p less than 0.02) and made more saccades during smooth pursuit (p less than 0.02) than either comparison group. When saccades were assigned to subcategories based on direction and position error, only the frequency of 'catch-up' saccades differentiated schizophrenic patients from the comparison groups (p less than 0.05). Smooth pursuit gain was negatively correlated with saccadic frequency among all three subject groups. Eye velocity preceding saccades was significantly lower among the schizophrenic patients, but pre or post saccadic position error did not differ among the three groups. Discrete analysis of the fine structure of visual pursuit tracking may lead to a better understanding of eye movement abnormalities in schizophrenia.