Does pursuit abnormality in schizophrenia represent a deficit in the predictive mechanism?
Although an abnormality of smooth pursuit eye movement has been consistently noted in schizophrenia, the underlying ocular motor pathophysiology is unknown. It is unclear whether the abnormality represents deficits in processing of information provided by the moving target, generation of pursuit eye movements, or other ocular motor and related cognitive processes. To evaluate the ability to process information provided by a moving target, saccadic accuracies were studied in step-ramp and single step tasks. Schizophrenic (with and without tardive dyskinesia [TD]) and normal subjects made equally accurate initial corrective saccades to the moving target. Thus, when the target jumped and then smoothly moved (creating a position and a velocity error on the retina), the patients were able to process retinal motion information and generate a normally accurate saccadic response. After the initial corrective saccade, both groups followed the target with a combination of pursuit eye movements and occasional catch-up saccades. During this period, the retinal velocity error is minimal because the eye approximates the target motion, and the major source of target motion information both for the smooth pursuit and saccadic responses is extra-retinal (i.e., predictive mechanism). The accuracies of catch-up saccades were significantly lower in the schizophrenic patients than in the normal subjects. During this period, overall pursuit performance, measured by pursuit gain, was also significantly worse in the patients. Accuracies of subsequent catch-up saccades, but not initial corrective saccades, significantly predicted the pursuit gain. Low pursuit gain was associated with high numbers of saccades per time spent in pursuit, which were similar in both schizophrenic subgroups (i.e., with and without TD), but were only significantly higher in the patients with TD than in the normal subjects. These preliminary data suggest that schizophrenic patients are able to process retinal motion information but have difficulties in using extra-retinal motion information to generate an appropriate saccadic response.