Predictive eye and hand movements are differentially affected by schizophrenia

Abstract

Schizophrenic patients are known to have problems suppressing reflexive eye movements. This is considered to indicate a dysfunction in prefrontal cortex. As the eye and hand motor systems are tightly coupled, we investigated whether predictive hand movements and eye-hand coordination are unimpaired in schizophrenic patients. Saccades and hand movements of 19 patients during an acute schizophrenic episode and 19 controls were measured in a task in which the predictability of target timing was varied. Schizophrenic patients generated more anticipatory and less visually triggered saccades than controls with both non-predictable and predictable target timing. Anticipatory saccades in the wrong direction were clearly directed towards previous target positions, indicating that they are indicators of erroneous prediction rather than of fixation instability. In contrast to saccades, the number of anticipatory and visually triggered hand movements was the same in patients as in controls. As a consequence, patients took longer to initiate a hand movement after a saccade than controls. Schizophrenic patients show increased predictive saccadic activity, but no qualitative changes in predictive saccades. Since prediction itself was not disturbed, the patients' deficit rather lies in the suppression or gating of anticipatory saccades than in their generation. This may be explained by a selective dysfunction of the basal ganglia oculomotor loop. As predictive hand movements were unimpaired, the problems in eye-hand coordination as expressed by a longer initiation time of hand movements relative to saccades are a direct consequence of impaired predictive saccadic behaviour.