The effects of dark adaptation on pursuit tracking dysfunction in psychotics with impaired vestibular suppression.

Smooth pursuit eye tracking performance, evaluated by electronic processing and subjective ratings, was assessed in actively psychotic psychiatric patients and normal controls under conditions of light and dark adaptation and was related to subjects' ability to suppress vestibular nystagmus induced by caloric irrigation. Patients' tracking performance was significantly inferior during the light-adapted condition on all tracking measures. Dark-adaptation eliminated group differences based on electronic analyses, but subjective ratings--which showed a variable relationship to the other tracking measures--did not reflect this improvement. Vestibular nystagmus suppression was significantly impaired in patients relative to controls and this impairment, coupled with smooth pursuit dysfunction and marked improvement in tracking performance during dark-adaptation, suggests that cerebellar dysfunction contributes significantly to tracking dysfunction in psychotic patients.