Impairment of voluntary saccades and facilitation of reflexive saccades do not co-occur in Parkinson's disease.

Reflexive saccades (fast eye movements) and voluntary saccades activate overlapping parts of the oculomotor system. It is assumed that striatal dopamine depletion in Parkinson's disease (PD) only affects the voluntary saccadic system and that the often-reported facilitation of the reflexive saccadic system in PD is secondary to impairment of the voluntary saccadic system. If this assumption is correct, facilitation of reflexive saccades should co-occur with impaired performance of voluntary saccades in patients with PD. We measured reflexive and voluntary saccades in a group of patients with PD (both "on" and "off" medication) and a matching group of control subjects. Interestingly, performance measures showed strong positive correlations across reflexive and voluntary saccades in the PD group. Our results suggest that facilitation of reflexive saccades does not co-occur with impairment of voluntary saccades and that PD may affect the parts of the oculomotor system which are common to reflexive and voluntary saccade generation.