Instability of Oculomotor Control in Parkinson's Disease Without Freezing of Gait: Evidence From Reflexive and Voluntary Saccade Variability.

Parkinson's disease (PD) is associated with alterations in both voluntary and reflexive eye movements; however, the characteristics of oculomotor variability across task contexts remain incompletely understood. This study investigated prosaccade, antisaccade and fixation parameters in 15 individuals with early- to midstage PD, assessed in the on-medication state and 15 age- and sex-matched neurologically healthy controls. Eye movements were recorded during structured saccadic tasks and during free-viewing of dynamic video stimuli using high-resolution binocular eye tracking. Compared with controls, participants with PD exhibited significantly higher prosaccade error rates and an increased peak velocity-to-amplitude ratio. Trial-to-trial variability, quantified using coefficients of variation, was consistently elevated in the PD group across multiple saccade parameters. During the video-viewing condition, changes in saccade metrics following video exposure were observed in the control group but not in the PD group, whereas fixation-based measures did not reliably differentiate groups. Together, these findings indicate that increased variability and reduced consistency of saccadic execution are prominent features of oculomotor control in PD without freezing of gait, particularly during reflexive saccade tasks. The results underscore the value of variability-based analyses for probing sensorimotor control in PD and motivate future work to examine their task dependence, longitudinal stability and relevance across disease stages.