Reduced sensitivity to trial pacing in Parkinson's disease saccadic eye movements.

Abstract

The involuntary and stereotyped kinematics of saccadic eye movements have inspired many optimisation models, and allowed for saccades to be a useful clinical diagnostic. Faster velocity, "supra-optimal", saccades have been difficult to elicit and of more limited clinical use (e.g. opsoclonus). Small velocity increases (~5%) can be elicited via explicit reward paradigms. Recent, but limited data in young healthy adults, also suggests that faster saccades can be facilitated by simply increasing the pacing of trials (reduced inter-trial intervals). One explanation of this pacing effect is that the accumulated implicit reward of landing on target increases with faster pacing, and that this is mediated by dopaminergic neurons in the basal ganglia. We asked whether effects of pacing on saccadic velocities are decreased in Parkinson's disease as reward sensitivity is impaired in these patients. We recorded saccades from 10 young controls, 10 age-matched controls and 10 Parkinson's disease patients to targets that stepped to a new location at intervals ranging from 0.1-1.1s after the primary saccade to the preceding target location. Healthy young and old controls showed small, but robust increases in saccade velocities with increased trial pacing (+3.7% and +3.6%, respectively), comparable to previously reported findings. Interestingly, there was no significant difference with age. In contrast, Parkinson's disease patients showed no velocity modulation with trial pacing (pacing slope = -0.02, p > 0.05), suggesting the implication of the reward system in the increased saccade pacing effect. We also conclude that changes in saccade velocity with pacing may usefully distinguish Parkinson's disease from healthy controls. Meeting abstract presented at VSS 2018