Self-paced saccades and saccades to oddball targets in Parkinson's disease

Patients with Parkinson's disease (PD) manifest difficulty in initiation and execution of movements, particularly when movements are sequential, simultaneous or repetitive. Eye movements are particularly effective in evaluating motor impairments. We utilized a series of saccadic eye movement paradigms to explore the ability of 13 patients with mild-moderate PD and 13 age-matched healthy controls to self-pace saccades between two continuously illuminated targets, before and after an externally cued tracking period, and respond to unexpected changes in task demand. The latter was explored by measuring saccadic responses to unexpected "oddball" targets that appeared during a well-learned reciprocating sequence of saccades, in either the opposite direction to that expected or at twice the anticipated extent. Results indicated that all participants demonstrated a marked increase in saccade amplitudes from the externally cued saccade tracking to the self-paced saccades. Unexpectedly, this difference was magnified in PD patients. Self-paced saccades before externally cueing were also more frequent than requested in the PD group, but timing improved following external cueing. The second key finding was that while patients were able to respond to unexpected changes in target amplitude, performance was more variable (in terms of latency and accuracy) when responding to unexpected changes in target direction. Hence, beneficial effects of external cueing on the timing of self-paced saccades may be mediated through cortical regions, placing less emphasis on striatal regions known to be compromised in PD. Additionally, responding to changes in saccade direction (but not amplitude) may rely on basal ganglia circuitry.