The role of anticipation and prediction in smooth pursuit in Parkinson's disease

Patients with Parkinson's disease (PD) have difficulties in self-guided (internally driven) movements. The basal ganglia provide a non-specific internal cue for the development of a preparatory activity for a given movement in the sequence of repetitive movements. Controversy surrounds the questions whether PD patients are capable of (a) anticipating their own movements before an external trigger appears (anticipation) and (b) predicting movement velocity once the moving target shortly disappears from the visual scene (prediction).

In previous studies PD patients performed visually (externally) driven movements with predictable velocities and directions (phase lag of sinusoidal pursuit or latency of pursuit ramps or trapezoid ramp) showing no deficits in predictive pursuit.

We examined internally generated smooth pursuit eye movements in 20 PD patients and 20 age-matched healthy controls by systematically varying extinction periods of a trapezoidally moving target (16°/s) in 4 paradigms (initial extinction, extinction after a short ramp, mid-ramp extinction vs. no blanking; modified after Barnes & Collins 2008). The same was examined in a ramp paradigm at gaze straight ahead. This internal (extra-retinal) mechanism plays a role in both predictable and randomised pursuit responses.

When compared with controls PD patients showed (i) a decreased smooth pursuit gain (without extinction), (ii) normal anticipatory pursuit (no differences in pursuit latency, initial extinction), (iii) decreased extra-retinal predictive pursuit velocity (midramp target extinction) as assessed by the larger velocity difference between the midramp and the short ramp paradigm.

While anticipatory smooth pursuit latency appears normal PD patients appear to have deficits in the acquisition and storage of velocity information of target motion, i.e. they cannot develop sufficient internal drive according to expected future target motion.