Altered perceptual sensitivity to kinematic invariants in Parkinson's disease

Abstract

Ample evidence exists for coupling between action and perception in neurologically healthy individuals, yet the precise nature of the internal representations shared between these domains remains unclear. One experimentally derived view is that the invariant properties and constraints characterizing movement generation are also manifested during motion perception. One prominent motor invariant is the "two-third power law," describing the strong relation between the kinematics of motion and the geometrical features of the path followed by the hand during planar drawing movements. The two-thirds power law not only characterizes various movement generation tasks but also seems to constrain visual perception of motion. The present study aimed to assess whether motor invariants, such as the two thirds power law also constrain motion perception in patients with Parkinson's disease (PD). Patients with PD and age-matched controls were asked to observe the movement of a light spot rotating on an elliptical path and to modify its velocity until it appeared to move most uniformly. As in previous reports controls tended to choose those movements close to obeying the two-thirds power law as most uniform. Patients with PD displayed a more variable behavior, choosing on average, movements closer but not equal to a constant velocity. Our results thus demonstrate impairments in how the two-thirds power law constrains motion perception in patients with PD, where this relationship between velocity and curvature appears to be preserved but scaled down. Recent hypotheses on the role of the basal ganglia in motor timing may explain these irregularities. Alternatively, these impairments in perception of movement may reflect similar deficits in motor production.

Figure 1. Experimental design and stimuli.

(a) Subjects viewed a white light spot on the computer screen moving in elliptical trajectories. They were asked to modify its motion until it appeared to move most uniformly. (b) Ellipse eccentricity (ε) (c) Velocity profiles for the ellipse with medium eccentricity (ε = 0.835).

Figure 2. Mean β_f values chosen by PD patients and controls, for each of the three different eccentricities (A) and across the effect of tracing speed (B).

Error bars denote SE.

Figure 3. Mean β_f values chosen by PD patients and controls for each of the three tracing-durations (A) and across the effect of eccentricity (B).

Error bars denote SE.

Figure 4. Scores of the difference index for each patient plotted against his/her total motor UPDRS score.

(A) and against a composite scores for all left- and right-sided motor UPDRS items (B). * Statistically significant at p<0.05.