Parkinson's disease shows perceptuomotor asymmetry unrelated to motor symptoms

Abstract

Evidence of neglect symptoms in Parkinson's disease (PD) have been reported during visuoperceptual tasks and linked to side of disease onset. The goal of this study was to determine if in PD perceptual asymmetry is also evident in perceptuomotor tasks without visual input. The task was to point to a remembered straight-ahead (SA) target in peripersonal space. During baseline pointing, a bias left of SA was evident in PD patients and right of SA in healthy controls. To evaluate whether this was linked to a proprioceptive bias in PD, pointing during axial twisting of the trunk was tested. Axial rotation (±15°, 1° s −1) of the lower-body about shoulders fixed against rotation induced a non-veridical perception of upper-body rotation and lower-body stationarity. Pointing endpoints were shifted right of the actual SA during clockwise (CW) lower-body rotation and left of SA during counter-clockwise (CC) rotation, despite the fact that the shoulders and head were not rotated. In PD patients, endpoints relative to SA were shifted less during CW than CC rotation of the lower-body, whereas controls showed symmetrical pointing. Levodopa did not significantly change this bias. Both hands were tested in each subject and bias appeared regardless of hand used. Neither disease progression nor side of disease onset was linked to the direction or size of pointing bias. These findings suggest that PD manifests a contraction of left external hemispace relative to right hemispace, which affects generation and execution of motor commands throughout disease progression.