Body position sense in Parkinson's disease

Background: We are investigating whether side of onset of Parkinson’s disease symptoms affects movement through the world. Previous experiments (e.g., Lee et al., 2001) have shown that those with left-sided body onset (LPD) show more visuospatial problems than those whose symptoms started on the right (RPD). We believe these problems contribute to both falling and “freezing” (akinesia paradoxica). A 72-year-old male whose PD symptoms began on the left took part in a landmark bisection task. He reported no visual problems, but presented with an unconscious 14-degree tilt to the right. After righting, he would slowly return to his tilted position. We used a landmark bisection task presented on a large screen (1.4×1m) with a viewing distance of 2 metres. Landmark bisection is a visual task, not compromised by the motor problems in PD. The participant had to state the side of any mis-bisection (left or right). This participant was tested with the tilt, and with corrected posture. The LPD group made 30% errors in this task compared to 11% in the RPD group. Our age-matched, healthy control group made 5%. When tilted, the participant performed as well as the control group (5%). After postural correction, the participant reverted to the performance of a typical LPD participant (30%). There is little known about why a minority of PD patients sometimes develop such unconscious tilting. However, on the basis of this patient we argue that the postural tilt is an attempt to correct faulty visual input by the proprioception system.