Dopaminergic contributions to distance estimation in Parkinson's disease: a sensory-perceptual deficit?

Recent research has found that perceptual deficits exist in Parkinson's disease (PD), yet the link between perception and movement impairments is not well understood. Inaccurate estimation of distance has the potential to be an underlying cause of movement impairments. Alternatively, those with PD may not be able to perceive their own movements accurately. The main objectives of these studies were to evaluate (1) whether distance estimation is influenced by static perception compared to perception during movement in PD, (2) how visual motion processing contributes to distance estimation during movement, and (3) how dopaminergic medication contributes to these distance estimation deficits. Thirty-seven participants (19 individuals with PD, 18 age-matched healthy control (HC) participants) estimated distance to a remembered target in a total of 48 trials, in 4 randomized blocks. Estimation conditions included: (i) static perception (laser): participants pointed with a laser, (ii) active dynamic perception (walk): participants walked to the estimated position, (iii) passive dynamic perception (wheelchair): participants were pushed in a wheelchair while they gave their estimate. PD patients completed this protocol twice; once OFF and once ON dopaminergic medication. Participants with PD and HC did not differ in judgment accuracy during the static perception (laser) condition. However, those with PD had greater amounts of error compared to HC participants while estimating distance during active dynamic perception (walk). Interestingly, those with PD significantly underestimated the target position compared to healthy control participants across all conditions. Individuals with PD also demonstrated greater variability in their judgments overall. There was no significant influence of dopaminergic medication in any of the conditions. Individuals with PD demonstrated distance estimation deficits only when required to actively move through their environment. In contrast to estimations made with movement, neither static perception (laser) nor passive dynamic perceptions (wheelchair) revealed significant differences in the magnitude of error between the two groups. Thus perceptual estimation deficits appear to be amplified during movement, which may be suggestive of an underlying sensory processing deficit which leads to a problem integrating vision and self-motion information.