The effects of visual feedback during a rhythmic weight-shifting task in patients with Parkinson's disease.

Augmented visual feedback (VF) may offer benefits similar to those of rhythmic external cues in alleviating some mobility-related difficulties in individuals with Parkinson's disease (PD). However, due to an impaired ability to reweigh sensory information under changing circumstances, subjects with PD may be rather vulnerable to incongruity of visual information. In the present study, we investigated whether VF is indeed effective in improving motor functioning in a weight-shifting task during upright stance, and whether subjects with PD are affected more by incongruent VF than healthy controls. Participants performed sideways swaying motions based on tracking of real-time and delayed VF - the first providing congruent, and hence more accurate, visual information than the latter. We analyzed center-of-pressure signals patterns for 28 individuals with PD and 16 healthy, age- and gender-matched controls by estimating task accuracy, movement pattern variability, and normalized movement amplitude. For conditions without feedback and with real-time feedback, subjects with PD performed lateral swaying motions with greater error (F(1, 42)=12.065, p=.001) and with more variable movement patterns than healthy controls (F(1, 24)=113.086, p<.001). Error change scores revealed that patients with PD were nevertheless still able to use VF to improve tracking performance (t(24)=-2.366, p=.026). However, whereas controls were able to adapt to a certain amount of visual incongruity, patients with PD were not. Instead, movement amplitude was significantly reduced in this group (F(1.448, 60.820)=17.639, p<.001). By reducing movement amplitude, subjects with PD appear to resort to a 'conservative' strategy to minimize performance breakdown.