Increased use of target cues during visuo-motor tracking in Parkinson's disease.

The effect of temporarily suppressing the visual display of either the target or actual movement trajectory upon the accuracy of visuo-motor tracking was studied in patients with Parkinson's disease (PD) and healthy subjects. Subjects made wrist movements to superimpose a movement cursor upon a target cursor on a VDU screen. The tracking of slow ramp and sinewave target waveforms was investigated. Trials involving the three conditions of visual suppression, namely, target suppressed (TS), movement suppressed (MS) and non-suppressed (NS) were ordered randomly. In TS and MS trials, respectively, the target or movement cursor disappeared from the subject's view for a 4 s period whilst in NS trials both the target and movement cursors were continuously present. Prior to experimental trials, subjects initially practised a series of NS movements. Tracking errors were analysed by ANOVA for group, suppression condition and waveform effects. The tracking performance of the PD patients, during each form of suppression condition, was worse than that of healthy subjects. Both TS and MS elicited significant reductions in accuracy across groups and waveforms. TS induced a more pronounced impairment of tracking accuracy in the PD group than in the control group suggesting that parkinsonians exhibit an abnormally increased reliance upon visual information of the required trajectory during the present visuo-motor tracking tasks. By contrast, there was no between-group effect of MS in these tasks, suggesting that PD patients show a comparable dependence upon visual feedback of their own movements to that shown by controls.