Force transition control within a movement sequence in Parkinson's disease.

An experiment was performed which examined movement planning and force transition control in six patients with Parkinson's disease (PD) during a sequence of five finger taps at either a fast (200 ms) or slow (600 ms) temporal speed. The patients acted as their own controls and performed finger taps under three task conditions: (1) where all taps had to be of the same force intensity: no stress; (2) where it was known that one of the taps had to be executed with an augmented force: stress simple reaction time (SRT); and (3) where it was not known prior to initiation which one of the taps was to be more forcefully produced: stress choice reaction time (CRT). Reaction time data revealed a between-condition effect where stress SRT was faster than stress CRT and no stress was faster than both. Under both speed conditions, the interval after the stressed tap was profoundly lengthened. It was found that the lengthening was due to increases in both lift and dwell times for the slow tapping rate. In contrast, at the fast tapping rate, proportionately more of the interval increase was due to the increase in lift time. These findings suggest that patients with Parkinson's disease, when performing under a motor program mode, have difficulty in initiating a sequence and making a transition to lighter force levels after a stressed tap.