Visual control of isometric force in Parkinson's disease.

The current article reports an investigation of the influence of visual feedback on force production in Parkinson's disease (PD) that required subjects to maintain a constant amount of isometric force with their index finger and thumb with and without visual feedback. Eight PD and eight matched control subjects produced force at 5, 25 and 50% of their maximal voluntary contraction for 20 s. In conditions of full vision, the force trajectory and force target were viewed on the computer monitor. In the no visual feedback condition, visual feedback of the force trajectory vanished after the initial 8 s of the trial. The results showed that under the vision condition PD subjects produced levels of maximal and submaximal force that were similar to controls. Approximately 1.5-2.5 s following the removal of visual feedback, the force level in both subject groups decreased to steady-state levels. There was no difference in the time between visual feedback removal and the beginning of force decay in PD. There was a larger amount and faster rate of force decay after visual feedback removal in PD subjects compared to the controls. It is proposed that the increased force decay in PD does not result from sensory reflex deficits but from higher order sensory-motor memory processes.