We studied grip force performance in 10 patients who were in a stable state after
implantation of bilateral stimulating electrodes in the subthalamic nuclei (STN) to
counter drug-resistant or drug-induced symptoms of advanced Parkinson's disease. The
patients used a precision grip to lift an object which recorded grip force development
and lift dynamics. Lifting was performed with stimulation on and with stimulation
off under optimal medication. In the „off“ state the patients showed the profound
bradykinesia and excessive levels of grip force development associated with Parkinson's
disease from its early stages. Stimulation improved both the rate of grip force development
and the speed of the lifting phase. The excessive levels of grip force, however, were
even more marked with STN stimulation on. It is suggested that this results from a
failure to modify stored motor programs developed over a long period under the influence
of bradykinesia, leading to an inappropiately prolonged duration of grip force development
when this influence is removed by STN stimulation. Thus although STN stimulation achieved
a dramatic improvement in rates of force development and lifting dynamics, it does
not restore, and may even worsen, the ability to match lifting parameters to actual
conditions.
