P47. A cerebellum-dependent velocity-duration tradeoff ensures the precision of fast finger movements

Abstract

We have previously argued that cerebellum-dependent short-term saccadic plasticity is a reflection of a mechanism whose purpose is the maintenance of saccadic precision in the face of trial by trial changes in saccade velocity due to motor, cognitive or neuronal “fatigue”. This is achieved by adjusting saccade duration according to the velocity expected in a given trial, a velocity-duration tradeoff that is ensured by continuously updating a Purkinje-cell simple spike population code (for review, Prsa & Thier, Europ J Neurosci 2011;33:2114–28). To test if the notion of a cerebellum-controlled velocity-duration tradeoff is of general validity, we studied precise index finger movements in healthy subjects and patients suffering from global cerebellocortical degeneration. Subjects were asked to make a long series of rapid alternating up and down movements of their index finger about the MCP joint in order to move a cursor, representing the finger tip’s vertical position, into narrow target zones on a monitor in front of the subject. During the experiment the hand was kept in a comfortable horizontal resting position and movements of the distal finger joints were prevented by a splint. Finger position was measured with the search coil technique. To keep the participants motivated during the task, we provided continuous feedback on task performance. Healthy subjects were able to ensure precise pointing movements despite trial to trial as well as long-term variations in finger velocity by adjusting duration accordingly. This velocity-duration tradeoff was deteriorated in patients, explaining the much larger endpoint variability. These findings suggest that dysmetria of movement, the hallmark of cerebellar disease, is in general a consequence of the inability to fine tune movement duration. In other words, these observations on finger movements support the notion, originally suggested by work on saccades, that the cerebellum contributes to movement control by deploying a fast and precise velocity-duration tradeoff mechanism.