Basal ganglia and cerebellar impairment differentially affect the amplitude and time scaling during the performance of forearm step tracking movements.

Forearm step tracking movements of different amplitudes were analysed in nine patients with cerebellar and five patients with parkinsonian symptoms in comparison to six normal subjects. Movements were made under two instructions: 1) track "as fast as possible" and 2) "as precisely as possible". Movement duration, acceleration duration and peak velocity increased with increasing amplitude. The ratio between acceleration and deceleration duration was independent of movement amplitude in the parkinsonian patients and the normal subjects. In the cerebellar patients, however, this ratio decreased with increasing amplitude. This impairment of the relative timing between acceleration and deceleration duration implies that, in contrast to parkinsonian patients, cerebellar patients cannot anymore utilize physiological scaling mechanisms when performing movements of different amplitudes.