POSTURAL RESPONSES TO CHANGING TASK CONDITIONS IN PATIENTS WITH CEREBELLAR LESIONS

Objective-To investigate the role of the cerebellum in postural adapta tion for changes to the stimulus type of support surface displacements (backward translations v ''toes up'' rotations). Methods-A group of 1 3 patients with chronic, isolated lesions of the cerebellum and 15 con trol subjects were tested. Automatic postural responses of the medial gastrocnemius and anterior tibial muscles were recorded. The first par adigm consisted of 10 rotational perturbations followed by 10 backward translations of the platform, and 10 backward translations followed b y 10 rotations. The second paradigm consisted of 18 rotations and two randomly interposed translational perturbations, and 18 translations w ith two rotations randomly interposed. Results-When the type of pertur bation changed from an expected translation to an unexpected rotation and vice versa both control subjects and cerebellar patients showed an immediate and significant change in the response amplitude of the med ial gastrocnemius and at the same time an immediate and significant ch ange in the response amplitude of the anterior tibial muscles. Neither controls nor cerebellar patients showed effects of prediction in surf ace displacements of unexpected types of perturbation. Both controls a nd cerebellar patients showed no gradual increase in the gastrocnemius response in subsequent trials of surface translations following a blo ck of 10 surface rotations and no gradual increase in the response amp litude of the anterior tibial muscle in subsequent trials of surface r otations following a block of 10 surface translations. Conclusions-Des pite postural hypermetria, the integrity of the cerebellum does not se em critical for adaptation of postural synergies to changing stimulus types of surface displacements. The present results support previous f indings suggesting that the main role of the cerebellum in automatic p ostural responses may be gain control.