PERTURBED STEP INITIATION IN CEREBELLAR SUBJECTS - 1 - MODIFICATIONS OF POSTURAL RESPONSES

Recent experiments in healthy subjects have demonstrated that automati c postural responses can be suppressed when subjects are instructed to step instead of maintain stance in response to the surface translatio n. The aim of the present study was to investigate the role of the cer ebellum in coordinating this interaction between the central command t o step and peripherally triggered automatic postural responses. Eight subjects with cerebellar degeneration and eight control subjects were instructed to either maintain stance or step forward in response to a backward translation. In order to determine whether prediction of pert urbation amplitude assisted suppression of postural responses, three p latform translations were pre sented in both a serial (predictable) an d a random (unpredictable) order. Cerebellar subjects were able to sup press their initial postural responses to the same amount as control s ubjects when instructed to step forward in response to backward transl ations, despite their hypermetria and inability to scale responses to predictable perturbation amplitudes. Control, but not cerebellar, subj ects scaled the size of their postural responses to predictable pertur bation amplitudes. The perturbation amplitude, however, had no effect on the size of early automatic responses when they were suppressed by instruction to step. The size of the suppressed postural response was independent of predictability of perturbation amplitudes in both contr ol and cerebellar subjects. The dynamic interaction between automatic postural responses to an external perturbation and anticipatory postur al adjustments for step initiation seems independent of prediction of perturbation amplitude and the integrity of the cerebellum. Although c erebellar subjects show larger-than-normal magnitude and variability o f postural responses and an inability to scale the size of responses t o predictable perturbation amplitudes, the cerebellum does not seem to be critical for suppression of the early postural response with a cen trally intended movement.