Acquisition of anticipatory postural adjustments in a bimanual load-lifting task: normal and pathological aspects.

Anticipatory adjustments of forearm posture are associated with a voluntary load-lifting movement in bimanual load-lifting tasks. Three aspects of these adjustments are analyzed: their goal, their central organization, and their acquisition. The goal of the anticipatory adjustment in this task is to minimize the perturbation of forearm posture that occurs during unloading. The central organization is based on two parallel controls responsible, respectively, for the lifting movement of the moving forearm and the anticipatory postural adjustment of the postural forearm, their coordination depending on a central timing signal. The acquisition of the anticipatory postural adjustment was tested using a paradigm where the voluntary movement performed by one hand triggered, via an electronic switch, the load release of the postural forearm. It was achieved after 40-60 trials and was not graded as a function of the voluntary movement parameters, but of the disturbance of the postural arm about to occur. The learned anticipation was not transferred when, after a first acquisition session with one forearm as the postural forearm, a second learning session was performed with the other forearm as the postural forearm. The acquisition was tested in Parkinsonian and in hemiparetic patients with capsular lesion. The highest acquisition deficit was observed in hemiparetic patients, when the contralateral forearm was the postural forearm; the deficit was less important when the ipsilateral arm was postural. Surprisingly, the anticipatory postural adjustments in hemiparetic patients were rather well preserved when the natural load-lifting task was tested. These results suggest that the basal-ganglia SMA circuit and M1 premotor areas are important in the acquisition process.