Normal prism adaptation but reduced after-effect in basal ganglia disorders using a throwing task.

Prism adaptation is a form of visuomotor learning in which the visual and motor systems need to be adjusted because a visual perturbation is produced by horizontally displacing prisms. Despite being known for over two centuries, the neuronal substrates of this phenomenon are not yet completely understood. In this article the possible role of the basal ganglia in this kind of learning was analysed through a study of Huntington's and Parkinson's disease patients. A throwing technique requiring the use of open loop feedback was used. The variables analysed were visuomotor performance, adaptation rate and magnitude, and the after-effect. The results clearly showed that both Huntington's and Parkinson's disease groups learned at the same rate as control subjects. In addition, despite having a disturbed visuomotor performance, both experimental groups showed the same adaptation magnitude as the control group. Finally, the after-effect, which is measured after removing the prisms, is reduced in both patients groups. This reduction leads to a disruption in the normal adaptation-after-effect correlation found in normal volunteers. These results suggest that basal ganglia are not involved in this type of open-looped visuomotor learning. The large number of patients studied as well as the similarity of the findings between both populations support this hypothesis. By contrast, there is an impairment in the after-effect on both basal ganglia patient populations. This impairment may be the result of the deterioration of the perceptual recalibration process involved in visuomotor learning.

Learning motor synergies makes use of information on muscular load.

Prism adaptation, a form of procedural learning, requires the integration of visual and motor information for its proper acquisition. Although the role of the visual feedback has begun to be understood, the nature of the motor information necessary for the development of the adaptation remains unknown. In this work we have tested the idea that modifying the arm load at different stages of the adaptation process, and the ensuing change of motor information perceived by the subjects, would modify the final properties of the adaptation. We trained a set of subjects to throw balls to a target while wearing prism glasses and varied the weight of their arms at different time points during the task. We observed that the acquisition of the adaptation was not affected by the change in load. However, its persistence (i.e., the aftereffect) was reduced when tested under a weight condition different from the training trials. Furthermore, when the training weight conditions were restored later during testing, a second, late aftereffect was unmasked, suggesting that the missing aftereffect did not disappear but had remained latent. Our results show that the internal representation of a motor memory incorporates information about load conditions and that the memory stored under a specific weight condition can be fully retrieved only when the original training condition is restored.