Learning motor synergies makes use of information on muscular load

Prism adaptation, a form of procedural learning, requires the integration o f visual and motor information for its proper acquisition. Although the rol e of the visual feedback has begun to be understood, the nature of the moto r information necessary for the development of the adaptation remains unkno wn. In this work we have tested the idea that modifying the arm load at dif ferent stages of the adaptation process, and the ensuing change of motor in formation perceived by the subjects, would modify the final properties of t he adaptation. We trained a set of subjects to throw balls to a target whil e wearing prism glasses and varied the weight of their arms at different ti me points during the task. We observed that the acquisition of the adaptati on 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 unmaske d, suggesting that the missing aftereffect did not disappear but had remain ed latent. Our results show that the internal representation of a motor mem ory incorporates information about load conditions and that the memory stor ed under a specific weight condition can be fully retrieved only when the o riginal training condition is restored.