Modifications with aging in the role played by vision and proprioception for movement control.

Young and older adults performed manual aiming movements to a visible target for either 40 or 200 trials. Under each level of practice, half of the subjects practiced the task under normal visual conditions (proprioception + vision [PV] condition), whereas the other half were not permitted to see their ongoing movement toward the target (proprioception-only [P] condition). Each acquisition trial was followed with knowledge of results (KR). After the last acquisition trial, all subjects were transferred to a common task in which only the target to be reached was visually available, with no KR. During acquisition, the younger subjects were found to be spatially more accurate than their older counterparts, and this was so regardless of the number of acquisition trials. Withdrawing KR during the transfer test did not modify the spatial accuracy of the subjects who had trained under the P condition. This indicates that the subjects had a reliable reference of the movement to be realized. Withdrawing vision of the moving hand and KR in the transfer test caused a significant increase in spatial error for both the older and the younger subjects. However, the increase in error was less pronounced for the older than for the younger subjects. In fact, the older subjects performed as well in the transfer test as the subjects who had trained in the P condition. This pattern of results suggests that in the transfer test, the older subjects could still guide their movements with the proprioceptive information that was available during both acquisition and transfer. However, such was not the case for the younger subjects. This suggests that, unlike the younger subjects, the older subjects could still rely on the proprioceptive cues available during acquisition in the PV condition. These results are taken to indicate that practicing with numerous sources of afferent information, as was the case in the PV condition, resulted in an integrated reference store for the younger subjects. In contrast, while practicing the task in the PV condition, the older subjects appeared to process independently from each other the different sources of sensory information available.