Specificity of practice results from differences in movement planning strategies.

Withdrawing visual feedback after practice of a manual aiming task results in a severe decrease in aiming accuracy. This decrease in accuracy is such that participants are often less accurate than controls who are beginning practice of the task without visual feedback. These results have been interpreted as evidence that motor learning is specific to the sources of afferent information optimizing performance, because it could be processed at the exclusion of other sources of afferent information. The goal of the present study was to test this hypothesis. To reach our goal we evaluated whether online visual feedback prevented kinesthetic information to be used for: (1) eliminating movement anisotropy resulting from difference in limb inertia when aiming in different directions and (2) creating an internal model of limb mechanics. Participants practiced a manual aiming task with or without visual feedback and with knowledge of results. After this acquisition phase, participants performed two transfer tests. The first transfer test was performed without visual feedback and/or knowledge of results. The second transfer test was similar to the first one but participants initiated their movements from a different starting base. The results showed strong specificity effects in that withdrawing visual feedback resulted in large pointing bias and variability. However, the results of the two transfer tests showed that the processing of visual feedback did not prevent the processing of kinesthetic information used to eliminate movement anisotropy or to create an internal model of limb mechanics. Rather, specificity of practice effects resulted from participants using the same motor plan in transfer as they did in acquisition even though they had no longer access to visual feedback to modulate their movement online. These results indicate that during acquisition participants adopted different movement planning strategies depending on the source of afferent information available.