Context Versus Aiming Under Uncertainty When Both Feedforward and Feedback Control Are Engaged.

Theories of human motor learning commonly assume that movement plans are adjusted in response to the sensory feedback received about their success or failure. The degree to which movement errors drive changes in feedforward motor plans is further assumed to scale inversely with sensory uncertainty. However, support for these assumptions comes primarily from experiments that limit feedback corrections during an ongoing movement. In contrast, we have recently shown that when this restriction is relaxed, a different pattern of behavior emerges. Participants gradually adjust their reaching movements in response to a perturbation from trial-to-trial, following a consistent and incremental envelope of error reduction. Riding on top of this gradual learning envelope, participants also exhibit large and abrupt changes in their initial reach direction that are strongly correlated with the uncertainty level of the sensory feedback experienced on the previous trial, but are insensitive to the size and direction of the movement error made on that trial. A class of models in which sensory uncertainty influences an aiming process best accounted for this pattern. Here, we examine the possibility that uncertainty acts as a contextual cue to shunt motor processes to one of many context-specific internal models.