Learning adapted feedback responses to unpredictable force fields during reaching

Motor adaptation results from the acquisition of novel representations in the nervous system allowing improvements of performance across a broad range of tasks. To date, there has been much emphasis on the acquisition of such patterns for predictable disturbances, and previous work highlighted that both prior and feedback control mechanisms share knowledge about limb and environmental dynamics. Here we investigated whether healthy adults could learn to produce adapted feedback responses to unpredictable force field disturbances applied during reaching to address whether sensory feedback during movement could be used to adjust movement control. We instructed 18 healthy adults to perform reaching movements towards a visual target with a robotic handle (KINAMR, BKIN Technologies). On a random subset of trials (probability 1/3), we applied orthogonal (lateral force proportional to forward velocity), or curl field (proportional and orthogonal to hand speed) perturbations of clockwise and counter-clockwise directions randomly interspersed with unperturbed trials. We found a reduction in path length across force field trials for both perturbations and directions, indicative of better compensation for the applied disturbance. An analysis of average surface activity from the main muscles involved in lateral corrections indicated that there was no systematic increase in co-activation, and highlighted a down-regulation of perturbation-related responses which accounted for the improvement in online correction. Furthermore, the measured force at the handle became gradually better correlated with the commanded force, re-calculated offline based on the velocity profile of each trial. This increase in correlation was also observed in a standard adaptation paradigm performed as a control experiment (n = 8 participants), which confirms that it is a metric sensitive to adaptation. Together these observations indicate that feedback responses to unpredictable disturbances were tuned to the specific force profile experienced within each perturbation trials, and displayed similar improvements as in standard adaptation to predictable disturbances. These observations highlight that the nervous system adapts feedback control strategies within a trial to the experienced disturbances, and are consistent with the hypothesis that the brain idea that the nervous system performs very fast, possibly online, adaptation to environmental dynamics