Task instructions and the need for feedback correction influence the contribution of visual errors to reach adaptation.

Previous research has questioned whether motor adaptation is shaped by an optimal combination of multisensory error signals. Here, we expanded on this work by investigating how the use of visual and somatosensory error signals during online correction influences single-trial adaptation. To this end, we exposed participants to a random sequence of force-field perturbations and recorded their corrective responses as well as the after-effects exhibited during the subsequent unperturbed movement. In addition to the force perturbation we artificially decreased or increased visual errors by multiplying hand deviations by a gain smaller or larger than one. Corrective responses to the force perturbation clearly scaled with the size of the visual error, but this scaling did not transfer one-to-one to motor adaptation and we observed no consistent interaction between limb and visual errors on adaptation. However, reducing visual errors during perturbation led to a small reduction of after-effects and this residual influence of visual feedback was eliminated when we instructed participants to control their hidden hand instead of the visual hand cursor. Taken together, our results demonstrate that task instructions and the need to correct for errors during perturbation are important factors to consider if we want to understand how the sensorimotor system uses and combines multimodal error signals to adapt movements. Significance Statement We investigated the factors influencing visual and proprioceptive feedback contributions to movement control and adaptation. While online corrections increased with the size of visual errors, this scaling did not transfer one-to-one to adaptation. Instead, we observed a consistent relationship between limb displacement during perturbation and subsequent after-effects which was independent of the visual error. However, adaptation was slightly reduced when visual errors were artificially decreased and this influence of vision was modulated by the task-instruction given to participants. Our results demonstrate that task-related factors, such as the need to correct movement errors and the instructions, have to be considered to advance our understanding of how the sensorimotor system uses multisensory feedback to adapt motor commands.