Visual modulation of proprioceptive reflexes during movement.

Previous research has demonstrated that feedback circuits such as reflexes can be tuned by setting their gains prior to movement onset during both posture and movement tasks. However, such a control strategy requires that perturbation contingencies be predicted during movement planning and that task goals remain fixed. Here we test the hypothesis that feedforward regulation of reflex circuits also occurs during the course of movement in response to changes in task goals. Participants reached to a visual target that was occasionally jumped on movement initiation, thus changing task goals. Reflex responses were elicited through a mechanical perturbation on the same trial, 100 ms after the target jump. Impedance to the perturbation was tuned to the direction of the preceding jump: reflex responses increased or decreased depending on whether the perturbation opposed or was consistent with the target jump. This modulation, although sensitive to the direction of the jump, was insensitive to jump amplitude, as tested in a follow-up experiment. Our findings thus suggest that modulation of reflex circuits occurs online, and is sensitive to changes in visual target information. In addition, our results suggest a two-level model for visuo-motor control that reflects hierarchical neural organization.