Suppression of proprioceptive feedback control in movement sequences through intermediate targets

Simple movements can be seen as building blocks for complex action sequences, and neural control of an action sequence can be expected to preserve some control features of its constituent blocks. It was previously found that during single-joint elbow movements to a single target, the proprioceptive feedback control is initially suppressed, and we tested this feedback suppression in a two-segment sequence during which subjects momentarily slowed down at an intermediate target at a 30° distance (first segment) and then immediately moved another 30° to the final target (second segment). Either the first or second segment was unexpectedly perturbed; the latency of the earliest response to the perturbation in the muscle surface electromyogram was analyzed. The perturbations were delivered either at the onset of each segment or about 0.1 s later. We found that in both segments, the response latency to the late perturbation was shorter than the latency to the early perturbation, which suggests that the proprioceptive feedback control is suppressed in the beginning of each segment. Next, we determined the latency of the response to unexpected perturbations in 30° movements to a single target. We found that the response latency was not significantly different in the movement to a single target and in each segment in the sequence. This result suggests that the initial suppression of the proprioceptive feedback control in movements to single targets is preserved in movements through intermediate targets and supports the idea of modular organization of neural control of movement sequences.