Late integration of vision and proprioception during perturbed reaches

Abstract
The motor system corrects rapidly, but selectively, for perturbations to ongoing reaching movements, depending on the constraints of the task. To account for such sophistication, it has been postulated that corrections are based on an estimated limb state that integrates all sensory changes caused by the perturbation, taking into account their processing delays. Here we asked if information from different sensory modalities is integrated immediately, or processed separately in the early phase of a response. We perturbed the estimated state of the limb with both uni-modal and bi-modal visual and proprioceptive perturbations without changing the actual limb state. For visual perturbations, a cursor representing the hand was shifted to the left or the right relative to the true hand location. For proprioceptive perturbations, the biceps or triceps muscles were vibrated, which induced illusory limb state changes to the right or the left. In the bimodal condition the perturbations to vision and proprioception were either congruent or incongruent in their directions. Response latencies show that it takes ~100 ms longer to respond to unimodal visual perturbations than to unimodal proprioceptive perturbations. Responses to bimodal perturbations show that it takes an additional ~100 ms beyond the response to unimodal visual perturbations for inter-modal consistency to impact the response. These results suggest that visual and proprioceptive signals are initially processed separately for state estimation and only combined at the level of the limb's motor output, instead of being immediately integrated into a single state estimate of the limb.