Conflicting visual and proprioceptive reflex responses during reaching movements

The visually and mechanically induced corrective motor responses of hand position during reaching movements were investigated. Subjects performed reaching movements on a robotic manipulandum where the hand position was presented to the subjects by means of a projected display. On random reaching trials the subject's hand position was mechanically perturbed relative to the predicted hand trajectory. The visual representation of the hand position was either perturbed in the same direction as the hand, mirrored relative to the hand, or not perturbed at all. The visually induced reflexive responses were still elicited after a mechanical perturbation, whether or not the information agreed with the mechanical perturbation. The visually induced reflexes contributed to limb stiffness after 200 ms from the onset. If the visual and mechanical errors were consistent, the restoring force to a perturbation (or the effective stiffness) was increased at long latencies. The results suggest that on short time scales, error signals from different sensory modalities are processed separately, combined only at the output stage.