Ariff G, Donchin O, Nanayakkar T, Shadmehr R (2002) A real-time
state predictor in motor control: study of saccadic eye movements during unseen
reaching movements. Journal of Neuroscience 22:7721-7729.
Abstract Theoretical motor control predicts that because of delays in sensorimotor pathways, there should exist a neural system in the brain that uses efferent copy of commands to the arm, sensory feedback, and an internal model of the arm's dynamics to predict future state of the hand, i.e., a forward model. We tested this theory under the hypothesis that saccadic eye movements, tracking an unseen reaching movement, would reflect the output of this state predictor. We found that in unperturbed reaching movements, saccade occurrence at any time t consistently provided an unbiased estimate of hand position at t+196ms. To investigate the behavior of this predictor during feedback error control, we applied 50ms random force perturbations to the moving hand. Saccades showed a sharp inhibition at 100ms post-perturbation. At ~170ms, there was a sharp increase in saccade probabilities. These post-perturbation saccades were an unbiased estimator of hand position at saccade time t+150ms. The ability of the brain to guide saccades to future position of the hand failed when a force field unexpectedly changed the dynamics of the hand immediately post perturbation. Behavior of the eyes suggest that during reaching movements, the brain computes an estimate of future hand position based on an internal model that relies on real-time proprioceptive feedback. When an error occurs in reaching movements, the estimate of future hand position is recomputed. The saccade inhibition period that follows the hand perturbation may indicate the length of time it takes for this computation to take place.