Limited contribution of primary motor cortex in eye-hand coordination: A TMS study

Abstract

The ability to track a moving target with the eye is substantially improved when the target is self-moved compared with when it is moved by an external agent. To account for this observation, it has been postulated that the oculomotor system has access to hand efference copy, thereby allowing to predict the motion of the visual target. Along this scheme, we tested the effect of transcranial magnetic stimulation (TMS) over the hand area of the primary motor cortex (M1) when human participants (50% females) are asked to track with their eyes a visual target whose horizontal motion is driven by their grip force. Wereasoned that, if the output ofM1is used by the oculomotor system to keep track of the target, on top of inducing short latency disturbance of grip force, single-pulse TMS should also quickly disrupt ongoing eye motion. For comparison purposes, the effect of TMS overM1was monitored when subjects tracked an externally moved target (while keeping their hand at rest or not). Inbothcases, resultsshowednoalterations insmoothpursuit, meaningthat its velocitywasunaffected within the25–125msepochthat followed TMS. Overall, our results imply that the output ofM1has limited contribution in driving the eye motion during our eye-hand coordination task. This study suggests that, if hand motor signals are accessed by the oculomotor system, this is upstream of M1.