Corticospinal control of wrist muscles during expectation of a motor perturbation: a transcranial magnetic stimulation study.

Voluntary movement is often perturbed by the external forces in the environment. Because corticospinal (CS) control of wrist muscles during preparation of voluntary movement has been extensively studied without variation in the external forces, very little is known about the way CS control adapts when subjects expect motor perturbations. Here, we studied the CS control of wrist muscles during expectation of an imposed wrist extension. Subjects were instructed either to compensate (COMP) the perturbation (applied at variable delays) or not to intervene (NINT). In a quarter of all trials at random, in the time window when perturbation might occur, TMS was applied over contralateral M1. Motor evoked potentials (MEPs) were measured in the FCR (flexor carpi radialis) and ECR (extensor carpi radialis) muscles, as well as the silent period (SP) in the FCR. Following the perturbation, we found a larger long-latency stretch reflex in COMP than in NINT. During the expectation of the perturbation, MEP amplitudes did not differ across conditions in FCR. However, those evoked in ECR were greater in COMP than in NINT condition. Moreover in the FCR, the silent period lasted longer in NINT. Thus, we showed a selective effect of the prepared reaction on the anticipatory tuning of CS excitability and cortical inhibition in the agonist/antagonist muscles. This tuning clearly differed from the tuning during voluntary movement preparation without variation in the external forces. This shows that the tuning of the CS system during motor preparation depends on the dynamical context of movement production.