Nonspecific Inhibition of the Motor System during Response Preparation

Motor system excitability is transiently inhibited during the preparation of responses. Previous studies have attributed this inhibition to the operation of two mechanisms, one hypothesized to help resolve competition between alternative response options, and the other to prevent premature response initiation. By this view, inhibition should be restricted to task-relevant muscles. Although this prediction is supported in one previous study (Duque et al., 2010), studies of stopping ongoing actions suggest that some forms of motor inhibition may be widespread (Badry et al., 2009). This motivated us to conduct a series of transcranial magnetic stimulation (TMS) experiments to examine in detail the specificity of preparatory inhibition in humans. Motor-evoked potentials were inhibited in task-irrelevant muscles during response preparation, even when the muscles were contralateral and not homologous to the responding effector. Inhibition was also observed in both choice and simple response task conditions, with and without a preparatory interval. Control experiments ruled out that this inhibition is due to expectancy of TMS or a possible need to cancel the prepared response. These findings suggest that motor inhibition during response preparation broadly influences the motor system and likely reflects a process that occurs whenever a response is selected. We propose a reinterpretation of the functional significance of preparatory inhibition, one by which inhibition reduces noise to enhance signal processing and modulates the gain of a selected response.