Premovement inhibition can protect motor actions from interference by response-irrelevant sensory stimulation

Suppression of corticospinal excitability is reliably observed during preparation for a range of motor actions, leading to the belief that this preparatory inhibition is a physiologically obligatory component of motor preparation. The neurophysiological function of this suppression is uncertain. We restricted the time available for participants to engage in preparation and found no evidence for preparatory inhibition. The function of preparatory inhibition can be inferred from our findings that sensory stimulation can disrupt motor output in the absence of preparatory inhibition, but enhance motor output when inhibition is present. These findings suggest preparatory inhibition may be a strategic process which acts to protect prepared actions from external interference. Our findings have significant theoretical implications for preparatory processes. Findings may also have a pragmatic benefit in that acoustic stimulation could be used therapeutically to facilitate movement, but only if the action can be prepared well in advance.

Shortly before movement initiation, the corticospinal system undergoes a transient suppression. This phenomenon has been observed across a range of motor tasks, suggesting that it may be an obligatory component of movement preparation. We probed whether this was also the case when the urgency to perform a motor action was high, in a situation where little time was available to engage in preparatory processes. We controlled the urgency of an impending motor action by increasing or decreasing the foreperiod duration in an anticipatory timing task. Transcranial magnetic stimulation (TMS; experiment 1) or a loud acoustic stimulus (LAS; experiment 2) were used to examine how corticospinal and subcortical excitability were modulated during motor preparation. Preparatory inhibition of the corticospinal tract was absent when movement urgency was high, though motor actions were initiated on time. In contrast, subcortical circuits were progressively inhibited as the time to prepare increased. Interestingly, movement force and vigour were reduced by both TMS and the LAS when movement urgency was high, and enhanced when movement urgency was low. These findings indicate that preparatory inhibition may not be an obligatory component of motor preparation. The behavioural effects we observed in the absence of preparatory inhibition were induced by both TMS and the LAS, suggesting that accessory sensory stimulation may disrupt motor output when such stimulation is presented in the absence of preparatory inhibition. We conclude that preparatory inhibition may be an adaptive strategy which can serve to protect the prepared motor action from external interference.