Abstract
A transient period of corticospinal suppression is reliably observed approximately 200 ms prior to movement initiation across diverse contexts of movement. However, the motor system may forego that suppression when preparation is under time constraints. Given this, the physiologic function of premovement suppression is unclear. Here, we investigated urgency effects on corticospinal suppression throughout the time course of an anticipatory timing task. Participants performed timing actions under high and low temporal constraints while single-pulse transcranial magnetic stimulation (TMS) was applied at different times throughout action preparation. Our results confirmed that corticospinal suppression is observed under low-urgency preparation but absent under high urgency scenarios. Critically, we found no evidence that preparatory suppression could be temporally shifted or compressed under high urgency, but rather, corticospinal excitability remained at baseline levels before engaging a period of excitation leading up to action. Moreover, responses prepared in the absence of premovement suppression were more likely to be disrupted by external perturbations. These results suggest that preparatory suppression might be a protective, but not obligatory mechanism employed by the central nervous system to shield motor actions from interference of external events when time allows. Together, these results suggest that when temporal constraints are high, the motor system can prioritise speed over stability, favouring excitatory drive at the expense of inhibitory gating. Given these data, we further consider processes of inhibition and excitation individually and propose a descriptive model which may account for the absence of preparatory suppression under time pressure to act.
