The time-course of action control: measuring conditioned action tendencies and action suppression using transcranial magnetic stimulation.

Stimuli associated with a response can prime the motor system for action. While these action tendencies can be advantageous, reducing the demands of routine decisions and actions, they can be counter-productive when goals change. It is therefore important to understand how automatic motor preparation is brought under control. We used transcranial magnetic stimulation (TMS) to investigate the neurophysiological signatures of conditioned action tendencies in the motor system. Participants were trained to respond to target images appearing in a stream of other images. We then delivered TMS to the primary motor cortex to measure motor-evoked potentials (MEPs) as an index of cortico spinal excitability (CSE). Critically, participants were instructed to withhold the previously trained response to target cues. Despite this, the target cues increased CSE shortly before the timepoint at which a response would have typically been made (median RT). This was followed by distinct CSE suppression at later timepoints. TMS also triggered a motor response that may have otherwise been withheld in a time-sensitive manner. These results provide new evidence about the time-course of action tendencies triggered by conditioned cues and suggest that cue-elicited elevation of CSE is reined in when task goals change.