Preparatory inhibition: Impact of choice in reaction time tasks

By applying transcranial magnetic stimulation (TMS) over primary motor cortex (M1) to elicit motor-evoked potentials (MEPs) in muscles of the contralateral hand during reaction time (RT) tasks, many studies have reported a strong global suppression of motor excitability during action preparation, a phenomenon called preparatory inhibition. Several hypotheses have been put forward regarding the role of this broad suppression, with the predominant view that it reflects inhibitory processes assisting action selection. However, this assumption is still a matter of debate. Here, we aimed at directly addressing this idea by comparing MEPs in a task that required subjects to select a finger response within a set of predefined options (choice RT task: left or right index finger abduction) or when subjects simply had to provide the same finger response on every trial, in the absence of choice (simple RT task). Moreover, we minimized any effect that could be associated with other forms of inhibition. In both versions of the task, TMS was applied on both M1 (double-coil protocol) at several time points between the go signal and the left or right index finger response, eliciting MEPs bilaterally in the prime mover (index finger agonist) and in an irrelevant muscle (pinky agonist). Overall, MEP suppression was moderate in this study compared to past research; it was only found for the irrelevant muscle. As such, MEPs in the index agonist were facilitated when elicited in a responding hand (e.g. left MEPs preceding left responses) and remained mostly unchanged in a non-responding hand (e.g. left MEPs preceding right responses). In contrast, MEPs were almost always suppressed in the pinky muscle when elicited in the non-responding hand. This finding contrasts with previous studies where preparatory inhibition usually concerns both relevant and irrelevant muscles. Yet importantly, the suppression was more consistent in the choice than in the simple RT task, supporting the view that preparatory inhibition may assist action selection.