TMS reveals distinct patterns of proactive and reactive inhibition in motor system activity

Response inhibition is our ability to suppress or cancel actions when required. Deficits in response inhibition are linked with a range of psychopathological disorders including addiction and OCD. Studies on response inhibition have largely focused on reactive inhibition-stopping an action when explicitly cued. Less work has examined proactive inhibition-preparation to stop ahead of time. In the current experiment, we studied both reactive and proactive inhibition by adopting a two-step continuous performance task (e.g., "AX"-CPT) often used to study cognitive control. By combining a dot pattern expectancy (DPX) version of this task with transcranial magnetic stimulation (TMS), we mapped changes in reactive and proactive inhibition within the motor system. Measured using motor-evoked potentials, we found modulation of corticospinal excitability at critical timepoints during the DPX when participants were preparing in advance to inhibit a response (at step 1: during the cue) and while inhibiting a response (at step 2: during the probe). Notably, motor system activity during early timepoints was predicted by a behavioural index of proactive capacity and could predict whether participants would later successfully inhibit their response. Our findings demonstrate that combining TMS with a two-step CPT such as the DPX can be useful for studying reactive and proactive inhibition, and reveal that successful inhibition is determined earlier than previously thought.