Individual differences in intracortical inhibition during behavioural inhibition.

The time required to abort an initiated response can be measured as the Stop Signal Reaction Time (SSRT). We determined whether GABAergic activity in the primary motor cortex (M1), measured using paired-pulse Transcranial Magnetic Stimulation (TMS) was related to SSRT. GABAergic activity in M1 was assessed by measuring Short-Interval Intracortical Inhibition (SICI). In two experiments, participants (males and females) completed the Stop Signal Task while we measured SICI from the first dorsal interosseous muscle. In Experiment 1, SICI was measured at fixed time points after Stop signal onset on Stop trials (50ms, 100ms, 150ms, 200ms), and at corresponding time points for Go trials. In Experiment 2 SICI was measured at fixed time points before the end of the SSRT interval (125ms, 75ms, 25ms) on Stop trials, and at corresponding time points for Go trials. In each experiment, 30 participants were classified as fast stoppers or slow stoppers based on a median split of their SSRTs. Fast stoppers had more SICI than slow stoppers, both when executing a response (Go trials) and when inhibiting a response (Stop trials). Indeed, the correlation between mean SICI and SSRT on successful Stop trials was 0.81. Experiment 2 showed that for fast stoppers (relative to baseline) there was reduced SICI on Go trials and recovery of SICI on Stop trials. Slow stoppers however, showed reduced SICI on Stop and Go trials relative to baseline. Our results show that individuals who are faster at stopping not only show more GABAergic activity in M1, but can more effectively control M1 GABAergic activity to inhibit motor cortical excitability when stopping a response and disinhibit excitability when executing a response.