Dual-Coil Transcranial Magnetic Stimulation Reveals Temporal Dynamics of Bilateral Corticomotor Excitability During Response Inhibition

A changing environment may suddenly require some parts of a multi-component response to be cancelled, while others continue. Such partial cancellation consistently produces a behavioural delay in the remaining component. This delay may reflect a three-step process of non-selective neural inhibition of all response components, functional uncoupling of components, and selective initiation of the remaining response. However, most neurophysiological evidence supporting this hypothesis has been recorded from muscles of a single hand, without direct comparison between response components. We aimed to simultaneously record - and therefore directly compare - corticomotor excitability (CME) in the cancelled and responding hands using a dual-coil technique not yet applied in this context. Human participants received transcranial magnetic stimulation to both primary motor cortices 1ms apart while performing a bimanual response inhibition task. Motor evoked potentials (MEPs) were recorded from both first dorsal interosseous (FDI) muscles as a measure of CME during partial cancellation. An equivalent reduction in CME was evident for both the responding and cancelled FDI muscles 175 ms after the stop cue during successful partial cancellation. The responding FDI subsequently exhibited an increase in CME above levels in the cancelled hand, leading to the unimanual response. This study reveals, for the first time, the temporal dynamics of CME for both response components simultaneously during a bimanual response inhibition task. Our results provide strong evidence that neural modulation during sudden partial cancellation can be viewed in light of the stop-change framework as a sequential non-selective stop, uncouple and switch, then selectively go process.