On how the motor cortices resolve an inter-hemispheric response conflict: an event-related EEG potential-guided TMS study of the flankers task.

An important aspect of human motor control is the ability to resolve conflicting response tendencies. Here we used single-pulse transcranial magnetic stimulation (TMS) to track the time course of excitability changes in the primary motor hand areas (M1(HAND)) while the motor system resolved response conflicts. Healthy volunteers had to respond fast with their right and left index fingers to right- and left-pointing arrows. These central target stimuli were preceded by flanking arrows, inducing premature response tendencies which competed with correct response activation. The time point of maximum premature activation was individually measured as peak latency of the lateralized readiness potential (LRP) in the EEG. In the subsequent TMS experiment, single pulses were applied to left or right M1(HAND) during the same flanker task. The amplitude of the motor evoked potentials in the contralateral first dorsal interosseus muscle was taken as an index of corticospinal excitability. Guided by the previous LRP measurement, magnetic stimuli were applied 0-90 ms after the individual LRP peak, to cover the epoch of conflict resolution. When flankers were incompatible with the target, excitability of the prematurely activated M1(HAND) gradually decreased during this 90 ms period. This decrease was paralleled by a mirror-symmetrical increase in excitability in the other M1(HAND). These results show that the inappropriate response tendency is cancelled in one M1(HAND) simultaneously with activation of the correct response in the other. This integrated implementation of response activation and cancellation at the level of the M1(HAND) presumably represents a downstream effect orchestrated by premotor brain regions.