Motor-cortex excitabilities change mirror-symmetrically when response tendencies induced by flanking stimuli are cancelled. An ERP-guided TMS study

An important aspect of response selection is the ability to suppress inappropriate response tendencies. Is the inappropriate response tendency cancelled before the activation of the appropriate response, or do these two processes go in parallel? To address this question, we tracked the time courses of excitability of the primary motor hand areas (M1-HAND), by applying transcranial magnetic stimulation as a probe in each trial while healthy volunteers performed a two-choice reaction time task to right- and left-pointing arrows with their right and left index fingers. The central target stimulus was preceded by flanking stimuli that were neutral or were arrows, compatible or incompatible to the target. These flankers induce premature tendencies to respond which compete with correct response activation. The time point of maximum flanker-induced activation was determined in the individual Lateralized Readiness Potential (LRP). As displayed schematically in the left-sided figure, single suprathreshold transcranial stimuli were applied 0, 30, 60, or 90ms after this time-point, inducing motor evoked potentials in the contralateral first dorsal interosseus muscle and thereby probing corticospinal excitability.

The major result is depicted in the second figure. The M1-HAND activated by incompatible flankers showed a linear decrease in excitability during the 90ms period following maximal flanker-induced response activation. This decrease was paralleled by a mirror-symmetrical increase in excitability in the M1-HAND that was contralateral to the hand required for responding to the target.

These results show that cancellation of the inappropriate response tendency and activation of the correct response occur simultaneously, like the two scales of a balance.