In the Flanker response congruency task, motor evoked potential amplitude increases in the chosen hand prior to response, but does not change in the unchosen hand

The coloured Flanker task examines preparatory motor processes, providing insights into stimulus-interference (distracting stimuli requiring the same response) and response-interference (distracting stimuli triggering competing responses). Continuous measurement of the responses – using dynamometers and electromyography (EMG) – provides information about ongoing motor processes in both hands simultaneously, from stimulus onset until response production. The present study investigated the representations of chosen and unchosen hand responses using dual simultaneous transcranial magnetic stimulation (TMS) over left and right primary motor cortices (M1), evoking EMG activity in first dorsal interosseous and thenar muscles. We hypothesised to find decreased activation of the unchosen hand (associated with the flankers) and increased activation of the chosen hand (associated with the target), starting about 150 ms before the manual reaction time. Fifteen adults performed the coloured Flanker task. We delivered, simultaneously, two single-pulses of TMS, one over left and one over right M1 within a time window from target presentation to the mean RT in the prior block. In the chosen hand, motor evoked potential (MEP) amplitudes started increasing 156 ms before the reaction time, with no differences across stimulus and response interference conditions. In the unchosen hand, MEP amplitudes did not differ significantly from baseline throughout response preparation. These findings suggest that preparatory motor processes of the chosen hand seem not to be influenced by stimulus or response interference. Double-coil stimulation, coupled with continuous bilateral measurement of motor responses using grip force, EMG, and MEPs, provides a valuable alternative to discrete measures of stimulus and response interference.