Transcranial magnetic stimulation of the primary motor cortex modulates response interference in a flanker task

In a typical flanker task, responses to a central target (“S” or “N”) are modulated by whether the flankers are compatible (“SSSSS”) or incompatible (“NNSNN”), with increased reaction times and decreased accuracy on incompatible trials. The role of the motor system in response interference under these conditions remains unclear, however. Here we show that transcranial magnetic stimulation (TMS) of the left primary motor cortex modulates the amount of flanker interference depending on the hand used for the response. Left motor TMS delivered at 200 ms after the onset of the array increased interference from incompatible flankers (“SSNSS”) when the target response was associated with the contralateral motor response (i.e. for “N” responses with the right hand), relative to when responses were to targets using the (left) hand ipsilateral to the site of TMS. Interestingly, under identical conditions, the degree of flanker interference was reduced when the TMS pulse was applied later in time. The analyses of the TMS-induced motor evoked potentials pointed to motor activity varying in the same conditions. We discuss the implications for understanding response interference and the role of the primary motor cortex in response selection.