Magnetic stimulation of the dorsal premotor cortex modulates the Simon effect

In choice reaction tasks, subjects typically respond faster when the relative spatial positions of stimulus and response match than when they do not match. A prominent explanation attributes this 'Simon effect' to automatic response activation elicited by spatial correspondence, which facilitates or competes with the controlled selection of the response demanded by the stimulus. To test this account, we applied repetitive transcranial magnetic stimulation (rTMS) on the dorsal premotor cortex (PMd), as this area may subserve the inhibitory control of automatic response activation. Temporary interference with PMd was predicted to release the automatic activation from inhibition and thereby enhance the Simon effect. The results confirmed a modulation for trials following an incompatible trial, providing new evidence for competition between automatic and controlled response activation as a mechanism underlying the Simon effect.