Response priming in the Simon paradigm - A transcranial magnetic stimulation study

The Simon effect refers to the finding of faster responses when stimulus an d response locations correspond than when they do not, although a nonspatia l stimulus feature is task-relevant. These performance differences are usua lly accounted for by response priming processes directly induced by the tas k-irrelevant stimulus location. The present study investigated neural mecha nisms of response priming in a Simon task at the level of the motor cortex with the help of transcranial magnetic stimulation (TMS) and motor evoked p otentials (MEPs) in both arms. A single TMS was applied contralateral or ip silateral to the requested response at the time point where response primin g was at a maximum. The MEP effects depended on the stimulated hemisphere. Over the left hemisphere, MEP areas were larger when TMS was applied over t he primed motor cortex. However, reduced MEPs for the nonprimed hemisphere fell short of significance. Over the right hemisphere, only a MEP reduction for nonprimed left-hand responses was present. Therefore, we conclude that mainly excitatory activation underlies response priming in a Simon task, w hereas the role of inhibitory processes is tentative.