Control over response priming in visuomotor processing: a lateralized event-related potential study.

In a typical Simon task responses are faster when the task-irrelevant stimulus location corresponds to the response location than when it does not. In the case of noncorrespondence it is assumed that externally triggered and internally selected responses are in conflict. Crucially, such conflict appears to be subject to contextual modulations as induced by the immediately preceding event, i.e., the Simon effect was found to be absent when a conflict trial preceded the current event (Stürmer et al. 2002, JEP:HPP). Here, we examined two possible accounts of this context effect in terms of early suppression of externally triggered S-R coding at a premotoric level versus late suppression at a motoric level. Lateralized event-related brain potentials (L-ERPs) were recorded in a Simon task and analyzed as a function of the correspondence sequence. L-ERP activity started earliest over occipito-parietal brain areas and revealed location-based S-R priming irrespective of the prior correspondence context. By contrast, when a noncorresponding trial preceded, such location-based priming was absent in L-ERP activity over the motor cortex (MC). Thus, in support of the late suppression view L-ERPs suggest a clear dissociation in function between externally triggered visuomotor functions within the dorsal stream and the MC reflecting context-controlled response activation.