The Simon effect refers to the finding of faster responses in case stimulus and response positions correspond than when they do not. Dual route models propose that spatial stimulus features prime the corresponding response via a direct route, whereas response selection occurs via an indirect route. The Simon effects depends on the correspondence condition of the predecessor, that is, the Simon effect is absent after a noncorresponding predecessor, it only shows up after a corresponding predecessor. We account for this finding by executive control over direct route priming achieved by dorsolateral prefrontal cortex (DLPFC) functioning.
Recent studies showed an increase of spatial conflicts by Cannabis. For example, the execution of antisaccades and spatial working memory was hampered indicating the involvement of DLPFC. Therefore, we investigated whether Cannabis interferes with executive control over reponse priming. A double-blind study was run with healthy adults getting delta-9-THC, Cannabis or a placebo. Event-related brain potentials were recorded in the Simon task, and the lateralized readiness potential (LRP) was calculated as an indicator of specific hand activation. Usually, in noncorresponding conditions there is an early incorrect activation in the LRP replaced by correct response hand activation later on. This early LRP lateralization is seen to reflect response conflict and does not occur after a noncorresponding predecessor. In line with our assumption a Simon effect after a noncorresponding predecessor was present with medication affecting also the early LRP lateralization. Moreover, delta-9-THC and Cannabis enlarged early attention related ERP components but reduced the later P300.