THE INTERACTION OF STIMULUS-RELATED AND RESPONSE-RELATED PROCESSES MEASURED BY EVENT-RELATED LATERALIZATIONS OF THE EEG

The present study focused on the relationship between movement- and st imulus-related asymmetries of the electroencephalogram (EEG). In seven tasks the same bilateral stimuli containing asymmetric information we re presented but response requirements differed. Three functionally di stinct asymmetries were found: (1) an asymmetry over the motor cortex prior to unimanual movements, (2) an asymmetry over the posterior cort ex beginning about 20 ms after the start of the movement, and (3) an e arly increase of negativity contralateral to a relevant stimulus (200- 300 ms after stimulus onset) that was maximal at temporo-parietal site s but was also visible at central sites. Although related to stimulus side, this asymmetry was modulated by response requirements: it was la rgely abolished with simple responses, smaller with nogo than with Go stimuli and occurred twice when a sequence of simple and choice respon ses was required. Therefore, the early temporo-parietal asymmetry most probably reflects an interface between sensory and movement-related p rocesses.