RESPONSES TO IRRELEVANT PROBES DURING TASK-INDUCED NEGATIVE AND POSITIVE SHIFTS

The functional significance of task-induced negative and positive cort ical shifts were tested with the probe-stimulus method. Both shifts we re induced within the same experimental situation in three variants of a CNV paradigm, where a slow positive wave (a variant of P300) appear ed following S2. In Experiment I and II, S2 called for making or withh olding a motor response (go/no-go); in Experiment III, S2 informed the subject about the correctness of a previous guess. Irrelevant probe-s timuli were applied in conjunction with the task during the CNV, the p ost-S2 positivity and the intertrial interval (ITI). The probe-evoked vertex EPs were smaller during the post-S2 positivity as compared to t he CNV and the ITI. This was true not only for the motor task but also for the guessing task, where the effect is unlikely to have been cont aminated by motor potentials. This indicates that positive shifts have an inhibitory effect on the processing of irrelevant probe-stimuli an d possibly on information-processing in general. (C) 1998 Elsevier Sci ence B.V.