Interactions between P300 and passive probe responses differ in different visual cortical areas

The regulation of firing thresholds of cortical neurons was suggested as on e of the mechanisms underlying the generation of the P300 component in the human event-related potential. According to this hypothesis, the detection of an important stimulus produced the widespread inhibition of "irrelevant" networks, interrupting their ongoing activity and facilitating the analysi s of selected information. In the present experiment, the responsiveness of visual cortex was evaluated during the P300 potential by using additional, probing stimuli. Large separation of the cortical visual fields permitted separate analysis of the input and more advanced stages of processing. Resp onses were recorded from Fz, Cz, Pt and Oz scalp sites. P300 waves were evo ked by visual, mentally counted stimuli in a standard "odd-ball" procedure. Visual probes were delivered 200, 300, 400, 500, 700 and 1000 ms later. No responses to the probes were required. Significant suppression of response s to the probes delivered less than 400 ms after target stimuli was found i n Oz and Pt but not in Ct or Fz. The suppression was not proportional to th e voltage levels from which probe responses started. In Fz and Cz, latencie s of probe responses were elongated if probes were delivered less than 400 ms after target stimuli. The results suggest that probe responses suppresse d by the P300 potential in occipital and parietal cortex may be restored in frontal areas. In these areas the P300 potential could delay probe respons es instead of suppressing them.