The regulation of firing thresholds of cortical pyramidal cells has been suggested as one of the mechanisms underlying the generation of the P300 component of the human event-related potential. According to this hypothesis, the detection of an important stimulus produces a widespread inhibition of 'irrelevant' networks, interrupting the ongoing cortical activity and facilitating the analysis of the important information. In the present experiment, target stimuli in a standard 'odd-ball' paradigm were used as important events. The cortical responsiveness was measured using the responses to additional probing stimuli delivered 400 ms and 1,000 ms after target and non-target stimuli. The subjects were asked to count mentally the target stimuli and ignore the non-targets and the probes. The level of 'irrelevant' cortical activity was manipulated using additional visual noise stimulation. Event-related potentials were recorded at Fz, Cz, Pz and Oz scalp sites. Our results showed that the noise reduced the initial responses to target and non-target stimuli in Oz, Pz and Cz but not in Fz recordings. The noise reduced the probe responses in Oz and Pz but not in Cz and Fz recordings. The amplitudes of P300 components were not affected by the noise. The target stimuli reduced the subsequent probe responses in Pz and Cz but not in Oz and Fz recordings. Thus, the effects of noise and target detection were not identical in the different regions of cortex. The other important outcome of our study was that the target stimuli suppressed the effects of noise. The effect of noise on probe responses was significant in the non-target but not in the target trials. The effect of noise was significant if the probes were delivered 1,000 ms after 'odd-ball' stimuli, but it was insignificant when the delay was only 400 ms. Such results support the hypothesis that important information reduces cortical responses to other, irrelevant stimuli.