Time course of human 40 Hz EEG activity accompanying P3 responses in an auditory oddball paradigm.

In order to quantify the time course of auditory P3-related gamma activity, root mean square (RMS) values were calculated from band-filtered (30-45 Hz) target and non-target responses in an auditory oddball experiment. Evoked (phase locked) gamma activity was evaluated from the time domain averages, whereas induced (not necessarily phase locked) activity was analyzed on the basis of single trials. Gamma RMS values were integrated across different time windows, namely the prestimulus, N50/P50, N100, pre P3, P3 and post P3 window. The single trial P3 window hereby was defined by a maximum amplitude criterion. In accordance with other studies, we found a pronounced increase of evoked gamma activity in the time window up to 80 ms after stimulus onset. In contrast, induced gamma activity as revealed by single trial analysis decreased markedly after stimulus presentation. Starting with the P3 window, induced activity recovered to baseline level for the non-target trials, whereas it remained significantly suppressed for the target responses.