Human auditory event-related processes in the time-frequency plane

Sensory stimuli produce phase-locked and non-phase-locked changes in brain activity as indexed by EEG and MEG. Time-frequency methods such as wavelets, when carefully applied, allow simultaneous analysis of both types of activity. Here we used wavelets of different time and frequency resolutions in combination with spatial mapping to identify these processes. We found that auditory stimulation leads to a pattern of large-magnitude power increases and small-magnitude power decreases. The power increases, ranging from the theta to the beta frequency band, were accounted for by the transient auditory responses P50m, N100m and P200m. Following these responses, we observed a power reduction of non-phase-locked activity which occurred 250-500 ms after stimulus onset in the 14-24 Hz frequency range and could be localized to the temporal and parietal brain areas. These results indicate that auditory event-related processes consist not only of the well-established transient responses but also of power reductions of ongoing, non-phase-locked brain processes.