Phase synchronization of the ongoing EEG and auditory EP generation

Objective: We investigated the role of phase synchronization of the spontaneous electroencephalogram (EEG) in auditory evoked potential (EP) generation in a sample of healthy individuals. Methods: Auditory responses were obtained from 20 healthy subjects following a double stimulus paradigm, using two identical tone bursts (S1 and S2) separated by 0.5 s. Single-trial auditory evoked potentials were decomposed into sinusoidal, exponentially decaying/increasing components using the piecewise Prony method (PPM). Pre- and post-stimulus phase histograms were compared to determine the degree of phase synchronization produced by auditory stimulation. Results: Analysis of single responses revealed that the S1 stimuli produced phase synchronization in the 2–8 Hz frequency range, with little or no concomitant amplitude increase. A significantly reduced phase effect was seen in response to S2 stimuli. Conclusions: Stimulus-induced phase synchronization of the ongoing EEG is a major mechanism for the generation of auditory EP components with a latency in the 50–250 ms range. Significance: The fact that the EP components accessed here are generated through phase synchronization implies that the ensemble-averaged EP will not resemble the single trial response, and it would certainly be misleading to consider the single trial response as an amplitude-scaled version of the ensemble average.