Visual and auditory evoked phase resetting of the alpha EEG

An increasing number of studies over the last decade or so have suggested that evoked potential (EP) morphology is partially due to a reorganization of the phase of the ongoing EEG. Phase resetting is common to non-linear oscillatory systems in response to a perturbation and has been observed in a number of biological systems such as circadian rhythms and the ECG. However, it has not been studied adequately in the context of EP research. Five clinically normal male volunteers (age 25-32) were subjected to randomly occurring light flashes for approximately 1 h. EEG recordings were obtained from a midline parieto-occipital site (POz) referenced to linked ears. An additional five male subjects (age 26-36) participated in an auditory P3 study of the effects of nimodipine, a calcium channel blocker, on brain electrical activity. Four of the latter subjects were diagnosed as crack cocaine abusers and one was negative for crack use. The single trials were bandpass filtered in the 8-13 Hz band and the phase angle at the moment of stimulation was computed. We examined the relationship between initial phase angle and both latency and amplitude of the first two post-stimulus negative peaks. The results demonstrate that these peaks undergo phase and (pre-stimulus) amplitude sensitive latency reorganization during presentation of both visual flash stimuli and auditory non-target oddball stimuli in a P3 experiment.