Dynamics of narrow-band EEG phase effects in the passive auditory oddball task

Evidence suggests that the component frequencies of the electroencephalogram (EEG) are dynamically adjusted to provide particular brain states at stimulus occurrence, and that these facilitate cortical processing of the stimulus. We examined relationships between stimulus intensity, the phase of narrow-band EEG activity at stimulus onset, and the resultant event-related potentials (ERPs) in a passive auditory oddball task, using a novel conceptualization of orthogonal phase effects (cortical negativity vs. positivity, negative driving vs. positive driving, waxing vs. waning). EEG responses to the standard stimuli (50 vs. 80 dB, varied between subjects) were analysed. Prestimulus narrow-band EEG activity (in 1-Hz bands from 1 to 13 Hz) at Cz was assessed for each trial by digital filtering. For each frequency, the cycle at stimulus onset was used to sort trials into four phases, for which ERPs were derived from both the filtered and unfiltered EEG activity at Fz, Cz and Pz. Preferred brain states at various frequencies were indicated by 16-34% differential occurrence within the orthogonal phase dimensions explored. The preferred states were associated with smaller N1, N2 and N3, larger P2 and P3, shorter N1, P2, N2 and P3 latencies, and some intensity effects. These effects reflected the operation of three separate phase-influenced mechanisms, involving anticipatory potentials and prestimulus/poststimulus amplitudes in various EEG frequencies. Results indicate that, even in paradigms with a slightly varying interstimulus interval, brain dynamics provide preferred brain states at the moment of stimulus presentation, which differentially affect the EEG correlates of stimulus processing.