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Abstract
The family of late positive waves which occur at latencies of 250 to 500 msec (P300) has been the most widely studied endogenous event related potential of the brain. We performed experiments with human subjects by applying repetitive acoustical tone bursts; every third or fourth signal was omitted. The endogenous potential related to the omitted stimulation was first averaged. The averaged record was then transformed to frequency domain with Fourier analysis in order to obtain the frequency characteristics. We observed frequency selectivities between 1–2.5 Hz, 3–8 Hz and between 8–13 Hz. After this procedure, single epochs of prestimulus EEG and endogenous potentials were pass-band filtered in the above frequency ranges. The comparison of filtered prestimulus EEG and of single endogenous responses gave the following results:
(1) In delta and theta frequency ranges the EEG prior to omitted stimulation showed in most of the trials a phase-reordering; the P300-wave seems to be a continuation of the ordered and stationary pre-EEG.
(2) In some cases the P300-wave depicts an amplitude-enhancement against the delta and theta components of the pre-EEG.
(3) Alpha-enhancement and alpha-blocking also make important contributions to the organization of the endogenous event-related potentials.