The hippocampal EEG-activity and a systems analytical interpretation of averaged evoked potentials of the brain

Evoked Potentials in the hippocampus dorsalis are measured using chronically implanted and freely moving cats in applying auditory stimulations in the form of tone bursts of 3000 Hz. The hippocampal evoked potentials are characterized by 3 positive (I, III, V) and 3 negative peaks (II, IV, VI). Peaks I to VI have latencies of about 10, 28, 50, 75, 95 and 125 msec. These responses are then analyzed with a Laplace transform in order to obtain the hippocampal frequency characteristics. The amplitude frequency characteristic depicts resonance maxima of EEG-amplitude in theta (3–8 Hz) and beta (18–32 Hz) frequency ranges. A resonance in the frequencies of 3–8 Hz was expected because of the spontaneous hippocampal theta activity. Therefore this finding emphasize the reliability of the mathematical method used. On the other hand the existence of a hippocampal beta selectivity is highly remarkable and apparently the hippocampal activity is regulated at least by two different systems. The use of the mathematical method (Laplace transform) indicates that the simple knowledge of the latencies and the number of potential waves(usuallydenoted as P1, P2, ..., N1, N2 ...) cannot allow exact statements on mechanisms causing the formation of these peaks. Rather the slope and slope changes of the waves are determining. Different waves in the transient evoked response can be generated from a mechanism having only one resonant maximum in the frequency domain.

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