ON THE AVERAGE EEG RESPONSE ACTIVITIES IN RELATION TO THE INTENSITY OF FLICKER STIMULATION

Abstract

Autocorrelograms and their frequency spectra of normal adult EEGs during flicker light stimulation of about7.5-14cps frequency were studied by weak, intermediate and strong stimulations respectively. In addition, the average evoked responses to flashing light in relation to alpha dominant and low voltage fast EEG patterns were also observed to elucidate the physiological significance of peaks at the high harmonic frequency of the flicker stimulation in the spectra.A superimposition of a regular damped oscillation of approximately10 cps frequency and an aperiodic random phenomenon were suggested in the autocorrelogram of the control resting state before delivery of stimulation.Though there was considerable variety, the oscillation in the autocorrelogram during the flicker stimulation showed some decay at first in amplitude but thereafter there was no decay and the amplitude remained constant in response to the stimulation at such time, peaks occurred at the stimulating frequency and/or its high harmonic in the frequency spectra of the autocorrelograms.In a typical instance, the height of the peak at the stimulating frequency and its high harmonic increased proportionally with the logarithm of the intensity of the stimulation, whereas the peak that had been predominant at about10cps in the control state decayed proportionally suggesting alpha blocking phenomenon. Hereat a distinct low voltage fast pattern was observed in this EEG ink records.In some instances, the height of the peak in response to the stimulation was decreased by the strong stimulation, though it was increased with weak to intermediate stimulations. In other instances, however, the height of the peak was attenuated with weak to intermediate stimulations but increased by strong stimulation.An opposite change to alpha blocking phenomenon was also brought out in some instances by the stimulation with a different frequency from that of alpha wave in the control state.A hitherto established linear theory on EEG activity was developed further to aid the physiological interpretation of the above findings (see Appendix).The oscillation in the autocorrelogram during the stimulation was not always sinusoidal in its wave form, but was a saw teeth like wave or two or three swells of different forms were spaced in time in some instances suggesting the occurrence of a prominent peak at the high harmonic frequency. As two or three average responses were linked to the flashing light with different latencies and amplitudes, the above peak of the high harmonic wave did not seem to be the repetitive response of this frequency but appeared to be several responses of the stimulating frequency.Generally, the above average cerebral flash responses during low voltage EEG pattern with low alpha wave were lower than those during alpha dominant EEG, although an opposite evidence in a certain cerebral region was observed to suggest local difference in the cerebral activity.