Abstract
Nonlinear characteristics of the oscillators generating alpha rhythm, which are called α-oscillators in this paper, have been studied by presenting photic stimuli of various frequencies to subjects. Monopolar recording of the EEG was taken from Oz based on 10-20 system. From power spectra of the EEG, the presence of an entrainment region of alpha rhythm has been confirmed by photic stimuli around the intrinsic alpha frequency. The α-oscillators have the following nonlinear characteristics: (1) The entrainment region is bigger for stronger photic stimuli. (2) This entrainment of alpha rhythm has hysteresis. (3) The hysteresis region below the intrinsic alpha frequency is broader than the region above it. By calculating cross spectra between the EEG and the photic input, the phase delay (phase difference) of the evoked potentials in relation to the photic stimuli has been obtained. Patterns of phase change of the evoked potentials around the intrinsic alpha frequency can be classified into the following three types. (1) Gradually changing type. (2) N-shaped type with reversal of phase. (3) Steeply changing type. As the intensity of photic stimulus declines, type of phase change shifts from type (1) to type (3) via type (2).
