Spectral and bispectral analysis of the EEG rhythms in basal conditions and during photic stimulation

The aim of the present study is the quantification of the relationships and the phase coupling among spectral peaks in the EEG signal at different sites of the scalp. 10 normal subjects underwent the study. The multi-channel EEG signal was recorded during basal conditions and during photic stimulation. The stimulation frequency (SF) has been chosen related to the single subject's spontaneous alpha rhythm (SF = alpha, SF = 2alpha, SF = alpha/2) and not related to the alpha rhythm (SF = 14 Hz). Spectral and bispectral analysis put into evidence that, in basal conditions, with closed eyes, the spontaneous alpha rhythm is generated by independent oscillators in the occipital and frontal regions. In addition the beta rhythm in the spectra seems an harmonic component linked to the former one. During photic stimulation the spontaneous alpha rhythm is drastically decreased, and the harmonics are lowered, while frontal and occipital responses seem to synchronize. In addition the frontal lobe seems able to generate sub-harmonics which could be related to the genesis of generalized seizures in predisposed subjects.