Functional aspects of evoked alpha and theta responses in humans and cats. Occipital recordings in "cross modality" experiments.

A systems theoretical approach was used to compare possible functional roles of theta (4-7 Hz) and alpha (8-15 Hz) response components of brain evoked potentials. These response components were described earlier by Başar (1980). We recorded EEG and evoked potentials (EPs) from occipital scalp locations in 11 subjects. We used auditory and visual stimuli as inadequate and adequate stimuli, respectively ("cross-modality" measurements). The combined EEG-EP epochs were analysed in frequency domain with fast Fourier transform and adaptive digital filters. Alpha (8-15 Hz) response components turned out to be dependent on whether the stimulus was adequate or not (median amplitude with inadequate vs. adequate stimulation: 1.9 muV vs. 4.0 muV). Theta (4-7 Hz) response components were less dependent on stimulus modality (inadequate vs. adequate stimulation: 2.1 muV vs. 2.8 muV). In EP recordings the occipital alpha response almost disappeared in the first 250 ms following auditory stimulation. Comparable behaviour was observed in similar experiments with recordings from the cat visual cortex (area 17) and with occipital magnetoencephalographic recordings. Taking into account the above-mentioned previous reports on intracranial recordings in primary sensory areas of the cat brain and preliminary results of magnetoencephalographic measurements, we propose the following hypothesis: alpha responses in a time window of about 250 ms after stimulation might predominantly reflect primary sensory processing whereas the theta responses in the first 250 ms after stimulation might be more involved in supra-modality - or cross-modality - associative-cognitive processing.