Evoked potentials and rhythmic afterpotentials in relation to pre-stimulus EEG activity during quiet waking state.

The relationship between the frequency-characterized EEG activity preceding the stimulus presentation and the amplitude-characterized evoked potentials (EPs) and rhythmic afterreactions (RARs) was investigated. The experiments were performed on the somatosensory cortex of cats during quiet waking state. It was found that upon one and the same parameters of stimulation the amplitude of the EPs and the RARs depended significantly on the frequency of the spontaneous EEG activity recorded one second before the stimulus. A decrease of the frequency of the on-going EEG favoured the appearance of the high-amplitude early and late components of the evoked potentials and of the rhythmic afterpotentials. Conversely, the probability of appearance of low-amplitude EPs increased and that of RARs decreased when the frequency of the on-going EEG increased.