Brain Response Susceptibility

Abstract

The expression “excitable physiological system” was introduced by Sato and his coworkers (Sato 1963; Sato et al. 1971; Sato et al. 1977). These authors studied the relation of visual evoked potentials to EEG by comparing the power spectra of the spontaneous activity with the driven rhythmic activity of the brain. They have found that the occipital recording of a relaxed human subject displays a rhythmic spontaneous activity prior to photic flicker stimulation. However, the shape and frequency positions of the power maxima in the power spectrum of this activity are similar to the frequency characteristics obtained by application of visual stimuli, thus indicating the system’s selective excitability. Accordingly, the excitability of a physiological system may be considered as one of the most important basic transfer functions of the system.

Various brain structures depict spontaneous rhythmic activity in a wide frequency range between 1 Hz and 1000 Hz. Without application of external sensory stimulation, the spontaneous activity of a given brain structure can often show a frequency-stable and high-magnitude electrical activity. If such regular spontaneous waves can be detected in the electrical activity of a defined brain structure during a determined stage, it is expected that upon external sensory stimulation, this structure has a response susceptibility in the same frequency channel (for example 40 Hz activity of the hippocampus and cortex, 70 Hz activity of the inferior colliculus, 250–300 Hz activity of the cerebellar cortex; 1–3 Hz activity of all the brain structures during SWS; 50–60 Hz activity of the reticular formation, etc.). Başar (1980)