Fourier analysis of broad spectral EEG from a fluctuation point of view

It is believed that the EEG is the most reliable method of evaluating brain function, but neither quantitative nor qualitative studies of the EEG have been carried out over the entire range of frequencies. Analysis of limited frequency bands of the EEG has not disclosed the whole of neuronal activity. The aim of this study is to clarify the upper limit of EEG frequency. Our EEG analytic system is composed of a high fiedlity preamplifier and signal processor with a frequency response within -3 dB below 20 kHz. Thirty adult cats were used for these experiments. The upper limit of the frequency varied in different structures: 6.9 +/- 0.8 kHz (+/- SEM) in motor cortex, 4.1 +/- 0.3 kHz in the hippocampus, 2.9 +/- 0.5 kHz in the amygdala, 9.3 +/- 0.6 kHz in the ventrolateral nucleus of the thalamus, and 9.9 +/- 0.5 kHz in the midbrain reticular formation. Three different types of amplitude spectra were characterized in bilogarithmic graphs. These types are named types f, f + L, and L corresponding to 1/f or Lorentzian fluctuation. In conclusion, the upper limit of frequency and the spectral types correspond to the neuronal specificity of different brain regions. Their physioanatomic significance is discussed.