ABSTRACT
How can knowledge of the deterministic dynamics of a biological system help the medical practitioner? A very simple example of the application of a most basic level of knowledge is the analysis of electroencephalographic (EEG) signals. EEG recordings are made for a variety of clinical reasons, and by doing so, they provide a coarse measure of the mental state of the subject. The main weakness of EEG measurement, and particularly surface electrode EEG, is that such measurement can only provide a crude measurement of the actual activity of the brain. The measured surface EEG is a spatial average of the electrical activity of a vast number of individual neurons (which we will come to in a later chapter) diffused through cranial fluid, skull and skin. Nonetheless, such an imprecise measurement can still be useful. Since the
very earliest experiments with measuring EEG, it has been observed that the dominant frequency of electrical activity is associated with specific neural functions. The typical EEG signal is (rather arbitrarily) divided into five separate frequency bands δ (0.5-4 Hz), θ (4-8 Hz), α (8-13 Hz), β (13-22 Hz), γ (22-40 Hz). Of course, the actual division is not so clear-cut. Moreover, different sources will indicate slightly different frequency bands. Nonetheless, the definition of α to δ rhythm based on these frequencies is sufficient for us. Incidentally, the names α, β, γ and so on are assigned to different frequency bands based on the order in which they were first described. It does not, of course, coincide with the actual numerical bandwidths. Finally, we should not think that any activity outside these bands, below 0.5 Hz or above 40 Hz, can also be included in this definition. The numerical values here are just guidelines. While this scheme provides nothing more than an approximate classification
of activity of different frequencies, it is widely held that certain frequency bands are most commonly associated with certain types of activity. Broadly speaking, the five frequency bands are generally described in the following terms.
