Topography

A. Dynamic Measures that Depend on Spatial Location

EEG recorded from a single electrode pair is fully characterized by its time dependence, as in Eq. (1). In spatially extended systems, however, dynamic behavior generally depends on both time and spatial location, the usual independent variables of dynamical systems. Thus, multichannel recordings potentially introduce many new measures of brain dynamic behavior. Amplitude, phase, and frequency may vary with scalp or cortical spatial location, for example.

B. Spatial Distribution of Alpha Rhythms

Alpha rhythms have been recorded from nearly the entire upper cortical surface (ECoG), including frontal and prefrontal areas. High-resolution EEG scalp recordings also show widespread distribution of alpha rhythms over nearly the entire scalp in healthy, relaxed subjects. EEG clinical populations differ, typically involving patients who are older, have neurological problems, and may be anxious during recording. These factors all tend to work against production of robust, widespread alpha rhythms. Second, the clinical definition of alpha is based on raw waveforms rather than spectra. Often, alpha is identified simply by counting the number of zero crossings of recorded waveforms. This can sometimes provide a misleading picture because raw EEG composed of broad frequency bands can appear very "non-alpha" to visual inspection, even though its amplitude spectrum shows substantial contribution from the alpha band. Such alpha rhythms may consist of mixtures of both localized and widely distributed activity.

Larger amplitude frontal alpha often occurs as subjects become more relaxed, for example, by employing relaxation or meditation techniques. Alpha rhythms of unusually large amplitude or exhibiting frontal dominance may be associated with mental retardation and some types of epilepsy. Large amplitude and dominant frontal alpha rhythm may also be recorded in some coma and anesthesia states. In summary, frontal alpha rhythms of moderate amplitude are common in healthy relaxed subjects with closed eyes, but very large frontal alpha is associated with disease or anesthesia. The physiological relationships between these disparate alpha phenomena are unknown, but they appear to share some underlying physiological mechanisms since their frequencies and widespread distributions are similar.

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