The EEG discussed here is sometimes called “quantitative EEG” and is used for basic research on brain electrical activity during cognitive or affective processing. The EEG signal recorded from the scalp is composed of multiple sine waves cycling at different frequencies. Fourier analysis decomposes the EEG into these different sine waves and estimates the spectral power (in mean square microvolts) at each frequency. This results in information regarding the contribution of each individual frequency to the entire EEG spectrum at a particular electrode site. Power is thought to reflect the excitability of groups of neurons. Power values are usually totaled across frequency bins to form measures of power in a specific frequency band (see frequency band section). The power data should be normalized (e.g., natural log transformation) because EEG power data are usually positively skewed (Davidson, Jackson, & Larson, 2000).
