ABSTRACT
The suppression of the alpha rhythm, which can be observed in response to a variety of different tasks, is the best known EEG phenomenon. Because the alpha rhythm is most evident in relaxed wakefulness, the implication is that alpha reflects mental inactivity rather than active cognitive processing. However, more recent evidence suggests that activity in the alpha frequency range reflects active information processing, and that different frequencies within the alpha frequency band have quite different functions. Upper alpha, which is a 2 Hz band above the individually determined alpha frequency (IAF), reflects the processing of sensory-semantic information, whereas lower-1 alpha, in the range of IAF-4 Hz to IAF-2 Hz, is associated with attentional processes. The lower-2 alpha band, which falls just below IAF (IAF-2Hz to IAF), seems to reflect expectancy. The general conclusion is that different EEG frequencies reflect different types of cognitive processes. KEYWORDS: Alpha; Brain oscillations; Pace maker; Attention; Memory
1. INTRODUCTION
Alpha is an oscillatory EEG activity, within a range of about 8-13 Hz, which has been known since the early days of EEG research. It can best be observed over posterior regions of the head during relaxed wakefulness with closed eyes (cf. Niedermeyer, 1993a). As compared to other EEG phenomena it is characterized by the following outstanding features: (i) spectral analysis reveals that alpha is the dominant frequency in the human EEG, and with the exception of extremely low frequencies (below about 2.5 Hz) shows maximal power; (ii) it is well documented that alpha desynchronizes (becomes suppressed) in response to a variety of different tasks; and (iii) Alpha is the only frequency in the human scalp EEG that clearly and undoubtedly reflects an oscillatory process. Because of its outstanding features, many researchers have assumed that alpha may play a key role in understanding other EEG phenomena (Başar, 1997; Başar et al., 1997a,b). Before considering the functional meaning of alpha in more detail, we first
review some of its most important properties.
