Waking and the Reticular Activating System in Health and Disease by Edgar Garcia-Rill

Author:Edgar Garcia-Rill [Garcia-Rill, Edgar]
Language: eng
Format: azw3, pdf
ISBN: 9780128016329
Publisher: Elsevier Science
Published: 2015-04-27T16:00:00+00:00

Alpha Rhythm

The “alpha rhythm” is the best-known 10 Hz rhythm with a history stemming from the earliest days of the electroencephalogram (EEG). Alpha waves were originally described by Hans Berger, which resulted when the subjects being EEG recorded closed their eyes. When the subjects opened their eyes, the EEG frequency increased to beta and gamma, and alpha waves were “blocked.” Alpha waves are also reduced or “suppressed” when the subjects become drowsy or fall asleep, and the EEG shifts to theta and lower-frequency waves. The frequency range for alpha was originally set at 7.5–12.5 Hz, and theta waves were said to be 4–7 Hz. It is not clear how correlated, if at all, cortical EEG theta waves are to hippocampal theta waves, which are in the 6-10 Hz range. However, in human magnetoencephalography (MEG) recordings, the peak of the cortical theta wave is closer to 8 Hz, while the peak of the alpha wave is closer to 10 Hz with eyes closed (Llinas et al., 2001).

In sleep studies, subjects who are awake manifest beta/gamma activity, then shift to alpha upon closing the eyes, and later drift into sleep that exhibits mainly delta and slow waves in the 0.5–4 Hz range. Slower frequencies below 10 Hz signal drowsiness and the transition towards frank sleep, while faster frequencies above 10 Hz signal more activation and awakening. Alpha waves themselves are most evident when the eyes are closed. Why? Alpha waves are thought to signal relaxation or calmness, and they are the easiest waves to entrain when using biofeedback. Why?

Our hypothesis is that occipital alpha waves are manifested when we eliminate one of our most important sensory systems, the visual system. When we eliminate the constant flow of visual input flowing into our brains by closing the eyes, alpha waves are expressed because it is the basic idling speed of the awake brain. If we become drowsy, we shift into theta and delta waves towards sleep, but if we open our eyes, the EEG immediately shifts to beta and then gamma when we focus attention. That is, the 10 Hz rhythm that in the EEG is referred to as occipital “alpha” waves is actually the speed of the awake but unchallenged brain at its “resting state.” Any slower and we shift into drowsiness; any faster and we begin to pay attention and cogitate.

As we will discuss in Chapter 9, the EEG amplifier has inappropriate band-pass filters for detecting events as fast as action potentials, which occur in the 1–2 ms range. EEG amplifiers more faithfully reflect the activity of slow potentials such as dendritic postsynaptic potentials and intrinsic membrane oscillations, which occur in the 10–20 ms range. That is, the EEG is a measure of dendritic potentials and membrane oscillations, not of action potential frequencies (Murakami and Okada, 2006). That means that EEG recordings exhibit large-amplitude, low-frequency waveforms when large ensembles of cells are “beating together,” rather than separately, such as when low-amplitude, high-frequency activity is evident. The most simple way to think

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