Alcoholism and Human Electrophysiology

by National Institute of Health

Electroencephalography (EEG), the recording of electrical signals from the brain, provides a noninvasive measure of brain function as it is happening. Research using EEG, as well as event-related potentials (ERPs) and event-related oscillations (EROs), which measure brain activity in response to a specific stimulus, have shown that the brain activity of alcoholics and nonalcoholics differs in some characteristic ways. These differences are consistent with an imbalance between excitation and inhibition processes in the brains of alcoholics.

Researchers studying the effects of alcohol use on the brain are aided by techniques that yield images of the brain's structure and reveal the brain's activity as it is happening. Tools that produce images of the brain's structure, such as magnetic resonance imaging (MRI) and positron emission tomography (PET), are described in other articles in this issue. This article examines the techniques of electrophysiological brain mapping, which best reveals brain activity as it occurs in time, in fractions of seconds.

Electroencephalography is the recording ("graph") of electrical signals ("electro") from the brain ("encephalo"). Each nerve cell (neuron) in the brain produces a tiny electrical charge; when a number of neurons become active, the sum of these tiny electrical charges can be detected on the surface of the scalp. Small electrodes placed on the scalp detect this electrical activity, which is magnified and recorded as brain waves (neural oscillations). These brain waves illustrate the activity as it is taking place in various areas inside the brain.

In the purely resting state, brain waves often are randomly active. However, when a person perceives or responds to a sensory or cognitive stimulus, groups of neurons fire together, and the EEG is no longer random. The activity that is related to processing of the stimulus always occurs at the same time after the stimulus (is time-locked). This time-locked (event-related or evoked) activity is embedded in background random EEG that is not related to the stimulus processing. In order to see the tiny event-related activity, the EEG is averaged across multiple identical occurrences or trials (whenever the blue triangle occurs in a series of red squares); activity that is random with respect to the stimulus cancels out with each presentation of the stimulus, whereas the time-locked activity that occurs at the same time on every trial increases in the average. The waveform produced after averaging across identical trials is called an event-related potential (ERP). If one were to make a movie of the brain activity involved in the mental processing of a stimulus as it happens in real time, one would first see early fast activity that is related to sensory reception occurring in the visual cortex; this would then be followed by slower activity related to higher cognitive function (identification and attention to a blue triangle in a series of squares), which involves activity in the parietal and frontal lobes. The fast and slow neural oscillations that underlie the ERP, called event-related oscillations (EROs), represent sensory and cognitive functions.

This article reviews research indicating that alcoholics manifest aberrant resting EEGs, ERPs, and EROs, and discusses the significance of these findings.

The Resting Electroencephalogram

The resting EEG is the recording of ongoing spontaneous brain electrical activity while the person being examined is relaxing (the person's eyes may be open or closed). It is made up of oscillations that are described in terms of frequency, which is the number of times a wave completes its cycle per unit of time. Frequency is measured in Hertz (Hz), which is the number of cycles of the wave per second. EEG oscillations also are described by the magnitude of their voltage measured in microvolts (µV [millionths of a volt]).

Typically, EEGs are divided into the following frequency bands: delta, theta, alpha, beta, and gamma, with each frequency reflecting a different degree of brain activity. In healthy adults, medium and fast frequencies are predominant in the awake resting EEG, with only sparse occurrence of low frequencies and high frequencies. The resting EEG is stable throughout healthy adult life and is highly heritable.

Theta (3.5-7.5 Hz)

Theta rhythm is largest in the back region of the brain when a person is resting (resting or tonic theta) and in the front region when the person is actively engaged in mental activity (active or phasic theta). The normal adult waking EEG record contains relatively little theta rhythm. Investigators have reported that tonic theta increases in several neurological states, such as Alzheimer's disease; tonic theta also increases when cognitive activity decreases. Tonic theta power in alcoholics has been examined in the Collaborative Study on the Genetics of Alcoholism (COGA). Researchers compared eyes-closed resting theta (3-7 Hz) power in 307 alcohol-dependent study participants and 307 age- and gender-matched control subjects. The alcohol-dependent group had higher resting theta power at all scalp locations. In both male and female alcoholics, the increased theta power was most prominent at parietal regions; in males this prominent theta also extended more forward to the central regions. Correlation of drinking variables (such as recency of last drink and quantity of drinks in a typical week) with theta power revealed no group-specific differences.

Elevated tonic theta power in the EEG may reflect a deficiency in the information processing capacity of the central nervous system (CNS). Resting theta power has been reported to increase with age, as well as in Alzheimer's patients. A theta power increase also may be an electrophysiological indicator of the imbalance in the balance of excitatory and inhibitory neurons in the cortex.

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