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Electroencephalography (EEG) is a technique for recording electrical activity from the scalp related to cortical activity. It reflects the electrical activity of the brain at the level of the synapse (Nunez & Srinivasan, 2006). It records the product of the changing excitatory and inhibitory currents. Action potentials contribute very little to the EEG. However, since changes at the synapse do influence the production of action potentials, there is an association of EEG with spike trains (Whittingstall & Logothetis, 2009).

EEG measures the electrical activity of the brain as found on the scalp.

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electroencephalography (EEG): a technique for recording electrical activity from the scalp, which measures the electrical activity of the brain at the level of the synapse

The EEG was first demonstrated in humans by Hans Berger in 1924, and results were published 5 years later (Berger, 1929/1969). Since the neurons of the brain and their connections are constantly active, EEG can be measured during both waking hours and sleep. In fact, EEG serves as an objective measure of depth of sleep (see Figure 2.10).

EEG can be measured with only two electrodes or as a high-density array of more than 200 electrodes. EEG activity has been used to infer brain processing. The actual measure of EEG is the difference between the signals at any two electrodes. Traditionally, the second or reference electrode was placed at a location not considered to produce electrical signals, for example, the ear lobe. Today, a common practice is to average the signals in all of the electrodes available and compare that with each specific electrode.

An EEG “cap” that holds the electrodes is placed over the subject’s head.

Don Tucker

Some aspects of the EEG signal may appear almost random, while other fluctuations appear periodic. Using signal processing techniques, it is possible to determine the major frequency and amplitude seen in the signal. Amplitude refers to how large the signal is, and frequency refers to how fast the signal cycles, measured in cycles per second, or Hertz (Hz). Over the years, researchers have noticed that specific patterns of EEG activity were associated with a variety of psychological states (see Figure 2.11). When an individual is relaxed with his or her eyes closed, high-amplitude regular activity is seen in the EEG at a frequency of 8 to 12 Hz. Alpha activity in the 8 to 12 Hz range was the first pattern of EEG activity Hans Berger noted. If the person begins to perform some mental activity such as mental arithmetic, lower-amplitude EEG is seen at a higher frequency, above 20 Hz, and is referred to as beta activity.

EEG oscillations are one way in which information is transferred in the brain (Knyazev, 2007). For example, theta oscillations are associated with memory performance (Liebe, Hoerzer, Logothetis, & Rainer, 2012). Theta is also involved in coordinating emotional information between the limbic areas and the frontal areas of the brain. Delta oscillations are seen in sleep and motivational processes such as drug use. Drugs such as cocaine produce changes in a number of EEG frequency bands. Alpha oscillations, on the other hand, are involved in inhibiting the activity of various brain areas.

In recent years, researchers have become interested in the processing of a percept (Singer, 2009; Singer & Gray, 1995; Tallon-Baudry & Bertrand, 1999). For example, when one sees a black and white spotted Dalmatian dog against a black and white background, there is usually a subjective experience of having the image “pop out.” Associated with this perception is a burst of EEG gamma activity. Figure 2.12 compares the amount of EEG gamma activity in those individuals trained to see the Dalmatian as compared with those who were not trained.

Figure 2.10 Electroencephalogram as an Objective Measure of Sleep Stages

Source: P. Hauri, Current Concepts: The Sleep Disorders, 1982, Kalamazoo, MI: Upjohn.

Figure 2.11 Depiction of Specific Patterns of Electroencephalography Activity

Source: Hugo Gambo (2005), Wikipedia.

event-related potentials (ERPs): also known as evoked potentials (EP), they show electroencephalography (EEG) activity in relation to a particular event

evoked potentials (EP): also known as event-related potentials (ERPs), they show electroencephalography (EEG) activity in relation to a particular event