This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

The EEG and Evoked Potentials

From about 1940 to 1970, there were two main technologies for studying brain activity: (1) the EEG (electroencephalograph or "brain wave" machine); and (2) single-cell recording, in which activity of individual neurons was monitored with tiny wire electrodes or glass pipettes (small tubes). Both are still being used in neuroscience research. Single cell recording has been upgraded to multiple-cell recording, using implanted "chips" which can record from large populations of cells. Meanwhile, techniques for scanning the brain have improved markedly.

What does the EEG measure?

The EEG or "brain wave" machine has been around for over 60 years. Its usefulness to psychologists has never been greater, thanks to new computer techniques. The EEG machine records tiny electrical voltages from the brain, probably representing the averaged electrical activity of millions of neurons. Most of the electrodes are on the scalp, but one of the wires is attached to the ear lobe, which serves as an electrically neutral spot to compare with the scalp. The scalp may be covered with a dozen or more electrodes, each with a gooey electrolyte paste between the electrode disk and the scalp to maintain good electrical contact.

What is an evoked potential?

When a person is suddenly stimulated, or performs an action, or thinks about something, there are characteristic fluctuations in the EEG. This is called an evoked potential (EP) because it is an electrical potential triggered or "evoked" by stimulation. Evoked potentials can be measured on the EEG and correlated with psychological events such as the perception of a color or the comprehension of word meaning.

An example of evoked potential research appears in Chapter 7 (Cognition) in the section on thinking. Subjects in an experiment were asked to read sentences like, "He spread the butter on the bread." For some subjects, the sentence ended with an unlikely word, such as "socks." ("He spread the butter on the socks.") This group showed a change in their evoked potentials that reflected their surprise or the need to re-process the meaning of the sentence, given the unexpected ending.

Evoked potential research started in the 1950s and 1960s. Like all brain research involving technology, evoked potential research benefited greatly from the increased power and decreased price of computers since then. Today, relatively inexpensive computers can extract more information from the noisy EEG signal. A research group in San Francisco, led by Alan S. Gevins, uses computer pattern recognizing techniques to detect "shadows of thought"-rapidly changing areas of activity in the brain-in EEG recordings.

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