In the 1930s, Berger, a German psychiatrist, first discovered electrical brain waves. Any stimulus produces electrical responses in the brain called evoked potentials. Recording these electrical responses, and averaging to remove noise, displays brain activity in response to the stimulus every millisecond (thousandths of a second).

When the stimulus is a cognitive or intellectual task, the recording is a cognitive evoked potential. This examines the intellectual workings of the brain. A selective attention task involves presentation of two stimuli, one less frequently than the other. Auditory stimuli are tones of high and low pitches presented over headphones. Visual stimuli are different letters presented on a computer screen. When asked to differentiate between the two stimuli, the less frequent stimulus produces the P300, a positive electrical wave in the brain about 3/10 of a second after the stimulus. A small wave indicates problems with attention. A delayed wave indicates problems with information processing.

Brain function may be abnormal only in certain parts of the brain most involved in a specific intellectual task. Recent technological advances make it possible to record and analyze brain electrical activity from a large number of scalp electrodes. With at least 32 monitoring channels, it is possible to draw brain maps of the electrical activity. Earlier, brain mapping using X-rays made it possible to see brain structure (CT scan). Brain mapping using radioactive chemicals made it possible to study brain function over minutes (PET scan). Brain mapping of electrical activity makes it possible to study brain function over thousandths of a second.

Testing is in a private room with advanced computer equipment. Application of a cap with 31 electrodes and of electrodes on both ears and near both eyes is painless. Presentation of two different stimuli occurs, with instructions to press a button with presentation of the less frequent stimulus. Recording of the EEG occurs. The doctor interprets millions of pieces of information stored on computers.