It is apparent, then, that the cooperation of the patient is needed, and that the patient should be properly prepared physically and psychologically in order to obtain an accurate and useful record of brain activity. The electroencephalograph is an extremely sensitive instrument, and readings can be greatly influenced by the actions and physiologic status of the subject. It is particularly valuable in diagnosing epilepsy, brain tumor, and other diseases of and injury to the brain. “Electrical silence,” or no evidence of brain activity, when demonstrated once and then again in 24 hours, has been taken as one of the criteria of death.Įlectroencephalography is widely used in studying brain function and in tracing the connections between the parts of the central nervous system. Rhythmic slow waves of 4 to 7 hertz are called theta waves. Irregular slow waves of 2 to 3 hertz, called delta waves or the delta pattern, are normally found in deep sleep and in infants and young children, but they indicate an abnormality in the awake adult. Some persons with mild deviations from normal may have no evidence of cerebral disease, while others with readings within normal ranges may be suffering from a serious disorder. One should use the word “normal” with caution when speaking of EEG readings. During sleep the brain cells generate higher voltage electrical waves, but the rhythm is slowed down to 2 or 3 hertz, sometimes with short “sleep” spindles of about 15 hertz. In a normal EEG the frequencies are predominately within the range of alpha and beta rhythms at the rate of 8 to 30 hertz (cycles per second). These waves are blocked in the same way as are alpha waves, by opening the eyes.
The beta waves, obtained from the central and front parts of the head, are more closely related to the sensory-motor parts of the brain. These waves are blocked by excitement or by opening the eyes. Most of the recorded waves in a normal adult's EEG are the occipital alpha waves, which are best obtained from the back of the head (occipital region) when the subject is resting quietly, but not asleep, with the eyes closed. Age and degree of consciousness also cause the wave patterns to differ. The rate, height, and length of the waves vary in different parts of the brain, and each individual has a unique and characteristic pattern.
The impulses are of sufficient magnitude to move an electromagnetic pen that records the brain waves. The recording of changes in electric potentials in various areas of the brain by means of electrodes placed on the scalp, on the brain surface, or within the brain itself, and connected to a vacuum tube radio amplifier, which amplifies the impulses more than a million times.
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