Brain waves are recordings of fluctuating electrical changes in the brain. To obtain such a recording, electrodes are positioned on the surface of a surgically exposed brain (an electrocorticogram, ECoG) or on the outer surface of the head (an electroenephalogram, EEG). These electrodes detect electrical changes in the extracellular fluid of the brain in response to changes in potential among large groups of neurons. The resulting signals from the electrodes are amplified and recorded. Brain waves originate from the cerebral cortex, but also reflect activities in other parts of the brain that influence the cortex, such as the reticular formation. Because the intensity of electrical changes is directly related to the degree of neuronal activity, bran waves vary markedly in amplitude and frequency between sleep and wakefulness.
Brain waves are classified as alpha, beta, theta, and delta waves. Alpha waves are recorded most easily from the posterior regions of the head and have a frequency of 8-13 cycles per second. They occur when a person is awake but resting, with the eyes closed. These waves disappear during sleep, and if the wakeful person's eyes open, higher frequency beta waves replace the alpha waves.
Beta waves have a frequency of more than 13 cycles per second and are usually recorded in the anterior region of the head. They occur when a person is actively engaged in mental activity or is under tension.
Theta waves have a frequency of 4-7 cycles per second and occur mainly in the parietal and temporal regions of the cerebrum. They are normal in children, but do not usually occur in adults. However, some adults produce theta waves in early states of sleep or at times of emotional stress.
Delta Waves have a frequency below 4 cycles per second and occur during sleep. They originate from the cerebral cortex when it is not being activated by the reticular formation.
Brain wave patterns can be useful for diagnosing disease conditions, such as distinguishing types of seizure disorders (epilepsy) and to locate brain tumors. Brain waves are also used to determine when brain death has occurred. Brain death, characterized by the cessation of neuronal activity, may be verified by an EEG that lacks waves (isoelectric EEG). However, drugs that greatly depress brain functions must be excluded as the cause of the flat EEG pattern before brain death can be confirmed.