The process of recording change in voltage potentials across the scalp in humans, and subcutaneously in animals. EEGs record resting potentials, or the electrical activity of the brain when it is not actively engaged in a task. There are five well-documented bands of resting potentials, divided by the frequency of the waveform. Different EEG frequency bands are indicative of the underlying state of activity and alertness in the brain.
Alpha waves occur in the 8 to 13 Hz frequency range, and generally exhibit much greater amplitude than any of the other frequency bands. Alpha is most prominent when one is resting or relaxing, but not asleep. Alpha is characterized by consistently sinusoidal waveform.
EEGs are intended to measure the brain's activity during various stages of rest. When the brain is engaged in an active intellectual process, Alpha waves are replaced by what is known as Beta waves. Beta waves are faster than Alpha (> 14 Hz) and also much lower amplitude. They are, unlike Alpha, very non-uniform and random in their waveform.
Theta waves are of lower frequency and higher amplitude even than alpha. They are exhibited when an individual is exhausted or entering the first stage of sleep. There is no clear line dividing alpha and theta, however certain epidemiological studies have given this distinction some merit. Individuals suffering from stroke, aneurysm, or other brain lesions often exhibit theta in excessive abundance. Additionally some neurochemical conditions such as Alzheimer's Disease and Attention Defecit Disorder cause similar shift away from Alpha and towards Theta. ADD can even be treated by training an individual to increase Alpha and reduce Theta using biofeedback.
Delta is the slowest frequency band of all. It is evident only in the later stages of sleep (or in cases of serious, serious brain damage).
Gamma waves, only recently elicited in controlled laboratory experiments, are also known as the Cognitive Binding Frequency. They are present when multiple modalities of one's brain are engaged in cooperative efforts. For example, when one is required to push a square key when a red light comes on, and a triangular key when a green light comes on, it will be possible to record Gamma waves between the occipital lobe and the parietal lobe.
Another use of the electroencephalograph is to measure changes in voltage across the scalp when the subject is asked to do a specific task, such as avoiding a shock. When using the machine to collect data on such functional phenomenon, one is no longer recording EEGs, but Event Related Potentials, formerly known as Evoked Response Potentials and generally referred to as ERPs.
While recording brain waves has a decidedly futuristic sound to it, there are great obstacles to using brain recordings to do any of the myriad things we'd like to do. One of the biggest problems is the skull. Everyone's is a different thickness, and since bone is a great source of impedance this will result in great variability between subjects. Another drawback is that electric signals interfere with each other, meaning that the recording is merely a measure of general electrical activity, not the activation of any neuron or node. This makes it very difficult to locate the site of production for a given wave or waveform. Despite these obstacles, EEGs are useful for a broad range of experimental, diagnostic and therapeutic purposes.