Electroencephalogram (EEG)

The EEG is a recording of the brain’s electrical activity, in most cases made from electrodes over the surface of the scalp. It may also be made from electrodes placed directly over the surface of the brain or from needle electrodes inserted into the brain. The recordings are the summation of volume conductor fields produced by millions of interconnecting neurones. The neurone components producing the currents are the dendrites, axons and cell bodies. The architecture of the brain is not uniform but varies with different locations. Thus the EEG can vary depending on the location of the recording electrodes.

Sensory information is transmitted to the brain by frequency modulated trains of action potentials which cause neurone activity in particular regions of the brain depending on the type of sensory information and the site of stimulus in the body. Similarly the decision to initiate a movement, in response to sensory information, arises in various parts of the brain, depending on the type of movement and its location in the body, and gives rise to electrical activity at the corresponding sites.

EEGs show continuous oscillating electric activity. The amplitude and the patterns are determined by the overall excitation of the brain which in turn depends on the activity of the reticular activating system in the brain stem. Amplitudes on the surface of the brain can be up to 10 mV, those on the surface of the scalp range up to 100 µV.frequencies range from 0.5 to 100 Hz. The pattern changes markedly between states of sleep and wakefulness. Distinct patterns are seen in epilepsy. Five classes of wave groups are described: alpha, beta, gamma, delta and theta.

Alpha waves contain frequencies between 8 and 13 Hz with amplitude less than 10 µV. They are found in normal people who are awake and resting quietly, not being engaged in intense mental activity. Their amplitude is highest in the occipital region. When the person is asleep, the alpha waves disappear. When the person is alert and their attention is directed to a specific activity, the alpha waves are replaced by asynchronous waves of higher frequency and lower amplitude.

Beta waves have a frequency range of 14 to 22 Hz, extending to 50 Hz under intense mental activity. They have their maximum amplitude (less than 20 µV) on the parietal and frontal regions of the scalp. There are two types: beta I waves, lower frequencies which disappear during mental activity, and beta II waves, higher frequencies which appear during tension and intense mental activity.

Gamma waves have frequencies between 22 and 30 Hz with an amplitude of less than 2 µV peak-to-peak and are found when the subject is paying attention or is having some other sensory stimulation.

Theta waves have a frequency range between 4 to 7 Hz with an amplitude of less than 100 µV. They occur mainly in the parietal and temporal regions in sleep and also in children when awake, and during emotional stress in some adults, particularly during disappointment and frustration. Sudden removal of something causing pleasure will cause about 20 s of theta waves.

Delta waves have a frequency content between 0.5 and 4 Hz with an amplitude less than 100 µV. They occur during deep sleep, during infancy and in serious organic brain disease. They will occur after transections of the upper brain stem separating the reticular activating system from the cerebral cortex. They are found in the central cerebrum, mostly the parietal lobes.