Electroencephalography is the neurophysiologic measurement of the electrical activity of the brain by recording from electrodes placed to the scalp, or in the special cases on the cortex. The resulting traces are known as an electroencephalogram (EEG) and represent so-called brainwaves. This device is used to assess brain damage, epilepsy and other problems. In some jurisdictions it is used to assess brain death. EEG can also be used in conjunction with other types of brain imaging.
Neuroscientists and biological psychiatrists use EEGs to study the function of the brain by recording brainwaves during controlled behavior of human volunteers and animals in lab experiments. Theories to explain sleep often rely on EEG patterns recorded during sleep sessions. In addition, the procedure is used clinically to assist in the diagnosis of epilepsy.
The recording is obtained by placing electrodes on the scalp, usually after preparing the scalp area by light abrasion and application of a conductive gel to reduce impedance. Each electrode is connected to an input of a differential amplifier (one amplifier per pair of electrodes), which amplifies the voltage between them (typically 1,000–100,000 times, or 60–100 dB of voltage gain), and then displays it on a screen or inputs it to a computer. The amplitude of the EEG is about 100 µV when measured on the scalp, and about 1-2 mV when measured on the surface of the brain.
The electrode-amplifier relationships are typically arranged in one of three ways:
- Common reference derivation
- One terminal of each amplifier is connected to the same electrode, and all other electrodes are measured relative to this single point. It is typical to use a reference electrode placed somewhere along the scalp midline, or a reference that links both earlobe electrodes.
- Average reference derivation
- The outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each amplifier.
- Bipolar derivation
- The electrodes are connected in series to an equal number of amplifiers. For example, amplifier 1 measures the difference between electrodes A and B, amplifier 2 measures the difference between B and C, and so on.
This distinction has become void with the advent of computerized or paperless EEGs, which record all electrodes against an arbitrary reference and will calculate the above montages post hoc.
EEG has several limitations. Scalp electrodes are not sensitive enough to pick out individual action potentials, the electric unit of signaling in the brain, or whether the resulting electrical activity is releasing inhibitory, excitatory or modulatory neurotransmitters. Instead, the EEG picks up synchronization of neurons, which produces a greater voltage than the firing of an individual neuron. Secondly, EEG has limited anatomical specificity when compared with other functional brain imaging techniques such as functional magnetic resonance imaging (fMRI). Some anatomical specificity can be gained with the use of EEG topography, which uses a large number of electrodes to triangulate the source of the electrical activity.
EEG has several strong sides as a tool of exploring the brain activity. The time resolution is very high. As other methods for researching brain activity have time resoloution between seconds and minutes, the EEG has a resolution down to sub-millisecond. The brain is thought to work through its electric activity. EEG is the only method to measure it directly. Other methods for exploring functions in the brain do rely on blood flow or metabolism which may be decoupled from the brain electric activity. Newer research typically combines EEG or MEG with MRI or PET to get high temporal and spatial resolution.
Historically four major types of continuous rhythmic sinusoidal EEG waves are recognized (alpha, beta, delta and theta). There is no precise agreement on the frequency ranges for each type.
- Delta is the frequency range up to 4 Hz and is often associated with the very young and certain encephalopathies and underlying lesions. It is seen in deep sleep.
- Theta is the frequency range from 4.5 Hz to 8 Hz and is associated with drowsiness, childhood, adolescence and young adulthood. This EEG frequency can sometimes be produced by hyperventilation. Theta waves can be seen during hypnagogic states such as trances, hypnosis, deep day dreams, lucid dreaming and light sleep and the preconscious state just upon waking, and just before falling asleep.
- Alpha (Berger's wave) is the frequency range from 8.5 Hz to 12 Hz. It is characteristic of a relaxed, alert state of consciousness and is present by the age of two years. Alpha rhythms are best detected with the eyes closed. Alpha attenuates with drowsiness and open eyes, and is best seen over the occipital (visual) cortex. An alpha-like normal variant called mu is sometimes seen over the motor cortex (central scalp) and attenuates with movement, or rather with the intention to move.
- sensorimotor rhythm (SMR) is a middle frequency (about 12-16Hz) associated with physical stillness and body presence.
- Beta is the frequency range above 12 Hz. Disorganized, low amplitude beta is often associated with active, busy or anxious thinking and active concentration. Rhythmic beta is associated with various pathologies and drug effects.
- Gamma is the frequency range above 40 Hz (approximately 30-80 Hz to be precise). Gamma rhythms appear to be involved in higher mental activity, including perception and consciousness.
Rhythmic slow activity in wakefulness is common in young children, but is abnormal in adults. In addition to the above types of rhythmic activity, individual transient waveforms such as sharp waves, spikes, spike-and-wave complexes occur in epilepsy, and other types of transients occur during sleep.
In the transition from wakefulness, through Stage I sleep (drowsiness), Stage II (light) sleep, to Stage III and IV (deep) sleep, first the alpha becomes intermittent and attenuated, then disappears. Stage II sleep is marked by brief bursts of highly rhythmic beta activity (sleep spindles) and K complexes (transient slow waves associated with spindles, often triggered by an auditory stimulus). Stage III and IV are characterized by slow wave activity. After a period of deep sleep, the sleeper cycles back to stage II sleep and/or rapid eye movement (REM) sleep, associated with dreaming. These cycles may occur many times during the night.
EEG under general anesthesia depends on the type of anesthetic employed. With halogenated anesthetics and intravenous agents such as propofol, a rapid (alpha or low beta), nonreactive EEG pattern is seen over most of the scalp, especially anteriorly; in some older terminology this was known as a WAR (widespread anterior rapid) pattern, contrasted with a WAIS (widespread slow) pattern associated with high doses of opiates.
In 1913, russian physiologist, Vladimir Vladimirovich Pravdich-Neminsky published (http://en.wikipedia.org/wiki/Electroencephalography#endnote_14) the first EEG and the evoked potential of the mammalian (dog).
German physiologist Hans Berger (1873–1941) began his studies of the human EEG in 1920. He gave the device its name and is sometimes credited with inventing the EEG, though others had performed similar experiments. His work was later expanded by Edgar Douglas Adrian.
In the 1950s, English physician Walter Grey Walter developed an adjunct to EEG called EEG topography which allowed for the mapping of electrical activity across the surface of the brain. This enjoyed a brief period of popularity in the 1980's and seemed especially promising for psychiatry. It was never accepted by neurologists and remains a primarily a research tool up to now.
- ^ Pravdich-Neminsky VV. Ein Versuch der Registrierung der elektrischen Gehirnerscheinungen (In German). Zbl Physiol 27: 951–960, 1913.
- International Federation of Clinical Neurophysiology (http://www.ifcn.info/)
- The EEG and Clinical Neuroscience Society (http://www.ecnsweb.com)
- American Clinical Neurophysiology Society (http://www.acns.org/)
- OpenEEG (http://openeeg.sourceforge.net) - An open source hardware and software project for building a personal EEG
- BrainStorm (http://neuroimage.usc.edu/brainstorm) - A shareware MATLAB software toolbox for electromagnetic functional brain imaging using EEG and Magnetoencephalography.