The investigation of the electrical activity of the brain, using electrodes applied to the scalp, and usually recorded as a tracing on paper (an electroencephalogram, or EEG). The EEG changes with the mental activity of the subject, and characteristic patterns of electrical activity (eg for sleep, coma, epileptic seizure) can be recognized. The alpha rhythm (c.10 Hz) appears with relaxation and eye closure. The delta rhythm (14 Hz) appears in deep sleep.
Electroencephalography is the neurophysiologic measurement of the electrical activity of the brain by recording from electrodes placed on the scalp or, in special cases, subdurally or in the cerebral cortex. The EEG is a brain function test, but in clinical use it is a "gross correlate of brain activity" .
EEGs are frequently used in experimentation because the process is non-invasive to the research subject. The EEG is capable of detecting changes in electrical activity in the brain on a millisecond-level.
Clinical use
EEG in various forms is most useful as a tool for monitoring and diagnosis in certain clinical situations:
epilepsy and syncope (fainting) sleep disorders coma and brain deathIt is sometimes useful in assessing dementia, when other examinations are equivocal. Current research is being done to determine if EEG may also be used to help monitor clinical depression treatment, but such studies are still in the clinical stages.
Research use
Neuroscientists and biological psychiatrists use EEGs to study the function of the brain by recording cerebral activity during controlled behavior of human volunteers and animals in lab experiments. Theories to explain sleep often rely on EEG patterns recorded during sleep sessions.
Methods
In conventional scalp EEG, 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.
The filtered signal is then output on paper (in older systems), or displayed on a computer screen. 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 derivationThis distinction has become void with the advent of digital or paperless EEGs, which record all electrodes against an arbitrary reference and will calculate the above relationships (called montages) post hoc.
Limitations
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 the activity of large groups 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.
Advantages
EEG has several strong sides as a tool of exploring the brain activity. As other methods for researching brain activity have time resolution between seconds and minutes, the EEG has a resolution down to sub-millisecond. As the brain is thought to work through its electric activity, EEG is the only method to measure it directly. Newer research typically combines EEG or MEG with MRI or PET to get high temporal and spatial resolution.
Activity types
Historically four major types of continuous rhythmic sinusoidal EEG activity are recognized (alpha, beta, delta and theta). This EEG frequency can sometimes be produced by hyperventilation.
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).
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;
Artifacts
Biological Artifacts
Signals in the EEG that are of non-cerebral origin are called artifacts. The EEG is nearly always contaminated by such signals. This is one of the reasons why it takes considerable experience to interpret EEGs clinically. The most common types of artifacts are:
Eye artifacts (including eyeball, ocular muscles and eyelid) EKG artifacts EMG artifacts Glossokinetic artifactsEyeball artifacts are caused by the potential difference between the cornea and retina, which is quite large compared to cerebral potentials. Purposeful or reflexive eye blinking also generates electromyographic potentials, but more importantly there is reflexive movement of the eyeball during blinking which gives a characteristic artefactual appearance of the EEG (see Bell's phenomenon).
Some of these artifacts are useful. Eye movements are very important in polysomnography, and is also useful in conventional EEG for assessing possible changes in alertness, drowsiness or sleep.
EKG artifacts are quite common and can be mistaken for spike activity. Because of this, modern EEG acquisition commonly includes a one-channel EKG from the extremeties. This also allows the EEG to identify cardiac arrythmias that are an important differential diagnosis to syncope or other episodic/attack disorders. Minor tongue movements can contaminate the EEG, especially in parkinsonian and tremor disorders.
External Artifacts
In addition to internal artifacts, there are many artifacts which originate from outside the patient. From a completely different source, within the United States, poor grounding of the EEG electrodes can cause a significant 60 Hz artifact (50 Hz in many other countries).
Artifact Correction
Recently, source decomposition techniques have been used to "correct" or "remove" EEG artifacts. These source decomposition models, in one way or another, assume the ability to "unmix" EEG signal into some number of independent sources. There is no proof yet as to the validity or preciseness of these methods since simulated EEG recordings are the only way to know beforehand, the exact properties of the uncontaminated signal. In reality, EEG is to some extent stochastic.
History
A brief timeline is given here . In 1890, Beck publishes an investigation of spontaneous electrical activity of the brain of rabbits and dogs which included rhythmic oscillations altered by light
In 1912, Russian physiologist, Vladimir Vladimirovich Pravdich-Neminsky published 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.
In 1934, Fisher and Lowenback first demonstrate epileptiform spikes. The same year, the first EEG laboratory opened at Massachusetts General Hospital.
Franklin Offner (1911-1999), professor of biophysics at Northwestern University developed a prototype of the EEG which incorporated a piezoelectronic inkwriter called a Crystograph (the whole device was typically known as the Offner Dynograph).
In 1947, The American EEG Society is founded and the first International EEG congress is held.
In the 1950s, English physician William Grey Walter developed an adjunct to EEG called EEG topography which allowed for the mapping of electrical activity across the surface of the brain.
User Comments Add a comment…