| Neurophysiology Background: What is EEG? History of EEG Over a hundred and twenty years ago, an English physician, Richard Caton (1842-1926) was the first to discover the presence of spontaneous electrical activity in the brains of animals. His discovery enabled a monumental advance in neuroscience, specifically neurophysiology. In a publication of his work in 1875, Caton described how he discovered these signals by placing electrodes directly on the exposed surface of the brains of rabbits and monkeys, and recording the currents with a galvanometer. Fifty years after Caton's discovery of electrical brain activity, the Austrian psychiatrist Hans Berger (1873-1941) was the first to record electrical signals from the brains of humans. He named these recordings the 'electroencephalogram' (EEG), derived from the Latin words 'electro' meaning electrical signals, 'encephal' meaning brain, and 'gram' meaning written. In 1929, after more than a thousand recordings from the brains of 76 human subjects, Berger published a systematic description of EEG (Berger, 1929). 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 has been primarily a research tool until very recently, when it has started to be used by neurologists and clinical neurophysiologists as a diagnostic tool. Generation of EEG Electroencephalography (or EEG activity) is the graphic depiction of the electrical potentials in the brain, recorded by electrodes placed on the surface of the scalp, or in special cases directly on the cortex. It is generated by the synchronous electrical activity of millions of neurons of the brain, usually of the cerebral cortex. The resulting traces are known as an electroencephalogram (EEG) and represent so-called brainwaves. This measurement is used to assess brain dysfunction or damage and, in some cases, it is used to assess brain death. EEG can also be used in conjunction with other types of brain imaging such as MRI, fMRI and PET. This procedure is used in research (by neuroscientists or specifically, neurophysiologists) to study the function of the brain by recording brainwaves during controlled behavior of human volunteers and animals in laboratory experiments. Theories to explain sleep often rely on EEG patterns recorded during sleep sessions. In addition, the procedure is used clinically (by clinical neurophysiologists and by neurologists) to assist in the diagnosis of brain disorders, particularly epilepsy. Recording of EEG Electrode Placement Initially, in the 1930s, EEGs were recorded by applying electrode plates directly on the scalp. However, one problem with this recording technique was that the very minute EEG signals were often distorted or obscured by electrode movement artefacts. This was overcome with the introduction of a floater type electrolyte that required an electrolyte paste to be applied between the electrode and skin. The electrolyte allowed the small EEG currents to be more easily transferred to the electrodes. All the electrodes in use today are based upon this electrode type. Typical EEG studies place electrodes over bilateral frontal, temporal, central, parietal and occipital areas of the brain. This placement provides information relating to left versus right hemisphere, as well as information within each hemisphere concerning the functioning of different brain areas. Varieties of strategies have been used to select electrode placements sites. Approximately half of the EEG studies in current literature use the 10-20 System designed for use with adults and children in which the location of the electrode is specified in terms of its proximity to particular brain regions and its hemispheric location (Jasper, 1958). The use of a standard system enables between-laboratory and between-experiment comparisons to be made. In recent years, an increased interest in the precise functional and anatomical generators of activity in the brain has necessitated the sampling of electrical fields at a higher spatial frequency. Accordingly, the 10-20 system has been enhanced by the use of non-standard locations and a higher density of electrodes. The Geodesic Sensor Net and the Electrocap are two such electrode systems.. Amplification 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. In the 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. In the 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. In the 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. Types of EEG Rhythms The human EEG rhythms comprise continuous rhythmic sinusoidal waves. These are conventionally classified into different frequency ranges or frequency bands. Historically, four major frequency bands are recognised. Although one frequency dominates the EEG recorded at any time, other frequencies are still present at trace levels. The analysis of the bands in spontaneous or resting EEG provides insights into brain development and dysfunction, and the non-invasive procedure can be applied repeatedly in patients and normal subjects without risks or limitations. Hence, EEG is now a widely used research and clinical technique, particularly useful for diagnosis in neurological and psychiatric disorders and in neurophysiological research. Delta Frequency Band: Ranging from 1-4 Hz. Distribution: Usually widespread (across several lobes) and bilateral (across both hemispheres). Mental State: Delta waves or rhythms are associated with deep dreamless sleep, trance or unconscious states. Physiology: No movement. low level of arousal. This EEG frequency can be produced due to certain encephalopathies and underlying lesions. Rhythmic slow activity in wakefulness is common in young children, but abnormal in adults. Theta Frequency Band: Ranging from 4 - 7 Hz. Distribution: Usually widespread (across several lobes) and lateralized (involves one hemisphere). Mental State: Theta waves or rhythms are associated with a restful mental state. Physiology: Day-dreaming, encoding and retrieval of memories. This EEG frequency is also detected during hyperventilated states as well as hypnagogic states such as trances, hypnosis, deep day-dreaming, lucid dreaming, light sleep, and the preconscious state just upon waking and just before falling asleep. Alpha Frequency Band: Ranging from 8 - 12 Hz. Distribution: Usually regional (involves one entire lobe) and bilateral; common in occipital (visual) cortex. Mental State: Alpha waves, also known as Berger's Wave, are associated with a more relaxed and reflective mental states. Physiology: Passive observation of external surroundings, active and anticipatory suppression of external distractions. This EEG frequency is best detected when the eyes are closed. The alpha waves attenuate with drowsiness and open eyes and are best seen over the occipital (visual) cortex. Beta Frequency Band: Ranging from 13 - 30 Hz. Distribution: Usually regional or localized (within one lobe) and lateralized. Overall State: Beta waves or rhythms are associated with a more alert or active (possibly anxious) mental state. Physiology: Active observation of external surroundings. Rhythmic beta with a dominant set of frequencies is associated with various pathologies and drug effects. Sensorimotor Rhythm (SMR) Frequency: This is a middle frequency band ranging from 12 - 16 Hz. Physiology: SMR rhythms are associated with physical stillness and body presence. Gamma Frequency Band: Ranging from approximately 30 - 80 Hz. Distribution: Usually very localized (one electrode location) and lateralized Mental State: Gamma waves or shythms are associated with highly active and agitated mental states. Physiology: Higher mental activity, including perception and consciousness, high-level information processing, concentrating, problem solving, learning, emotional processing. Types of Complex EEG Patterns EEG during Sleep The EEG activity during sleep comprises various cycles, each of which includes the rhythmic activity described above as well as individual transient waveforms. 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 it 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 several times during the night. EEG under Anesthesia The EEG activity under general anesthesia depends on the type of anesthetic employed. With halogenated anesthetics and intravenous agents, a rapid (low beta or alpha), non-reactive EEG pattern is seen over most of the scalp, particularly the anterior scalp. This, in older terminology, is known as a WAR (widespread anterior rapid) pattern. In contrast, with high doses of opiates, a slow EEG pattern is seen over most of the scalp, which in older terminology was known as a WAIS (widespread slow) pattern. EEG during Epilepsy The EEG activity during epilepsy and certain other neuropsychiatric disorders includes individual transient waveforms such as sharp waves, spikes, spike-and-wave complexes in addition to the various types of rhythmic activity. Types of EEG Activity Ongoing EEG Activity This is the EEG activity that is not associated with a particular occurrence, such as a sensory stimulus, a cognitive event or a motor response. When measuring the activity related to a particular occurrence (such as ERPs or induced activity), the ongoing activity is usually considered to be noise. However it can be informative regarding the current mental state of the individual (e.g. wakefulness, alertness) and is often used in sleep research and in research and diagnostic tests for certain neurophysiological disorders, including epilepsy. Certain types of oscillatory activity, such as alpha waves, are part of the ongoing activity. Induced EEG Activity This is the EEG activity that is related to or 'induced by' a particular occurrence, such as a sensory stimulus, a cognitive event or a motor response. However this is different from event-related potentials (see ERP) which are the signals related to a particular occurrence that are obtained by averaging the portion of the ongoing EEG activity that is accurately time-locked to the particular occurrence, so that the signal components that are the same in each single measurement are conserved and all other signal components average out, eliminating ongoing brain activity. In contrast, induced activity refers to the signals that are related to a particular occurrence, but are different each time and would therefore cancel out during signal averaging. For example, a stimulus or cognitive event may induce strong oscillations in various frequency bands, due to the synchronization of ongoing activity, but this activity might have a different phase in each single measurement. Strengths of the EEG EEG has several strengths as a tool for exploring brain function. Foremost, the time resolution of EEG is very high, enabling brain activity to be tracked more accurately. While other methods for recording and assessing brain activity have a time resolution between seconds and minutes, the EEG has a resolution down to sub-milliseconds. Second, the brain is thought to work through its electric activity and EEG is the only method that enables its direct measurement. Other methods for exploring functions in the brain rely on blood flow or metabolism, which may be decoupled from the brain electric activity. Current research typically combines EEG or MEG with MRI or PET to obtain high temporal and spatial resolution. Limitations of the EEG EEG has several limitations. Firstly, scalp electrodes are not sensitive enough to pick out the individual action potential (the electric unit of signaling in the brain) of a single neuron, or pick out whether the resulting electrical activity is releasing inhibitory, excitatory or modulatory neurotransmitters. Instead, the EEG picks up the 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). However, some anatomical specificity can be gained with the use of EEG topography or BESA (Brain Elecctrical Source Analysis), which uses a large number of electrodes to triangulate the source of the electrical activity. ____________________________________________________________________________________________________ |
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| Dr Aditi Shankardass |
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| Consultant Neurophysiologist |
| © Aditi Shankardass |