electroencephalography electroencephalography electroencephalography (eeg) is a method to record an electrogram of the spontaneous electrical activity of the brain. the biosignals detected by eeg have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex and allocortex. it is typically non-invasive, with the eeg electrodes placed along the scalp (commonly called "scalp eeg") using the international 10-20 system, or variations of it. electrocorticography, involving surgical placement of electrodes, is sometimes called "intracranial eeg". clinical interpretation of eeg recordings is most often performed by visual inspection of the tracing or quantitative eeg analysis. electroencephalographyepileptic spike and wave discharges monitored eeg voltage fluctuations measured by the eeg bioamplifier and electrodes allow the evaluation of normal brain activity. as the electrical activity monitored by eeg originates in neurons in the underlying brain tissue, the recordings made by the electrodes on the surface of the scalp vary in accordance with their orientation and distance to the source of the activity. furthermore, the value recorded is distorted by intermediary tissues and bone which act in a manner akin to resistors and capacitors in an electrical circuit. this means not all neurons will contribute equally to an eeg signal with an eeg predominately reflecting the activity of cortical neurons near to the electrodes on the scalp. deep structures within the brain further away from the electrodes will not contribute directly to an eeg, these include the base of the cortical gyrus, mesial walls of the major lobes, hippocampus, thalamus and brain stem. a healthy human eeg will show certain patterns of activity which correlate with how awake a person is. the range of frequencies one observes are between 1 and 30 hz and amplitudes will vary between 20 and 100 μv. the observed frequencies are subdivided in various groups, these are alpha (8–13 hz), beta (13–30 hz), delta (0.5–4 hz) and theta (4–7 hz). alpha waves are observed when a person is in a state of relaxed wakefulness and are mostly prominent over parietal and occipital sites. during intense mental activity beta waves are more prominent in frontal areas as well as other regions. if a relaxed person is told to open their eyes one observes alpha activity decreasing and an increase in beta activity. theta and delta waves are not seen in wakefulness and if they are it is a sign of brain dysfunction. eeg can detect abnormal electrical discharges such as sharp waves, spikes or spike-and-wave complexes that are seen in people with epilepsy, thus it is often used to inform the medical diagnosis. eeg can detect the onset and spatio-temporal (location and time) evolution of seizures and the presence of status epilepticus. it is also used to help diagnose sleep disorders, depth of anesthesia, coma, encephalopathies, cerebral hypoxia after cardiac arrest, and brain death. eeg used to be a first-line method of diagnosis for tumors, stroke and other focal brain disorders, but this use has decreased with the advent of high-resolution anatomical imaging techniques such as magnetic resonance imaging (mri) and computed tomography (ct). despite limited spatial resolution, eeg continues to be a valuable tool for research and diagnosis. it is one of the few mobile techniques available and offers millisecond-range temporal resolution which is not possible with ct, pet or mri. derivatives of the eeg technique include evoked potentials (ep), which involves averaging the eeg activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory). event-related potentials (erps) refer to averaged eeg responses that are time-locked to more complex processing of stimuli; this technique is used in cognitive science, cognitive psychology, and psychophysiological research. history the first human eeg recording obtained by hans berger in 1924. the upper tracing is eeg, and the lower is a 10 hz timing signal. hans berger in 1875, richard caton (1842–1926), a physician practicing in liverpool, presented his findings about electrical phenomena of the exposed cerebral hemispheres of rabbits and monkeys in the british medical journal. in 1890, polish physiologist adolf beck published an investigation of spontaneous electrical activity of the brain of rabbits and dogs that included rhythmic oscillations altered by light. beck started experiments on the electrical brain activity of animals. beck placed electrodes directly on the surface of the brain to test for sensory stimulation. his observation of fluctuating brain activity led to the conclusion of brain waves. in 1912, ukrainian physiologist vladimir vladimirovich pravdich-neminsky published the first animal eeg and the evoked potential of the mammalian (dog). in 1914, napoleon cybulski and jelenska-macieszyna photographed eeg recordings of experimentally induced seizures. german physiologist and psychiatrist hans berger (1873–1941) recorded the first human eeg in 1924. expanding on work previously conducted on animals by richard caton and others, berger also invented the electroencephalograph (giving the device its name), an invention described "as one of the most surprising, remarkable, and momentous developments in the history of clinical neurology". his discoveries were first confirmed by british scientists edgar douglas adrian and b. h. c. matthews in 1934 and developed by them. in 1934, fisher and lowenbach first demonstrated epileptiform spikes. in 1935, gibbs, davis and lennox described interictal spike waves and the three cycles/s pattern of clinical absence seizures, which began the field of clinical electroencephalography. subsequently, in 1936 gibbs and jasper reported the interictal spike as the focal signature of epilepsy. 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 that incorporated a piezoelectric inkwriter called a crystograph (the whole device was typically known as the offner dynograph). in 1947, the american eeg society was founded and the first international eeg congress was held. in 1953 aserinsky and kleitman described rem sleep. in the 1950s, 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. this enjoyed a brief period of popularity in the 1980s and seemed especially promising for psychiatry. it was never accepted by neurologists and remains primarily a research tool. chuck kayser with electroencephalograph electrodes and a signal conditioner for use in project gemini, 1965 an electroencephalograph system manufactured by beckman instruments was used on at least one of the project gemini manned spaceflights (1965-1966) to monitor the brain waves of astronauts on the flight. it was one of many beckman instruments specialized for and used by nasa. the first instance of the use of eeg to control a physical object, a robot, was in 1988. the robot would follow a line or stop depending on the alpha activity of the subject. if the subject relaxed and closed their eyes therefore increasing alpha activity, the bot would move. opening their eyes thus decreasing alpha activity would cause the robot to stop on the trajectory. in october 2018, scientists connected the brains of three people to experiment with the process of thoughts sharing. five groups of three people participated in the experiment using eeg. the success rate of the experiment was 81%. clinical utility epilepsy an eeg recording setup using the 10-10 system of electrode placement. eeg is the gold standard diagnostic procedure to confirm epilepsy. the sensitivity of a routine eeg to detect interictal epileptiform discharges at epilepsy centers has been reported to be in the range of 29-55%. given the low to moderate sensitivity, a routine eeg (typically with a duration of 20–30 minutes) can be normal in people that have epilepsy. when an eeg shows interictal epileptiform discharges (e.g. sharp waves, spikes, spike-and-wave, etc.) it is confirmatory of epilepsy in nearly all cases (high specificity), however up to 3.5% of the general population may have epileptiform abnormalities in an eeg without ever having had a seizure (low false positive rate) or with a very low risk of developing epilepsy in the future. when a routine eeg is normal and there is a high suspicion or need to confirm epilepsy, it may be repeated or performed with a longer duration in the epilepsy monitoring unit (emu) or at home with an ambulatory eeg. in addition, there are activating maneuvers such as photic stimulation, hyperventilation and sleep deprivation that can increase the diagnostic yield of the eeg. other brain disorders an eeg might also be helpful for diagnosing or treating the following disorders: brain tumor brain damage from head injury brain dysfunction that can have a variety of causes (encephalopathy) inflammation of the brain (encephalitis) stroke sleep disorders it can also: distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders and migraine variants differentiate "organic" encephalopathy or delirium from primary psychiatric syndromes such as catatonia serve as an adjunct test of brain death in comatose patients prognosticate in comatose patients (in certain instances) or in newborns with brain injury from various causes around the time of birth determine whether to wean anti-epileptic medications. clinical setting epilepsy monitoring unit (emu) at times, a routine eeg is not sufficient to establish the diagnosis or to determine the best course of action in terms of treatment. in this case, attempts may be made to record an eeg while a seizure is occurring. this is known as an ictal recording, as opposed to an interictal recording which refers to the eeg recording between seizures. to obtain an ictal recording, a prolonged eeg is typically performed accompanied by a time-synchronized video and audio recording. this can be done either as an outpatient (at home) or during a hospital admission, preferably to an epilepsy monitoring unit (emu) with nurses and other personnel trained in the care of patients with seizures. outpatient ambulatory video eegs typically last one to three days. an admission to an epilepsy monitoring unit typically lasts several days but may last for a week or longer. while in the hospital, seizure medications are usually withdrawn to increase the odds that a seizure will occur during admission. for reasons of safety, medications are not withdrawn during an eeg outside of the hospital. ambulatory video eegs, therefore, have the advantage of convenience and are less expensive than a hospital admission, but the disadvantage of a decreased probability of recording a clinical event. epilepsy monitoring is often considered when patients continue having events despite being on antiseizure medications or if there is concern that the patient's events have an alternate diagnosis, e.g.: psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders, migraine variants, stroke etc. in cases of epileptic seizures, continuous eeg monitoring helps to characterize seizures and localize/lateralize the region of brain from which a seizure originates. this can help identify appropriate non-medication treatment options. in clinical use, eeg traces are visually analyzed by neurologists to look at various features. increasingly, quantitative analysis of eeg is being used in conjunction with visual analysis. quantitative analysis displays like power spectrum analysis, alpha-delta ratio, amplitude integrated eeg, spike detection can help quickly identify segments of eeg that need close visual analysis or in some cases be used as surrogates for quick identification of seizures in long term recordings. eeg may be used to monitor the depth of anesthesia, as an indirect indicator of cerebral perfusion in carotid endarterectomy, or to monitor amobarbital effect during the wada test. eeg can also be used to predict the depth of anesthesia based on the dynamics of the alpha-band. intensive care unit (icu) eeg can also be used in intensive care units for brain function monitoring to monitor for non-convulsive seizures/non-convulsive status epilepticus, to monitor the effect of sedative/anesthesia in patients in medically induced coma (for treatment of refractory seizures or increased intracranial pressure), and to monitor for secondary brain damage in conditions such as subarachnoid hemorrhage (currently a research method). in cases where significant brain injury is suspected, e.g. after cardiac arrest, eeg can provide some prognostic information. if a patient with epilepsy is being considered for resective surgery, it is often necessary to localize the focus (source) of the epileptic brain activity with a resolution greater than what is provided by scalp eeg. in these cases, neurosurgeons typically implant strips and grids of electrodes or penetrating depth electrodes under the dura mater, through either a craniotomy or a burr hole. the recording of these signals is referred to as electrocorticography (ecog), subdural eeg (sdeeg), intracranial eeg (iceeg), or stereotactic eeg