EEG Glossary The first attempt to systematically propose a syllabus for Activation procedure Any procedure designed to modu- electroencephalographers was made by O’Leary and Knott late EEG activity, for instance to enhance physiologi- who in 1955 published in the EEG Journal “Some Minimal cal waveforms or elicit abnormal paroxysmal activity. Essentials for Clinical Electroencephalographers” [1]. In the Examples include eye closing, hyperventilation, photic following decades, with the EEG being increasingly used in stimulation, natural or drug-induced sleep, sensory stimu- the experimental and clinical field, need to adopt a language lation (acoustic, somatosensory, or pain). as common as possible between various laboratories world- Activity, EEG An EEG wave or sequence of waves of wide became even more pressing. In fact, the multiplicity of cerebral origin. terms generated (and sometimes still generates) confusion Alpha band Frequency band of 8–13 Hz inclusive. Greek and misinterpretations, promoting misdiagnosis and making letter: α. it difficult to compare data between different laboratories. Alpha rhythm Rhythm at 8–13 Hz inclusive occurring To overcome this risk, in 1974 “A Glossary of Terms,” most during wakefulness over the posterior regions of the head, commonly used by “Clinical Electroencephalographers,” was generally with maximum amplitudes over the occipital published in the EEG Journal; this glossary was the result of areas. Amplitude varies but is mostly below 50 μV in the the work of a group of experts from the International Federation adult, but often much higher in children. Best seen with of Clinical Neurophysiology (IFCN) led by Chatrian [2]. the eyes closed, during physical relaxation and relative Thanks to this document, it was for the first time officially mental inactivity. Blocked or attenuated by attention, established which term had to be used to correctly describe a especially visual, and mental effort. specific EEG element, avoiding the proliferation of different Alpha variant rhythms An EEG rhythm recorded most terms to indicate the same pattern. Furthermore, the use of prominently over the posterior regions of the head that terms deemed inappropriate was strongly discouraged. differs in frequency, but resembles in reactivity, the alpha Similarly, the American Electroencephalographic Society rhythm. (later to become the American Clinical Neurophysiology Alpha wave Wave with duration of 1/8–1/13 s Society) wrote its own glossary, published as first edition in (77–125 ms). 1979, subsequently revised in 1984, 1994 [3], 2006 [4], and Amplitude, EEG Is a measure of the change of EEG sig- 2016 [5]. nals with respect to the mean value, usually measured in The IFCN International Glossary was also revised and microvolts (μV), and often expressed as the difference republished in 1999 by Noachtar et al. [6], basing entirely on between the maximum and minimum deviation (i.e., the proposal made by Chatrian et al. [2], but updating the peak-­to-­peak), or in rectified EEG from baseline to peak. terminology according to changes due to technological evo- For a variable EEG activity or modulating sinusoidal lution. The last, updated and revised version of the IFCN rhythm a range can be provided. glossary has been published in 2017, including a proposal Amplitude-integrated EEG (aEEG) Involves custom- for the report format of the EEG findings [7]. The most rel- ized display of EEG activity following signal processing evant entries of the “revised glossary of terms most com- which includes an asymmetric band-pass filter (2–15 Hz), monly used by clinical electroencephalographers” by the logarithmic amplitude display, rectification, smoothing, IFCN, so far the single most authoritative, comprehensive, and time compression (such that several hours can be and updated EEG glossary available in the literature, are viewed on a screen). Widely used in neonatal intensive reported in the next pages (portions of the text reproduced care unit monitoring, for example, of infants suffering a with permission from [7]). hypoxic ischemic encephalopathy. © Springer Nature Switzerland AG 2019 805 O. Mecarelli (ed.), Clinical Electroencephalography, https://doi.org/10.1007/978-3-030-04573-9 806 EEG Glossary Analog-to-digital conversion (AD conversion) either normal or abnormal, appears and from which such Transformation of a continuous, analog signal EEG into underlying pattern is distinguished. its digital representation (a discontinuous series of dis- Background slowing The frequency of the background crete amplitudes) rhythm which is below the normal value for age and state. Anterior (slow) dysrhythmia A normal EEG activity seen Band Range of EEG frequency in a spectrum for a given in term and near term infants at 32–44 weeks of post men- recording or epoch, i.e., delta, theta, alpha, beta, gamma strual age; characterized by bilateral frontal delta waves bands and high frequency oscillations. (50–100 μV), seen in isolation or brief runs, typically syn- Basal electrode Any electrode located in proximity to the chronously and symmetrically. base of the skull. Application, electrode The process of establishing fixa- Benign epileptiform transient of sleep (BETS) Use of tion and electrical connection between an electrode and term discouraged. A normal variant. Small sharp spikes the subject’s scalp or brain. of very short duration (<50 ms) and low amplitude Arrhythmic activity A sequence of EEG waves with an (<50 μV), often followed by a small theta wave, occurring inconstant periodicity. in the temporal regions during drowsiness and light sleep. Arousal Change from a lower to a higher level of alertness Beta band Frequency band of 14–30 Hz inclusive. Greek as manifest in EEG activity. letter: β. Array, electrode A regular arrangement of electrodes over Beta rhythm or activity Any EEG rhythm between 14 and the scalp or brain, or within the brain substance. 30 Hz (wave duration 33–72 ms). Most characteristically Artifact (1) A physiological potential difference due to recorded over the fronto-central regions of the head dur- an extracerebral source present in EEG recordings, such ing wakefulness. Amplitude of fronto-central beta rhythm as eye blinks and movements, electrocardiogram (ECG) varies but is mostly below 30 μV. Blocking or attenuation or muscle contractions (EMG). (2) A modification of the of the beta rhythm by contralateral movement or tactile EEG caused by extracerebral factors, such as instrumen- stimulation is especially obvious in electrocorticograms. tal distortion or malfunction, movement of the patient, or Other beta rhythms are most prominent in other locations ambient electrical noise. or are diffuse, and may be drug-induced (for example, Asymmetry Unequal amplitude, frequency or morphology alcohol, barbiturates, benzodiazepines, and intravenous of EEG activity in channels over homologous areas of the anesthetic agents). two hemispheres. To be considered abnormal in practice Bilateral Involving both sides of the head (or body). if the amplitude difference exceeds 50% or frequency dif- Bilateral independent periodic discharges (BIPDs) ference is equal to or greater than 1 Hz in the posterior BIPDs are two bilateral independent (i.e., asynchronous) dominant rhythm, although it should be recognized that surface-­negative bi- or di-, tri- or polyphasic complexes these are essentially arbitrary values. It can be quantified consisting of spike, sharp, or polyspike components, by quantitative EEG (qEEG) measures, such as the Brain with variably following slow waves, lasting 60–600 ms Symmetry Index (BSI). (typically 200 ms) that occupy at least 50% of a stan- Asynchrony The noncoherent occurrence of EEG dard 20 min EEG. Amplitude ranges from 50 to 150 μV activities over regions on the same or opposite sides (occasionally up to 300 μV), which may be asymmetric, (hemispheres) of the head. For example, two similar usually recurring at 0.5–2 c/s (but very occasionally with waveforms occurring at separate electrodes or channels, intervals of up to 10 s). BIPDs are broadly distributed and but not simultaneously due to a time lag between the waveform morphology stays fairly constant for a given channels. patient and EEG, with intervening background activity Attenuation Reduction in amplitude of EEG activity (for usually attenuated and slow. Most BIPDs are ephemeral example, the alpha rhythm is usually attenuated or blocked phenomena that usually resolve within weeks. They occur on eye opening). May occur transiently in response to a with acute marked focal destructive lesions (for example, physiological or other stimulus, such as electrical stimula- cerebral infarcts, tumors, or herpes simplex encephalitis) tion of the brain, or more permanently as a result of patho- or more subacute/chronic pathologies (for example, epi- logical conditions, such as cerebral atrophy or ischemia. lepsy and vascular compromise). Augmentation Increase in amplitude of EEG activity (for Biphasic wave Complex consisting of two wave compo- example, the alpha rhythm is characteristically augmented nents developed on alternate sides of the baseline. upon closing the eyes). Bipolar derivation (1) Recording from a pair of explor- Background activity Any underlying EEG activity rep- ing electrodes. (2) Method of organizing the linkages of resenting the setting in which focal or transient activity, electrodes to recording channels. EEG Glossary 807 Bipolar montage Multiple bipolar channels, with no elec- Complex A sequence of two or more waveforms having