Definition and history
The electroencephalogram (EEG) is the measurement of the electrical activity emanating from the surface of the brain clinically known as the cortex. It was the earliest ‘functional imaging’ technique and remains the technique of choice for many clinical and research applications where precision time resolution is required. It remains the ‘gold standard’ in terms of demonstrating and localising the hallmark electrical disturbances evident in the cortices of epilepsy sufferers.
The first observations of the electrical activity of the brain were by Caton who reported that he had detected currents from electrodes placed on the skull of exposed brain in rabbits and monkeys (Caton 1875). A half-century later Hans Berger recorded the first human electroencephalogram from electrodes placed on the scalp.
Over the next ten year period he recorded EEG from a wide variety of patients and published his findings between 1929-38 in the Archiv fur Psychiatrie und Nervenkrankheiten . These were translated into English by Gloor in 1969 (Gloor 1969). He later described α- and β- activity in the physiological and pathological state and recorded absence attacks with regular rhythmic high-voltage slow waves at a rate of 3 per second. However, he failed to demonstrate or report the associated spike which he had erroneously considered an artefact.
His first recordings were marred by artefacts and his earliest publications were received with scepticism. Some five years following his first publication, Adrian and Mathews in the UK (as well as others) confirmed his discovery of the ‘alpha rhythm’ by demonstrating their findings to the Physiological Society thus ensuring that his original findings were validated.
Many researchers began to apply the technique to the investigation of clinical disorders, and Berger’s original observation that epileptic seizures were accompanied by major electrical disturbances in the brain was confirmed (Berger 1930)(Tudor, Tudor et al. 2005). Walter was the first to demonstrate the link between slow focal waves in the EEG and cerebral tumour (Walter 1936).
Grass and Gibbs were the first to apply spectral techniques to EEG data, and Grey Walter went on to develop the first practical Frequency Analyzer (Grass A.M 1938; Walter 1943). By using photographic superimposition Dawson delineated the first evoked potentials and subsequently designed the first ‘averager’ in 1951 (Dawson 1951). Beck, working in Krakow, Poland, explored the electrical activity of the brain in much greater detail and made significant contributions which informed modern techniques of source localisation of sensory functions in the brain as well as to overall knowledge of the EEG (Brazier 1961). Earlier, Cybulski, (Beck’s teacher and friend) made his unique contributions to the knowledge base by publishing recordings of the spontaneous electrical activity of the cortex of dog and monkey with subsequent demonstration of the induction of an epileptic seizure in the dog. His contributions to science were described by Pawlik and Konturek in their 2006 review of his work (Pawlik, Konturek et al. 2006). Russian scientists were also active in the investigation of EEG. Kauffman in St Petersburg was the first to demonstrate that an epileptic attack was accompanied by abnormal cortical discharges. Neminsky was the first to publish observations on the frequency oscillations in the EEG and reported oscillations from the brain of the dog at 12-14 per second, and those from the surface of the brain and dura at 12-20 per second rising to 35 per second. He also reported that in asphyxia these oscillations slowed to 4-7 per second. (Brazier 1961). Neminsky published his first photographs of the encephalograph in 1912, unbeknown to Cybulski who published his in 1914.
The development of all of these techniques has continued to this day and the advent of digital technology has promoted ever more sophisticated analyses of the EEG. An emerging field is the use of EEG in the fMRI to provide optimal spatial and temporal resolution. Systems have been designed specifically for this purpose and are currently being largely applied in the areas of psychophysiological research.