Electroencephalogram (EEG) is a recording of the continuous electrical activity of the brain made by electrodes positioned on the scalp. It has many applications for clinical practice and both basic and applied research. The science of recording, analyzing, and interpreting EEG is part of a larger science called psychophysiology, which has its roots in both medicine and psychology. EEG is an important tool in cognitive neuroscience, the field of study that seeks to link human cognition and behavior with specific brain structures and processes. EEG has been used in the diagnosis of epilepsy and other neurological disorders and can also be used as a marker for the presence of numerous developmental abnormalities including, but not limited to, sensory and motor disorders. More recently, EEG has been used to study a variety of biologically based psychological disorders including depression, anxiety, and attention-deficit hyperactivity disorder (ADHD). EEG has also been used experimentally to learn about the cortical mechanisms involved in arousal, vigilance, mood regulation, and even higher cognitive functions such as language and mathematics. With the recent inventions of the related technologies of event-related potentials (ERPs), high-density electrode arrays, and computer-assisted topographic analysis (brain mapping), EEG promises to be a major tool in the neurosciences for years to come.
The biological source of an EEG recording is the postsynaptic membrane potentials of millions of pyramidal neurons that help to compose the human brain, or neocortex. The cells are organized into functional groups called microcolumns, which act as one unit when processing information. Because groups of neurons fire together, their tiny voltages summate and produce enough electrical activity to pass through the resistive mediums of brain tissue, skull, and scalp. Pyramidal cells, named for their triangular shape, are aligned so that their bodies are perpendicular to the surface of the scalp. This orientation means that for cells on the gyri (bumps) and some in the sulci (valleys) of the cortical surface their electrical fields project out to the scalp where they can be recorded.
The basic science underlying the EEG is that whenever an electrical current is passed through a circuit, its amplitude (measured on the y-axis of a graph) can be measured continuously at any point in time (measured in the x-axis) by a device generally known as a galvanometer. Early recordings were made with a few metallic electrodes that were filled with conductive paste and attached to the scalp, with the resultant deviations in voltage plotted with an attached ink pen
on a continuously scrolling drum or sheet of paper. Modern EEG recording is accomplished with multiple electrodes (as many as 256 at once), often positioned with the aid of an elastic cap, and the data are collected and analyzed entirely by microcomputer.
Hans Berger invented the method of human EEG in the 1920s, based upon previous work in animals by Richard Caton in the 1870s. These individuals demonstrated that the brain, like any living system, generates electrical potentials with regular patterns. The dominant patterns are labeled according to frequency and are called delta (.5–4 Hertz [Hz]), theta (4−7 Hz), alpha (8−13 Hz), and beta (13−30 Hz). Berger showed that these patterns are sensitive to both external cues and the internal states of the individual, such as level of arousal. For instance, alpha is present in conditions of relaxed wakefulness and can be suppressed by concentration on a difficult cognitive task. Delta and theta are hallmarks for the deeper stages of sleep, and abnormalities in the frequencies have been demonstrated in children with attention problems, depressed patients, and a variety of other disorders.
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