<unk>  <unk>  is a method to record an electrogram of the electrical activity on the scalp that has been shown to represent the macroscopic activity of the surface layer of the brain underneath. It is typically non-invasive, with the electrodes placed along the scalp. Electrocorticography, involving invasive electrodes, is sometimes called "intracranial<unk> ".<unk>  measures voltage fluctuations resulting from ionic current within the neurons of the brain. Clinically,<unk>  refers to the recording of the brain's spontaneous electrical activity over a period of time, as recorded from multiple electrodes placed on the scalp. Diagnostic applications generally focus either on event-related potentials or on the spectral content of<unk> . The former investigates potential fluctuations time locked to an event, such as 'stimulus onset' or 'button press'. The latter analyses the type of neural oscillations (popularly called "brain waves") that can be observed in<unk>  signals in the frequency domain.<unk>  is most often used to diagnose epilepsy, which causes abnormalities in<unk>  readings. It is also used to diagnose sleep disorders, depth of anesthesia, coma, encephalopathies, and brain death.<unk>  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,<unk>  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<unk>  technique include evoked potentials (EP), which involves averaging the<unk>  activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory). Event-related potentials (ERPs) refer to averaged<unk>  responses that are time-locked to more complex processing of stimuli; this technique is used in cognitive science, cognitive psychology, and psychophysiological research.
Electroencephalography
Quantitative electroencephalography
Magnetoencephalography
Stereoelectroencephalography
Echoencephalography
Amplitude integrated electroencephalography
Polysomnography
Hemoencephalography