An instrument with a variable number of channels designed to record electrical activity (in microvolts) generated in the upper layer of the cortex from electrodes attached to the scalp. In comparison with fMRI, EEG allows more direct and real time measurements of electrical signals generated by brain tissue. These signals are mainly derived from intra- and extra-cellular currents, or field potentials, elicited by the activation of excitatory and inhibitory synapses. Spatial and temporal summation is necessary, however, to generate signals strong enough to be detected from outside the skull. In the past, electrodes were glued to the head with collodion and electrode jelly injected through a small hole in the center of an electrode. Nowadays, such arduous preparation has been largely replaced by the use of multiple scalp electrodes (32, 64 or 128) embedded in electrode caps. The signal from the electrodes is measured either with a bipolar arrangement or relative to a reference electrode on the earlobes or mastoids. EEG with humans was first demonstrated by the psychiatrist Hans Berger (1873-1941) in 1924. However, the first person to record electrical activity from the brain was Richard Caton (1842-1926) in 1875. He did so by placing unipolar electrodes on the surface of both hemispheres, or one electrode on the cerebral cortex (or on the exposed gray matter of animals), and the other on the surface of the skull, and recording currents by means of optical (reflecting) magnification of a galvanometer invented by Lord Kelvin (1824-1907).
See Brain (neuro-) imaging, EEG stage 1 sleep, EEG stage 2 sleep, EEG 3 stage sleep, EEG 4 stage sleep, Event-related (brain) potentials (ERPs), Evoked activity (or responses), Functional magnetic imaging (fMRI), Human Connectome Project (HCP), Magnetoencephalograpy (MEG), Mutual gaze, NREM sleep, Optical imaging, REM sleep, Wakefulness