Auditory-evoked far fields averaged from the scalp of humans.

AVERAGING, when used to extract extremely small signals from relatively high levels of background noise, allows detection of evoked potentials at surprisingly large distances from the site of neuronal activity. In animals, volume-conducted potentials from brain-stem auditory structures can be recorded throughout the brain (Grinnell, 1963; Bullock et ah, 1968; Jewett, 1970) and even from the scalp (Jewett, 1970). In humans, auditory-evoked potentials can be detected at the vertex due to neural activity from subcortical structures (Jewett, Romano and Williston, 1970) as well as from primary cortical areas (Vaughan and Ritter, 1970). Under the conditions of skull and scalp boundaries and intracranial inhomogeneities, recordings from a single electrode position offer little indication as to the location of a wave's neural generator(s), whereas multiple recording sites may be more informative.