Visual and Brainstem Auditory Evoked Potentials in Neurology

Historically, Richard Caton (1875) discovered evoked potentials and the electroencephalogram (EEG) at the same time. The evoked potentials provided a useful tool for neurophysiological research (Shagass, 1976). Evoked potentials are the voltage changes generated in the brain, the sense organ and the pathway leading to the brain in response to an external stimulus. This stimulus has to be somewhat above its subjective detection threshold so as to be clearly discernible, as well as to be of abrupt onset and /or offset so, that discrete volley is set up in the afferent pathway, hence capable of eliciting distinct cortical potential changes (Kriss, 1980). Chiappa et al., 1987 defined an evoked potential as the record of the electrical activity produced by groups of neurons within the spinal cord, brain stem, thalamus or cerebral hemispheres following stimulation of one or another specific system by means of visual, auditory, or somatosensory input. EEG recording may be obtained in a relatively direct manner by amplifying and displaying the activity picked up by electrodes placed on the scalp (Shagass, 1976), but most evoked potential (EP) activity due to their low amplitude, 0.1-20 microvolts relative to the normal background activity of EEG, their activity can not be clearly displayed, however with the development of micro-computers in the seventies (special signal averaging process) helping the extraction of the evoked potential from the EEG activity which is of much greater amplitude leading to their widespread clinical use (Kimura, 1985).