It is well known that the sensory cortex responds to peripheral input, and that sensory stimuli evoke a cortical response. In fact, in cases of myoclonic epilepsy, the response to a peripheral stimulus can be detected using standard electroencephalogram (EEG ) recording. In normal subjects, however, the responses are of much lower amplitude. The normal EEG activity and the normal “noise” in the recording devices are of high enough voltage to mask any evoked response. With the onset of averaging technology, these small potentials became detectable. The procedure is to time-lock a peripheral stimulus to another which triggers the sweep of a computer of average transients. Any evoked potential that recurs with a fixed relationship to this peripheral stimulus is summed by the computer. All non-related potentials, or random waves, are progressively diminished and eventually cancel out. Thus, with adequate averaging, smaller and smaller evoked potentials can be extracted from the background activity.
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