A 40-Hz auditory potential recorded from the human scalp (hearing tests/auditory evoked potentials/40-Hz brain waves/sensory processing)

Computer techniques readily extract from the brainwaves an orderly sequence of brain potentials locked in time to sound stimuli. The potentials that appear 8 to 80 msec after the stimulus resemble 3 or 4 cycles of a 40-Hz sine wave; we show here that these waves combine to form a single, stable, composite wave when the sounds are repeated at rates around 40 per sec. This phenomenon, the 40-Hz event-related potential (ERP), displays several properties of theoretical and practical interest. First, it reportedly disappears with surgical anesthesia, and it resembles similar phenomena in the visual and olfactory system, facts which suggest that adequate processing of sensory information may re- quire cyclical brain events in the 30- to 50-Hz range. Second, la- tency and amplitude measurements on the 40-Hz ERP indicate it may contain useful information on the number and basilar mem- brane location of the auditory nerve fibers a given tone excites. Third, the response is present at sound intensities very close to normal adult thresholds for the audiometric frequencies, a fact that could have application in clinical hearing testing. When a person cannot, or will not, tell the interested observer what he hears, it has in the past been difficult to obtain trust- worthy measurements of hearing. The recent advent of elec- trophysiological tests of hearing and comprehension has radi- cally changed this situation (1, 2). In these tests one records the listener's brainwaves while sounds are presented via loud- speaker or earphones; the sounds generate electric responses within the brain that are readily extracted by a computer (Fig. lA). The resulting sequence of brain potentials, known as event- related potentials (ERPs), begins within a millisecond or two of stimulus delivery and continues thereafter for a half second or more. We deal here with the series ofwaves that appears 8-80 msec after stimulus delivery, the so-called middle-latency re- sponse (MLR). We report a way to extract it from the brain- waves, describe several properties of the response so obtained, and, among other things, show it to predict adult auditory thresholds in a highly reliable manner.