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 require cyclical brain events in the 30-to 50-Hz range. Second, latency and amplitude measurements on the 40-Hz ERP indicate it may contain useful information on the number and basilar membrane 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 trustworthy measurements of hearing. The recent advent of electrophysiological tests of hearing and comprehension has radically changed this situation (1, 2). In these tests one records the listener's brainwaves while sounds are presented via loudspeaker or earphones; the sounds generate electric responses within the brain that are readily extracted by a computer (Fig. 1A). The resulting sequence ofbrain potentials, known as eventrelated 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 response (MLR). We report a way to extract it from the brainwaves, describe several properties of the response so obtained, and, among other things, show it to predict adult auditory thresholds in a highly reliable manner. METHODSAny group of instruments that competently generates acoustic signals, amplifies brain waves, and averages ERPs (Fig. 1A) can be used to demonstrate the phenomenon to be described here. The response was, in fact, discovered by one ofus (P.J.T.), using a stimulator, amplifier, and computer built in the laboratory (3). Since then we have used four different types of signal generators, several types of earphones (including a hearing aid transducer) to produce the monaural clicks or tone bursts (usually 6-msec duration, 2-msec rise and fall) employed in this study, and four different commercial brands of averaging computers. The brain waves, usuallv recorded between electrodes at forehead and earlobe of the ear stimulated, were amplified at a gain as near to 105 and with bandpass as near to 10-100 Hz as the settings of the particular amplifier in use allowed.Subjects were 20 adu...