Neural generators of the brain-stem auditory evoked potentials (BAEPs) in the rhesus monkey

Brain-stem auditory evoked potentials (BAEPs) were recorded from rhesus monkeys in response to click stimuli. The BAEPs in the monkey have only 4 major peaks, while 5 major peaks can be identified in the human BAEP. The responses from electrodes placed at the vertex and the mastoid recorded either differentially or separately using non-cephalic references were compared to concomitant recordings from the auditory nerve, the cochlear nucleus, and the inferior colliculus, including the lateral lemniscus and the brachium of the inferior colliculus. It was found that the response from the intracranial portion of the auditory nerve lags behind the far-field response recorded from the mastoid by about 0.2 msec. The major component of the potentials recorded from the lateral lemniscus appeared with a latency that was close to that of the fourth peak in the BAEP. Deafferentation of the inferior colliculus resulted in only small changes in the BAEP, indicating that the contributions to the BAEP from the inferior colliculus and more central structures of the ascending auditory pathway are insignificant. It is concluded that the main reason for the difference between the human BAEP and the BAEP in the rhesus monkey is the smaller size of the monkey's head and the resulting shorter auditory nerve. The result is that the auditory nerve in the monkey only generates one peak in the far-field response, while in man the auditory nerve gives rise to both of the two earliest peaks in the BAEP; thus, the BAEP in man has 5 peaks while that in the monkey has only 4 peaks.