Responses from the brainstem at the entrance of the eighth nerve in human to contralateral stimulation.

Recordings were made from the exposed eighth nerve (N VIII) and the lateral brainstem near the root entry zone of N VIII to contralateral click stimulation in patients undergoing microvascular decompression operations to relieve hemifacial spasm or trigeminal neuralgia. Similar recordings were made in patients undergoing operations to remove acoustic tumors using a retromastoid approach to the cerebellopontine angle. The waveform of the response that was recorded using a monopolar electrode placed on the intracranial portion of the eighth nerve was similar to the potentials recorded from the lateral surface of the brainstem near the entrance of the eighth nerve, and consisted of a positive deflection with a latency of about 4 ms, and sometimes a second and smaller positive peak with a slightly shorter latency than that of peak V in the BAEP could be seen in such recordings. It was concluded that the potentials are generated in brainstem structures, most likely the cochlear nucleus. It is assumed that when the potentials are recorded from the exposed eighth nerve, the nerve passively conducts the activity from the active structure to the recording site. The latency of the main positive peak in these potentials does not bear any direct relationship to any identifiable component of the farfield evoked potentials when these potentials are recorded from the vertex or the earlobe of the stimulated ear and a noncephalic reference is used. It seems likely that the potential is generated by fibers that originate in cells in the cochlear nucleus on the stimulated side, and that the initial positivity reflects the termination of these fibers in the cochlear nucleus on the side from which the recording is made.