Compound Action Potentials Recorded from the Intracranial Portion of the Auditory Nerve in Man: Effects of Stimulus Intensity and Polarity

Abstract

Compound action potentials (CAP) were recorded from the intracranial portion of the eighth nerve in patients with normal hearing who were undergoing neurosurgical operations for cranial nerve disorders (trigeminal neuralgia and hemifacial spasm). Brainstem auditory-evoked potentials were recorded intraoperatively to ensure that no noticeable changes occurred in conduction in the auditory nerve as a result of surgical dissections. The CAP recorded from the middle portion of the exposed intracranial portion of the eighth nerve in response to clicks of high intensity (100-110 dB peak equivalent SPL, or pe SPL) had a triphasic shape, as is commonly seen in monopolar recordings from long nerves. A second negative peak (N²) could be identified in some patients. There was little difference in the waveform of the CAP in response to condensation and rarefaction clicks, and in some patients the waveform of the CAP remained the same over a range of stimulus intensities (from 105 to 75 dB pe SPL), whereas in others the negative peak of the CAP became much broader in response to stimuli with intensities of less than 85 dB. In some patients the N² peak became dominant as the stimulus intensity was decreased. At low stimulus intensities, the response consisted of a single, broad negativity. The latency-intensity curves for the N¹ peak had different slopes in different patients. In those individuals in whom there was a noticeable difference between the latency of the N¹ peak in response to clicks of opposite polarity, the latency-intensity curves of the responses to rarefaction clicks were steeper than those of the responses to condensation clicks, and the latency of the N¹ peak to condensation clicks became shorter than that to rarefaction clicks at intensities below 85–90 dB pe SPL. The latency-intensity curves for the N² peak were usually less steep than those of the N¹ peak, but in some patients the curves for these two peaks had similar slopes. The amplitude of the N¹ peaks showed a steep increase in click intensities at 95 and 105 dB, and a much less steep course for intensities below 95 dB. The amplitudes of the N² peak reached a plateau in the range 95–105 dB, and decreased more rapidly than the N¹ peak below 95 dB.

Key Words:

• Auditory nerve
• Evoked potentials
• Intraoperative recordings