Responses from the exposed human auditory nerve to pseudorandom noise

Whole-nerve responses to lowpass-filtered noise (2.5 kHz) and broadband click stimuli were recorded from the exposed intracranial portion of the eighth nerve in patients with normal hearing who were undergoing neurosurgical operations to relieve vascular compression of cranial nerves V, VII, and VIII. Cross-correlograms between the response and the noise showed a large degree of individual variation. When noise of 95 dB SPL was used, the correlograms in some patients had a large peak that represented a positive correlation between a negative nerve potential and the rarefaction phase of the sound and that peak could usually be identified at a delay that was close to the latency of the main negative peak in the response to high-intensity broadband click sounds (3.0-3.5 ms). The amplitude of these components decreased when the sound intensity was decreased, and, at stimulus intensities below 80 dB, the correlograms became dominated by components at longer delays. In other patients, peaks of a similar amplitude appeared in the cross-correlograms between delays of 2 and 12 ms in the entire range of sound intensities that were studied (65-105 dB SPL). In all patients the location of the peaks was little affected by the stimulus intensity in the range studied. All reproducible peaks that appeared at delays longer than 2.8 ms shifted towards longer delays when the recording electrode was moved from a location near the porus acusticus to a more central location (near the brainstem) on the exposed intracranial portion of the eighth nerve, indicating that components at longer delays than 2.8 ms result from propagated neural activity in the auditory nerve. It is assumed on the basis of the results that these correlograms are measures of phase-locking of neural activity to a complex stimulus sound (noise).