Injuries to the auditory nerve: a study in monkeys.

The effect of traction injury to the eighth nerve on the compound action potentials (CAPs) recorded from the exposed nerve in response to tonebursts and click sounds was compared to responses elicited by continuous tones and noise that were amplitude-modulated with pseudorandom noise. Responses to the continuous sounds were presented in the form of cross-correlograms between the averaged response, with the averager locked to the periodicity of the noise and one period of the noise. While the wave form of these cross-correlograms was similar to the wave form of the response to the same, but unmodulated, stimulus when presented in short bursts when the nerve was intact, these two measures changed in different ways as a result of slight injury to the nerve. Thus, the N1 peak of the CAP in response to clicks and tones or noisebursts became prolonged when the nerve was stretched, while the first peak in the cross-correlograms was essentially unchanged. The second peak in the cross-correlogram was obliterated, but the N2 peak in the CAP was clearly present, although attenuated. More severe injury resulting in a nearly total conduction block, however, produced similar changes in the CAPs and cross-correlograms.