Mechanical studies of the cochlea often reveal dramatic dependencies on the physiological condition of the cochlea. This is particularly true in the basal turns of the cochlea. In the more apical turns, the differences between healthy and impaired cochleae appear smaller, but there is considerable debate about how ‘normal’ the conditions have been when any of the observations have been made. The main problem here is that the apical mechanics experiments have not been backed up by sufficient proof of the cochlea’s condition. The present study provides some baseline data relating to the physiological condition of the cochlea during invasive apical turn experiments. The data are in the form of compound action potential (CAP) audiograms recorded at various stages through individual in vivo experiments.

CAP audiograms were measured from deeply anaesthetised guinea-pigs, which were killed humanely at the end of the experiments. The animals were anaesthetised using 30 mg kg^-1 sodium pentobarbitone, I.P., and 0.6 ml kg^-1 Hypnorm, I.M. (each ml of Hypnorm contains 10 mg fluanisone and 0.315 mg fentanyl citrate). Tone-evoked CAPs were recorded from the bony niche near the round window of the cochlea. The CAP thresholds (N₁-P₁ amplitudes ▓le│ 5 µV) were assessed using the onset responses to stimuli above 2 kHz. The thresholds for lower frequency stimuli were determined from the sustained (periodic, phase-locked) neural responses, after the cochlea’s microphonic potentials had been attenuated by averaging the responses to stimuli varying by 180 deg in phase (cf. Dallos, 1973).

Seven animals provided complete CAP audiograms at four key points during the experiments: (a) following minimal surgery, (b) after more extensive surgery, as needed to expose the apical aspects of the cochlea, (c) after shaving a hole into the apical turn of cochlea, and (d) after tearing Reissner’s membrane and placing several reflective microbeads on the apical cochlear partition. The audiograms showed that hearing losses were restricted to high frequencies (▓ge│ 12 kHz) during the initial approach to the apical cochlea; these losses indicate damage to the basal parts of the cochlea, and could probably be avoided by taking more care during the opening of the ventral bulla. Opening the apical cochlea caused an average threshold elevation of only 5 dB. This loss was restricted to low-frequency stimuli (300 and 500 Hz), indicating that the damage was restricted to the apical turns of the cochlea. Tearing the apical-turn Reissner’s membrane caused no additional threshold losses in the short term (n = 1), but led to losses averaging between 8 and 16 dB (from high to low frequencies) after several hours (n = 6). The lack of an immediate threshold change probably indicates that the low-frequency CAPs are insensitive to very localised damage, while the longer term changes probably reflect widespread changes in the ionic composition of the cochlea’s endolymph and perilymph.This work was supported by Defeating Deafness (the Hearing Research Trust) and the Royal Society.

Dallos, P. (1973). The Auditory Periphery: Biophysics and Physiology. Academic, New York.