Evidence for Nonlinear Vibrations in the Cochlea from Mössbauer Experiments

The Mössbauer technique has been applied to the measurement of vibration of the basilar membrane in the squirrel monkey's cochlea. Both steady‐state and transient responses have been recorded in the 7–8 kHz range of the cochlea. The steady‐state response indicates that the basilar membrane vibrates nonlinearly from frequencies of stimulation near or greater than the characteristic frequency. The nonlinearity can be observed at the lowest levels of stimulation, 70–80 dB SPL, for which measurements could be made. The nonlinearity extends to lower frequencies and the baslar‐membrane transfer function tends to broaden as SPL is increased. Rapid post‐mortem changes occur in the cochlea. The amplitude of the transfer ratio (basilar membrane/malleus) decreases 10–15 dB over a period of several hours, with a downward shift of 1.5–3 kHz in the characteristic frequency of the basilar membrane at a given location. The low‐frequency slope of the transfer ratio settles to 6 dB/oct by six hours after death. The slope of the phase of the transfer function increases as the characteristic frequency decreases. The transient response was studied using acoustic clicks of approximately 150 μsec in duration, presented in sequences of 100 000 to 400 000. The transient response has an early component which has a fast decay and a second component which has an extremely slow rate of decay and displays nonlinear behavior.