Evidence from Mössbauer experiments for nonlinear vibration in the cochlea

The Mössbauer technique has been applied to the measurement of vibration of the basilar membrane in the squirrel monkey cochlea. Both steady-state and transient responses have been recorded in the 7–8-kHz locus of the cochlea. The steady-state response indicates that the basilar membrane vibrates nonlinearly for 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 basilar membrane transfer function tends to broaden as SPL is increased. Rapid postmortem changes occur in the cochlea: (1) 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; (2) the low-frequency slope of the transfer ratio settles to 6 dB/octave by 6 h after death; (3) the slope of the phase of the transfer function increases the characteristic frequency decreases; (4) the basilar membrane vibrates linearly after death. The transient response was studied using acoustic clicks of approximately 150-μ 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 which displays nonlinear behavior. Preliminary results for the vibration of Reissner's membrane in the 13-kHz region of the cochlea are also reported.