Middle-ear response in the chinchilla and its relationship to mechanics at the base of the cochlea

TLDR

The malleus and stapes responses to sinusoidal acoustic stimulation were measured in anesthetized chinchillas using the Mössbauer technique. Malleus tip velocities reach ~2 mm/s at 100‑dB SPL (500–6000 Hz) with a 6 dB/octave low‑frequency slope, and opening the bulla boosts low‑frequency responses by up to 16 dB; low‑frequency sensitivity surpasses prior cat data, phase‑frequency curves follow minimum‑phase theory, stapes responses are similar but 7.5–10.7 dB lower, and basilar‑membrane displacement tracks stapes acceleration below 150 Hz and velocity between 150–2000 Hz.

Abstract

The responses of the malleus and the stapes to sinusoidal acoustic stimulation have been measured in the middle ears of anesthetized chinchillas using the Mössbauer technique. With ‘‘intact’’ bullas (i.e., closed except for venting via capillary tubing), the vibrations of the tip of the malleus reach a maximal peak velocity of about 2 mm/s in responses to 100-dB SPL tones in the frequency range 500–6000 Hz; vibration velocity diminishes toward lower frequencies with a slope of about 6 dB/oct. Opening the bulla widely increases the responses to low-frequency stimuli by as much as 16 dB. At low frequencies, malleus response sensitivity with either open or intact bullas far exceeds all previous measurements in cats and matches or exceeds such measurements in guinea pigs. Whether measured in open or intact bullas, phase-versus-frequency curves closely approximate those predicted from the magnitude-versus-frequency curves by minimum phase theory. The stapes responses are similar to those of the malleus, except that stapes response magnitude is lower, on the average, by 7.5 dB at frequencies below 2 kHz and 10.7 dB at 2 kHz and above. Comparison of the responses of the middle ear with those of the basilar membrane at a site 3.5 mm from the stapes indicates that, at frequencies below 150 Hz, the basilar membrane displacement is proportional to stapes acceleration. At frequencies between 150 and 2000 Hz, basilar membrane displacement is proportional to stapes velocity.