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

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.

Structures that contribute to middle-ear admittance in chinchilla

“…This dominance is supported by the large increase in low-frequency admittance when the impedance of the air spaces is "shorted-out" (Møller 1965;Guinan & Peake 1967;Dallos 1970) by the open hole in the bullar wall ( Figures 3B and 4). Similar large-increases in middle-ear mobility linked to opening the chinchilla middle-ear air spaces have been noted before (Dallos 1970;Drescher & Eldredge 1974;Ruggero et al 1990). …”

“…Our airspace sealed and hole-open measurements have many features in common with the umbo velocity measurements (Fig. 14) of Ruggero et al (1990). The frequency dependence and the relative magnitudes of our intact air-space and open-bullar hole measurements are similar, although the finer frequency resolution of our measurements better defines the small notch associated with cavity resonances in the sealed and vented middle ear and the large minimum introduced by opening the hole in the cavity wall.…”

“…While preliminary acoustic admittance (Dear 1987) 1 , cochlear potential (Dallos 1970) and ossicular-motion measurements (Ruggero et al 1990) are all consistent with this view, past studies have not determined which middle-ear structures contribute to the sensitivity to low-frequency sound.…”

“…While there are no published measurements of middle-ear admittance in chinchilla in the different middle-ear conditions that we investigated, there are measurements of sound-induced umbo velocity in the chinchilla with the middle-ear air spaces sealed and with an open bullar hole (Ruggero et al 1990). Our airspace sealed and hole-open measurements have many features in common with the umbo velocity measurements (Fig.…”

“…2. large tympanic-membrane (TM) and ossicular dimensions for an animal of its size (Vrettakkos et al 1988; Rosowski 1991a&b, 1994, and 3. increased mechanical sensitivity to low-frequency (f < 1000 Hz) acoustic stimuli relative to other animals with similar hearing ranges (Ruggero et al 1990). …”