Middle ear function and cochlear input impedance in chinchilla
Schematic of the animal preparation. The bulla was opened superiorly and posteriorly, and a hole approximately in diameter was made in the vestibule for the fiber-optic pressure sensor. The cartilaginous ear canal was cut and a brass tube was glued in the bony ear canal to allow repeatable couplings of the earphone delivering the sound stimuli. A built-in reference microphone measured sound pressure in the ear canal. A laser Doppler vibrometer was aimed at reflective beads placed on the posterior crus and the footplate of the stapes.
in six animals (individual data: thin lines; mean: thick solid line; 95% CI: shaded areas). Data points with a were not included; frequencies for which in at least one of the individual data point were marked by a circle (see Sec. II).
controls. (A) Stability: comparison of measured in chinchilla No. 5 before (solid line) and after (dotted line) taking the pressure sensor out of the vestibule and recalibrating it. Differences were less than 2 dB in magnitude and less than 0.01 cycles in phase. (B) Effect of interrupting the ossicular chain: in chinchilla No. 8 with an intact ossicular chain (solid line), and after interrupting the incudo-stapedial joint (dotted line). (C) Effect of plugging the vestibular hole on : in one animal (chinchilla No. 9), after sealing around the pressure sensor with a gel of high molecular weight (dashed line), there was a limited increase in . After removing the gel (dotted line), went back to baseline (solid line). The 95% CI (shaded areas) represent the variability of repeated measurements in the same condition.
SVTF in six animals. (A) SVTF before the vestibular hole was made (individual data: thin lines; mean: thick solid line; 95% CI: shaded areas). (B) Mean effect of introducing the vestibular hole on SVTF in six animals. We computed the change in magnitude and phase before the hole was made and afterward with the pressure sensor in place. Changes in magnitude were significant only in a small region around 6 kHz (asterisks).
in six animals (individual data: thin lines; mean: thick solid line; 95% CI: shaded areas) from simultaneous measurements of and . was resistive (roughly flat magnitude, phase close to 0) up to at least 10 kHz.
Effect of an ear canal correction (to account for the differences between and —see Sec. II) on and SVTF. (A) Mean before (dotted line) and after (solid line, from Fig. 2) correction. (B) Mean SVTF in the intact vestibule condition before (dotted line) and after [solid line, from Fig. 4(A)] correction.
Model predictions of the effect of the vestibular hole. (A) Simple model of the middle ear as a Norton equivalent circuit, providing volume velocity to the parallel combination of the inner ear load and the impedance of the hole . The Norton equivalent is composed of an ideal volume-velocity source and the output impedance of the middle ear . (B) Predictions of our simple model with a diameter hole partially plugged by a pressure sensor (equivalent ) were at least qualitatively similar to the experimental changes we found in and [see text and Figs. 3(C) and 4(B)]: The changes in were limited to low frequencies (solid and dashed-dotted lines); the changes in were small (dashed and dotted lines).
Comparison of our measurements of (solid lines) with other studies. (A) In chinchilla: comparison with a previous study by Décory (1989) and Décory et al. (1990): dashed line. The two data sets were very similar at stimulus frequencies of 500 Hz–3 kHz and 12–20 kHz, but different at other frequencies. (B) In other mammals: comparison with studies in cat, guinea pig (from Décory, 1989 and Décory et al., 1990: dotted and dashed-dotted lines), gerbil (from Olson, 2001: dashed line), and human temporal bone (from Puria et al., 1997: dashed-double dotted line). All these measurements were carried out with open holes in the bullar walls. The septum of the middle ear (Møller, 1965; Huang et al., 1997) was intact in the cat study.
Comparison of our measurements of SVTF (solid line) with previous studies in chinchilla by Songer and Rosowski (2007a): dotted line and Ruggero et al. (1990): dashed line. The differences in magnitude between ours and the Ruggero et al. (1990) study may be due to differences in the experimental setup (see text).
Comparison of our measurements of (solid lines) with other studies. (A) In chinchilla: comparison with the prediction of a model by Songer and Rosowski (2007a): dotted line and computations by Ruggero et al. (1990): dashed line, who used their own measurements and Décory’s (1989) measurements in other animals [see Fig. 8(A)]. (B) In other mammals: comparison with studies in cat (from Lynch et al., 1982: dotted line), gerbil (from de La Rochefoucauld et al., 2008: dashed line), and human temporal bone (from Aibara et al., 2001: dashed-double dotted line).
An average chinchilla audiogram (thick solid line, from Miller, 1970) was compared to the free-field sound pressure required to produce a constant level of (thin dotted line), (thin dashed line), and (thin dashed-dotted line).
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