Development of wide-band middle ear transmission in the Mongolian gerbil

Abstract

Stapes vibrations were measured in deeply anesthetized adult and neonatal (ages: 14 to 20 days) Mongolian gerbils. In adult gerbils, the velocity magnitude of stapes responses to tones was approximately constant over the entire frequency range of measurements, 1 to 40 kHz. Response phases referred to pressure near the tympanic membrane varied approximately linearly as a function of increasing stimulus frequency, with a slope corresponding to a group delay of 30 μs. In neonatal gerbils, the sensitivity of stapes responses to tones was lower than in adults, especially at midfrequencies (e.g., by about 15 dB at 10-20 kHz in gerbils aged 14 days). The input impedance of the adult gerbil cochlea, calculated from stapes vibrations and published measurements of pressure in scala vestibuli near the oval window [E. Olson, J. Acoust. Soc. Am. 103, 3445-3463 (1998)], is principally dissipative at frequencies lower than 10 kHz. Conclusions-(a)middle-ear vibrations in adult gerbils do not limit the input to the cochlea up to at least 40 kHz, i.e., within 0.5 oct of the high-frequency cutoff of the behavioral audiogram; and (b) the results in both adult and neonatal gerbils are inconsistent with the hypothesis that mass reactance controls high-frequency ossicular vibrations and support the idea that the middle ear functions as a transmission line.

Stapes Vibration in the Chinchilla Middle Ear: Relation to Behavioral and Auditory-Nerve Thresholds

“…At face value, the wide bandwidth and the approximately linear phase-frequency characteristics of the average stapes-velocity transfer function in the present study resemble those of a lossless transmission line, as suggested for other species (Wilson and Bruns 1983;Puria and Allen 1998;Olson 1998;Overstreet and Ruggero 2002). Alternatively, the same characteristics could be produced by a combination of multiple resonances in the tympanic membrane (Fay et al 2006).…”

“…Acoustic stimuli were tone bursts, digitally synthesized with a Tucker-Davis Technologies System II under computer control and delivered at 80-90 dB SPL via a modified Radio Shack Super Tweeter 40-1310B (Chan et al 1993;Overstreet and Ruggero 2002) either through a closed system or free field (from a position lateral to the tympanic membrane). In the closed system, the tweeter was coupled to a 56-mm-long rubber tube terminated with a speculum tip which abutted against the bone surrounding the tympanic membrane.…”

“…Two of us proposed that the outer and middle ears jointly act as a broadband transmission system and that the high-frequency limit of hearing is imposed by the cochlea . Ruggero and Temchin presented strong comparative evidence that Bthe highfrequency limit of the audiogram in great part is determined internally in the cochlea by the highfrequency arms of the frequency-threshold tuning curves of auditory-nerve fibers with the highest characteristic frequencies.^That proposal was inspired by the realization that, in gerbil, stapes velocity (Overstreet and Ruggero 2002) and pressure magnitudes in scala vestibuli (Olson 1998) are relatively constant over a frequency range which substantially exceeds the bandwidth of the gerbil audiogram (see Fig. 3 of ).…”

Low-Frequency Finite-Element Modeling of the Gerbil Middle Ear

“…Ravicz and Rosowski (2004), Ruggero (2002), andOverstreet et al (2003) dealt exclusively with high-frequency responses. Rosowski et al (1999), Olson and Cooper (2000), and Ravicz and Rosowski (2004) looked at both low and high frequencies.…”

Flexibility within the middle ears of vertebrates

“…The controversial theory that the mammalian middle ear works as a 'transmission line' 72,92,93 requires that the conducting elements be regarded as a series of masses coupled by springs, each mass-stiffness pair being matched to the load impedance of the cochlea. The flexible joints between the ossicles then act as shunting stiffnesses which counteract the effect of the ossicular masses, the result of which is wide-band and potentially 'lossless' sound transmission at the cost of acoustic delay.…”