Spontaneous Basilar Membrane Oscillation and Otoac

Authors: AL Nuttall K Grosh J Zheng E de Boer Y Zou T Ren

Publish Date: 2004/07/30

Volume: 5, Issue: 4, Pages: 337-348

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Abstract

A spontaneous otoacoustic emission SOAE measured in the ear canal of a guinea pig was found to have a counterpart in spontaneous mechanical vibration of the basilar membrane BM A spontaneous 15kHz BM velocity signal was measured from the 18kHz tonotopic location and had a level close to that evoked by a 14kHz 15dB SPL tone given to the ear Lowerfrequency puretone acoustic excitation was found to reduce the spontaneous BM oscillation SBMO while higherfrequency sound could entrain the SBMO Octaveband noise centered near the emission frequency showed an increased narrowband response in that frequency range Applied pulses of current enhanced or suppressed the oscillation depending on polarity of the current The compound action potential CAP audiogram demonstrated a frequencyspecific loss at 8 and 12 kHz in this animal We conclude that a relatively highfrequency spontaneous oscillation of 15 kHz originated near the 15kHz tonotopic place and appeared at the measured BM location as a mechanical oscillation The oscillation gave rise to a SOAE in the ear canal Electric current can modulate level and frequency of the otoacoustic emission in a pattern similar to that for the observed mechanical oscillation of the BMTo the best of our knowledge detailed measurements of a spontaneous vibration of the basilar membrane BM in a mammalian ear have not been reported Since one hypothesis for the generation of spontaneous emissions SOAEs involves “uncontrolled or underdamped” oscillations of the cochlear amplification mechanisms an idea that originated with Gold 1948 such measurements could provide valuable clues about the amplification process Perhaps this would enable one to distinguish spontaneous emissions linked to the somatic mechanics of the outer hair cells OHCs He and Dallos 2000 from those that might arise from spontaneous motion of the stereocilia Martin et al 2003 In this report we describe a relatively highfrequency SOAE from a guinea pig ear which had a counterpart as a basilar membrane spontaneous oscillation SBMOSpontaneous otoacoustic emissions SOAEs are sounds that are emitted from a human or animal ear They are believed to originate from oscillations of the organ of Corti in particular the basilar membrane Starting with Kemp 1979ab and Wilson 1980 who reported that subjects with normal hearing produced them SOAEs were systematically studied as being characteristic of a normal functioning cochlea Zenner and Ernst 1995 The healthy organ of Corti possibly possesses a positive feedback mechanism to amplify soundevoked vibrations this mechanism may have an innate tendency toward spontaneous oscillation eg by pathology Ruggero et al 1983It has also been proposed that SOAEs are due to inhomogeneities of the organ of Corti causing multiple internal reflections in the cochlea Kemp 1979a b Shera 2003 These inhomogeneities may be morphological variations such as the presence or absence of OHCs Lonsbury–Martin et al 1988 Hilger et al 1995 or more subtle variations in the mechanoelectric properties of individual cells Such variations could be involved when spontaneous emissions are “provoked” by loud sound exposure Clark et al 1984 Powers et al 1995 The proposed mechanism of multiple reflection requires the presence of intracochlear amplification Discovery of an SBMO with an accompanying SOAE would provide the “missing link” between cause and effectThe SOAEs of many animals have been studied and characterized Humans tend to have a large number of relatively lowfrequency puretonelike SOAEs the frequencies of which have a nearly constant frequency spacing Zweig and Shera 1995 The SOAE levels and frequencies are functions of the mechanical status of the cochlea such as lymphatic pressure Wilson 1980 or circadian and menstrual rhythms Velenovsky and Glattke 2002 Bell 1992 and can be suppressed by activity of the olivocochlear efferent system Collet et al 1990 Velenovsky and Glattke 2002Animal models of SOAEs to allow for mechanical studies have not been found Evans et al 1981 report on aged guinea pig with an SOAE at about 1 kHz Ohyama et al 1991 found a high incidence of lowfrequency SOAEs in awake guinea pigs However in our experience an anesthetized guinea pig will rarely evidence a SOAE and when it occurs the frequency is about 1 kHz In 20 years of cochlear physiological studies by author ALN the total number of such animals is about six It is probable that anesthesia greatly reduces the level of guinea pig SOAEs Brown et al 1990 Ohyama et al 1991 Lowfrequency SOAEs in the guinea pig have been used to study the effect of heartbeat on cochlear mechanics Talmadge et al 1993 Ren et al 1995Lowfrequency vibrations in the apical cochlear turn of the guinea pig have been recorded in vivo Keilson et al 1993 These vibrations would not be efficiently detected in the basal turn where the characteristic frequencies are above 12 kHz Teich et al 1994 have reported the only other example of a highfrequency SBMO of which we are aware In their study the magnitude spectrum of the BM velocity from an observation made through the cat round window membrane showed “spurious” puretone motion There was no measurement of ear canal sound to demonstrate that the SBMO had a SOAE counterpartThe subject was a pigmented guinea pig strain 2NCR obtained from the Charles River Laboratory weighing 300 g Until the day of the experiment it was housed in facilities approved by the American Association for Accreditation of Laboratory Animal Care at the Oregon Health Science University The experimental protocols described in this report were approved by the Committee on the Use and Care of Animals Oregon Health Science University The animal was anesthetized using ketamine 40 mg/kg im and xylazine 10 mg/kg im Supplemental doses of ketamine and xylazine were given on a schedule or as needed judging by leg withdrawal to a toe pinch

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