An experimental study into the acousto-mechanical effects of invading the cochlea

Abstract

The active and nonlinear mechanical processing of sound that takes place in the mammalian cochlea is fundamental to our sense of hearing. We have investigated the effects of opening the cochlea in order to make experimental observations of this processing. Using an optically transparent window that permits laser interferometric access to the apical turn of the guineapig cochlea, we show that the acousto-mechanical transfer functions of the sealed (i.e. near intact) cochlea are considerably simpler than those of the unsealed cochlea. Comparison of our results with those of others suggests that most previous investigations of apical cochlear mechanics have been made under unsealed conditions, and are therefore likely to have misrepresented the filtering of low-frequency sounds in the cochlea. The mechanical filtering that is apparent in the apical turns of sealed cochleae also differs from the filtering seen in individual auditory nerve fibres with similar characteristic frequencies. As previous studies have shown the neural and mechanical tuning of the basal cochlea to be almost identical, we conclude that the strategies used to process low frequency sounds in the apical turns of the cochlea might differ fundamentally from those used to process high frequency sounds in the basal turns.

Phase-Locked Responses to Tones of Chinchilla Auditory Nerve Fibers: Implications for Apical Cochlear Mechanics

“…Furthermore, there is no evidence disputing the conclusion that, at the apex of the cochlea, the frequency tuning of IHCs (Dallos 1985(Dallos , 1986) "results more from the micromechanical properties of the cochlea than from intrinsic properties of the cells themselves" (Kros 1996). Therefore, it is likely that the CF-specific nonlinearities illustrated in Figures 2 and 5 are present at the input to the stereocilia of apical IHCs and that their antecedents in organ of Corti vibrations in chinchilla were not detected because of abnormalities due to surgical damage (such as puncturing of Reissner's membrane; Dong and Cooper 2006). In fact, some recordings at apical cochlear sites in chinchilla (Fig.…”

“…2.3D of Cooper 2004) and guinea pig (Dong and Cooper 2001;Zinn et al 2000) give hints of CF-specific nonlinearity. Another possibility is that a micromechanical "second filter" is interposed between the vibrations of the BM/organ of Corti/tectorial membrane complex and the excitation of ANFs (Dong and Cooper 2006).…”

“…Accounts of vibrations at apical sites of the cochleae of live guinea pigs (Cooper 2006;Dong and Cooper 2006;Zinn et al 2000) and chinchillas Rhode and Cooper 1996) are inconsistent, differing in essential details including the nature of nonlinearity (compressive in chinchilla but expansive in guinea pig, according to Zinn et al (2000)) and its relationship to frequency tuning (i.e., level dependent in guinea pig (Zinn et al 2000) but independent of level in chinchilla ). The data for chinchilla Rhode and Cooper 1996) are probably not representative of those in intact cochleae because they were obtained after puncturing Reissner's membrane, a procedure that elevates low-frequency compound action potential (CAP) thresholds (Dong and Cooper 2006).…”

“…The data for chinchilla Rhode and Cooper 1996) are probably not representative of those in intact cochleae because they were obtained after puncturing Reissner's membrane, a procedure that elevates low-frequency compound action potential (CAP) thresholds (Dong and Cooper 2006). Even in studies in which vibrations were measured without rupturing Reissner's membrane and CAP thresholds were not elevated (in guinea pig; Dong and Cooper 2006;Zinn et al 2000), dysfunction may have gone undetected since lowfrequency CAP thresholds are insensitive to apical cochlear damage (Cooper and Rhode 1995;Harris 1979;Johnstone et al 1979;Zinn et al 2000).…”

Auditory Nerve Excitation via a Non-traveling Wave Mode of Basilar Membrane Motion

“…This supra-CF plateau in the BM motion, where the BM moves up and down together (with the same phase) has been observed in the apex and the base (Cooper and Rhode 1996;Narayan et al 1998;Ruggero et al 2000;Ren and Nuttall 2001;Dong and Cooper 2006;Rhode 2007). The physical basis for the BM plateau-this piston-like mode of BM motionis not well understood, but it is likely related to a Correspondence to: Elizabeth S. Olson & Department of Otolaryngology, Head and Neck Surgery · Columbia University · 630 West 168th Street, P&S 11-452 New York, NY 10032, USA Phone: +1-212-305-3993; e-mail: eao2004@columbia.edu compressive pressure that dominates the traveling wave pressure in the supra-CF region.…”