Effect of the Attachment of the Tectorial Membrane on Cochlear Micromechanics and Two-Tone Suppression

Abstract

The mechanical stimulation of the outer hair cell hair bundle (HB) is a key step in nonlinear cochlear amplification. We show how two-tone suppression (TTS), a hallmark of cochlear nonlinearity, can be used as an indirect measure of HB stimulation. Using two different nonlinear computational models of the cochlea, we investigate the effect of altering the mechanical load applied by the tectorial membrane (TM) on the outer hair cell HB. In the first model (TM-A model), the TM is attached to the spiral limbus (as in wild-type animals); in the second model (TM-D model), the TM is detached from the spiral limbus (mimicking the cochlea of Otoa(EGFP/EGFP) mutant mice). As in recent experiments, model simulations demonstrate that the absence of the TM attachment does not preclude cochlear amplification. However, detaching the TM alters the mechanical load applied by the TM on the HB at low frequencies and therefore affects TTS by low-frequency suppressors. For low-frequency suppressors, the suppression threshold obtained with the TM-A model corresponds to a constant suppressor displacement on the basilar membrane (as in experiments with wild-type animals), whereas it corresponds to a constant suppressor velocity with the TM-D model. The predictions with the TM-D model could be tested by measuring TTS on the basilar membrane of the Otoa(EGFP/EGFP) mice to improve our understanding of the fundamental workings of the cochlea.

Swept source optical coherence tomography for in vivo imaging and vibrometry in the apex of the mouse cochlea

“…In contrast, at the characteristic frequency, 10 dB SPL sound stimuli evoked deflections of ~3 nm. These data provide an estimate of the range of stereociliary bundle movements evoked during normal sound transduction, and will help to test and guide the development of accurate cochlear models [4,5]. Furthermore, these preliminary data demonstrate the potential for 040010-3 this approach to be used in estimating the mechanical properties of specific bundle proteins using transgenic mice as well as understanding the in vivo implications of altered bundle mechanics.…”

Simple oscillator model of frequency dependence of cochlear two-tone suppression

“…The transmission line model reproduces the mechanical dynamics in the cochlea well [12][13][14][15][16][17], and Fig. 1 schematically illustrates the discretized transmission line model of the cochlea.…”

“…In Ref. [17], temporal responses to 2TS were calculated as AM signals including the probe component f p and distortion components f p AE n f s (n is an integer) within a cochlear transmission line model. This indicates that temporal responses cannot be calculated when the probe frequency is lower than the suppressor frequency because the distortion components f p À f s become negative under this condition.…”

“…To simulate phasic suppression in the BM, Ref. [17] showed that a BM response can be computed using a combination of both the probe and suppressor tone frequencies as an AM signal. Thus, an AM signal is representative of phasic suppression in the BM.…”

“…Transmission line models can calculate the fluid and mechanical dynamics of the cochlea, and some transmission line models have simulated 2TS using active elements that include simple saturation functions [12][13][14][15][16][17]. To simulate phasic suppression in the BM, Ref.…”