Direction of wave propagation in the cochlea for internally excited basilar membrane

Otoacoustic emissions reveal the micromechanical role of organ-of-Corti cytoarchitecture in cochlear amplification

The intricate, crystalline cytoarchitecture of the mammalian organ of Corti presumably plays an important role in cochlear amplification. As currently understood, the oblique, Y-shaped arrangement of the outer hair cells (OHCs) and phalangeal processes of the Deiters cells serves to create differential "push-pull" forces that drive the motion of the basilar membrane via the spatial feedforward and/or feedbackward of OHC forces. In concert with the cochlear traveling wave, the longitudinal separation between OHC sensing and forcing creates phase shifts that yield a form of negative damping, amplifying waves as they propagate. Unlike active forces that arise and act locally, push-pull forces are inherently directional-whereas forward-traveling waves are boosted, reverse-traveling waves are squelched. Despite their attractions, models based on push-pull amplification must contend with otoacoustic emissions (OAEs), whose existence implies that amplified energy escapes from the inner ear via mechanisms involving reverse traveling waves. We analyze hybrid local/push-pull models to determine the constraints that reflection-source OAEs place on the directionality of cochlear wave propagation. By implementing a special force-mixing control knob, we vary the mix of local and push-pull forces while leaving the forward-traveling wave unchanged. Consistency with stimulus-frequency OAEs requires that the active forces underlying cochlear wave amplification be primarily local in character, contradicting the prevailing view. By requiring that the oblique cytoarchitecture produce predominantly local forces, we reinterpret the functional role of the Y-shaped geometry, proposing that it serves not as a push-pull amplifier, but as a mechanical funnel that spatially integrates local OHC forces.

Inner hair cell stereocilia displacement in response to focal stimulation of the basilar membrane in the ex vivo gerbil cochlea

The inner hair cells in the mammalian cochlea transduce mechanical signals to electrical signals that provide input to the auditory nerve. The spatial-temporal displacement of the inner hair cell stereocilia (IHCsc) relative to basilar membrane (BM) displacement is central to characterizing the transduction process. This study specifically focuses on measuring displacement of the stereocilia hair bundles in the radial dimensions where they are most sensitive. To simplify the mechanical response of the cochlear partition, a mechanical probe was used to drive the BM. Optical imaging was used to measure radial displacement of the inner hair cell stereocilia local to the probe in ex vivo gerbil cochleae. The mechanical probe displaced the BM in the transverse direction using sinusoidal stimuli with frequencies ranging from 10 Hz to 42.5 kHz. IHCsc displacement measurements were made in the radial dimension as a function of their longitudinal location along the length of the BM. The results were used to quantify the frequency response, longitudinal space coupling, traveling wave velocity, and wavelength of the radial displacement of the stereocilia. The measurements were centered at two best frequency locations along the BM: Proximal to the round window (first turn), and in the second turn. At both locations, frequency tuning was seen that was consistent with published place maps. At both locations, traveling waves were observed simultaneously propagating basal and apical from the probe. The velocity of the traveling waves at the center frequency (CF) of the location was higher in the first turn than in the second. As the stimulus frequency increased and approached CF for a location, the traveling wavelength decreased. Differential motion of the BM and IHCsc was observed in the second turn as the stimulus frequency increased toward CF. The longitudinal coupling measured in this study was longer than observed in previous studies. In summary the results suggest that the shape of the wave patterns present on the BM are not sufficient to characterize the displacement of the IHCsc.

Hydromechanical Structure of the Cochlea Supports the Backward Traveling Wave in the Cochlea In Vivo

The discovery that an apparent forward-propagating otoacoustic emission (OAE) induced basilar membrane vibration has created a serious debate in the field of cochlear mechanics. The traditional theory predicts that OAE will propagate to the ear canal via a backward traveling wave on the basilar membrane, while the opponent theory proposed that the OAE will reach the ear canal via a compression wave. Although accepted by most people, the basic phenomenon of the backward traveling wave theory has not been experimentally demonstrated. In this study, for the first time, we showed the backward traveling wave by measuring the phase spectra of the basilar membrane vibration at multiple longitudinal locations of the basal turn of the cochlea. A local vibration source with a unique and precise location on the cochlear partition was created to avoid the ambiguity of the vibration source in most previous studies. We also measured the vibration pattern at different places of a mechanical cochlear model. A slow backward traveling wave pattern was demonstrated by the time-domain sequence of the measured data. In addition to the wave propagation study, a transmission line mathematical model was used to interpret why no tonotopicity was observed in the backward traveling wave.

The Coda of the Transient Response in a Sensitive Cochlea: A Computational Modeling Study

In a sensitive cochlea, the basilar membrane response to transient excitation of any kind–normal acoustic or artificial intracochlear excitation–consists of not only a primary impulse but also a coda of delayed secondary responses with varying amplitudes but similar spectral content around the characteristic frequency of the measurement location. The coda, sometimes referred to as echoes or ringing, has been described as a form of local, short term memory which may influence the ability of the auditory system to detect gaps in an acoustic stimulus such as speech. Depending on the individual cochlea, the temporal gap between the primary impulse and the following coda ranges from once to thrice the group delay of the primary impulse (the group delay of the primary impulse is on the order of a few hundred microseconds). The coda is physiologically vulnerable, disappearing when the cochlea is compromised even slightly. The multicomponent sensitive response is not yet completely understood. We use a physiologically-based, mathematical model to investigate (i) the generation of the primary impulse response and the dependence of the group delay on the various stimulation methods, (ii) the effect of spatial perturbations in the properties of mechanically sensitive ion channels on the generation and separation of delayed secondary responses. The model suggests that the presence of the secondary responses depends on the wavenumber content of a perturbation and the activity level of the cochlea. In addition, the model shows that the varying temporal gaps between adjacent coda seen in experiments depend on the individual profiles of perturbations. Implications for non-invasive cochlear diagnosis are also discussed.

The fluid-structure-electrical interaction in the cochlea enable the basilar membrane, one of the most important structures in the cochlear partition, to display different dynamic patterns depending on the frequency content of the incoming sound. Interestingly, in a healthy cochlea the motion of the basilar membrane shows echoes upon an impulse acoustic stimulation delivered to the ear canal. The delay, duration, and shape of these echoes vary from one cochlea to another. A hypothesis that irregularities of the properties of the cochlear partition coherently scatter acoustic waves and generate echoes is examined. These irregularities are posited to arise, for example, the damage of the sensory cells or the natural randomness in the morphology of the cochlear partition. Here we build a physiologically-based mathematical model to understand the echoes observed in experiments by introducing irregularity to the properties of the sensory cells. We found that the patterns of the echoes depend on the individual profiles of the irregularities. Our work suggest that the ear canal recording, which is correlated to the dynamics of the basilar membrane, can be used as a non-invasive tool not only to diagnose the intracochlear damage but also to interpret these data given its idiosyncratic origin.

Light-induced vibration in the hearing organ

The exceptional sensitivity of mammalian hearing organs is attributed to an active process, where force produced by sensory cells boost sound-induced vibrations, making soft sounds audible. This process is thought to be local, with each section of the hearing organ capable of amplifying sound-evoked movement, and nearly instantaneous, since amplification can work for sounds at frequencies up to 100 kHz in some species. To test these fundamental precepts, we developed a method for focally stimulating the living hearing organ with light. Light pulses caused intense and highly damped mechanical responses followed by traveling waves that developed with considerable delay. The delayed response was identical to movements evoked by click-like sounds. This shows that the active process is neither local nor instantaneous, but requires mechanical waves traveling from the cochlear base toward its apex. A physiologically-based mathematical model shows that such waves engage the active process, enhancing hearing sensitivity.

Basilar membrane vibration is not involved in the reverse propagation of otoacoustic emissions

To understand how the inner ear-generated sound, i.e., otoacoustic emission, exits the cochlea, we created a sound source electrically in the second turn and measured basilar membrane vibrations at two longitudinal locations in the first turn in living gerbil cochleae using a laser interferometer. For a given longitudinal location, electrically evoked basilar membrane vibrations showed the same tuning and phase lag as those induced by sounds. For a given frequency, the phase measured at a basal location led that at a more apical location, indicating that either an electrical or an acoustical stimulus evoked a forward travelling wave. Under postmortem conditions, the electrically evoked emissions showed no significant change while the basilar membrane vibration nearly disappeared. The current data indicate that basilar membrane vibration was not involved in the backward propagation of otoacoustic emissions and that sounds exit the cochlea probably through alternative media, such as cochlear fluids.