Edward Porsov

Reverse wave propagation in the cochlea

Otoacoustic emissions, sounds generated by the inner ear, are widely used for diagnosing hearing disorders and studying cochlear mechanics. However, it remains unclear how emissions travel from their generation sites to the cochlear base. The prevailing view is that emissions reach the cochlear base via a backward-traveling wave, a slow-propagating transverse wave, along the cochlear partition. A different view is that emissions propagate to the cochlear base via the cochlear fluids as a compressional wave, a fast longitudinal wave. These theories were experimentally tested in this study by measuring basilar membrane (BM) vibrations at the cubic distortion product (DP) frequency from two longitudinal locations with a laser interferometer. Generation sites of DPs were varied by changing frequencies of primary tones while keeping the frequency ratio constant. Here, we show that BM vibration amplitude and phase at the DP frequency are very similar to responses evoked by external tones. Importantly, the BM vibration phase at a basal location leads that at a more apical location, indicating a traveling wave that propagates in the forward direction. These data are in conflict with the backward- traveling-wave theory but are consistent with the idea that the emission comes out of the cochlea predominantly through compressional waves in the cochlear fluids.

Localization of the Cochlear Amplifier in Living Sensitive Ears

Background To detect soft sounds, the mammalian cochlea increases its sensitivity by amplifying incoming sounds up to one thousand times. Although the cochlear amplifier is thought to be a local cellular process at an area basal to the response peak on the spiral basilar membrane, its location has not been demonstrated experimentally. Methodology and Principal Findings Using a sensitive laser interferometer to measure sub-nanometer vibrations at two locations along the basilar membrane in sensitive gerbil cochleae, here we show that the cochlea can boost soft sound-induced vibrations as much as 50 dB/mm at an area proximal to the response peak on the basilar membrane. The observed amplification works maximally at low sound levels and at frequencies immediately below the peak-response frequency of the measured apical location. The amplification decreases more than 65 dB/mm as sound levels increases. Conclusions and Significance We conclude that the cochlea amplifier resides at a small longitudinal region basal to the response peak in the sensitive cochlea. These data provides critical information for advancing our knowledge on cochlear mechanisms responsible for the remarkable hearing sensitivity, frequency selectivity and dynamic range.

Fast Reverse Propagation of Sound in the Living Cochlea

The auditory sensory organ, the cochlea, not only detects but also generates sounds. Such sounds, otoacoustic emissions, are widely used for diagnosis of hearing disorders and to estimate cochlear nonlinearity. However, the fundamental question of how the otoacoustic emission exits the cochlea remains unanswered. In this study, emissions were provoked by two tones with a constant frequency ratio, and measured as vibrations at the basilar membrane and at the stapes, and as sound pressure in the ear canal. The propagation direction and delay of the emission were determined by measuring the phase difference between basilar membrane and stapes vibrations. These measurements show that cochlea-generated sound arrives at the stapes earlier than at the measured basilar membrane location. Data also show that basilar membrane vibration at the emission frequency is similar to that evoked by external tones. These results conflict with the backward-traveling-wave theory and suggest that at low and intermediate sound levels, the emission exits the cochlea predominantly through the cochlear fluids.

Auditory Test Result Characteristics of Subjects with and without Tinnitus

Tinnitus is the perception of sound that does not have an acoustic source in the environment. Ascertaining the presence of tinnitus in individuals who claim tinnitus for compensation purposes is very difficult and increasingly becoming a problem. This study examined the potential to observe differences in loudness and pitch matches between individuals who experience tinnitus versus those who do not. This study follows a previous pilot study we completed that included 12 subjects with and 12 subjects without tinnitus. The current study included 36 subjects with and 36 without tinnitus. Results of this study revealed no significant differences between groups with regard to decibel sensation level (SL) loudness matches and within-session loudness-match reliability. Between-group differences revealed that the tinnitus subjects had (1) greater decibel sound pressure level loudness matches, (2) better between-session loudness-match reliability, (3) better pitch-match reliability, and (4) higher frequency pitch matches. These findings support the data from our pilot study with the exception that decibel SL loudness matches were greater for the tinnitus subjects in the pilot study. Tinnitus loudness and pitch matching may have some value in an overall battery of tests for evaluating tinnitus claims.

Reverse Propagation of Sounds in the Intact Cochlea

to the editor: In a recent study, [Meenderink and van der Heijden (2010)][1] investigated an important mechanism involving the reverse propagation of sounds in the cochlea by measuring group delays of distortion products (DPs). The cochlear microphonic potential (CM) from the round window and the

Annexin A5 is the Most Abundant Membrane- Associated Protein in Stereocilia but is Dispensable for Hair-Bundle Development and Function

The phospholipid- and Ca2+-binding protein annexin A5 (ANXA5) is the most abundant membrane-associated protein of ~P23 mouse vestibular hair bundles, the inner ear’s sensory organelle. Using quantitative mass spectrometry, we estimated that ANXA5 accounts for ~15,000 copies per stereocilium, or ~2% of the total protein there. Although seven other annexin genes are expressed in mouse utricles, mass spectrometry showed that none were present at levels near ANXA5 in bundles and none were upregulated in stereocilia of Anxa5−/− mice. Annexins have been proposed to mediate Ca2+-dependent repair of membrane lesions, which could be part of the repair mechanism in hair cells after noise damage. Nevertheless, mature Anxa5−/− mice not only have normal hearing and balance function, but following noise exposure, they are identical to wild-type mice in their temporary or permanent changes in hearing sensitivity. We suggest that despite the unusually high levels of ANXA5 in bundles, it does not play a role in the bundle’s key function, mechanotransduction, at least until after two months of age in the cochlea and six months of age in the vestibular system. These results reinforce the lack of correlation between abundance of a protein in a specific compartment or cellular structure and its functional significance.