Cochlear aging disrupts the correlation between spontaneous rate- and sound-level coding in auditory nerve fibers

The spiking activity of auditory nerve fibers (ANFs) transmits information about the acoustic environment from the cochlea to the central auditory system. Increasing age leads to degeneration of cochlear tissues, including the sensory hair cells and stria vascularis. Here, we aim to identify the functional effects of such age-related cochlear pathologies of ANFs. Rate-level functions (RLFs) were recorded from single-unit ANFs of young adult (n = 52, 3-12 months) and quiet-aged (n = 24, >36 months) Mongolian gerbils of either sex. RLFs were used to determine sensitivity and spontaneous rates (SRs) and were classified into flat-saturating, sloping-saturating, and straight categories, as previously established. A physiologically based cochlear model, adapted for the gerbil, was used to simulate the effects of cochlear degeneration on ANF physiology. In ANFs tuned to low frequencies (<3.5 kHz), SR was lower in those of aged gerbils, while an age-related loss of low-SR fibers was evident in ANFs tuned to high frequencies. These changes in SR distribution did not affect the typical SR versus sensitivity correlation. The distribution of RLF types among low-SR fibers, however, shifted toward that of high-SR fibers, specifically showing more fast-saturating and fewer sloping-saturating RLFs. A modeled striatal degeneration, which affects the combined inner hair cell and synaptic output, reduced SR but left RLF type unchanged. An additional reduced basilar membrane gain, which decreased sensitivity, explained the changed RLF types. Overall, the data indicated age-related changes in the characteristics of single ANFs that blurred the established relationships between SR and RLF types.NEW & NOTEWORTHY Auditory nerve fibers, which connect the cochlea to the central auditory system, change their encoding of sound level in aged gerbils. In addition to a general shift to higher levels, indicative of decreased sensitivity, level coding was also differentially affected in fibers with low- and high-spontaneous rates. Loss of low-spontaneous rate fibers, combined with a general decrease of spontaneous rate, further blurs the categorization of auditory nerve fiber types in the aged gerbil.

Anatomy of the Human Osseous Spiral Lamina and Cochlear Partition Bridge: Relevance for Cochlear Partition Motion

The classic view of cochlear partition (CP) motion, generalized to be for all mammals, was derived from basal-turn measurements in laboratory animals. Recently, we reported motion of the human CP in the cochlear base that differs substantially from the classic view. We described a human soft tissue "bridge" (non-existent in the classic view) between the osseous spiral lamina (OSL) and basilar membrane (BM), and showed how OSL and bridge move in response to sound. Here, we detail relevant human anatomy to better understand the relationship between form and function. The bridge and BM have similar widths that increase linearly from base to apex, whereas the OSL width decreases from base to apex, leading to an approximately constant total CP width throughout the cochlea. The bony three-dimensional OSL microstructure, reconstructed from unconventionally thin, 2-μm histological sections, revealed thin, radially wide OSL plates with pores that vary in size, extent, and distribution with cochlear location. Polarized light microscopy revealed collagen fibers in the BM that spread out medially through the bridge to connect to the OSL. The long width and porosity of the OSL may explain its considerable bending flexibility. The similarity of BM and bridge widths along the cochlea, both containing continuous collagen fibers, may make them a functional unit and allow maximum CP motion near the bridge-BM boundary, as recently described. These anatomical findings may help us better understand the motion of the structures surrounding the organ of Corti and how they shape the input to the cochlear sensory mechanism.

Contralateral efferent reflex effects on threshold and suprathreshold psychoacoustical tuning curves at low and high frequencies

Medial olivocochlear efferent neurons can control cochlear frequency selectivity and may be activated in a reflexive manner by contralateral sounds. The present study investigated the significance of the contralateral medial olivocochlear reflex (MOCR) on human psychoacoustical tuning curves (PTCs), a behavioral correlate of cochlear tuning curves. PTCs were measured using forward masking in the presence and in the absence of a contralateral white noise, assumed to elicit the MOCR. To assess MOCR effects on apical and basal cochlear regions over a wide range of sound levels, PTCs were measured for probe frequencies of 500 Hz and 4 kHz and for near- and suprathreshold conditions. Results show that the contralateral noise affected the PTCs predominantly at 500 Hz. At near-threshold levels, its effect was obvious only for frequencies in the tails of the PTCs; at suprathreshold levels, its effects were obvious for all frequencies. It was verified that the effects were not due to the contralateral noise activating the middle-ear muscle reflex or changing the postmechanical rate of recovery from forward masking. A phenomenological computer model of forward masking with efferent control was used to explain the data. The model supports the hypothesis that the behavioral results were due to the contralateral noise reducing apical cochlear gain in a frequency- and level-dependent manner consistent with physiological evidence. Altogether, this shows that the contralateral MOCR may be changing apical cochlear responses in natural, binaural listening situations.

Can a Static Nonlinearity Account for the Dynamics of Otoacoustic Emission Suppression?

This study investigates whether time-dependent compression mechanisms in the cochlea are necessary to explain dynamic properties of otoacoustic emissions (OAEs). Dynamic properties of click-evoked OAEs (CEOAEs) have been observed in temporal suppression; the effect where the CEOAE magnitude is reduced when a click is presented less than 10 ms before the test click. A time-domain model of the cochlea that represented the basilar membrane (BM) as a cascade of coupled bandpass filters was used to investigate the cochlear origin of temporal suppression in CEOAEs. The model, implemented with a time-invariant nonlinearity, was able to simulate temporal suppression, but was unable to account for the exact time scale and magnitude of the effect. The results suggest that temporal overlap of BM impulse responses can account for suppression in CEOAEs, but that an additional time-dependent cochlear gain mechanism may be needed to account the high suppression maxima at inter-click intervals larger than zero.

Tracing Distortion Product (DP) Waves in a Cochlear Model

In many cases a cochlear model suffices to explain (by simulation) the properties of waves in the cochlea. This is not so in the case of a distortion product (DP) set up by presenting two primary tones to the cochlea. A three-dimensional model predicts, apart from a DP wave traveling in the apical direction, a DP wave that travels from the region of overlap of the two tone patterns towards the stapes-setting the stapes in motion so as to produce an otoacoustic emission at the DP frequency. Experimental research has shown, however, that the actual DP wave in the cochlea appears to travel in the opposite direction, from near the stapes to the overlap region. This feature has been termed "inverted direction of wave propagation" (IDWP). The forward wave could result from an unknown process such as a "hidden source" near the stapes. In the present study we have disproved this notion, by using a one-dimensional model of the cochlea. It is found that both reverse and forward waves are set up by the source of nonlinearity, in the same way as has been published in an earlier work. The present results reveal that IDWP in the data corresponds to the region where the DP wave, originally created as a reverse wave but reflected from the stapes, has received so much amplification that it starts to dominate over the reverse wave. Hence we conclude that IDWP in a one-dimensional model is a direct manifestation of cochlear amplification.