By James Dewey, Ph.D.
The ability to discriminate sounds that are similar in frequency is critical for auditory tasks like understanding speech. In mammals, this ability largely arises from how sound is converted into patterns of vibration within the cochlea—the fluid-filled, spiral structure that houses the organ of Corti and its sensory cells (called inner and outer hair cells).
The researchers’ custom-built optical coherence tomography system captured this cross-sectional OCT image of the live mouse cochlea. Click here for more details. Credit: Dewey, Shera/JARO
Due to the cochlea’s mechanical properties, sounds at different frequencies cause vibrations that peak at different locations along its length, with low frequencies vibrating the cochlea’s apex and high frequencies vibrating its basal end. Active mechanical amplification provided by the outer hair cells further enhances and sharpens these vibration patterns.
For a given cochlear location, this ensures that the stimulation of the inner hair cells (the cells that primarily communicate with the auditory nerve) is sharply tuned to a narrow range of frequencies. This means that information about individual frequency components in a sound is carried by distinct populations of neurons, which improves the overall frequency resolution of the auditory system.
Though the cochlea’s mechanical frequency tuning plays an incredibly important role in hearing, it cannot be directly assessed in humans, as invasive techniques are needed to access the cochlea and observe how its structures vibrate in response to sound. While perceptual methods have been developed to infer cochlear tuning, these methods are very time consuming and may be strongly influenced by processing that occurs at higher levels of the auditory system.
Since cochlear frequency tuning is thought to typically be degraded in ears with common forms of hearing loss, a fast, objective, and noninvasive technique for assessing frequency tuning in a clinical setting would be ideal. In our study published in the Journal of the Association for Research in Otolaryngology in April 2023, we demonstrated that sounds emitted by the ear—called otoacoustic emissions (OAEs)—may provide such a noninvasive window onto cochlear frequency tuning. OAEs are a byproduct of the amplification process mediated by outer hair cells and are often measured in clinical hearing screenings.
We specifically examined distortion-product OAEs (DPOAEs), which are evoked by two tones at closely spaced frequencies and exhibit a tuned, bell-shaped pattern when the frequency of one stimulus tone is held fixed while the other is varied. Motivated by a theoretical link between this pattern and the sharpness of cochlear frequency tuning, we compared DPOAEs with cochlear vibrations measured directly in mice.
Remarkably, we found a strong quantitative agreement between the sharpness of the bell-shaped pattern observed in DPOAEs and the tuning of cochlear vibrations. This agreement was preserved over a wide range of stimulus intensities and at cochlear locations tuned to either relatively low or high frequencies. Through a simple computational model, we determined that the tuning of DPOAEs results from constructive and destructive interference of DPOAE energy as it propagates out of the cochlea, and that this interference pattern is controlled by the sharpness of cochlear frequency tuning.
While it is possible that DPOAEs could be used to noninvasively infer cochlear frequency tuning in humans, more studies are necessary to validate this approach in other species. Additional experimental and modeling work is also needed to determine how the relationship between DPOAE and cochlear tuning is impacted by different types of hearing loss.
Even if DPOAEs cannot be used to accurately assay cochlear tuning under all circumstances, this work will undoubtedly lead to a better understanding of how these signals are generated and travel out of the ear. This will ultimately enhance the clinical utility of DPOAE measurements for detecting and diagnosing hearing loss.
A 2020 and 2022 ERG scientist, James Dewey, Ph.D., is an assistant professor of otolaryngology–head and neck surgery at the Keck School of Medicine of the University of Southern California.
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