New Cochlear Mechanisms Improve Human Hearing

The human ear has a complex, previously unidentified set of "modes," which Yale physicists have found. These modes place significant limitations on how the ear can detect a remarkable range of sound frequencies in between, amplify faint noises, and withstand loud blasts. Using a generic mock-up of a cochlea, a spiral-shaped organ in the inner ear, the researchers applied existing mathematical models to uncover a new layer of cochlear complexity. The results provide a new understanding of the extraordinary accuracy and capacity of human hearing.

Benjamin Machta, an assistant professor of physics in Yale's Faculty of Arts and Science, stated, "We set out to understand how the ear can tune itself to detect faint sounds without becoming unstable and responding even in the absence of external sounds." We discovered a novel set of low-frequency mechanical modes that the cochlea probably supports while investigating this(1^✔ ✔Trusted Source
Hair Cells in the Cochlea Must Tune Resonant Modes to the Edge of Instability without Destabilizing Collective Modes

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Cochlea Amplifies and Tunes Sound for Precise Hearing

In humans, sound is converted into electrical signals in the cochlea. People are able to detect sounds with frequencies across three orders of magnitude and more than a trillion-fold range in power, down to tiny vibrations of air.

Once sound waves enter the cochlea, they become surface waves that travel along the cochlea’s hair-lined basilar membrane.

“Each pure tone rings at one point along this spiral organ,” said Asheesh Momi, a graduate student in physics at Yale’s Graduate School of Arts and Sciences and the study’s first author. “The hair cells at that location then tell your brain what tone you are hearing.”

Those hairs do something else as well: They act as mechanical amplifiers, pumping energy into sound waves to counteract friction and help them reach their intended destinations. Pumping in just the right amount of energy — and making constant adjustments — is crucial for precise hearing, the researchers said.

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But that is simply one set of hearing modes within the cochlea, and it is well-documented. The Yale team discovered a second, extended set of modes within the organ.

In these extended modes, a large portion of the basilar membrane reacts and moves together, even for a single tone. This collective response constrains how hair cells respond to incoming sound and how the hair cells pump energy into the basilar membrane.

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“Since these newly discovered modes exhibit low frequencies, we believe our findings might also contribute to a better understanding of low-frequency hearing, which is still an active area of research,” said Isabella Graf, a former Yale postdoctoral researcher who is now at the European Molecular Biology Laboratory in Heidelberg, Germany.

Graf and Machta have collaborated on a series of studies in recent years that used mathematical models and statistical physics concepts to understand better biological systems, such as a pit viper’s sensitivity to temperature change and the interplay between phases of matter that come into contact with cell membranes.

Reference:

1. Hair Cells in the Cochlea Must Tune Resonant Modes to the Edge of Instability without Destabilizing Collective Modes- (https://journals.aps.org/prxlife/abstract/10.1103/PRXLife.3.013001)

Source-Eurekalert