Yale University physicists have discovered a new set of ‘modes’ in the human ear that put constraints on how the ear amplifies faint sounds, tolerates noisy blasts, and discerns a range of sound frequencies in between.
The researchers revealed a new layer of cochlear complexity by applying existing mathematical models to a generic mock-up of a cochlea.
Modes in the ear refer to mechanical modes in the cochlea that help the ear amplify sounds, distinguish frequencies, and tolerate loud noises.
The physicists believe the new set of low frequency mechanical modes they found in the cochlea may contribute to a better understanding of low frequency hearing.
They published their study on 2 January 2025 in the open access journal Physical Review X Life, PRX Life, which publishes research at the intersection of physics and biology.
“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,” co-senior author and Assistant Professor of physics in Yale’s Faculty of Arts and Science Benjamin Machta said in Yale News.
“But in getting to the bottom of this we stumbled onto a new set of low frequency mechanical modes that the cochlea likely supports.”
In humans, sound is converted into electrical signals in the cochlea. People can detect sounds with frequencies across three orders of magnitude.
Sound waves enter the cochlea and 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 the study’s first author, Mr Asheesh Momi, a graduate student in physics in Yale’s Graduate School of Arts and Sciences.
“The hair cells at that location then tell your brain what tone you are hearing.”
The hairs also act as mechanical amplifiers, pumping energy into sound waves to counteract friction and help them reach their intended destinations. The researchers said that pumping in the right amount of energy and making constant adjustments were crucial for precise hearing.
They said this was one well-documented set of hearing modes within the cochlea.
Discovered second set of modes
The team discovered a second, extended set of modes in the cochlea. In the extended modes, a large portion of the basilar membrane reacts and moves together, even for a single tone, they said.
This collective response places constraints on how hair cells respond to incoming sound and how the hair cells pump energy into the basilar membrane.
“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 another author, Ms Isabella Graf, a former Yale postdoctoral researcher now at the European Molecular Biology Laboratory in Heidelberg, Germany.
Graf and Machta previously collaborated on a series of studies that used mathematical models and statistical physics concepts to better understand biological systems.
These included a pit viper’s sensitivity to temperature change and the interplay between phases of matter that come into contact within cell membranes.
Other co-authors were Mr Michael Abbott of Yale and Mr Julian Rubinfien of Harvard. Machta, Momi, and Abbott are part of Yale’s Quantitative Biology Institute.
The National Institutes of Health, a Simons Investigator award, and the German Research Foundation supported the research.
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