Tiny eardrum sounds may help sync visual, auditory perception

Around 2012, Jennifer Groh and her colleagues began a series of experiments investigating the effect of eye movements on auditory signals in the brain. It wasn’t until years later that they noticed something curious in their data: In both an animal model and in people, eye movements coincide with ripples across the eardrum.

The finding, published in 2018, seemed “weird,” says Groh, professor of psychology and neuroscience at Duke University — and ripe for further investigation. “You can go your whole career never studying something that is anywhere near as beautifully regular and reproducible,” she says. “Signals that are really robust are unlikely to be just random.”

A new experiment from Groh’s lab has now taken her observation a step further and suggests the faint sounds — dubbed “eye movement-related eardrum oscillations,” or EMREOs for short — serve to link two sensory systems. The eardrum oscillations contain “clean and precise” information about the direction of eye movements and, according to Groh’s working hypothesis, help animals connect sound with a visual scene.

“The basic problem is that the way we localize visual information and the way we localize sounds leads to two different reference frames,” Groh says. EMREOs, she adds, play a part in relating those frames.

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he brain, and not the eyes, must generate the oscillations, Groh and her colleagues say, because they happen at the same time as eye movements, or sometimes even before. To learn more about the oscillations, the team placed small microphones in the ears of 10 volunteers, who then performed visual tasks while the researchers tracked their eye movements.

The EMREO data showed that horizontal and vertical eye movements generate unique sounds, such that the researchers could calculate the specific direction and angle of a participant’s eye movements based on their EMREOs. The data also revealed that EMREOs contain “some information about the starting position of the eyes,” Groh says. The group published their results in Proceedings of the National Academy of Sciences in November.

The EMREOs do not affect a person’s ability to detect sound, according to findings published in the Journal of Neuroscience in November. That research came from Felix Bröhl, a postdoctoral researcher at the Max Planck Institute for Human Development, and his doctoral supervisor, Christoph Kayser, professor of neuroscience at Bielefeld University. The pair also replicated Groh’s 2018 discovery, and they are exploring if EMREOs shape other dimensions of hearing, such as spatial hearing.

“It makes sense that [an EMREO] doesn’t change the way we at least detect sounds,” says Brohl, but “so far, we can’t say what the functional specificity is.”

Whatever the purpose, this phenomenon is not limited to people; in the 2018 study, Groh and her colleagues also found EMREOs in rhesus macaques. The waveform frequency of the oscillations was higher in monkeys, they later demonstrated in a 2023 study, and the wave period, length of the EMREO and other features showed greater differences among individual monkeys than in people.

One reason for the observed differences might be that the signal in monkeys is “clearer,” says Stephanie Lovich, a graduate student in Groh’s lab who worked on the 2023 study, because many more of these experiments have been run in monkeys than in people. Regardless, the presence of EMREOs in both primates and humans suggests they might exist across other sections of the animal world.

“If our theory is right, there should be signals related to the pinna orientation in species that can move their ears,” such as rabbits, deer and mice, Groh says.

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heir next frontier is to explore the relationship between ear anatomy and eardrum oscillations, Lovich says. Muscles of the middle ear might also contribute to EMREOs, according to a study published in Hearing Research last March. Pulling on the tensor tympani muscle in particular can cause delays and attenuation in middle ear transmission, which could affect sound localization cues, the researchers found.

“The tensor tympani is kind of intriguing to us, actually, because of all the muscles in the human body, the role of the tensor tympani is not established,” says lead investigator Sunil Puria, associate professor of otolaryngology-head and neck surgery at Harvard Medical School.

Groh says she is now interested in analyzing eardrum oscillations in people with hearing issues, specifically middle-ear muscle or tensor-tympani-related conditions.

“Once we know something about the underlying mechanism,” Groh says, “it will set us up to more directly test the perceptual consequences.”