By coiling up like a snail shell, the human inner ear concentrates the energy of sound, increasing sensitivity in the bass range, researchers have found. In different mammals, the tightness of the coil affects the range of frequencies a species can hear.
The coiling of the cochlea — the fluid-filled structure of the inner ear that detects sound and turns it into nerve signals — guides sound the way a whispering gallery does, explains Daphne Manoussaki, a mathematician at Vanderbilt University in Nashville. The whispering-gallery effect is heard inside some rotundas, where the shape of the dome carries sound from one end to the other, so that two persons standing at opposite ends can hear each other whisper.
But the cochlea has an added twist, Manoussaki and her collaborators have now shown. Its spiral curves less tightly at the outer end, where sound waves enter, and then tightens up more and more toward the center. As sound travels inside the cochlear canal, it bounces off the inner walls. The increasingly curved walls shift more of the energy in the sound’s waves to the outer edge of the canal, computer simulations revealed.
“The coiling has an extra effect, of throwing the energy more toward the outside wall,” says study coauthor Richard Chadwick of the National Institute on Deafness and Other Communication Disorders in Bethesda, Md. The result looks a bit like what happens during a washing machine’s spin cycle, when centrifugal forces flatten clothes toward the drum — except that in the ear there aren’t any actual centrifugal forces involved.
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Because the energy is focused, sound-sensing hair cells inside the cochlear canal will vibrate more if they are toward the outer side than will the cells on the inner side. It appears that the cells then detect a shear in the movement of the cells’ hairs. The shear will be especially boosted toward the center of the cochlea, which specializes in low-frequency hearing.
Manoussaki’s collaborators also compared the geometry of the cochlea across a dozen different species of mammals. Using high-resolution CT scans, the researchers measured the change in coiling tightness between the beginning and the end of the cochlea. The larger that change, the lower the frequency a species can hear, the researchers found.
In rats, for example, the cochlea coils about three times as tightly at the center than at the entrance, while in gerbils it coils almost seven times as tightly. Correspondingly, rats are almost deaf below a few hundred hertz, while gerbils can hear frequencies as low as 56 hertz — handy when you live in a low-frequency world of tunnels.
In humans, the coiling gets more than eight times as tight, and our hearing range extends even lower than gerbils’. But elephants and cows, whose coiling tightens by almost a factor of nine, do even better. Other anatomical differences between species also play a role in bass sensitivity, but the researchers say that the change in coiling tightness accounts for 70 percent of the variation across species.
The results, which appear in the April 22 Proceedings of the National Academy of Sciences, should put some older theories to rest, says coauthor Darlene Ketten, a functional anatomist at the Woods Hole Oceanographic Institution in Massachusetts. In particular, her team found no correlation between the length of the unrolled cochlea and the lowest frequencies heard, though she says another correlation is true: “Typically, the longer the cochlea, the more octaves you can hear.”
And coiling, she adds, doesn’t just allow the cochlea to fit in the tight space available in the skull, as some previously believed. DavidMountain, a biomedical engineer at BostonUniversity, agrees. “Most researchers have assumed that the spiral shape of the cochlea was to save space,” he comments. “Most previous theoretical studies of the cochlea made the simplifying assumption that the cochlear duct is straight because it was believed that the effects of the curvature were minimal.”