Hearing ability is also determined by shape, scientists at the Vanderbilt University have shown. They report that there is a direct connection between the curvature of the cochlea and the threshold of hearing in several mammals.
The team established the surprisingly important role of the shape of the cochlea, the snail-shell-shaped organ in the inner ear that converts sound waves into nerve impulses that the brain deciphers.
The scientists said that the relationship will be useful in conservation to estimate the impact that the noises of human activities are having on animals like Siberian tigers, polar bears and marine mammals that won't sit still for hearing tests.
It also can provide new information about the hearing of extinct mammals, like mammoths and saber-toothed tigers, and, in so doing, may contribute new insights into how the sense of hearing evolved.
"It turns out that it is the curvature of the cochlea, not its size, that is highly correlated to the low-frequency hearing limit," said Daphne Manoussaki, assistant professor of mathematics at Vanderbilt University, who headed the new study with Richard S. Chadwick, a section chief at the National Institute on Deafness and Other Communication Disorders (one of the National Institutes of Health, or NIH).
Spiral-shaped cochleae are exclusive to mammals. Birds and reptiles generally have plate-like or slightly curved versions of this critical organ, limiting the span of octaves that they can hear. Animals with tightly coiled cochleae tend to have greater hearing ranges, but previous attempts to associate these auditory effects with the physical characteristics of the cochlea have proven unsatisfactory because they did not take a critical acoustic effect into account.
In 2006 Manoussaki and her NIH collaborators published a paper proposing that the helical shape of the cochlea enhances low-frequency sounds through an effect analogous to the well-known "whispering gallery effect" in which soft sounds that travel along curved walls in a large chamber remain loud enough that they can be heard clearly on the opposite side of the room.
When sound waves enter the ear, they strike the eardrum and cause it to vibrate. Tiny bones in the ear amplify and transmit these vibrations to the fluid in the cochlea, creating pressure waves that travel along a narrowing canal in the coiled tube-like organ. The canal is one of two main chambers that are created by an elastic membrane that runs the length of the cochlea.
The mechanical properties of this "basilar" membrane vary from very stiff at the broad, outer end to increasingly flexible toward the inner end as the chambers narrow. The basilar membrane's graded properties cause the waves to grow and then die away. Different frequencies peak at different positions along the membrane.
Sensory cells are attached to the basilar membrane and have tufts of tiny hairs called stereocilia that stick up into adjacent structures in the canal. As the basilar membrane moves it tilts the sensory cells, causing the stereocilia to bend. The motion generates electric signals that travel along the auditory nerve to the brain. As a result, the sensory cells near the outer end of the cochlea detect high-pitched sounds, like the notes of a piccolo, while those at the inner end of the spiral detect lower-frequency sounds, like the booming of a bass drum.
This mechanical ordering of response from high to low frequencies works in the same fashion whether the cochlear tube is laid out straight or coiled in a spiral. But Manoussaki's calculations predicted that the spiral shape causes the energy in the low-frequency waves to accumulate against the outside edge of the chamber. This uneven energy distribution, in turn, causes the membrane to move more toward the outer wall of the chamber, enhancing the bending of the stereocilia. The enhancement is strongest at the apex of the spiral, where the lowest frequencies are detected.
Manoussaki and her collaborators calculated that the increase in the sound pressure level can be as much as 20 decibels, equivalent to the difference between the aural ambience of a quiet restaurant and a busy street.
"The idea that the cochlea's curvature has a significant effect on hearing has been quite controversial for many years," said Darlene R. Ketten, a senior scientist at Woods Hole Oceanographic Institution and assistant professor at the Harvard Medical School, who participated in the current study.
"Curvature was often dismissed or, when examined, the theories were not entirely satisfactory. Now we have a theory that we have confirmed with a number of concrete examples using real ear shapes and hearing abilities," Ketten added.
Ketten provided Manoussaki and her collaborators with high-resolution CT scans of the cochleae of a number of different species of land and marine mammals. Together with her biophysicist colleagues, Manoussaki analyzed these shapes and found that low- frequency hearing limits of species ranging from mice to cats to cows to whales varied in step with the ratio of the radii of curvatures at their cochlea's base to that of its apex. This ratio varies from about two to nine: The larger it is the lower the frequencies that the animal can hear.
"What I like about this is that a macroscopic feature of the ear has such a major effect on our hearing. As colleagues have pointed out, so much research today is done at the genetic and cellular level that you don't often see cases like this where simple geometry proves to be so important," said Manoussaki.
The study is published online in the Proceedings of the National Academy of Sciences.