Study links low-frequency hearing to shape of the cochlea

Shape matters, even in hearing.

Specifically, it is 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 – which proves to be surprisingly important.

A study published online last week in the Proceedings of the National Academy of Sciences establishes a direct link between the cochlea’s curvature and the low-frequency hearing limit of more than a dozen different mammals.

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,” says 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.