The cochlea in the inner ear, where sound is transmitted to the brain, has a spiral shape resembling a snail shell.  It’s not just to save space, researchers have found.

For many years, physiologists assumed that the coiled shape of the cochlea simply saved space.  Six years ago, though, scientists found that the coiling has an auditory enhancement function: it improves perception of low-frequency vibrations, like having a mega-bass booster in your head (see 2/28/2006).  Now, another function has been found for the peculiar spiral shape: it helps you locate sound vertically.

PhysOrg reported on work in China that tentatively identifies the spiral shape as critical for detecting the vertical orientation of a sound source.   Having two ears helps us locate the direction of sound horizontally; that’s because the sound waves arrive at each ear a tiny fraction of a second apart, and the brain can use that delay to help us perceive stereo sound.  That’s fine and good for a concert hall or headphones, but what if the sound is at different distances overhead?  Two ears don’t help in that case, because each ear hears the sound at the same time.  Bats, in particular, need that critical information as they hunt insects in the dark.

Measurements of “beamforming” showed the researchers that a coiled cochlea carries more information than a straight cochlea when detecting vertically-displaced sounds.  They believe the brain uses this extra information for vertical sound orientation.  They plan to test this hypothesis further.

“The finding that vertical sound localization can be improved purely by geometric changes supports the argument that the cochlea’s coiled shape is useful not just for conserving space,” they said.  “The results could be helpful for designing cochlear implants and echolocation systems, in which sound waves are used to detect objects.”

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