How Does Our Ear Reflect the Sounds?

A healthy ear reacts to the sounds it receives, emitting soft sounds in response. These sounds can be detected by sensitive microphones, which enable doctors check newborns' hearing, as a deaf ear doesn't reflect the sounds. A new study published in the "Proceedings of the National Academy of Sciences" and made by a team at the University of Michigan and Oregon Health and Science University reveals that, oppositely to the current scientific concept, the echo doesn't have the same route as the sound that entered the ear, confirming a previous controversial research.

"The former wisdom on how otoacoustic emissions left the ear was that there was a backward-traveling wave going along the structure of the cochlea in the same way as the forward-traveling sound wave. These measurements show that is not the case," said co-author Karl Grosh, a professor in the U-M departments of Mechanical Engineering and Biomedical Engineering.

"The next step is to develop tools to find out where hearing damage is occurring. If we want to try to infer from the emission what's wrong with the ear, we have to understand how the emission is produced," said Grosh.

The experiments have revealed that the sounds coming out go through the fluid of the inner ear, rather than along the cochlea's basilar membrane. The cochlea, found in the inner ear, has a snail shape, and the basilar membrane splits the inner channel of the cochlea diametrically into two equal liquid-filled chambers.

Sound waves enter the inner ear and undulate along the basilar membrane, triggering auditory sensations in the organ of Corti, which sends auditory impulses to the brain via the auditory nerve. Sounds leaving the ear appear now to move through the fluid on both sides of the basilar membrane. The team employed laser interferometers to determine vibrations of the basilar membrane as a reaction to sound, at two points in the cochlea of gerbils (desert rats), fact that showed that backward-traveling waves do not use the membrane.

"Our new method can detect vibrations of less than a picometer, 1,000 times smaller than the diameter of an atom. The new data demonstrate that there is no detectable backward-traveling wave at physiological sound levels across a wide frequency range. This knowledge will change scientists' fundamental thinking on how waves propagate inside the cochlea, or how the cochlea processes sounds," said lead researcher Tianying Ren, associate professor at Oregon Health and Science University.