July 2016 Imperial College London and Proc National Academy of Sciences
The inner ear processes low-frequency sounds, important for speech and music perception, differently to high-frequency sounds, new research has found. The exact mechanism for how the inner ear processes these important sounds is poorly understood, as the organ is difficult to access in experiments. Knowing the mechanism could help scientists to understand what happens when the process doesn’t work, causing hearing loss. It could also allow engineers to borrow from nature and design new audio equipment.
The new research, by an international team London, Sweden and the US, has shown one vital difference in the way high- and low-frequency sounds are processed by the ear. When a sound wave reaches a part of the inner ear called the basilar membrane, it causes it to vibrate. This in turn activates hair cells on the membrane which amplify the sound, allowing us to hear faint sounds. The hair cells also convert the movement into an electrical signal that can be interpreted by the brain. The region of the basilar membrane that vibrates in response to low-frequency sounds does not react in the same way as the region that responds to high-frequency sounds. The team found that low-frequency sounds move the basilar membrane and the hair cells, but the activity of hair cells causes little additional membrane motion. In contrast, high-frequency sounds, which are used for example to pick out speech in a crowded room, are known to cause the hair cells to feedback to the basilar membrane, creating extra vibration there.