Brain Facts:
Hearing

Brain’s Goals

The brain has two major goals for sound information: to locate a sound in space, so you can look towards the sound’s source, and to identify the sound. Neither task is easy. Each is accomplished in different parts of the brain.

Brainstem

Sound information from the two ears is brought together in the neurons of the brainstem.

Cochlea

The outer ear transmits sound waves to an organ in the inner ear call the cochlea. The cochlea contains the ear’s sound-sensing cells, which are arranged in row along a long, coiled membrane. Sound pressure moves the fluid in the ear, causing the membrane to vibrate in different ways depending on the sound’s frequencies. This vibration activates the sensors, called hair cells because they have a bundle of fine fibers that stick up from the top of the cell like a punk hairdo. Movement of these fibers transforms the vibration signal into an electrical signal that can be understood by other neurons.

Loss of Hearing

One-third of people over sixty and half those over seventy have hearing loss. The most common cause is long-term exposure to loud noises. Baby boomers are losing their hearing earlier than their parents and grandparents did, presumably because our worlds are noisier than they used to be.

Noise

Noise causes hearing loss by damaging hair cells, which detect sounds in the inner ear. Hair cells have a set of thin fibers call the hair bundle extending from their surface that moves in response to sound vibrations. If the hair bundle moves too much, the fibers can tear, and that hair cell will no longer be able to detect sound. The hair cells that respond to high-pitched sounds (like a whistle) are most vulnerable and tend to be lost earlier than the hair cells that respond to low-pitched sounds (like a foghorn). That’s why noise-related hearing loss tends to begin with difficulty in hearing high-pitched sounds.