Cochlear Implants: Helping the Deaf to Hear

For the profoundly deaf, a device called the cochlear implant that has been around since the late 1980s may, in some cases, be the differencebetween hearing sounds and not hearing sounds. Unlike a hearing aid,that amplify sounds, the cochlear implant takes over the job ofprocessing sounds that damage nerves can no longer do. Some sixty thousand people world-wide have had cochlear implants installed andhave recovered some to a great deal of lost hearing.

What is Hearing?

The outer ear detects sound, which is the vibration of the air causedby anything from a person speaking to a CD player making music. Thesound then travels down the ear canal to the ear drum, a thin, coned shaped piece of skin about .4 inches in diameter. The ear drum vibrates when sound impacts it. The vibrations are transmitted to a series of bones in the middle ear, known as the malleus, the incus, and the stapes. These bones in turn transmit sound vibrations to the fluidfilled cochlea or inner ear.

Inside the cochlea, the basilar membrane, is a rigid surface that extends across its length. When the stapes moves in and out, itpushesand pulls on the basilar membrane. This force starts a wave moving along the surface of the membrane. The wave travels something like ripples along the surface of a pond, moving from the oval window down to the other end of the cochlea. The basilar membrane has a peculiar structure, made of 20,000 to 30,000 reed-like fibers thatextend across the width of the cochlea. A specific wave frequency will resonate perfectly with the fibers at a certain point, causing them to vibrate rapidly.

The organ of corti is a structure containing thousands of tiny hair cells. It lies on the surface of the basilar membrane and extends across the length of the cochlea. The wave energy in the basilar membrane is strong enough to move the organ of corti hair cells.When these hair cells are moved, they send an electrical impulse through the cochlear nerve. The cochlear nerve sends these impulses onto the cerebral cortex, where the brain interprets them.

What is Hearing Loss?