While implant technology has improved, says one doctor, "what's more remarkable is the ability of the brain to adapt to man-made, computer-processed signals . . . "

In normal hearing, sound waves enter the ear canal and strike the eardrum, which then vibrates. The vibrations travel along the tiny bones of the middle ear to the oval window, the membrane at the entrance to a coiled organ known as the cochlea.

The cochlea, shaped like a snail shell, is lined with hairs and filled with fluid. The vibrations against the oval window ripple through the fluid, causing the hairs to bend. This movement creates an electrical disturbance that nerve cells pick up and transmit to the brain, resulting in the sensation we know as hearing.

In some types of hearing loss, the cochlea's little hairs are damaged or absent. The cochlear implant essentially attempts to substitute for them.

A surgeon inserts the slender electrode array (with 12 to 22 electrodes), which curls inside the cochlea. A wire extends from the electrodes to a receiver under the skin above the ear. Just across it, a coil rests on the surface of the skin. The coil is wired to a tiny computer, called a speech processor, which hooks over the ear.

The speech processor has a microphone. When the microphone picks up sound, the speech processor analyzes it and codes it into a pattern of electrical impulses, which travel to the coil and are transmitted across the skin by radio waves. The implant receives the impulses and sends them into the cochlea, where the auditory nerve picks up the signal to take to the brain.

In the brain, the signal is perceived as sound, but it's not normal hearing. Especially at first, the sound can be metallic or cartoony, or just plain noisy. But as time goes by, the brain adapts, learning to separate meaning from clatter.

"The human inner ear has tens of thousands of channels," said Dr. Daniel J. Lee, director of the Cochlear Implant Program at the UMass Memorial Medical Center, in Worcester. "We're getting away with 22 or 12. Although the technology has improved, what's more remarkable is the ability of the brain to adapt to man-made, computer-processed signals and to be able to interpret that. It's remarkable testament to the adaptability of the brain."

The process of inserting the implant damages the cochlea so that any remaining natural hearing is destroyed. But the implant manufacturers are working to design devices that preserve hearing.

Most implant users have a hard time enjoying music. The implants can't process the subtleties and complexities of music (although manufacturers are working on improving this, too). When Brian Hubbard, a deaf-blind man from Newport, received his first cochlear implant, he enjoyed hearing high-pitched sounds he'd never heard before. But he was surprised that his favorite songs no longer sounded good.