Implant devices can give 'perception' of hearing to deaf, severely hearing-impaired

From: Portsmouth Herald News - Portsmouth,NH,USA - Dec 26, 2004

By Michael Keating
mkeating@seacoastonline.com

"Hearing" with a bionic ear or cochlear implant is not the same as what the hearing world takes for granted, according to a leading New England specialist.

"You could imagine that sound is used to communicate many different kinds of information," said Dr. Donald Eddington, director of the Cochlear Implant Research Laboratory at the Massachusetts Eye and Ear Infirmary in Boston, a teaching hospital for the Harvard Medical School. "Music communicates feeling, all kinds of emotions, and this is usually quite different from speech communication. With the cochlear implant the main goal is speech recognition."

Not everyone who is deaf or severely hearing-impaired can benefit from a cochlear implant, said Eddington. The amount and type of damage to the inner ear nerves are primary factors in determining whether a person can benefit from the technology, which has been developing since the mid-1960s, he said.

Some patients who are completely deaf or impaired find perception so distorted that they cant get any message out of it, while others who have some residual hearing can see lips and combine lip-reading cues giving them the ability to get meaning - a dramatic improvement - so they can communicate much more fluently, said Eddington

The key word here is "perception." Cochlear implants give a patient a "perception of hearing," which can be strikingly different than actual sounds which convey information.

"For most people receiving cochlear implants, their impact is very positive in improving their ability to communicate, but they are mostly using them in conjunction with lip-reading," said Eddington. "It becomes a very natural phenomena and they dont even realize theyre lip-reading anymore. When I first turn it on and ask, How does it sound?, some say, Perfect. Then when I ask them again with my hand covering my lips they realize they were lip-reading."

HOW IT WORKS

Cochlear implants consist of two parts - one that is implanted in the head and the other is worn externally behind the ear, said Eddington.

The external part has a microphone that senses sound and translates that into an electrical representation. This is processed by the external sound processor, which analyzes that signal and determines how to stimulate the 16 to 22 electrodes (tiny metal contacts) implanted in the inner ear that actually deliver the electronic stimulus to the nerve fibers, he said. The external processor determines how those electrodes should be stimulated and sends instructions to accomplish that to the internal packet of electronics implanted behind the ear. These internal electronics then send the electric stimuli to the electrodes as instructed.

The outer device also sends in the electronic power to operate the internal complex.

Tiny electrodes activate the remaining nerve fibers and its that activation that sends signals to the brain, which the brain interprets as sound, said Eddington. But is the "sound" conveyed through the cochlear implant the same "sound" heard by an otherwise normal hearing person?

"Thats the goal," said Eddington. "The auditory nerves are a bundle of about 30,000 nerve fibers. In normal hearing, some code high and some code low for frequency information." Think of the fibers as tiny lights and you would see consistent light patterns as they receive certain sounds or words, said Eddington.

"Ahh has a very specific pattern of nerve activity, and that is the pattern the brain has learned to label as Ahh," he said. "Its the same for E (and all the other sounds). These patterns arent there anymore for a person who is completely deaf, so the idea is to produce this pattern artificially. But the pattern produced by a cochlear implant is not identical to that of a normal-hearing persons and thats where the clarity problem comes in. Given that youre only working with 16 to 22 electrodes, even if all the nerve fibers were there it would be impossible to create the same pattern."

And thats where the research stands today, said Eddington "Were trying to develop patterns that are more like what a normal-hearing person has."

The best predictor of future success is the historical record, said Eddington. When the device was first introduced in the 1960s, only about 5 percent to 8 percent of patients using the implant without lip-reading were able to recognize single syllable words, he said. Today, the median score for this type of speech recognition is at about 40 percent, he said.

With both manufacturers of the devices and the National Institutes of Health supporting research, Eddington remains optimistic for advancing progress.

"There has been steady improvement over time and one expects that kind of improvement will probably continue," he said.

Web sites:

* Massachusetts Eye and Ear Infirmary : www.meei.harvard.edu/shared/oto/cochlear.php

* National Institute on Deafness and Other Communication Disorders : www.nidcd.nih.gov/health/hearing/coch.asp

1 Sound processor

* Captures sound from the environment

* Processes sound into digital information

* Transmits to the implant over a transmitting antenna, or headpiece, held in place by magnets in both the headpiece and implant

2 Implant

* Converts digital information into electrical signals

* Sends signals down tiny wires to the electrode array in the inner ear

3 Electrode array

* Delivers electrical signals through tiny contacts, or electrodes, to the hearing nerve

* The hearing nerve carries the sound information to the brain, where it is heard

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