POSTED 22 FEBRUARY 2007
Hearing is believing
After covering various experimental devices for replacing broken neurons, we come to the cochlear implant -- an on-the-market sound system that electrically stimulates the cochlea, which sends nerve signals that the brain interprets as sound.
Courtesy Michael Chorost
Author Michael Chorost was born with profound hearing loss in 1964, and then went completely deaf in 2001. It was a "very frightening day, but even within the first hour, when I realized my ears were going wrong, I immediately thought about cochlear implants," he says. "I was already familiar with the technology, so it felt exciting as well."
But the implant was no panacea -- at first. "It was completely different than I expected. I was very surprised, and not pleasantly so," he says. "Everyone kept reassuring me that if I kept working at it, listening, I would be able to make sense out of what sounded like gibberish." Over two years, he learned to accommodate to the implant; he says he is now "hearing pretty reliably." Chorost described his experience in Rebuilt: How becoming part computer made me more human."
One major drawback of cochlear implants is their one-way nature, Chorost says. In normal hearing, but not implants, the brain adjusts the cochlea in response to what it is hearing. "A prosthetic that simply sends information to the nerve ending has a serious limit because it is unable to participate in the circular loop of action and feedback that the body engages in. Hearing is not a passive process; the brain sends neural feedback to the ear, changing the hair cells, but the cochlear implant has no way of picking up that feedback and so is unable to refine its output."
Most patients get only one cochlear implant, but new evidence shows that two are much better for locating the source of sounds, as Ruth Litovsky, a researcher in the University of Wisconsin-Madison Waisman Center, recently reported.
In results presented Feb. 13 to the Association for Research in Otolaryngology, Litovsky reported on a study of 55 deaf children who received a second cochlear implant one to seven years after the first one. The children were placed inside a circle of loudspeakers, and asked to locate the source of sounds. When the test was repeated with one only one implant working, sound localization largely disappeared.
The test also produced a strong sign of how valuable the second implant was to these users, who did not "like to remove an implant," says Litovsky. "We have to barter for that, with M&Ms or something else that motivates them."
The results also highlighted the role of learning in neural prosthetics: With experience, the kids got better at locating sounds. "We're now seeing that the ability to localize sounds takes time to emerge," says Litovsky. "What seems to get better is the integration of the information from the two ears in the brain."
Learning by doing
The brain's ability to adapt -- to learn -- is something prosthesis makers should learn to exploit, according to Michael Merzenich, a neuroscientist at the University of California at San Francisco. He notes that the signal from an artificial cochlea is vastly simpler than that supplied by original equipment, where 32,000 hair cells supply 32,000 channels of information to the auditory nerve and then to the brain.
In contrast, an implant supplies at most 22 signals. Hearing with an implant, Merzenich says, "Is like playing Chopin with your forearms."
For an implant to work, Merzenich says, the brain must "collaborate in the recovery. Using the device effectively requires learning; the brain has to readjust, make use of the new input."
Adapted graphic from FDA
People like Chorost, who learned language before going deaf, have full access to language information, Merzenich says, but they need to associate past learning with the new inputs. "The brain collaborates in creating the new constructs. It is very much involved." Without changes in the brain's wiring, "the recovery does not occur."
The brain, as Merzenich and others have shown, changes as it learns, and these changes continue into adulthood, no matter what you may have heard to the contrary. The brain "is remodeled in detail as we acquire skills and abilities, it has a remarkable adaptive ability," Merzenich says. "One challenge is to use this incredibly powerful, self-organizing machine to facilitate the prosthetic device."
Cochlear implants have proven that the brain can make do with an extraordinarily limited sensory supply, Merzenich says. "In this apparently dramatic recovery, the sound perception itself is not normalized, but who cares?"
We normalized our bibliography!
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive