Now, however, scientists have shown in animals that it's possible to implant a tiny, ultra-thin electrode array in the auditory nerve that can successfully transmit a wide range of sounds to the brain. The studies took place at the University of Michigan Kresge Hearing Research Institute.
If the idea pans out in further animal and human studies, profoundly and severely deaf people would have another option that could allow them to hear low-pitched sounds common in speech, converse in a noisy room, identify high and low voices, and appreciate music — areas where cochlea implants, though a boon, have significant limitations.
"In nearly every measure, these work better than cochlear implants," says U-M researcher John C. Middlebrooks. He led a study requested by the National Institutes of Health to re-evaluate the potential of auditory nerve implants. Middlebrooks is a U-M Medical School professor of otolaryngology and biomedical engineering. He collaborated with Russell L. Snyder of the University of California, San Francisco and Utah State University.
The possible auditory nerve implants likely would be suitable for the same people who are candidates today for cochlear implants: the profoundly deaf, who can't hear at all, and the severely deaf, whose hearing ability is greatly reduced. Also, the animal studies suggest that implantation of the devices has little impact on normal hearing, offering the possibility of restoring sensitivity to high frequencies while preserving remaining low-frequency hearing.
Middlebrooks says it's possible that the low power requirements of the auditory nerve implants might lead to development of totally implantable devices. That would be an improvement over the external speech processor and battery pack cochlear implant users need to wear and often have to recharge daily.
If the initial success in animals is borne out in further tests, a human auditory nerve implant is probably five to 10 years away, he says.
The researchers used cats bred for laboratory use in their experiments. They measured brain processing of auditory signals in normal conditions, then compared deaf animals' brain responses to sounds using cochlear implants and then the direct auditory nerve implants. These measurements employed neuron -monitoring technology developed earlier at U-M. The scientists found their sensitive 16-electrode microarray resulted in several advantages over cochlear implants.
Approved by the Food and Drug Administration in 1984, cochlear implants have greatly benefited profoundly and severely deaf people. More than 100,000 implants have been performed worldwide in the last two decades, including more than 1,000 at U-M.
Like the new device, cochlear implants are small electrode arrays that receive signals from an external sound processor. They are designed to stimulate the auditory nerve and other cells to produce a sensation of hearing. But their location, separated from auditory nerve fibers by fluid and a bony wall, is a limitation.
"Access to specific nerve fibers is blunted," Middlebrooks says. "The effect is rather like talking to someone through a closed door."With the new intraneural stimulation procedure, that effect is eliminated, and there are other technical advantages, too.
"The intimate contact of the array with the nerve fibers achieves more precise activation of fibers signaling specific frequencies, reduced electrical current requirements and dramatically reduced interference among electrodes when they are stimulated simultaneously," Middlebrooks says.
Middlebrooks has talked with U-M surgeons in otolaryngology about surgical approaches in humans, and is working with U-M biomedical engineers on an intraneural device that can remain in place and be tested further in animals over the next two years. The devices need to be studied over time to see if they are safely tolerated by the auditory nerve.
"If our work continues to go very well, we might begin human trials in no less than five years," Middlebrooks says.Such a device might be used first in people whose cochleas are filled with bone and therefore aren't eligible for a cochlear implant, or people whose cochlear implants are no longer effective.
The University of Michigan has submitted a patent application for the procedure. Through its Office of Technology Transfer, it is seeking a commercialization partner to assist in bringing the technology to market.