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NIDCD-funded Research Explores Use of Laser to Stimulate Auditory Nerve
The cochlear implant is a great biomedical success story, enabling tens of thousands of people with profound hearing loss to communicate effectively in today’s society. Nevertheless, the ability to decipher speech in more complex environments – such as when there is a lot of background noise – has remained difficult for many users. For this reason, the National Institute on Deafness and Other Communication Disorders (NIDCD), one of the National Institutes of Health (NIH), is seeking ways to improve upon the current technology and provide a more realistic listening experience for people using cochlear implants.
In one such project, NIDCD-funded researchers from Northwestern University are investigating the development of a cochlear implant that uses light, not electrodes, to stimulate the auditory nerve. Although their work is still being conducted on laboratory animals, the goal is to develop a more precise implant that helps people who have profound hearing loss to distinguish speech in noisy environments.
Cochlear implants are electronic devices that process sounds from the environment, bypassing damaged portions of the inner ear to directly stimulate the auditory nerve. While they do not restore normal hearing, cochlear implants can give a person with profound hearing loss a useful representation of sounds in the environment and help him or her to understand speech. Roughly 100,000 people worldwide have received implants.
Today’s cochlear implants convert sound into electrical signals that travel to electrodes surgically placed inside the cochlea, the snail-shaped part of the inner ear. Because the cochlea detects sounds on a gradient from high to low frequency, signals from high-pitched sounds are sent to electrodes at the cochlea’s entrance while those from lower-pitched sounds are sent to electrodes inserted further into the cochlea. The electrodes, in turn, stimulate adjacent auditory nerve fibers, which send an electrical signal to the brain. Although the use of electrodes in cochlear implants has been very effective over the years, an ongoing limitation has been that the electrical current often spreads beyond the targeted cells, stimulating broad regions of nerve tissue and interfering with the signal that is relayed to the brain.
Principal investigator Claus-Peter Richter, M.D., Ph.D., and colleagues suggest that a cochlear implant that uses low-energy infrared light from a laser to target the auditory nerve could zero in on small populations of nerve cells with pinpoint precision. Because the regions of stimulation are much smaller than the region stimulated by a conventional implant, the laser implant will better correspond to the cochlea’s “frequency-based” layout, providing more accurate information about sounds in the environment to the brain. Increased accuracy and less spread to non-targeted cells may also make it possible to target additional relevant nerve populations, ramping up the amount of useful information flowing into the brain and improving the brain’s ability to decipher speech sounds from background noise, the researchers contend. In addition, the optical fiber that would replace the implant’s electrodes and guide the light into the cochlea need not be directly touching the targeted tissue to stimulate it, minimizing potential damage to the tissue.
The idea to use light to stimulate individual nerve cells was first discovered by researchers at Vanderbilt University, who reported in 2005 that infrared light emitted from a laser could stimulate the sciatic nerve (a large nerve running down the back of the leg) in rats without damaging the nerve tissue. Current uses of lasers in medicine include the removal of retinal tissue, tumors, and skin anomalies, such as warts and moles, as well as the closing off of small blood vessels, lymph vessels, or nerve endings.
So far, Dr. Richter and his team have demonstrated in gerbils that infrared light can stimulate the auditory nerve and that small populations of auditory nerve cells can be selectively activated with no damage to the tissue, even after several hours of stimulation. Although the researchers do not yet know how lasers stimulate nerve cells, they hypothesize that heat absorbed by water inside the tissue is the most likely cause.
Dr. Richter and his colleagues, who were awarded a five-year, $1.68 million contract in September 2006 by the NIDCD, are attempting to identify the optimal conditions – including wavelength, pulse length, pulse frequency, and optical fiber position – required to selectively stimulate auditory nerve cells in several animal models. The team will also study key safety issues, such as whether the cochlea can be stimulated over several months without experiencing damage from overheating. The eventual goal is to develop a hybrid implant that uses both electrodes and a laser for possible clinical testing in people. Development of the laser is being conducted by Aculight Corp., Bothell, WA, which is also funded by a grant from NIDCD.