Making Deaf Ears Hear with Light

A laser-based approach could make cochlear implants, which currently use electrical signals, more effective.

About 100,000 profoundly deaf people now hear with cochlear implants, which work by stimulating the auditory nerve with a string of electrodes implanted in the inner ear. While the devices enable many users to converse easily and use telephones, they still fall short of restoring normal hearing. Now scientists at Northwestern University are exploring whether laser-based implants could one day outperform today's electrical version.

Sound effects: An optical cochlear implant would bypass these inner ear hair cells (above), which are nonfunctional in many cases of profound deafness. Optical radiation would stimulate the neurons that are normally triggered by the motion of these microscopic fibers in response to sound vibrations. Credit: Steve Gschmeissner / Photo Researchers, Inc.

The mammalian ear uses neural firing rates as one way of encoding sound. As part of a project funded by the National Institute for Deafness and Other Communication Disorders (NIDCD), Claus-Peter Richter and his colleagues at Northwestern have demonstrated that they can control firing rates in the auditory nerve of animals using infrared laser radiation. They are now trying to establish that it's safe to use for long periods of time and that it can manipulate neural firing rates with enough precision to send useful information to the brain.

With conventional cochlear implants, electrical signals spread in the wet, salty environment of the body, muddying the signal. That makes it difficult to trigger specific populations of nerves inside the cochlea. Further complicating matters, simultaneous pulses in different locations merge with each other, stimulating the cochlea everywhere instead of in the desired locations.

Engineers work around the problem by triggering only one or two of the 16 or 24 electrodes in the inner ear at a time. It's done so rapidly that the user has the illusion that all of the electrodes are firing, but the result is still a relatively crude simulation of normal hearing. To many cochlear implant users, voices sound mechanical and music sounds washed out.

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An infrared laser, on the other hand, can be beamed at nerve fibers with pinpoint accuracy. Furthermore, the directional nature of laser light means that optical pulses in different places won't interfere with each other. The increased precision of neural stimulation would make voices and music sound more natural, and users would be able to converse in noisy environments more easily.

While it's not yet clear why infrared radiation can trigger activity in the auditory nerves, Richter hypothesizes that it heats the cells slightly, opening ion channels in the cell walls and sending an electrical signal down the length of the neuron.