A symphony of light: Mid-IR study could lead to new hearing implant

Laura S. Marshall, laura.marshall@laurin.com

Light has always been a best friend to the deaf. Without light, deaf people would not be able to see the hand shapes and movements that make up the sign languages of the world; a speaker’s face, with all the visual cues that help them to understand speech; or the written word when other types of communication fail. And a Northwestern University researcher is working on a device that could deepen that friendship.

Dr. Claus-Peter Richter has used mid-infrared light to stimulate auditory nerves in guinea pigs, which he said are established models for cochlear implant research because acoustical information is coded similarly in human and guinea pig auditory nerves. Richter is an associate professor in Northwestern’s department of otolaryngology, the director of resident research, an adjunct associate professor for biomedical engineering and communication sciences and disorders, and a fellow of the Hugh Knowles Foundation.

Dr. Claus-Peter Richter’s team at Northwestern University has created this schematic of a potential optical cochlear implant. In this design, an array of tiny optical sources is implanted in the inner ear, allowing pulses of mid-infrared light to stimulate the auditory nerve cells. Courtesy of Northwestern University.

He and his team studied pure tone tuning curves, which show the frequencies to which auditory fibers respond, gathered from the auditory processing centers in the guinea pigs’ brains. In the experiments, the tuning curves were both acoustically and optically evoked. “The results show that optically evoked neural activities are extremely selective and have a wide dynamic range,” Richter said.

The research could lead to a new form of implant, one that relies on light instead of electricity to help deaf people hear.

For many people with hearing loss, the problem is in the nerves. In sensorineural hearing loss, a malfunction somewhere along the vestibulocochlear nerve, in the inner ear, or in the brain’s auditory processing center leaves a person unable to receive or interpret auditory signals. The cause ranges from illness or drugs to trauma or exposure to loud noise.

Light-based solution

Today’s cochlear implant works by skipping over the parts of the ear that don’t work. “In individuals with severe to profound hearing loss,” Richter said, “cochlear implants bypass normal inner ear function by applying electrical current directly into the cochlea, thereby stimulating remaining, functional cochlear nerve fibers.”

The device consists of external parts: a microphone, a speech processor and a transmitter; and internal parts: a receiver and a stimulator, which is implanted into the skull and which converts the signals from the transmitter into electrical impulses and passes them along to an array of electrodes in the cochlea. The electrodes stimulate the auditory nerves, and the signals are sent to the brain for processing.

In a light-based implant, pulses of mid-infrared light would replace electrical stimulation. The advantage, according to Richter, would be selectivity.

“Pulsed mid-infrared optical stimulation has a fundamentally different interaction with tissue than electric current,” Richter noted. A problem with electrical stimulation, he said, is that it can spread through tissue, making the signal less clear. “For normal listeners, speech recognition improves with an increasing number of frequency bands available for the listener. For cochlear implant users, however, speech recognition scores increase only to a maximum of about ten electrode contacts, a finding which has been explained in part by the overlap in stimulation of spiral ganglion cells by the electrodes.

“One goal of implant device development is to design cochlear implants that can stimulate smaller independent populations of spiral ganglion cells, thereby improving performance.”

With mid-infrared stimulation, he added, “the radiation is confined to the optical path and spreads minimally in tissue.” The result? A clearer signal than has ever been possible with existing cochlear implant technology.

With that goal in mind, Richter said his group has teamed up with laser company Lockheed Martin Aculight and by March will have readied a four-channel stimulator device for chronic safety testing. If all goes smoothly, he said, the researchers hope to obtain an investigational device exemption, which permits testing in humans, by the end of 2010.