Light opens up a world of sound for the deaf
An interesting article about cochlear implants was published this week - in New Scientist!
In the current issue of the magazine, reporter Rachel Nowark describes how infrared light can stimulate neurons in the inner ear as precisely as sound waves, a discovery that could lead to better cochlear implants for deaf people.
A healthy inner ear uses hair cells that respond to sound to stimulate neurons that send signals to the brain. But hair cells can be destroyed by disease or injury, or can contain defects at birth, leading to deafness. In such cases, cochlear implants can directly stimulate neurons.
Rachel says, "The hearing provided by today's implants is good enough to enable deaf children to develop speech skills that are remarkably similar to hearing children's." She acknowledges that implant users still find it difficult to appreciate music, communicate in a noisy environment and understand tonal languages like Mandarin; but that's because the implants use only 20 or so electrodes, a small number compared to the 3000-odd hair cells in a healthy ear.
More sources of stimulation should make hearing clearer but more electrodes cannot be packed in because tissue conducts electricity, so signals from different electrodes would interfere. In contrast, laser light targets nerves more precisely and doesn't spread, which could allow an implant to transmit more information to the neurons.
To explore this idea, a team led by Claus-Peter Richter at Northwestern University in Chicago shone infrared light directly onto the neurons in the inner ear of deaf guinea pigs. At the same time, the researchers recorded electrical activity in the inferior colliculus, a relay between the inner ear and the brain cortex, producing a set of frequency "maps". These maps are a good indication of the quality of sound information sent to the brain.
Electrical stimulation of the inner ear by a cochlear implant produces blurred maps, but the light stimulation produced maps that were as sharp as those produced by sound in hearing guinea pigs, said Richter, who presented the findings at a conference this month.
It is a mystery how light stimulates the neurons, as they do not contain light-sensitive proteins. The phenomenon was discovered by surgeons attempting to "weld" nerves with heat from a laser. Richter says the heat that accompanies the light may play a role, and his team is now investigating the long-term effects of heating neurons.
Making fibre optics and lasers to target light in the inner ear is the next challenge.
Read more from the New Scientist here and more about Cochlear implants here.
(Photo source: Steve Gschmeissner/SPL)
