Researchers
at Northwestern University in Chicago have made a new discovery that
could lead to better cochlear implants for deaf people. They have
found that infrared light can stimulate neurons in the inner ear as
precisely as sound waves, reports New Scientist. A healthy inner ear
uses hair cells that respond to sound to stimulate neurons that send
signals to the brain. However, 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.
The
hearing provided by implants is good enough to enable deaf children
to develop speech skills that are remarkably similar to hearing
children's. However, implant users still find it tough to appreciate
music, communicate in a noisy environment and understand tonal
languages like Mandarin and 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. On the contrary, laser light targets nerves more
precisely and doesn't spread, which could allow an implant to
transmit more information to the neurons.
In order to explore
that idea, a research 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.
Richter
said that 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. While it's a mystery how light stimulates the neurons, as they
do not contain light-sensitive proteins, Richter hypothesizes the
heat that accompanies the light may play a role, and his team is now
investigating the long-term effects of heating neurons. The findings
were presented at the Medical Bionics conference in Lorne, in the
Australian state of Victoria, earlier this week.
