A few days in the dark can improve an animal's hearing, scientists report this week in the journal Neuron. This temporary loss of visual input seems to trigger favorable changes in areas of the brain that process auditory information, they say.
The finding suggests there may be a new way to help people with cochlear implants, tinnitus, and disorders that make it difficult to understand speech, says Patrick Kanold, a researcher at the University of Maryland and one of the study's authors.
"This won't help if you went to too many rock concerts," Kanold says. "But if this works in people, it might be useful for auditory processing disorders."
The study builds on research showing that people who are blind from birth can often do remarkable things with their other senses. "Blind people seem to have supernormal abilities in the auditory domain," Kanold says. "They are better in discriminating different frequencies or discriminating sound locations."
One explanation for these abilities is that when the brain has no visual input during early childhood, it tends to use areas that would ordinarily process visual information to process auditory information instead.
Scientists used to think that once early childhood was over, that kind of rewiring couldn't happen anymore. And that's why, several years ago, one of Kanold's colleagues was baffled by the result of an experiment that kept adult rodents in the dark for a few days.
Hey-Kyoung Lee, who is now at Johns Hopkins, was looking to see whether the light deprivation was affecting the animals' visual cortex. "And all of a sudden we started seeing stuff in the auditory cortex — which we did not really expect," she says.
To figure out what was going on, Lee teamed up with Kanold and got NIH funding for a number of experiments. In one test they kept mice in the dark for about a week. Then the researchers monitored electrical activity in the brain while the mice listened to tones of different pitches and volumes.
It turned out that mice kept in the dark had neurons that responded more strongly to both changes in pitch and changes in volume. "Their sensitivity to sounds had increased," Kanold says. And their brains had strengthened connections that carry auditory information.
Somehow, even though the mice were all grown up, a lack of input in the visual cortex seemed to be causing changes in the auditory cortex. "We were quite surprised to see the changes because there is no known anatomical connection that is directly between these two areas," Lee says. Also, she says, "it happened quite rapidly, which I really did not expect." And the effect persisted for weeks after the mice were back in the light.
The finding could be especially important for deaf people who receive cochlear implants, says Amal Isaiah, a surgeon at the University of Maryland and a co-author of the study. The brains of these people sometimes have difficulty processing the new auditory information coming from the implant, he says.
This problem is most common among people who were born deaf and didn't receive an implant until they were adults, Isaiah says. "Imagine yourself in that position, where you've never heard sound before and your brain is provided with that input," he says. "It doesn't know what to do next."
The mouse study suggests the possibility, Isaiah says, that spending some time in the dark might help the brain make sense of the sounds its hearing.
AUDIE CORNISH, HOST:
Some surprising news about hearing - scientists have found that just a few days without sight can improve the brain's ability to process sounds. The finding was in animals, but NPR's Jon Hamilton reports it could have implications for many people, including those who get cochlear implants.
JON HAMILTON, BYLINE: People who are blind from birth can often do remarkable things with their other senses. Patrick Kanold, at the University of Maryland, says hearing, for example, gets a measurable boost.
PATRICK KANOLD: Blind people seem to have super normal abilities in the audio domain. They are better in discriminating different frequencies or discriminating sound locations.
HAMILTON: Kanold says one explanation for this is that when the brain has no visual input during early childhood, it rewires circuits in the outer layer of the brain, called the cortex. He says areas that would ordinarily process visual information are instead used to process sounds.
KANOLD: There's a very early period in development when very large-scale changes of cortical wiring can take place so you can wire auditory information into the visual cortex, or vice versa.
HAMILTON: But scientists had thought that once early childhood was over, that kind of rewiring couldn't happen anymore, which is why one of Kanold's colleagues was baffled by something she was seeing in her lab. Hey Kyoung Lee was studying adult mice that were kept in the dark for a few days. She thought that might affect their visual cortex.
HEY KYOUNG LEE: And all of a sudden, we started seeing stuff in the auditory cortex, which we did not really expect.
HAMILTON: A lack of visual input seemed to be causing changes in an area of the brain that processes sounds. Lee, who is now at Johns Hopkins, says that was really puzzling at first.
LEE: And then it came to me one day that hey, wait a minute. Blind people are supposed to have better sound discrimination abilities; they also can read Braille. So I came to this conclusion, maybe this is how they can actually enhance their other senses.
HAMILTON: Even when blindness occurs after that critical period in early childhood. Lee teamed up with Patrick Kanold to find out whether the adult brain really could rewire itself in this way. Kanold says funding from the NIH allowed them to perform a series of experiments.
KANOLD: So what we are doing is take mice, place them in the dark for about one week. And then we put them in a sound chamber, present tones of various frequencies; and then record the electro activity of the brain.
HAMILTON: The mice heard tones with slightly different pitches ...
(SOUNDBITE OF TONES)
HAMILTON: ...and slightly different volumes.
(SOUNDBITE OF TONES)
HAMILTON: Kanold says they compared the electrical signals from neurons in mice who'd been in the dark with the signals from mice who'd lived normally, and they found that brain cells in the light-deprived mice responded more strongly to the changes in pitch.
KANOLD: In addition, neurons in mice that were in the dark are much better to detect very soft sounds. So their sensitivity to sounds has increased.
HAMILTON: Hey Kyoung Lee says somehow, even though the mice were adults, a lack of input in one area of the brain seemed to be causing changes in a completely different area.
LEE: We were quite surprised to see the changes because there is no known anatomical connection that's directly between these two areas.
HAMILTON: Yet, Lee says later tests showed that darkness was associated with stronger connections among cells that are important in processing auditory information.
LEE: The other surprising aspect of our work is that it happened quite rapidly, which I really did not expect.
HAMILTON: And the effect persisted for weeks after the mice were back in the light. Amal Isaiah is a surgeon at the University of Maryland and a co-author of the team's paper, which is published in the journal Neuron. He says the results suggest there may be ways to improve results for people who get cochlear implants. Isaiah says the greatest need is among people who are born deaf but don't get an implant until they are adults.
AMAL ISAIAH: Imagine yourself in that position where you've never heard sound before, and your brain is provided with that input. So it doesn't know what to do next.
HAMILTON: Isaiah says many of these people never get much benefit from the implants, but he says the mouse experiment suggests it's just possible that spending some time in the dark could help their brains make sense of the sounds they're hearing. And Isaiah says people who already use cochlear implants successfully might be able to do more with them, like listen to music.
Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.