February 6, 2014
Spending A Week In Darkness May Improve Hearing Loss
[ Watch the Video: Lights Out For Your Hearing ]
April Flowers for redOrbit.com - Your Universe Online
It has long been known that if you take away once sense from a person, the other four will become more finely tuned. A new study, published in Neuron, reveals that minimizing a person's sight for as little as a week may help improve the brain's ability to process hearing.
Stevie Wonder and Ray Charles – both blind musicians – are often cited by music experts as examples of how a lack of sight can heighten or enhance hearing. The mechanism behind this enhancement was not fully understood until now.
Hey-Kyoung Lee, an associate professor of neuroscience and researcher at the Mind/Brain Institute at the Johns Hopkins University, along with biologist Patrick Kanold, of the University of Maryland, College Park, collaborated to examine the relationship between vision and hearing in the brain. The team used mice to uncover how the neural connections in the area of the brain that manages vision and hearing work together to support each sense.
"In my opinion, the coolest aspect of our work is that the loss of one sense – vision – can augment the processing of the remaining sense, in this case, hearing, by altering the brain circuit, which is not easily done in adults," Lee said.
"By temporarily preventing vision, we may be able to engage the adult brain to now change the circuit to better process sound, which can be helpful for recovering sound perception in patients with cochlear implants for example," she said.
Healthy adult mice were placed in a darkened environment for about a week to simulate blindness. The researchers monitored the response to certain sounds, which were compared to a second group of mice kept in a traditional, naturally-lit environment.
The team observed a change in the brain circuitry for the mice that experienced the simulated blindness. This change occurred in the area of the brain called the primary auditory cortex, which processes sound and allows conscious perception of pitch and loudness.
To test this circuitry, the team played a series of one-note tones while testing individual neurons in the middle layer of the auditory cortex – a region that receives signals from the thalamus, which acts as a switchboard for sensory information.
The mice that experienced simulated darkness showed neurons that fired faster and more powerfully when the tones were played. They were also more sensitive to quiet sounds, could discriminate sounds better and developed more synapses, or neural connections, between the thalamus and the auditory cortex.
"Our result would say that not having vision allows you to hear softer sounds and better discriminate pitch," said Lee, an expert on how the brain processes vision. "If you ever had to hear a familiar piece of music with a loud background noise, you would have noticed that sometimes it seems the beat or the melody is different, because some of the notes are lost with the background. Our work would suggest that if you don't have vision you can now rescue these 'lost' notes to now appreciate the music as is."
The thalamocortical inputs are a set of connectors in the primary sensory areas of the brain, which are less flexible later in a human's life. The researchers concluded that when another sense is impaired, however, those connectors can be reactivated to support the sense that is lagging.
The researchers are hopeful that their findings will apply to humans as well.
"This makes me hopeful that we would see it in higher animals too," including humans, he said. "We don't know how many days a human would have to be in the dark to get this effect, and whether they would be willing to do that. But there might be a way to use multi-sensory training to correct some sensory processing problems in humans."
So far, the changes detected in the brain are reversible. After a few weeks in a normal light-dark environment, the mice that experienced simulated blindness eventually reverted to normal hearing. Lee and Kanold intend to look for ways to make the sensory improvement permanent in future research, which will also look beyond individual neurons to study broader changes in the way the brain processes sounds.