Hearing Loss

From Ray Charles to Daredevil, popular culture has long held that blindness can sharpen your hearing. Now, neuroscientists have found not only that this is true, but they've also discovered how it happens.

A new study, co-authored by Hey-Kyoung Lee, an associate professor at the Mind/Brain Institute at Johns Hopkins University, and Patrick Kanold, an associate professor at the University of Maryland, College Park, was published in the journal Neuron. It describes how depriving mice of light causes the mouse brain to hone its response to sounds.

The scientists placed lab mice in a dark chamber for one week, completely depriving the mice of visible light. Then, they measured how their responses to different sounds had changed.

After a week in total darkness, the mice showed an increased ability to hear soft sounds, to recognize the exact pitch of sounds, and to locate which direction a sound was coming from.

"Our result would say that not having vision allows you to hear softer sounds and better discriminate pitch," said Lee in a press release. "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."

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A Light in the Darkness

The team examined the primary auditory cortex (A1) in each mouse, which is the region of the brain that processes incoming sounds. They found that after a week of light deprivation, the auditory cortices of the mice were more active. Nerve cells, or neurons, in A1 were activated in response to sound at lower thresholds.

The cells were also activated more often and with greater sensitivity to the exact pitches of sound. “Thus, neurons could discriminate between tones of different frequencies better,” explained Kanold in an interview with Healthline.

A1 wasn’t the only part of the brain affected, either. All incoming sensory input, except for smell, is channeled through the thalamus, the brain’s relay station. The neurons that connect the thalamus to A1 are called thalamocortical inputs. In the light-deprived mice, the thalamocortical inputs were strengthened, giving A1 a stronger signal and more auditory information to work with.

Unfortunately for the mice, these changes didn’t last. “Thalamocortical [connections] reverted after one week in the light,” said Kanold. “We are planning follow-up studies to explore how to make these changes permanent.”

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To further test their theory that blindness improves hearing, the scientists took a new batch of mice and made them deaf. The new mice showed a similar pattern of increased activity in their primary visual cortex (V1). When they lost their hearing, their vision improved to help them better navigate their new, soundless environment.

This suggests that other senses could also be improved by temporarily blocking intact ones.

A Window of Opportunity for Change

What's interesting about this study is the fact that scientists were able to cause brain changes in adult mice at all. Conventional wisdom says that only the developing brain is plastic, or able to change easily. New studies are revealing that the adult brain may have more potential for growth than we realize, particularly in areas that have to do with learning and the formation of new memories.

“We believe that this shows that there is more potential for changing brain circuits than previously assumed,” said Kanold. “The nice thing here is that we don’t need drugs. However, we don’t know how long dark exposure would have to be in humans and if humans would be willing to this.”

The study offers hope for people who have suffered damage to brain regions that control the senses. It is also promising for people whose sensory areas never fully developed due to blindness or deafness at birth and who have had their vision or hearing restored later in life.

"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.”

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