Better Hearing From Darkness?

What do Ray Charles (Click on Ray for a Song) and Stevie Wonder  (Click on Stevie for a song) have in common?  Besides super music, both people were blind, which, according to recent studies, may enhance hearing not only for acuity, but for quality as well.  A few months go, Hearing International published an article about Echolocation where the blind use their extra acute hearing for mobility.  If a listener produces a sound, the person may be able to use the time delay and loudness differences between production and reception to determine the distance of reflective surfaces.  Reflected sound from any source can provide the listener with distance and object property information using timbre and pitch variations. While this is interesting and has been around for quite some time in mobility training for the blind, there is scanty research to support the theory that hearing is more acute in those who are blind.  But we do know that some individuals have a special gift for music and mobility and often they are blind.

Recently, researchers at the University of Maryland (UM) and Johns Hopkins University  (JHU) have found what they have dubbed the “Ray Charles Effect.”  That is, based upon their research, a young blind child learns to hear things that others cannot. It is well known that young brains are malleable enough to re-wire some circuits that process sensory information and that the earlier we begin working with hearing-impaired children, the easier it is for them to recognize speech, i.e. Auditory – Oral programs.  Now, researchers, Patrick Kanold (UM) and Hey-Kyoung Lee (JHU) have shown that the brains of adult mice can also be re-wired, compensating for vision loss by improving their hearing, which may lead to treatments for human hearing loss. Minimizing a person’s sight for as little as a week may help improve the brain’s ability to process hearing.  The findings of Kanold and Lee published Feb. 5 in the peer-reviewed journal Neuron, may lead to treatments for people with hearing loss or tinnitus. These researchers feel that there is some level of interconnectedness of the senses in the brain that are being revealed by their studies.   The thought behind their research was that the adult brain might be flexible if it were forced to work across the senses rather than within one sense.

They used a simple but reversible technique to simulate blindness by placing adult mice with normal vision and hearing in complete darkness for six to eight days.  At the end of that time, the adult mice were returned to a normal light-dark cycle and though their vision was unchanged they heard much better than before the induced blindness.  The researchers played a series of one-note tones and tested the responses of individual neurons in the auditory cortex, a part of the brain devoted exclusively to hearing. Specifically, they tested neurons in a middle layer of the auditory cortex that receives signals from the thalamus, a part of the midbrain that acts as a switchboard for sensory information. The neurons in this layer of the auditory cortex, called the thalamocortical recipient layer, were generally not thought to be malleable in adults.  But the research team found that for the mice that experienced simulated blindness these neurons did, in fact, change.  For the mice placed in darkness, the tested neurons fired faster and more powerfully when the tones were played, were more sensitive to quiet sounds, and could discriminate sounds better. These mice also developed more synapses, or neural connections, between the thalamus and the auditory cortex. The mice that experienced simulated blindness eventually reverted to normal hearing after a few weeks in a normal light-dark cycle. The fact that the changes occurred in the cortex, an advanced sensory processing center structured about the same way in most mammals, suggests that flexibility across the senses is a fundamental trait of mammals’ brains, Kanold said.  Lee noted, “We can perhaps use this to benefit our efforts to recover a lost sense. By temporarily preventing vision, we may be able to engage the adult brain to change the circuit to better process sound.”

As has been known to audiologists for some time, Kanold explains that there is an early “critical period” for hearing, similar to the better-known critical period for vision. The auditory system in the brain of a very young child quickly learns its way around its sound environment, becoming most sensitive to the sounds it encounters most often. But once that critical period is past, the auditory system doesn’t respond to changes in the individual’s soundscape.  “This is why we can’t hear certain tones in Chinese if we didn’t learn Chinese as children,” Kanold said. “This is also why children get screened for hearing deficits and visual deficits early. You cannot fix it after the critical period.”  While they are not sure how many days in the dark higher animals would need to get the same effect, they are hopeful that multi-sensory training to correct some sensory processing issues may be a possibility.

These results are reminiscent of findings from sensory integration studies of the 70s that considered the benefits of simultaneous auditory/visual stimulation. While these studies showed promise, there was no explanation for the how and why of the effects, nor how to implement them clinically.  So far they cannot help any of us obtain the giftedness offered by some blind performers or mobility experts. But in the next phase of their five-year study, Kanold and Lee plan to look for ways to make the sensory improvements they have found in mice to be permanent and, hopefully, to look beyond individual neurons to study broader changes in the way the brain processes sounds.