By Meeri Kim
Special to The Washington Post
WASHINGTON, D.C. —
Try closing your eyes for a minute. Without the luxury of vision, your sense of hearing seems to kick into overdrive, with every rustle and stir sounding louder and clearer. Sit in darkness long enough and your brain may end up rewiring itself to turn you into a super-listener.
A new study has found that mice kept in total darkness compensated for the loss in vision with an improved sense of hearing and more auditory connections in the brain. Also, the mice were adults, challenging the belief that only young, developing brains can so readily re-mold.
In just one week, the rodents’ visually deprived brains had already adapted. Neurons responsible for processing sound fired stronger and faster, and could pick up on a wider range of tones compared with a control group. Their brains even grew more connections from the thalamus — sort of a gateway between our sense organs and the brain — to the auditory cortex.
When the mice were returned to a normal light-dark cycle, their brains returned back to normal within about a week.
This sensory compensation also extends to hearing loss. When mice were deafened, the opposite occurred: the link between the thalamus and the visual cortex strengthened.
The mechanisms of how these different brain areas manage to communicate remain a mystery.
“There’s no connection between the primary visual cortex and the primary auditory cortex, so how is this working?” said University of Maryland biologist Patrick Kanold, author of the study published online Wednesday in the journal Neuron. “It’s an important question.”
MIT neuroscientist Mark Bear, who was not involved in the study, agrees that there is no direct connection between the two areas of the brain — but there may be a roundabout route.
“Given enough synapses, every neuron in the brain connects to every other neuron in the brain,” said Bear, who found the results surprising and interesting.
Kanold hopes to explore this further with his collaborator, neuroscientist Hey-Kyoung Lee of Johns Hopkins University, citing this as a first experiment in a long line of future studies on the subject.
“The most exciting part is that these changes were happening in adults, since it is long known that the adult brain is less plastic than a child’s,” Lee said.”We were really, really shocked.”
Brain plasticity refers to the brain’s ability to re-mold and rewire itself throughout a lifetime. When we are young, our brains are still developing and are highly plastic — which is why, for example, learning new languages is much easier as a child. During that “critical period,” outside stimuli actively shapes the brain, which uses the input to strengthen or prune specific synapses.
Scientists once thought that after childhood, much of the brain became set in stone. Findings in the past few decades have proven that it remains more moldable than previously thought, even as we age. Lee and Kanold’s study adds another compelling piece of evidence supporting the existence of adult neuroplasticity.
Lee stumbled upon this interplay between different sensory regions of the brain several years ago, while working on an earlier experiment. At that time, she was curious about how visual experiences are encoded by the brain and decided to see if vision deprivation in mice alters their brain circuitry. After inducing blindness, the visual cortex didn’t change — but surprisingly, the other sensory areas of the brain did.
She wanted to explore this unexpected finding further and enlisted the help of her then-colleague Kanold, who is an auditory function expert.
“Her lab was next to my office, so it was very natural to start a collaboration,” Kanold said.
Next, Lee wants to explore whether there’s any way to make the changes more permanent to possibly help people with hearing loss — but she can’t say for certain that the same rewiring will happen in humans.
“Mice have poor vision, and they have ultrasonic hearing,” she said, compared with humans. “But we believe that in the human brain, the same principle should work.”
They can’t easily recreate the experiment with human subjects, but some behavioral studies have shown that indeed blindness leads to superhuman auditory abilities. For instance, the blind can localize sounds in 3-D space better and have the ability to discern pitch changes 10 times faster than sighted individuals. But as brain plasticity would dictate, performance often improves the younger the onset of blindness.
Neurologist Krish Sathian of Emory University thought the experiments were “very nicely done” and speculates that their findings could one day help patients.
“This study clearly reinforces the hope that such neuroplasticity can be harnessed for rehabilitative training of individuals with various kinds of sensory loss, such as visual or hearing loss,” he said.