Study finds that hearing sharpens with vision loss

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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 superlistener.

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 Patrick Kanold, University of Maryland biologist and author of the study published online Wednesday in the journal Neuron. "It's an important question."

Mark Bear, a Massachusetts Institute of Technology neuroscientist 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 Mr. Bear, who found the results surprising and interesting.

Mr. 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," Ms. 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.

Ms. 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 Mr. Kanold, who is an auditory function expert.


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