How deaf people's brains change to boost sight


How deaf people's brains change to boost sight

After studying the brains of congenitally deaf cats, the only animal apart from humans that can be born deaf, researchers proposed that the part of the brain normally used for hearing is reorganized to boost sight in deaf people, thus explaining their reported capacity for "supersight".

You can read how the researchers made their discovery in a paper that was published online in the journal Nature Neuroscience on 10 October.

The lead author was Dr Stephen G Lomber of The Centre for Brain and Mind at The University of Ontario in Canada, and his co-authors were Dr M Alex Meredith of the School of Medicine at Virginia Commonwealth University in the US and Dr Andrej Kral of Hannover Medical University's Institute of Audioneurotechnology in Germany.

When the brain doesn't receive input from one sense that is not working, it often compensates by boosting performance of another sense that is working: for example deaf and blind people often report enhanced ability in other senses. But what remains a bit of a mystery is how this happens neurologically.

As a result of what they found in congenitally deaf cats, Lomber and colleagues proposed that the brain is "plastic" enough to reallocate areas normally dedicated to one sense to further the performance of the remainder.

Lomber, who is an associate professor in the Department of Physiology and Pharmacology at the Schulich School of Medicine & Dentistry, and Department of Psychology in the Faculty of Social Science, both at The University of Ontario, explained that the brains of deaf people probably use the redundant auditory brain areas to boost visual performance in two ways: enhancing peripheral vision and detecting how fast things around them are moving.

For example, he said, " if you're deaf, you would benefit by seeing a car coming far off in your peripheral vision, because you can't hear that car approaching from the side; the same with being able to more accurately detect how fast something is moving".

"The brain wants to compensate for the lost sense with enhancements that are beneficial," said Lomber, adding that the "brain is very efficient, and doesn't let unused space go to waste".

He and his colleagues already had a hunch that this was the case: "it has been proposed that cross-modal reorganization of deaf auditory cortex may provide the neural substrate mediating compensatory visual function," they wrote, so they tested their hypothesis comparing congenitally deaf cats to hearing cats while they performed a series of psychophysically challenging tasks.

They found that the deaf cats had "have superior localization in the peripheral field and lower visual movement detection thresholds", compared to the hearing cats.

But when they performed reversible brain operations in the deaf cats that deactivated the posterior auditory cortex (the part that normally picks up peripheral sound), they found the cats lost their superior performance in peripheral vision, which led them to suggest the function stayed the same (to detect peripheral signals), but just switched from auditory to visual.

They also found that deactivating the deaf cats' dorsal auditory cortex made them lose their superior visual motion detection.

So Lomber and colleagues concluded that:

"Our results indicate that enhanced visual performance in the deaf is caused by cross-modal reorganization of deaf auditory cortex and it is possible to localize individual visual functions in discrete portions of reorganized auditory cortex."

Lomber and colleagues now want to investigate further how this phenomenon might affect deaf people who get cochlear implants. What happens to the brains of people who have been deaf all their lives when they suddenly start getting auditory signals to process; especially if the areas normally used to process hearing are being used to boost sight?

Lomber drew an analogy with letting a friend settle into a cottage you weren't using for a while. And then suddenly you find you need it back: but they have made themselves comfortable, rearranged the furniture, and made it suit their needs.

"They may not want to leave just because you've come back," explained Lomber.

The team are also hoping to investigate if these same changes happen in the brains of people were not deaf at birth but became deaf later in life. They want to know, for example, if experience of hearing stops the brain reorganizing the auditory cortex to boost another sense.

"Cross-modal plasticity in specific auditory cortices underlies visual compensations in the deaf."

Stephen G Lomber, M Alex Meredith & Andrej Kral.

Nature Neuroscience, Published online: 10 October 2010.

DOI:10.1038/nn.2653

Source: University of Western Ontario.

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