Why the Deaf Have Enhanced Vision

Deaf people with enhanced vision can thank otherwise idle brain cells for their heightened sense, a new study in cats suggests.

That's because the brain recruits cells normally devoted to hearing to help them see better, the research revealed.

"The brain is very efficient and it's not going to let this huge territory that is the auditory cortex and all the processing that it has go to waste," said study leader Stephen Lomber of Canada's University of Western Ontario. The auditory cortex is the part of the brain that controls hearing.

"So it makes sense that other senses will come in and colonize."

Deaf-Cat Experiments Reveal Brain's Wiring

In behavioral tests, Lomber and his team determined that domestic cats born deaf have better peripheral vision and motion-detection abilities than cats born with normal hearing—a finding that parallels visual test results in deaf people.

Next, the researchers used a surgical method called reversible deactivation to temporarily cool and render inoperative parts of the brain. This enabled the scientists to pinpoint which parts of the  brain were responsible for the enhanced visual abilities.

(Related: "Brain's 'Core' Revealed by First Hi-Res Wiring Map.")

"Reversible deactivation is very powerful because you can test an animal before you deactivate an area of the brain, again while the area is deactivated, and a final time when the brain is rewarmed afterward," said study team member Alex Meredith, a neuroscientist at Virginia Commonwealth University.

"It's like having a stroke without losing brain tissue."

The scientists found that when they cooled the part of the deaf cats' auditory cortex involved in peripheral hearing, the animals lost their peripheral vision advantage.

Likewise, when the scientists deactivated the part of the brain normally involved in discerning which direction a sound was coming from, the deaf animals fared no better than normal cats in visually detecting motion. (See brain pictures.)

"These visual functions [that are enhanced] don't just randomly redistribute" in the auditory cortex, Lomber said.

"They actually seem to take up residence in an auditory area that would perform a similar function."

Improved Treatment for the Deaf?

More studies will reveal if same is true in humans, the authors said.

But the cat experiments do explain "why, in deaf humans, some visual skills get better and others do not change at all," said Daphne Bavelier, a neuroscientist at the University of Rochester in New York who was not involved in the study.

The research, published October 11 in the journal Nature Neuroscience, also seems to explain why deaf individuals who receive a cochlear implant later in life don't regain as much of their hearing compared with people who receive the implants as young children.

A cochlear implant is a small electronic device surgically inserted under the skin that can give a sense of sound to a deaf person.

"If you delay the implantation, then the brain reorganization that occurred [in early life] is more or less locked in," study leader Lomber said. "The brain's lost the ability to reorganize a second time and push the visual functions out."

But Lomber and colleagues say the research could lead to improved cochlear implants that target specific regions of the auditory cortex, such as the part involved in understanding speech, for example.

(Read "Beyond the Brain" in National Geographic magazine.)

"If you can understand the changes that the brain is going to undergo and the areas that you really want to target with your signals, then you can create a next-generation type of cochlear implant that better serves the needs of the brain," Lomber said.

Bavelier, who wrote a commentary about the research in Nature Neuroscience, agreed.

Understanding how the auditory cortex functions in young deaf people is "critical to our understanding of how to maximize the chances of successful implant," she said.