The auditory portions of a deaf person's brain can learn to process touch and vision. Mapping these communications may advance the technology of cochlear implants.
Our experiences help shape our brains. So it might make sense that for a person born without hearing, the part of the brain that's meant to process audio would be underdeveloped. But according to a new study, those who have been deaf since birth actually use the sound-related part of the brain -- known as the primary auditory cortex -- to do even more heavy lifting than their hearing counterparts.
"They really feel touch in their auditory cortex," explained Christina M. Karns, lead author on a study published yesterday in The Journal of Neuroscience.
"What we found is, in people born deaf the hearing part of the brain processes touch and, to a lesser degree, vision," she says. "And this wasn't the case at all for the hearing people."
In the study, Karns and her colleagues at the University of Oregon strapped 13 deaf volunteers into a head apparatus that delivers a flash of light to the eye (to stimulate vision), along with two bursts of air to the face (to stimulate a touch response). They did the same with 12 hearing peers, and observed the brain responses in all subjects via fMRI.
Karns' team found that the deaf subjects were not only processing both the touch and visual responses through the hearing part of their brains, but they were also far more likely to associate these inputs through something called the "double flash illusion."
Using the head apparatus, researchers "flashed a light to each subject's eye once, and then at the exact same moment we touched their face with two little puffs of air, so that the flash and the puff-puff were right at the same time," Karns said, explaining her experiment. While deaf subjects reported seeing two flashes, their hearing peers didn't see anything out of the ordinary, just one burst of light.
"I had the idea that maybe in people born deaf, touch and vision would be interacting more closely because these two senses have had more time to get to know each other during development, without sound monopolizing the conversation in the auditory cortex," said Karns, "and it turned out to be true." (She noted that a similar double-flash illusion happens in hearing individuals, but the association tends to be between sound and vision, rather than touch -- a "beep-beep" triggers it, rather than a "puff-puff".)
Previous studies on the flexibility of the brain among deaf individuals have mostly focused on how the visual system interacts with the auditory cortex, Dr. Peter Hauser, a deaf neuropsychologist from the National Technical Institute for the Deaf at Rochester Institute of Technology and the NSF Science of Learning Center on Visual Language and Visual Learning, explained in an email.
In that light, Karns' study makes a major contribution to our understanding of the plasticity of the deaf brain. "It shows that the primary auditory cortex can assume more functions than traditionally believed," Hauser wrote.
Karns hopes that her findings will lead to a rethinking of aids and interventions for the deaf. For one, she says that her study could influence future successes in cochlear implants -- surgically implanted devices that deliver sound-input directly to auditory areas in the brain.
Deaf people who have a strong link between vision and touch in the hearing region of their brain may be less receptive to auditory inputs, which would make them less receptive to cochlear implants in turn, she explained. Such implants are already known to work best in children or adults who lost hearing late in life and had a chance to experience sound.
"This study shows us that deaf people [whose auditory cortex has been taken over by other senses] will need special training to be able to use the device, or they might respond better to a completely different approach," Karns added. "We just don't know yet." For that, more comprehensive studies are needed.