The brain spreads its sights in the deaf

People often assume that the deaf, because they live in a silent world, compensate by seeing more vividly or clearly than the hearing do. Yet scientists know little about the visual capacities of deaf people.

A new study finds that compared with hearing adults, people who have been deaf from birth display a unique pattern of activity in the brain’s visual system that may strengthen their peripheral vision.

A brain area implicated in tracking objects that move on the fringes of a person’s visual field exhibits elevated activity in deaf individuals, says a team of neuroscientists led by Daphne Bavelier of the University of Rochester. This brain response arises from deaf people’s unusually high reliance on peripheral vision, the researchers propose. For instance, they note, deaf people regularly scan their surroundings to compensate for the absence of acoustic cues and typically monitor the arm and hand motions of sign language with peripheral vision while looking at a conversation partner’s eyes.

The scientists report their findings in the Sept. 1 Journal of Neuroscience.

“This is exciting because it’s the first study to show a specific neural substrate for visual perception in the deaf,” remarks Karen R. Dobkins, a psychologist at the University of California, San Diego.

The researchers studied 9 deaf and 11 hearing adults, ages 18 to 27. Deaf participants had been born unable to hear, had deaf parents, and had acquired American Sign Language as their native language.

In a series of trials, each volunteer watched sets of moving dots either on the periphery or in the center of the visual field. The participants indicated when the dots’ brightness changed.

At the same time, a functional magnetic resonance imaging device monitored participants’ brain activity.

Deaf and hearing participants performed equally well on the simple task. However, earlier studies using tougher visual tasks—directed by study coauthor Helen Neville of the University of Oregon in Eugene—had found peripheral-vision advantages in deaf participants.

In the current study, as the volunteers monitored the peripheral visual field, greater activity appeared in a motion-sensitive area near the back of the brain in the deaf participants than in hearing ones, the scientists say. Moreover, signals between that motion-sensitive area and a region known to participate in spatial perception and attention were stronger in deaf than in hearing participants during peripheral-vision trials.

No group differences in overall activation of the motion-sensitive area emerged when the volunteers watched the center of the visual field.

However, in both the central-vision and peripheral-vision trials, deaf people displayed much greater activity on the motion-sensitive region of the brain’s left hemisphere than on the corresponding region of the right hemisphere. In contrast, hearing participants showed slightly more activity on the motion-sensitive region of the brain’s right side.

The results support an investigation directed by Dobkins and reported in the May 1999 Psychological Science indicating that motion perception is controlled by the left hemisphere in deaf adults and by the right hemisphere in hearing adults.

“It looks like midlevel areas of visual processing are altered in the deaf, probably fairly early in life,” Dobkins comments. The possibility that lifelong deafness fundamentally affects how people see the world “is the million-dollar question,” she adds.