People blind from birth can be taught to "see" images that are
conveyed as sounds, says a new study that calls into question a longstanding
belief about the limits of the human brain.
The blind have long used other senses -- including touch -- to compensate for their visual impairment. But a new study finds that when images are conveyed as sound, even the congenitally blind can distinguish among objects and their brains respond as if they are seeing. (Anacleto Rapping)
Devices that scan visual images and reinterpret regularities as sounds
were used to retrain the brains of congenitally blind people in a study
published this week in the journal Neuron. The authors -- at the Safra Center for Brain Science at Hebrew University in
Israel -- put people who had been blind since birth through 70 hours of
training with a visual-to-auditory sensory substitution device.
Initially, the subjects were able to distinguish among faces, houses,
everyday objects, body shapes and textures. Eventually, they were able to read
letters and words, identify facial expressions and locate people's positions.
In one video, a blind person is shown a picture of a woman with a ponytail and
identified the hairstyle.
Blind people have long used the capability to use another sensory
perception to compensate for blindness: Braille and blind walking canes allow
people without sight to read and navigate. But when the authors of the current
study put subjects in a brain scanner, they gained insight into the process by
which training with a sensory substitution device allowed the mind's eye to
"see." The human brain is a remarkably efficient and adaptable organ:
when an appendage such as a hand is amputated, or a sensory perception such as
sight is lost, the specialized regions of the brain in which input from the
hand or the eyes is processed are reassigned to other duties.
But scientists have long believed that the brain's adaptability is
limited by early conditions: when a person is born blind, the capacity of the
brain's visual cortex to process sight never develops, scientists have
believed. With that lost opportunity, a window is closed, and even if eyesight
were to be restored, the visual cortex, they believed, would forever remain
"blind" to images. Not so, the current study finds. When blind
subjects listened to the "soundscapes" that conveyed information
about a visual image, they showed activation in their visual cortex. In fact,
when sounds conveyed the shape of letters, subjects who had never
"seen" a letter showed activation in a patch of the left ventral
visual cortex that uniquely comes alive when people with normal vision read
letters and words.
"The adult brain is more flexible than we thought," said
co-author Amir Amedi. With the right training approaches and technologies, the
brains of people who have been blind for a long time -- even since birth --
might be "reawakened" to the task of processing visual information,
he said. For some, he added, such training might even restore some lost vision.
In fact, a 2008 study first offered tantalizing evidence that scientists were wrong in
their long-held belief that a congenitally blind person could never have normal
vision even if eyesight were restored. A young Indian woman identified only as
SRD was born blind of dense congenital cataracts, but her vision was restored
when she was 12 at the Iladevi Eye Clinic in Ahmedabad, India. Twenty years
after she regained her eyesight, a team from the Massachusetts Institute of
Technology discovered this rare instance of a congenitally blind person whose
vision was restored, and tested her extensively to see how she saw.
SRD's sight was virtually normal, though the MIT team found that she did
not use certain cognitive tricks that most sighted people use to make sense of
conflicting visual cues. Other interesting cases of visual impairment and the
cognitive strategies that the blind and the sighted use to make sense of their
world are discussed by neurologist Oliver Sacks in his book The Mind's Eye.
Source: Chicago Tribune.
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