Brain areas for vision are co-opted for locating sound in space.
Blind people have been shown to recruit visual areas of the brain to pinpoint the direction of sounds.
The finding, published this week in PLoS Biology1, gives the first clear link between superior hearing abilities in blind people and increased activity in the visual centres of their brains.
"These results tell us about the plasticity of the brain," says neuroscientist Franco Lepore of the University of Montreal, Canada. He believes that when people who were born blind, or went blind at a young age, use noise to navigate their environment, their mental processing adapts.
Previous experiments have shown that people who lose their sight at an early age often excel at non-visual tasks, such as speech perception, verbal memory and musical ability.
Some scientists have claimed that blind people can pinpoint sound in space better than the sighted. But other researchers have failed to find this advantage. Lepore and his colleagues decided to investigate why only some blind people are better at locating noises.
The team tested 19 people, 12 of whom were born blind or had lost their sight at a very early age. Each participant sat in front of a semicircle of 16 loudspeakers that emitted noise bursts lasting a fraction of a second.
The researchers played sound out of a random speaker and then asked their subjects to identify its source. Participants used both ears in one set of tests, and only one ear in another.
To Lepore's surprise, there were no differences among participants using both ears. "We think that the reason why blind people were not better with both ears open was the task was too easy. The speakers were too far apart," he says.
But in the one-eared tests, five of the blind people were better at spotting the position of sounds. These five localized the loudspeaker's position to within 15 degrees, even with one ear plugged.
None of the sighted subjects could do this; nor could seven of the blind ones. These participants always perceived the sounds as coming from the side of the semicircle closest to their open ear.
Imaging the difference
The team scanned the subjects' brains to find which areas were most active, using positron emission tomography, which uses harmless radioactive substances to pinpoint increased tissue activity.
Blind people with exceptional sound-locating abilities had increased activity in the visual cortex, the part of the brain devoted to processing what people see. None of the sighted subjects, and none of the blind subjects who lacked superior hearing skills, showed this activity.
"That's convincing, I think," says Marcel Zwiers, a neuroscientist at the University Medical Centre in Utrecht, the Netherlands, who has studied blind people's hearing abilities. The connection between the visual cortex and superior sound location provides a missing link in the previous debate, he says.
If someone goes blind at a young enough age, his or her brain may undergo a subtle reorganization, Lepore suggests. His team has found that people who lose their sight later in life lack any extraordinary visual cortex activity, further supporting this idea.
Gougoux, F. et al. PLoS Biol 3, e27 doi:10.1371/journal.pbio.0030027.sg001 (2005).
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Khamsi, R. Plastic brains help the blind place sounds. Nature (2005). https://doi.org/10.1038/news050124-6