Do blind people track sounds better?
Research now indicates that blind and sighted people display the same skill at locating a sound's origin when using both ears, but some blind people can home in on sounds more accurately than their sighted counterparts when all have one ear blocked. Canadian scientists describe the work in the Sept. 17 Nature.
Participants in the study were tested individually in a sound-insulated room. They faced 16 small, concealed loudspeakers arrayed in a semicircle a few feet away. With a headrest keeping their heads steady, the participants pointed to the perceived origins of the sounds.
The researchers tested eight blind people, who had been completely sightless from birth or since a very early age. They also tested three nearly blind persons, who had some residual vision at the periphery of their gaze; seven sighted people wearing blindfolds; and 29 sighted people without blindfolds. All participants were tested beforehand to ensure that their hearing was normal.
When restricted to one-ear, or monaural, listening, four of the eight blind people identified sound sources more accurately than did the sighted people, says study coauthor Michel Pare, a neuroscientist at the University of Montreal. The sighted people showed especially poor localization of sounds from the speakers on the side of the blocked ear.
In sighted people who can hear with both ears, "the brain learns to rely on binaural [stereo] cues. These data suggest that blind people haven't learned that and keep monaural cues as the dominant cues," says Eric I. Knudsen, a neurobiologist at Stanford University School of Medicine. "I find it surprising."
In blind people, some parts of the brain that would otherwise process visual images might be reorganized to process auditory messages, says Helen J. Neville, a cognitive neuroscientist at the University of Oregon in Eugene.
Ironically, such reorganization, also called plasticity, may account for the poor performance of the partly blind people in this study, says Tim P. Pons, a neuroscientist at Wake Forest University in Winston-Salem, N.C. These participants fared worse than either the blind or sighted groups, whether an ear was blocked or not. In these individuals, plasticity may have caused the visual cortex in the brain to devote itself not to hearing but to peripheral vision, the only sight remaining, Pons suggests.
In 1996, a U.S.-Japanese team found that blood flow in the visual cortex of blind people increased when they read Braille, a tactile activity, even though the main tactile sensory region sits apart from the visual cortex.
This and the Canadian report fuel an ongoing scientific debate about the hearing capabilities of the blind. "I think these studies show that folklore probably has some element of truth," Pons says. Such compensatory change in the brain "makes sense from a Darwinian perspective--to increase the capacity to use the remaining senses better increases your chances of survival," Neville says.
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|Article Type:||Brief Article|
|Date:||Sep 19, 1998|
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