Hearing better in the dark: blindness fuels ability to place distant sounds

Science News, Oct 16, 2004 by B. Bower

Whether they lose sight early or later in life, blind people estimate the location of many sounds more accurately than sighted individuals do, a new study finds. In lieu of visual cues, the blind typically learn to perceive subtle acoustic signals that help them navigate, concludes a research team led by neuroscientist Franco Lepore of the University of Montreal

Most studies of sound localization in blind individuals have examined how accurately they can reach out and touch, with a hand or a cane, the source of a nearby tone.

Lepore and his coworkers studied the ability, to tell whether two distant noise bursts, presented one after the other against a background of low-level noise, originated from the same or different locations. Participants included 14 individuals who had lest their vision before age 11 and 9 people who became blind after age 16. All had been blind for at least 20 years. Their blindness stemmed not from brain damage but from diseases of and injuries to their eyes. The researchers also tested 10 sighted, blind-folded volunteers.

Blind and sighted individuals performed comparably well when localizing sounds from about 3 meters away that were directly in front of them or just to either side of center front.

In contrast, blind individuals perceived sounds from the right or left and behind their bodies more accurately than sighted volunteers did. Those who had early-onset blindness were slightly better than those with late-onset blindness at localizing sounds coming from directly ahead or slightly right or left of that.

Both blind groups performed comparably well--and markedly better than sighted participants did--at discerning sounds directly in flout of their bodies that came from different distances, ranging from a to 4 m. Lepore and his coworkers report their findings in the Oct. 5 Current Biology.

"The ability to locate sounds that originate far from one's body is better in late-onset blindness than has often been assumed," Lepore says. For people who can't see, localizing distant sounds provides caudal input for many daily tasks, such as deciding when to cross a street, Lepore says.

Preliminary brain-imaging data indicate that parts of the visual cortex at the back of the brain contribute to superior sound-localization skills in people with early-onset, but not late-onset, blindness, according to Lepore and his coworkers. Ways in which people with later sight loss learn to discern sound locations deserve closer scrutiny, the researchers remark.

Lepore's results add to what's known about brain rewiring in the blind, says neuroscientist Alvaro Pascual-Leone of Harvard Medical School in Boston. In a prior study, his group found substantial activity in part of the visual cortex as early-onset blind people read Braille script.