PARIS: Just as bats bounce sound waves off objects to find their way in the dark, some blind humans spontaneously make clicking sounds with their mouths to navigate the world, scientists said Wednesday.
Not only that, but they adjust the speed and volume of the clicks when they need to zoom in on a hard to place object.
"Even though people have not been 'designed' to echolocate, they have adapted their brains extraordinarily well to detect faint echoes and to instinctively adjust emissions as the task changes," said Lore Thaler of Durham University in the UK, co author of a study published in the journal Proceedings of the Royal Society B.
The research team tracked down eight blind people with experience in echolocation for the study.
They placed the "expert echolocators" in a noise insulated room, then conducted experiments by placing a wooden disk at different angles from the person straight ahead, at 45, 90, or 135 degree angles to their left, or directly behind them, always at a distance of a metre (3.3 feet).
Standing in one place and without moving their heads, the subjects' task was to make clicking sounds with their mouths to determine whether the disk was present in the room or not.
At zero, 45 and 90 degree angles, the participants were correct 100 percent of the time, the researchers found.
Their success rate dropped to about 80 percent when the disc was placed at an 135 degree angle, just over their left shoulder.
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At 180 degrees, directly behind them, the participants were correct half the time.
For the larger, more difficult angles, the echolocators "increased the number of clicks they made and the intensity of the clicks," the team discovered.
"Our results clearly demonstrate that people, just like bats, adjust their emissions to situational demands."
The team also measured the echoes, and found they were "very faint".
"Indeed, based on previous research, people would have said that it should have been impossible for them to perceive these echoes," Thaler told.
The fact that blind people can teach themselves to echolocate had been documented before, he said, but never that they can adjust their sound emissions in sonar like fashion.
The clicks are made by pressing the tongue against the roof of the mouth, then quickly pulling the tongue down, creating a vacuum that "pops".
The sound travels, and bounces from surfaces and objects around the blind person, returning as echoes.
"What we have found is very useful when teaching people echolocation," said Thaler.
"Now we can tell them, for example, that if they find a task or situation difficult or they find it hard to hear an echo they should consider making the click louder and make some more clicks and this will help them hear the echoes better."
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