A blind woman stands facing the front of the room. Directly behind her, about three feet away is a pole with a wooden disc about the size of a coffee saucer. The woman doesn’t know it’s there. She begins making clicking sounds with her mouth, modulating the volume and then stopping after about four seconds. “Yes,” she says. Using only her mouth clicks, she has correctly determined the presence of the disc.
This woman, an anonymous study participant, is one of eight blind people who worked with Lore Thaler and her colleagues at Durham University in England to demonstrate their unique ability to echolocate. The skill is commonly associated with bats, which emit sounds of different quantities, intensities, durations, and frequencies, and then sense the delicate echos that reflect off objects. From the echoes bats build a high-resolution picture of their physical surroundings.
But as Thaler and her team have discovered, blind humans that echolocate can also detect nearly imperceptible echos off objects and instinctively adjust the loudness and number of pulses of their mouth clicks to improve detection — a dynamic that’s never been measured before. (They don’t adjust the frequency, making high-pitched to low-pitched clicks, for instance.)
The results not only underscore the brain’s remarkable ability to turn unusual sensory information into meaning, but shows that humans are capable of perceiving audio much quieter than anyone thought possible. Previous researchers had estimated that the human ear should be able to discern an echo from a mouth click as long as the echo was about 80 percent lower in intensity than the original click. But this new research finds that human echolocators are able to detect echos about 95 percent lower in intensity.
“Based on those previous reports, to detect the echoes should have been impossible,” Thaler, an associate professor at Durham University, told Seeker. Thaler is the first author on a paper describing the research, which appears in the journal Proceedings of the Royal Society B.
The study also revealed how quickly echolocators could pick out an object within a hundred milliseconds to just a few seconds depending the location of the disc.
Thaler has been studying human echolocators for the last eight years and said that she has personally verified 14 of them as expertly skilled at the task. But there may be more she doesn’t know about and said the skill may be underrepresented. One of the most well known echolocators is Daniel Kish, founder of the nonprofit World Access for the Blind, and an author on the paper. Kish taught himself to echolocate as a child and his foundation works to help others learn.
For the study, each participant was asked to stand in a noise-insulated room and identify whether or not a wooden disc had been placed in the room. In the real world, echolocators move their heads from side-to-side sending clicks in a variety of different directions, in the same way that a person visually scans a room.
But in this case, the researchers asked the participants to keep their heads straight ahead while clicking. Thaler said that she and her team weren’t trying to understand whether the blind person could identify where the disc was, but simply how they used their clicks to detect it.
The wooden disc, about 7 inches in diameter, was attached to a thin metal rod, which was half a centimeter thick. The disc acted as a sound reflector, and at random intervals was placed either in front of the person, or at 45, 90, 135, or 180-degree angles relative to the person’s face. Each time the pole and disc were moved, the participant was asked to cover his or her ears and hum so that they could not hear the movement.
Once the disc was in place, the researchers tapped the participant on the ankle using a very long rod, giving the cue to start clicking. Tiny microphones attached to near the participants ears recorded the clicks and echoes used to locate the disc.
All eight participants correctly identified the presence of the disc 100 percent of the time when it was placed in front of them and at 45 and 90-degree angles. It took some people just one click and others between three and five clicks to identify the disk. The loudness of the clicks was between 74 and 76 decibels — about as loud as an average conversation.