Visually impaired people who use echolocation to "sense" their environment -- in the same way bats and dolphins do -- are using the same parts of their brains that people with normal vision do, researchers have found.
The proof, ironically, came from a study that showed their echolocation can create perception mistakes about objects in their environment that are the same as those experienced by sighted people, researchers in Canada and Scotland found.
"Some blind people use echolocation to assess their environment and find their way around," says Gavin Buckingham of Heriot-Watt University in Scotland. "They will either snap their fingers or click their tongue to bounce sound waves off objects, a skill often associated with bats, which use echolocation when flying.
"However, we don't yet understand how much echolocation in humans has in common with how a sighted individual would use their vision."
To try and find out, the researchers conducted an experiment with three groups of people -- blind echolocators, blind people who did not use echolocation, and people with no visual impairment -- and asked them to judge the weight of three cubes of different sizes.
Despite the size differences, the cubes in fact were all of identical weight -- which set up the groups to possibly experience a "size-weight illusion" where the size of the cube might fool them into perceiving their weights as different, the researchers report in the journal Psychological Science.
"The blind group who did not echolocate experienced no illusion, correctly judging the boxes as weighing the same amount as one another because they had no indication of how big each box was," Buckingham explained. "The sighted group, where each member was able to see how big each box was, overwhelmingly succumbed to the 'size-weight illusion' and experienced the smaller box as feeling a lot heavier than the largest one."
Surprisingly, the researchers found, blind echolocators, whose echo technique was able to give them some idea of the boxes' size, experience the same illusion.
"This showed that echolocation was able to influence their sense of how heavy something felt," Buckingham says. "This resembles how visual assessment influenced how heavy the boxes felt in the sighted group."
The findings support earlier studies that have shown blind echolocators, when listening to their own echoes and processing them, use the same "visual" parts of their brains as sighted people.
It suggests echolocation can serve not just as a functional aid in helping visually impaired people move around in an environment, but could be considered a comparable sensory substitute for vision, the researchers say.