Daniel Kish has no eyes. He lost them to cancer at just 13 months of age, but you wouldn’t be able to tell from watching him. The 44-year-old happily walks round cities, goes for hikes, rides mountain bikes, plays basketball, and teaches other blind youngsters to do the same. Brian Bushway helps him. Now 28 years old, Bushway lost his vision at 14, when his optic nerves wasted away. But, like Kish, he finds his way around with an ease that belies his disability.

Both Kish and Bushway have learned to use sonar. By making clicks with their tongue and listening to the rebounding echoes, they can “see” the world in sound, in the same way that dolphins and bats can. They are not alone. A small but growing number of people can also “echolocate”. Some develop the skill late in life, like Bushway; others come to it early, like Kish. Some use props like canes to produce the echoes; others, just click with their tongues.

The echoes are loaded with information, not just about the position of objects, but about their distance, size, shape and texture. By working with these remarkable people, scientists have worked out a lot about the scope and limits of their abilities. But until now, no one had looked at how their brains deal with their super-sense.

Enter Lore Thaler from the University of Western Ontario. Together with Stephen Arnott and Melvyn Goodale, Thaler has found that Bushway and Kish process the echoes of their clicks with parts of their brain that are usually devoted to vision, rather than hearing.

Thaler wanted to scan Bushway and Kish’s brains as they responded to their own echolocation clicks, using a brain-scanning technique called functional magnetic resonance imaging (fMRI). These scans reveal parts of the brain that are active when people respond to certain triggers or perform certain tasks.

But there was a problem. MRI scanners are long claustrophobic tubes. Inside them, people have to wear ear protection and aren’t allowed to move their heads very much. They’re not very natural environments for sonar users. “I don’t like MRI’s, so that part was a drag,” said Kish.

Thaler worked with her two volunteers to come up with a solution. Kish says, “It was an honour to be in a position to advise and consult with well respected scientists. And, we were well fed.” Thaler ended up using small microphones to record the sound of the two volunteers’ clicks, and their echoes, from within their ears. She then played these recordings back to them inside the scanner. These recorded echoes were just as good as the real deal –Kish and Bushway could make out objects like a tent pole based on their recorded echoes.

As Kish and Bushway listened to a variety of different echoes, Thaler watched their brains in action, and compared them to two men who could see and couldn’t echolocate. When all four men listened to the recordings, their auditory cortex – the part of the brain responsible for hearing – lit up on the scans. That was expected.

But there was far more going on in Kish and Bushway’s brains. When he heard the sounds of click echoes, Bushway’s calcarine cortex – a part of the brain that normally deals with vision – lit up. Kish’s reacted even more strongly. And when they heard the sounds of echoes reflecting from moving targets, they showed activity in areas that deal with movement. Neither of the two sighted volunteers reacted in the same way – to them, the recordings were just noises.

Many studies have shown that the brains of blind people reorganise to adapt to their condition, and the areas used for vision take on new roles. But the calcarine cortex seemed to be specifically tuned to echoes, as opposed to other noises. It became far more active when Kish and Bushway heard the sounds of soft echoes than when they heard echo-less recordings, even though the auditory cortex reacted similarly to both sets of sounds. This suggests that both men have diverted a part of their brain, which would normally deal with sights, to handling the sound of echoes.

Both men responded in the same way, but Kish has thirty more years of experience with his sonar than Bushway, and started using it far earlier. It showed in the scans. Kish’s calcarine cortex was more strongly tuned to the sound of echoes, showing blazing activity compared to Bushway’s gentle simmer. It even showed the same handedness that the eyes of sighted people have for light. Echoes coming from the left triggered a response from his right calcarine cortex; those coming from the right triggered the left half.

Thaler chose to compare Kish and Bushway to sighted people, rather than blind ones who couldn’t echolocate, because it’s not clear if most blind people can echolocate to some extent, even if not consciously. That was a reasonable choice, but it makes it hard to say which of the patterns in the brain scans are relevant to echolocation, and which are simply due to being blind. However, the fact that the calcarine cortex only lit up when Kish and Bushway heard click echoes suggests that it’s specifically tied to sonar, rather than a more generally acute sense of hearing.

Thaler’s scans might explain why people who can see are generally not very good at echolocating. One obvious explanation is that blind people have more sensitive hearing, and are more finely attuned to echoes. But even though Kish and Bushway had similarly good hearing, the former had more sophisticated sonar than the latter. Alternatively, blind people use echolocation more frequently and might just be more practiced at it. A final, and more intriguing, possibility is that echolocation and vision compete for the same part of the brain – the calcarine cortex. Maybe, in humans at least, both senses cannot coexist easily with one another.

To Kish himself, the results weren’t surprising but he says, “I am enthusiastically hopeful that they will point the way toward a better and more realistic and accurate understanding of sensory processing and imaging, with a more respectful perspective on non-visual processing.”

Clearly, this study is just the beginning. There is still much to learn about this extraordinary sense, and Thaler only studied two people (although in fairness, echolocators aren’t exactly easy to recruit in large numbers). For now, it is enough to know that Kish and Bushway “use echolocation in a way that seems uncannily similar to vision… This has broad practical implications in that echolocation is a trainable skill that can potentially offer powerful and liberating opportunities for blind and vision-impaired people.”

That’s exactly what Kish and Bushway do today. As leaders of their organisation, World Access for the Blind, they travel the world teaching echolocation (they called it “FlashSonar” to blind teens and adults, and leading them on activities such as hiking and mountain biking. They have reached over 2,500 people in 18 countries. Kish says, “We expect this to add much creditibility to our own approach to teaching and working with blind students. In this respect, I hope it helps to make me a better Perceptual Mobility Therapist.”

Reference: Thaler, Arnott & Goodale. 2011. Neural Correlates of Natural Human Echolocation in Early and Late Blind Echolocation Experts. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0020162

More on blindness and visual disorders:

• Blind man navigates obstacle course perfectly with no visual awareness
• Faulty connections responsible for inherited face-blindness
• Cross-breeding restores sight to blind cavefish
• Gene therapy gives full colour vision to colour-blind monkeys

More on echolocation:

• How bats find water and why metal confuses them
• Ninja bat whispers to sneak up on moths
• Echolocation in bats and whales based on same changes to same gene
• Tiger moths jam the sonar of bats
• Moths mimic each others’ sounds to fool hungry bats

May 25th, 2011 by Ed Yong in Animal behaviour, Animal senses, Animals, Inside the brain, Neuroscience and psychology, Perception | 7 comments | RSS feed | Trackback >