May 27 2011

Human Echolocation

Remember the Marvel comic hero Daredevil? He was blinded by exposure to radiation, but that same exposure ramped up his other senses so that he could, in essence, “see” with his hearing. Daredevil used a form of echolocation in order to survey his surroundings, and was able to be an acrobatic crime-fighter as a result.

While I liked the character, I always thought the idea was far-fetched. (Yeah, I know – it’s a comic-book character.) But perhaps the idea is not as science-fiction as you might think. There are reported cases of blind humans who developed a form of echolocation – they even use clicks to generate sound for this ability.

Echolocation is the use of sound waves to bounce off of object and then form an image of the those objects from the sound waves that bounce back. This may seem extraordinary, but it is no more extraordinary then using light waves to form three-dimensional images of the world around us. It just takes a bit of brain processing. Bats are the most common animal to come to mind when one thinks of echolocation, but other creatures do it as well, such as dolphins.

A new study in PLoS One looks at two so-called human echolocation experts, one with early blindness and one with late blindness. They found that when the subjects listened to ordinary sound their auditory cortex was recruited. But when they listened to clicks used for echolocation, part of their visual cortex was also recruited. The pattern of cortex activated also depended on the location and movement of the objects reflecting the echo.

This suggests that human echolocation experts, to an extent, are actually “seeing” with echolocation – they are using the visual processing part of their brain to process sound. This makes sense on many levels. First, sound waves are already processed to a degree to detect direction, distance, and even size. Our ears are positioned so that sound waves will hit them at slightly different times from different directions, and our brains can process that information – comparing the timing of the sounds in both ears, to give us a sense of where the sound is coming from. If you combine this type of processing with a bit of visual processing, it makes sense that you can make a crude image of the environments from sound information alone.

This also demonstrates the plasticity of the brain – it can change its function based upon use. Also, parts of the brain that no longer have a function – like the visual cortex of someone who becomes blind – can be recruited to serve a new function.

The existence of human echolocation is also a nice bit of evidence for the plausibility of evolution. Deniers often have a difficult time imagining how sophisticated biological features can emerge. How can a bat ancestor evolve echolocation. What the cases of human echolocation demonstrate is that new abilities can emerge suddenly, sufficiently formed to be useful, even if extremely crude. This is due, in part, to coaptation – the use of a biological feature evolved for one purpose for a different purpose. In this case people are using their brain hardwiring evolved for ordinary processing of sound and vision in order to process sound like vision.

Now imagine humans who have moved to a niche that is very dark or to a nocturnal lifestyle (without artificial light). Those few who develop even crude echolocation would have a significant advantage, and would provide an evolutionary toe-hold into further refinement of the ability. Eventually you would have something like what bats have today.

A completely unrelated thought occurred to me. Reports of human echolocation indicate that they use mouth clicks, or taping of their hands, feet, or objects like a cane, in order to generate the sound used in echolocation. This is very limiting as the frequency of the clicks is very low, and so the amount of information is highly limited. I wonder if anyone has tried using an electronic device to generate clicks. This would be a trivial device to create – just a small box that creates clicks with a slider that adjust the frequency of the clicks, and a dial for volume. It would be interesting to see what the limits of human echolocation are using these artificial clicks.

In any case – this is one of the coolest neuroscience stories in a while – a comic-book superhero’s power is actually real (in a way).

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