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While driving home on Sunday I listened to another Skeptics Guide podcast. The ‘science, or fiction?’ segment included the statement that ‘scientists had found that a blind man was able to navigate flawlessly around obstacles without using any other senses.’ Science, or fiction, indeed. I was inclined to pick it as the ‘fiction’ statement, until I remembered reading something about the phenomenon known as ‘blindsight’.

Blindsight? It’s where someone who is clinically blind, & has no conscious recollection of seeing anything, nonetheless shows evidence of ‘seeing’ things at some subconscious level. Their blindness has to be due to some damage at the level of the brain ie their eyes function just fine, but the message doesn’t get through to the visual cortex.

And indeed, that’s just what the SG segment was about. (For once I got one right!) The patient they were describing, who’s identified as TN by the researchers – unfortunately our institutional subscription won’t let me access the original paper so all I can see is the abstract, very sad – has what’s known as ‘cortical blindness’, which means that his blindness is due to damage (from two strokes) to his visual cortex. His eyes and optic nerves function just fine, but the information they gather & transmit is not being processed in the part of the brain responsible for interpreting visual signals – the researchers confirmed this using brain imaging.

After confirming that TN really is clinically blind, here’s what the researchers did next:

They then persuaded TN to set his stick aside and walk down a corridor strewn with lab equipment. He was able to do so flawlessly, despite being unable to consciously see any of the obstacles. Head down and hands loose by his side, he twisted his body to slalom slowly but surely between a camera tripod and a swingbin, and neatly stepped around a random series of smaller items.

(The quote’s from NatureNews, so my apologies in advance if you can’t access it directly.)

It’s unusual to have an example of blindsight where the patient has damage to the visual cortex in both hemispheres of their brain. I’ve read other studies where scientists have tested the phenomenon in people who are affected on just one side. When such individuals are shown a picture, to their blind side only, they can often give at least some information about that picture, which suggests that some data from the retina are being processed by another, subconscious, part of the brain.

Apparently TN is able to interpret facial expressions – which he can’t ‘see’ in the normal sense – but the finding that he can also navigate his way around obstacles with a high degree of accuracy was unexpected. You could argue that he’s using echolocation, but the research team said ‘no’, becuase he was so accurate in his movement down a cluttered corridor. That doesn’t entirely rule echolocation out, because TN wasn’t blindfolded to block out all input from his eyes. It would be really interesting to know if he’d consent to repeating the corridor walk under such conditions.

But if TN is definitely using blindsight, think what this suggests about the subconscious goings-on in our brains. And after all, who hasn’t gone for a walk & arrived at the destination without any clear memory of what happened between A & B? (Or is that just me? That’s a worry!)