The video above seems completely unremarkable at first - man walks down a corridor, navigating his way around easily visible and conspicuous obstacles. But it's far from an easy task; in fact, it should be nigh-impossible. The man, known only as TN, is totally blind.
His inability to see stems from a failure in his brain rather than his eyes. Those work normally, but his visual cortex - the part of the brain that processes visual information - is inactive. As a result, TN is completely unaware of the ability to see and in his everyday life, he behaves like a blind person, using a stick to find his way around. Nevertheless, he can clearly make his way through a gauntlet of obstacles without making a single mistake.
TN was a doctor before two successive strokes destroyed his ability to see. The first one severely damaged the occipital lobe on the left side of his brain, which contains the visual cortex. About a month later, a second stroke took out the equivalent area on the right hemisphere. TN is one-of-a-kind, the only known patient with damage like this in the entire medical literature. The fibres that connect the occipital lobes on the right and left halves of the brain have also been severely damaged and tests reveal that no blood flows between these disconnected areas.
Alan Pegna from the University of Bangor in Wales was the first to study TN's abilities after he was recovering from this second stroke in a Swiss hospital. Pegna was the first to discover TN has an ability called blindsight, that allows him to unconsciously detect things in his environment without any awareness of doing so. He could correctly guess the emotions playing across the faces of other people. And as he did so, his right amygdala - an area of the brain involved in processing emotions - became active.
His visual cortex is a different story. This area, which normally lights up like a beacon when people view the world around them, failed to do so in TN's brain. With his consent, Beatrice de Gelder from Tilburg University put him through a series of visual tasks and scanned his brain using several different techniques. To a one, all of these tests failed to show any trace of activity in the visual cortex. For example, he repeatedly failed to spot bright circles of static or flickering light, displayed against a dark screen.
But not all the tests were letdowns. When de Gelder showed him a metre-long rod attached to a wall, he was able to tell whether it was horizontal, vertical or diagonal with 100% accuracy. He could discriminate between any two orientations of the rod, as long as they were about 25 degrees apart. It was a small victory, but an encouraging one and, actually, another famous blindsight patient called DB also did well on the same test.
In light of these results, de Gelder decided to see how TN would fare in a more realistic setting. Her team created the obstacle course of bins, boxes and tripods arranged randomly along a long corridor. To everyone's astonishment, TN negotiated it perfectly, never once colliding with an object despite lacking both guide and walking-stick. An experimenter followed him at all times in case he fell, but it never happened. The first time he completed the course, the assembled researchers burst into applause.
How does he do it? Some scientists have suggested that blind or blindfolded people can use sound to guide their movements, using an extremely crude version of the sophisticated echolocation wielded by bats. But de Gelder says that it's a very remote possibility - during TN's walk down the corridor, neither he nor the experimenter behind him made any noises loud enough to produce helpful echoes. And while echoes may be useful for large objects or spaces, humans aren't sensitive enough to them to be able to avoid smaller objects, as TN clearly does in the video.
TN's ability is unique among humans, but not among all animals. In 1974, Nick Humphrey at Cambridge University found that a monkey called Helen, who also had lesions in both her visual cortices, could also avoid obstacles in an open space. In Helen's case, Humphrey thought that her skill might stem from a small undamaged part of her left visual cortex.
But TN's more comprehensive damage rules that explanation out for him. Instead, de Gelder thinks that other parts of the brain outside of the visual cortex may be responsible. When de Gelder asked TN to look at pictures of bodies and faces and say if they were the right way up or upside-down, he only answered correctly at levels expected by chance. However, as he was looking at the images, electrodes recorded some activity in the left part of his brain, in areas near, but not within, the visual cortex.
This time round, TN failed to guess the expression on happy or fearful faces put in front of him, as he did in Segna's study. The researchers said that he seemed less willing to make rapid, spontaneous guesses and instead gave answers after much time and effort. But again, electrodes detected activity in the left half of the brain, near the back. Some mental cogs were indeed whirring to life.
The identities of these other pathways, which allow TN to be aware of his surroundings without any conscious sight, remain a mystery. But, perhaps, not for much longer.
Reference: Current Biology in press
More on perception:
- Log in to post comments
Little big man.
I suppose you could differentiate between some part of the brain being able to use the information to avoid the obstacles and echolocation by blindfolding the subject. If he can still navigate with his eyes closed or covered, he's not using them.
I believe he navigates with sound, using echolocation. It happens to me very often, that I can feel somebody standing right behind me, even though he makes no sound. Its probably because he's blocking noise coming from behind. It can be very little noise (smaller than from street through good windows, air flowing). It's like when you cover your ears - you can actually HEAR the silence, its different.
But yeah, could be anything, looking forward for explanation.
Sorry, me again.
Maybe if they could try few things:
- record him going in total dark, using infra camera
- move objects further away from each other, so that you know whether he dodges (or not) because he sensed object on his side, or in front of him (i mean that green book)
- try different shapes with different material (what about flat piece of another carpet?)
This is not perhaps as entirely mysterious as your entry implies. The paper gives a nicely characterised case, particularly by using DTI imaging to track white matter pathways and show the extent of the damage, which wasn't available for earlier case reports. But blindsight and related effects are well-established phenomena, with Larry Weiskrantz, one of the co-authors of this paper, being particularly prominent in this field over several decades.
Just because striate cortex is obliterated doesn't mean visual information doesn't reach other parts of the brain. For example, this person will almost certainly still have a pupil reflex. Most visual fibres from the optic tract reach a relay station in the thalamus called the lateral geniculate nucleus and travel thence to striate cortex (which is disrupted in this case). But some fibres directly project directly to the pretectal area in the midbrain which controls the pupil reflex. So it is quite possible for a person's pupils to constrict in response to light that they don't report seeing.
Still other fibres travel to another midbrain structure called the superior colliculus, which is capable of controlling much more interesting behaviour. It can generate eye movements to a visual stimulus and cause other orienting movements. There may be a useful ascending pathway from superior colliculus to parietal cortex which controls spatial awareness and directed movement. That cortical area normally gets much of its information from the neighbouring visual cortex but might rely more on other sources when that is not available, as in this case.
The authors are being rightly conservative about naming possible pathways because they don't have any evidence in favour of any particular one in this case. But candidate pathways are certainly known:
"In addition to the projections to the superior colliculus, the retina projects directly to at least ten other distinct sites in the thalamus, hypothalamus and midbrain and some of these structures send projections in turn to the cerebral cortex. Processing in all or some of these pathways could support many of the different kinds of visually guided behavior that have been called blindsight." (Danckert & Goodale, 2000. Blindsight: a conscious route to unconscious vision. Current Biology, 10, R64-R67, p. 64)
To rule out the use of auditory information would be reasonably straightforward. Just get the subject to wear an iPod, playing white noise to be scientific, but loud music would suffice.
If I recall my blindsight literature correctly (and I haven't read this report yet) I thought for sure that there are identified pathways directly from the thalamus to motion area V5 (or called MT). Not mysterious. But if so, these are better abilities than have been demonstrated before.
Now I have to go read!
Thanks for the context folks. I did feel that the paper was frustatingly conservative in suggesting possible explanations/pathways but that's an emotional reaction from me - as Michael said, I fully understand the reasons for being cautious.
Psycho - I think the objection to the echolocation idea is that it's unlikely that it could provide enough spatial resolution to be able to avoid some of the smaller obstacles like the boxes. In the example you cite of someone standing behind you, a person is considerably larger than any of the obstacles TN bypassed.
What I looked for in the article, but didn't see, was a report of that the patient himself thinks about this. Unless his speech has been badly affected, he should have some interior report which might add to the topic.
Noni
While it'd be interesting to read about how he felt while navigating the corridor, I actually wonder how informative that might be. I think the idea is that whatever pathways he's using lie outside conscious thought - he is, after all, functionally blind and has no conscious awareness of seeing. He doesn't have any other cognitive problem, by the way - just the blindness.
Has any one thought to ask TN how he does it?
It would, indeed, be interesting to hear what TN has to say. I did find this general claim, on the subject:
But, the hallway obstacle course isn't a task involving forced choice between given alternatives. I wonder if TN was simply asked to walk with the additional instruction "try to avoid any obstacles as you walk." I found this speculation from another article interesting:
When I was younger I'd get slight migranes; not much pain, but I'd see sparkles around the edge of my vision, which would slowly grow until they blocked my entire eyesight (I've heard them called "optic migraines"). I'd just have to sit and rest or nap and they'd go away. One particular time I had one and was unable to "see" at all due to sparklies in my vision, but someone handed me a glass of water and I grabbed it without thinking; I briefly experimented with 'seeing' and found that, the less I concentrated on what I was doing, the better I did (the more I did, the dizzier I got, so I didn't do it long). It really shows how "me" and "my body" tend to be two things operating cooperatively, not necessarily one-and-the-same.