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So why does thishappen? There was a complete map, a systematic map of the missing phantom hand on his face, draped on his face. So you have a medical mystery of sorts, the sort of mystery we saw with David, the patient with the Capgras syndrome, the sort of mystery that would have intrigued Sherlock Holmes, Conan Doyle or Berton Rouché. So what's going on?
To answer this question, you have to look at the anatomy of the brain again. The entire skin surface, touch signals, all the skin surface on the left side of the brain is mapped on to the right cerebral hemisphere on a vertical strip of cortical tissue called the post-central gyrus. Actually there are several maps but I'll simplify them and pretend there's only one map called the post-central gyrus. Now this a faithful representation of the entire body surface. It's almost as though you have a little person draped on the surface of the brain. It's called the Penfield homunculus, and for the most part it's continuous which is what you mean by a map, but there is one peculiarity and that is the representation of the face on this map on the surface of the brain is right next to the representation of the hand on this map, instead of being near the neck where it should be, so it's dislocated. Now nobody knows why, something to do with the phylogeny or the way in which the brain develops in early foetal life or in early infancy, but that's the way the map is.
So I realised that what's going on here is when you amputate the arm, the part of the cortex of the brain corresponding to the hand is not receiving any signals because you've removed the hand. So it's hungry for sensory input. So what happens is the sensory input from the face skin now invades the vacated territory corresponding to the missing hand, and that then is misinterpreted by higher centres in the brain and arising from the missing phantom hand. And that's why the patient says, every time you touch his face he says oh that's my phantom thumb you're touching, that's my phantom index finger, that's my phantom pinkie. In fact you can even put an ice cube on the face and the patient will say oh my thumb is ice cold. You can put a drop of hot water, in fact you put a drop of hot water and the water started trickling down the face, the patient will trace the trickle on his phantom with his normal hand following its path. On one occasion we had the patient raise his phantom and he was amazed to feel the trickle going uphill which is against the law of physics.
Now that's just a hypothesis but to test this idea, we used the brain imaging technique called MEG or magnetoencephalography which allows you to see which parts of the brain light up when you touch different parts of the body, and sure enough what we found was in this patient, Victor, unlike the normal brain when you touch his face, it activated not only the face area in the brain but also activated the hand region of the Penfield map in the brain. So there has obviously been this cross-wiring in the brain of this patient. And this is important because it allows you to link changes in brain anatomy, changes in brain maps, in the sensory map, with phenomenology, allows you to link physiology and psychology which is one of the goals of cognitive neuroscience, this discipline we're talking about.
The discovery also has broader implications. One of the things we were all taught as medical students is that co
OK so we have seen that after our arm amputation, the patient's brain gets cross-wired. The face input now i
Now, another aspect of this syndrome is whenever you hear a particular tone you see a particular colour. Some of these people also when they see numbers in black and white, like the number five or the number six or the number seven, what we call Arabic numerals - Indian numerals strictly speaking - when you see these numbers, every time you see a number it evokes a particular colour so 5 is always red, 6 is always green. It's always tinged green. 7 is always indigo, 8 is always yellow, so this again is another example of synesthesia and it's very common. We find it's about one in two hundred people have this so it's not as rare as people have thought it to be in the past.
Now why does this happen? Why does this mixing of signals occur? Well we, a student of mine, Ed Hubbard and I were looking at brain atlases, brain maps and we were struck by the fact that if you look at the fusiform gyrus, that's where the colour information is analysed. But amazingly the number area of the brain which represents visual graphemes of numbers, 5 6 7 8 and all of that, is right next to it also in the fusiform gyrus, almost touching it. So we said this can't be a coincidence. Maybe in some people there is some accidental cross-wiring. Just as after amputation we get cross-wiring between the face and the hand, in these people maybe there's a cross-wiring between the number and colour area in the fusiform gyrus. Of course the key difference is in the case of phantom limbs, it's the amputation that causes the reorganisation whereas in synesthesia given that it runs in families, we think it's caused by a gene or a set of genes which causes abnormal co
First of all many neurologists, neuroscientists in the past, even though the phenomenon was described by Galton a hundred years ago, it's been regarded mainly as a curiosity. It sounds crazy. They're just crazy or they're just faking it to draw attention to themselves, or maybe it's childhood memories. You played with refrigerator magnets, 5 was red, 6 was blue, 7 was green. Now it never made much sense to me because why does it then run in families? You have to say maybe the same magnets were passed round. So first thing we wanted to show this is a real phenomenon, it's the person genuinely sees red when he sees 5. It's not just imagination or memory. How do you show that?