So we devised a simple display on the computer screen, a number of 5s scattered on the screen, black 5s, just black and white. Embedded among those 5s are a number of 2s and these 2s form a shape like a triangle, a hidden shape, a triangle or a square. Now when all of us here, any one of us normals or less gifted individuals, looks at this pattern, you see nothing. But when a synesthete looks at it, who sees numbers as colour, he sees the 5s as red and the 2s as green so he looks at it and he says my god, instantly the 2s pop out and he says oh they're forming a triangle, they're forming a square. And they are much better at doing this than normal people so it's a clinical test for discovering synesthetes.

Now, the fact that synesthetes are actually better at this than normal people suggests that they can't be crazy. If they're crazy, how come they're better at it? So this shows this is a genuine sensory phenomenon. It also shows that it can't be memory association or something cognitive or a metaphor because - if that were true, how come he's able to see the triangle or the square pop out from the background? OK we have shown that this phenomenon is real, we in La Joya and as well as Jeffrey Gray and Mike Morgan and others here in London have done experiments to test the idea there's actual cross-wiring in the brain and have shown that in fact there's activation of the fusiform gyrus in the colour area just showing numbers to these people.

But then the next question is OK, here are some people with some quirk in the brain. They see numbers as colours so what's the big deal, why should I care? Well now I'm going to show you that you're all synesthetes but you're in denial about it. So what I want you to do is I want you all to imagine two shapes in front of you. I wish I had a slide but of course this is radio. One of them imagine is a shattered piece of glass with jagged edges, the other is like an amoeba, it's got undulating curvy shapes. And one of them I'm going to call a bouba, and the other is kiki - which is which? Now the amazing thing is 98% of people will pick the shattered piece of glass with the jagged edges, and say oh that's a kiki, and the undulating amoeboid shape, oh that's a bouba, even though they have never seen the shape before.

Why does this happen? Well I suggest it happens - I am going to talk about this in detail in my Oxford lecture - because you're all synesthetes.

Now we have talked about several syndromes, phantom limb, synesthesia, Capgras syndrome and these are quite bizarre and we have tried to explain it in terms of neural circuitry in the brain but there is a syndrome that is even more bizarre, and this is pain asymbolia. I'll tell you about a patient I saw in Vellore in India about ten years ago. He was quite normal, intellectually normal, mentally quite lucid, alert, attentive, his memory was normal, perception, everything was fine except I took a needle and pricked him with a needle to determine the intact, not to be sadistic but to determine the intactness of his pain pathways, to determine his pain sensations, and every time I poked him, of course, all of you here would say, ouch. He started giggling, telling me doctor, I feel the pain but it doesn't hurt. It feels very funny, like a tickle and he would start laughing uncontrollably.

Here is the ultimate irony - a human being laughing in the face of pain. OK, why would the patient do this? Well, seeing this patient made me ask an even more basic question, why does anybody laugh? Why do all of you here laugh, OK? Now a martian ethologist watching all of you here would be very suprised every now and then. It is a species-specific trait, laughter. Every now and then all of you here stop what you're doing, shake your head, make this funny staccato rhythmic sound, hyena-like sound. Why do you do it? Now clearly laughter is hard-wired, it's a "universal" trait. You see it every society, every civilization, every culture, society, has some form of laughter and humour - except the Germans.

So this suggests strongly it's hard-wired in the brain and this raises an interesting question. Why did it evolve in the brain, OK, how did it evolve through natural selection? When you look at all jokes and humour across societies, the common denominator of all jokes and humour despite all the diversity is that you take a person along a garden path of expectation and at the very end you suddenly introduce au unexpected twist that entails a complete re-interpretation of all the previous facts. That's called a punch-line of the joke. Now obviously that is not sufficient for laughter because then every great scientific discovery or every "paradigm shift" would be funny, and my scientific colleagues wouldn't find it amusing if I said their discoveries were funny.

OK, the key ingredient here is, it's not merely sufficient that you introduce a re-interpretation but the re-interpretation, the new model you have come up with should be inconsequential, it should be of trivial consequence. It sounds a bit abstract so let me illustrate with a concrete example. Here is a portly gentleman walking along, he is trying to reach his destination, but before he does that he slips on a banana peel and falls. And then he breaks his head and blood spills out and obviously you are not going to laugh. You are going to rush to the telephone and call the ambulance. But imagine instead of that, he walks along, slips on the banana peel, falls, wipes off the goo from his face, looks around him everywhere, and and then gets up, then you start laughing. The reason is I claim is because now you know it's inconsequential, you say, oh it's no big deal, there's no real danger here. So what I'm arguing is, laughter is nature's false alarm. Why is this useful from an evolutionary standpoint? So what you are doing with this rhythmic stocatto sound of laughter is informing your kin who share your genes, don't waste your precious resources rushing to this person's aid, it's a false alarm everything is OK. OK, so it's nature's OK signal.

OK, now what does this got to do with my patient in Vellore? Let me explain. When we examined his brain, when we do the CT scan we found there was damage to the region called the insular cortex on the sides of the brain. The insular cortex receives pain signals from the viscera and from the skin. That's where you experience the raw sensation of pain but it turns out there are many layers to pain. It's not just a unitary thing. From the insular cortex the message goes to the amygdala, which we encountered earlier in the context of the Capgras syndrome and then to the rest of the limbic system, and especially the anterior cingulate, where you respond emotionally to the pain, to the agony of pain and take the appropriate action. So my idea was, maybe what's happened on this patient is, the insular cortex is normal. That's why he says, doctor I can feel the pain, but the message, the wire that goes from the insular to the rest of the limbic system and the anterior cingulate is cut.

Therefore you have the two key ingredients you need for laughter and humour, namely one part of the brain signalling a potential danger, my god there is something painful here, but the very next instant the anterior cingulate says but I'm not getting any signal. Big deal, there is no danger here, forget it, OK. So you got the two key ingredients and the patient starts laughing and giggling uncontrollably, OK. So it is a disconnection similar to what we saw with the Capgras patient.