Now the problem with this explanation is that in science you can never explain one mystery with another mystery. Saying that synesthesia is just a metaphor doesn't explain a damn thing because we don't know what a metaphor is or how it's represented in the brain. And indeed as we go along, what I'd like to do is to turn it upside down and suggest the very opposite, that synesthesia is a sensory phenomenon whose neural basis you can discover in the brain and that in turn can give you an experimental foothold for understanding more elusive aspects of the mind such as what is a metaphor, so why has it been ignored? There's an important lesson here in the history of science. And I think in general it's fair to say that for a curious phenomenon, an anomaly to make it into mainstream science and have an impact, it has to fulfil three criteria, and that is first you have to show it's a real phenomenon. Second, you have to have a candidate mechanism that explains what it might be. And third it has to have broad implications. What's a big deal? So what, who cares? So for example if you take telepathy, OK telepathy has vast implications if true so the third criterion is fulfilled but the first criterion is not fulfilled, it's not repeatable. We don't even know if it's true, it's probably bogus. Another example would be bacterial transformation. If you take one species of bacteria - pneumococcus - and you incubate it with another species of bacterium, the second species actually becomes transformed into the first species and you can do this just extracting the chemical, the DNA, and then use that to induce the transformation and this was reliably repeatable. Many times it was repeated as published in a prestigious journal but people ignored it. OK why did they ignore it? Because nobody could think of a candidate mechanism. How can you possibly encode heredity in a chemical until Watson and Crick came along, described the double helical structure of DNA, described the genetic code and then people started accepting it, and recognised the importance of bacterial transformation.

So I'd like to do the same thing with synesthesia. First of all I'd like to show it's real, it's not bogus. Second, suggest candidate mechanisms, what's going on in the brain. And third, so what - why should I care? So I'm going to argue in fact synesthesia has very broad implications. It might tell you about things like metaphor and how language evolved in the brain, maybe even the emergence of abstract thought that us humans, human beings are very good at.

So first we need to show synesthesia is a real phenomenon. What we did was essentially develop a clinical test for discovering closet synesthetes, and how do you do that? First of all we found two synesthetes and these people saw numbers as colour, for example five as red and two as green, so we produced a computerised display on the screen which had a random jumble of fives on the screen and embedded among these fives are a number of twos, and the twos are arranged to form a shape like a triangle or a square or a circle. Now when you and I, anybody here in the audience who is not a synesthete looks at this display, it takes several seconds, as much as twenty or thirty seconds before you say oh I see all the twos, they are arranged to form a triangle or a square. Now when we showed this sample display to the two synesthetes, they immediately or very quickly saw the triangle or the square because the numbers are actually coloured for them, they see them conspicuously popping up from the background so this demolishes the idea that they're just crazy because if they're crazy, how come they're better at it than all of you normals? It also suggests that it's a genuine sensory effect because if it's just a memory association or something high level, how come they actually see the triangle? So we know the phenomenon is real and using this test and other similar tests, we are able to show that it's much more common than people have assumed in the past. In fact people have claimed that it's one in ten thousand. We find it's one in two hundred, probably two or three of you here in the audience who don't want to admit it.

So next what causes synesthesia? Well my students and I, especially Ed Hubbard, he and I were looking at brain atlases and we found if you look at a structure called the fusiform gyrus in the temporal lobes of the brain, it turns out that the fusiform gyrus has the colour area V4 which is described by Semir Zeki. This is the area which processes colour information but we were struck by the fact that the number area of the brain which represents visual numbers as shown by brain imaging studies, that number area is right next to it, almost touching the colour area of the brain so we said this can't be a coincidence, how come the most common type of synesthesia is number/colour and the number area and colour area are almost touching each other right next to each other in the same part of the brain? Maybe what's going on is these people have some accidental cross-talk, or cross-wiring, just as in my experiments on phantom limbs in my London lecture I showed that the face area becomes cross-wired with the hand area in the cortex, except in this case it happens not because of amputation but because of some genetic change in the brain. And now we've done imaging experiments on people with synesthesia and showed that if you show just black and white numbers, they get activation in the colour area.

Now the next question is why does this cross-wiring or cross-activation occur? Well remember I said it runs in families. Well this suggests there's a gene or set of genes involved. What might this gene be doing, this bad gene? Well one possibility is we are all born with excess connections in the brain. In the foetus there are many more redundant connections than you need and then you prune away the excess connections to produce the modular architecture characteristic of the adult brain, like Michelangelo chipped away everything that doesn't look like David to produce David. That's how you generate a brain. So I think what's happened in these people is that gene is defective and therefore there's defective pruing so there's cross-activation between adjacent areas of the brain - or there could be some kind of chemical imbalance that produced cross-activation between adjacent parts of the brain that are normally only loosely connected and this produces a hyperconnectivity between these parts of the brain.

Now what we found next was even more amazing. Take the same two synesthetes. Instead of showing them Arabic numbers- actually I should call them Indian numbers but it doesn't matter - Indian/Arabic numbers, you show them Roman numbers, Roman V which looks like a V or a 6. Guess what happens? They say oh I know it's a five but it doesn't look coloured, it's black and white so Roman numbers don't give colours. Now what does that prove? It's very important because it shows it's not the numerical concept that drives the colour but the visual appearance of the Indian/Arabic number and it fits with what I'm saying because the fusiform gyrus represents the visual appearance of numbers and letters and things like that, not the abstract concept of sequence or ordinality.

Where does that occur, the abstract idea of number? We don't know but a good guess is angular gyrus in the left hemisphere. We know that because when that's damaged in patients they can no longer - they're fluent in conversation, they are intelligent and all of that but they can't do even simple arithmetic. You say what's seventeen minus three, he'll say oh is it nine? Gets it completely wrong. So we think that abstract number concepts are represented in the angular gyrus and remember this chap's cross-wiring, is in the fusiform gyrus but in the visual appearance of a number and the colour.