We did quite a bit on a whole load of visual phenomena, including illusions, one might sound amazingly trivial, it was a café wall down the road which is tiled, it’s Victorian, and neatly done in rows, each parallel, and it’s sort of black and white, it’s like a chess board with alternate rows displaced by a half square, that is an exact description of it, and it’s got lines between the rows like grey lines, and we were puzzled by this because when you look at it, they don’t look parallel, they look like wedges, long, long wedges, alternate wedges, so that the first row might be shorter, apparently, to the right, the next one to the left, the next one to the right alternately. This was amazingly puzzling and because it actually appears to violate the principle of physics which is Curie’s principle, that’s Pierre Curie, the husband of Madame Curie, radium and all that, and his principle states that if you’ve got something symmetrical, that can’t produce or generate or create asymmetry, but here is a symmetrical pattern, this chess board thing, but it’s asymmetrical because you’ve got these great, long wedges in it, which are not symmetrical so how can a symmetrical pattern produce an asymmetrical illusion? The fact that it’s an illusion still means it should not violate the law of physics, one would think. So I spent quite a lot of time on that, we made models of it and played about with it and we could change the brightnesses, the thicknesses of the lines, and we developed about ten laws of the Café wall illusion, which briefly are these. If you make the lines between the rows darker or lighter than the squares, the illusion disappears. If you make the tile red and green, or any other colours of the same brightness, then the illusion disappears. By the way, there’s a lesson here. If you’ve got any phenomenon and you can make it disappear, you learn an awful lot about that phenomenon. It’s what switches something off which can be crucially important in physics or in psychology or perception. This is really important and this a major reason for the science centre, by the way, where children and real people can play about because they realise after a time that the phenomena only work under certain conditions, what are those conditions, why are those conditions critically important? And it’s understanding all of that that leads you to the answer to the original problem, in this case, why do these things look like wedges and it was by looking at when the phenomenon, the illusion, disappears, that we got to what I think is the answer, why it happens. You have to look at when something doesn’t happen in order to understand why it does happen. Why does it work? Well, it’s slightly complicated but imagine these half squares, you’ve got a black square and then below that a white square displaced by half a square so you’ve got a black edge, a neutral line and then a light edge but what I think is happening is, this is quite a subtle idea really, the brain doesn’t know, the simple minded bit of the brain doing the original analysis, that this is actually a grey line between them. It could be an error in its own processing and it’s really a question of exactly where edges are because it’s not precisely signalled visually so the brain has to sort of guess where a line is. Now, what happens is that the narrow line, what we call a mortar line, between the squares, get pulled together to avoid what the brain would see as an error in its signalling so actually the distortion is due to an error correcting system which, in that situation, produces an error and often when you’ve got a system for avoiding, reducing or removing error, that itself in certain situations produces an error. There’s no infallible way of avoiding errors. So what you could so is to tease out the error correction systems in the brain by looking at illusions, paradoxically. I’m not saying for all illusions but it’s true of many and it’s true of this one.