I’ve been thinking about the way you see colours that go with each number, and also colours for each day of the week. It’s called synaesthesia- but you probably know that- and you seem like the have number-colour synaesthesia (which is common). There are other kinds like sound-colour synaesthesia or even sound-taste synaesthesia (people who get a taste whenever they hear certain sounds!). Anyway we were talking about it at Burning Man, maybe, or at Christmas, and I seemed to be able to guess the same associations between numbers and colours as you actually see, even though I know I’m definitely not synaesthetic (did you know that synaesthesia is much more common in women than men?). So I thought what I was probably doing was remembering a synaesthetic association from childhood (did you know that synaesthesia is far more common in children?), and that was how I was getting a colour for each number- from memory .
So, next thought, is there a way to distinguish between someone who just has a memory of an association- or is just imaging an association- from someone who really is seeing actual colours when they are shown numbers? Is there, in other words, a test we can do to check if you are really synaesthetic? And of course there is, so I thought I’d write to you and tell you about it and you can have a go.
We’ll get to the test in a sec, but first here’s how it works. It works because colours are obvious. They jump out at you, they’re a kind of visual building block (there’s an early part of the visual system devoted to colours, and neural specialisation is always a good indicator of importance). So, say if you look at a collection of black things and some of them are red you don’t need to search – the red things just jump out at you. Something we (I mean psychologists) sometimes call ‘pop-out’ (technical, eh?). You can demonstrate this to yourself like this. Your job will be to look at a collection of black symbols, and spot the red coloured ones. Click on this link to open the image in a new window. What you should find is that you don’t have to look at them one by one (“what colour is that one? black. Move on. What colour is that on? black. Move on” etc etc), insteaad you just instantly spot the ones which are coloured red. They pop-out at you. No effort required, the answer is just delivered direct to your conscious awareness.
So here’s the test of synaesthesia (well, it’s a test of number-colour synaesthesia at least, which is one kind you have. Do you have any others?). If numbers really do create an actual experience of colour for you, then different numbers should pop out at you in the same way that different colours should. The trick is to control for the different shapes that numbers normally have. So what some very clever people did  was use two numbers that have the same shape, 5 and 2, but are each the reverse of each other upsidedown. Anyway, yes, the test. Your job is similar to last time: look at the image here and try and spot the numbers which are different from all the others. If you can do it as automatically and easily as with the first image then you are really synaesthetic and when you see numbers you really do get an honest-to-goodness perceptual experience of colour. If you can’t, well then it’s not a perceptual phenomenon, but more of a memory and imagination phenomenon (which doesn’t make it not real, it just makes it less unusual).
So this is one way you can confirm to yourself, or test someone else to see if they are properly synaesthetic. Another way is to put them in a brain scanner, and you can see that, for example, numbers really do activate the visual cortex .
I’ve put some links and references at the end here, in case you want to read more about it all. It’s really gripping stuff – I’m convinced this is going to be one of the areas of psychology where loads of progress is made in the next few years. The first Ramachandran paper  is a great place to start and the second one  gives some interesting thoughts on why synaesthesia occurs – perhaps because of excessive ‘cross-wiring’ between different bits of the brain, or, more accurately, a failure to remove cross wiring which everyone has in infancy (brain development after birth actually involves killing off brain cells that aren’t used- ‘neural pruning’- rather than growing new ones). This would also explain why synaesthesia is more common in the young. There’s all sorts of interesting things discussed in the paper, including the higher incidence of creativity reported in synaesthetes and the idea that synaesthesia helped humans develop language (because by connecting different senses it gives a kind of natural symbolism. Me and Matt talked about this more in the book [Hack #50]). The other paper I’ve put in the references is one by a guy called Benny Sannon, who discusses synaesthesia created by taking psychoactives, and his own “extensive, firsthand, experience” with Ayahuasca – a powerful kind of loopy juice which has traditional uses in the indigenous tribal cultures of the upper Amazonian. 
Anyway, hope this has been fun for you. I’ll give you a call soon
the taste of music (mindhacks.com) Fantastic bibliography of different researchers and their theories of synaesthesia (and links to many PDFs) here This guy has done out the numbers and the alphabet so you can see if you get how they look for him Matt Webb’s notes on the ‘phenomenology of synaesthesia’ paper
- Elias, L. J., Saucier, D. M., Hardie, C., & Sarty, G. E. (2003). Dissociating semantic and perceptual components of synaesthesia: behavioural and functional neuroanatomical investigations. Cognitive Brain Research, 16(2), 232-237. PDF of draft here
- Ramachandran, V.S. and Hubbard, E.M. (2001), Synaesthesia: Awindow into perception, thought and
language, Journal of Consciousness Studies, 8(12), pp. 3‚Äì34. PDF here
- Ramachandran, Vilayanur S. and Edward M. Hubbard. (2003). The phenomenology of synaesthesia. Journal of Consciousness Studies, 10(8): 49‚Äì57. PDF here
- Shanon, B. (2003). Three stories concerning synaesthesia – A commentary on Ramachandran and Hubbard. Journal of Consciousness Studies, 10, 69-74. PDF here