Chris: I'm not really sure why DS thought that 40 has a distinct color. He seemed to think of 40 as a stand-in for a generic large number: he said that both 100 and infinity are also gray, and that was because they are large numbers. One thing I'm interested in, though, is whether more numbers come to have colors over time - as a child matures. Not sure. In any case, there are definitely some adult synaesthetes who have individual responses to large numbers. Daniel Tammet, for instance, says that each number up to 10,000 has a distinct experiential profile for him. That said, I'm not at all sure that our DS counts as a number-color synaesthete. I showed him the pop-out image and he had some trouble even finding all the two's. So do I, but I don't think numbers have colors in the first place.
BB
