So, there is this one condition called synaesthesia, where basically perception gets crosslinked. Most commonly, numbers, letters, and words get colours coupled to them. This way around, I have a few friends who I know have it.

The more exotic varieties couple more or other senses to each other.

The whole thing gets Really Interesting, and ties in to quite a bit of philosophy as well, when you start coming near the really odd cases. Qualia are the philosophical term for "how things are perceived by us". Basically, it boils down to the following: if I see something red, is this intrinsic to the object, or something existing in my perceptive neurons only?

And so far, arguing about it has been more or less all there was. At least known to me.

Then I stumbled across one of the latest post at Making Light. They link to the story of a colourblind synaesthete. Who perceives colour through his synaesthesis that he DOES NOT perceive in the real world.

I don't really have much of a philosophical point of my own - I seem to get entangled whenever I try to start writing one - more than Go! And Read! The Making Light thread is a good starting point for this, and the comments are - as is normal over there - absolutely brilliant.