Now, being a loyal follower of all things psychological, as I'm sure you are, you'll have heard of synaesthesia by now. If not, trundle on over to Wikipedia and have a quick read.

Originally when this cross-sensory perception became known, researchers were keen to see if people were really having a really real experience or whether it was, to some extent, a fiction. How permanent actually is the connection between, for example, the number 32 and the smell of recently cut grass. Real synaesthetes passed the test and so researchers moved onto something more advanced.

Here in the latest research published in Neuron, a heavyweight-type journal, V S Ramachandran (Reith lecturer from 2003) and others have found further evidence that synaesthetes are experiencing something real and different from the rest of us.

Putting people into an fMRI scanner, the researchers found that synaesthetes showed greater activation in the colour perception region of the cortex. Not only that but different synaesthetes showed different patterns of activation suggesting they might each be having diverse experiences.
ScienceDaily (Press Release)
Individual Differences among Grapheme-Color Synesthetes: Brain-Behavior Correlations [Full article PDF]