Van Leeuwen, T. (2011). How one can see what is not there: Neural mechanisms of grapheme-colour synasthesia. PhD Thesis, Radboud University Nijmegen, Nijmegen.
People with grapheme-colour synaesthesia experience colour for letters of the alphabet or digits; A can be red and B can be green. How can it be, that people automatically see a colour where only black letters are printed on the paper? With brain scans (fMRI) I showed that (black) letters activate the colour area of the brain (V4) and also a brain area that is important for combining different types of information (SPL).
We found that the location where synaesthetes subjectively experience their colours is related to the order in which these brain areas become active. Some synaesthetes see their colour ‘projected onto the letter’, similar to real colour experiences, and in this case colour area V4 becomes active first. If the colours appear like a strong association without a fixed location in space, SPL becomes active first, similar to what happens for normal memories.
In a last experiment we showed that in synaesthetes, attention is captured by real colour very strongly, stronger than for control participants. Perhaps this attention effect of colour can explain how letters and colours become coupled in synaesthetes.