Synesthesia, in which letters or numbers elicit color perception, could be due to increased brain connectivity between relevant regions, or due to failure to inhibit feedback in cortical circuits. Diffusion tensor imaging now provides evidence for increased connectivity in word processing and binding regions of the brain. Hubbard comments (PDF here) on the article by Rouw and Scholte (PDE here):
If looking at this page of text causes you to see a cascade of colors, you have grapheme-color synesthesia, in which viewing letters and numbers in black and white elicits the experience of seeing colors... grapheme-color synesthesia occurs in as many as 2 out of every 100 people...To study this:
...the authors used diffusion tensor imaging (DTI), a neuroimaging technique that measures the diffusion of water molecules in the living human brain. Water molecules diffuse more easily parallel than perpendicular to the direction of white-matter fibers, because of the myelin sheaths and axonal membranes. By measuring relative differences in how easily water diffuses along different axes (termed fractional anisotropy), it is possible to infer the size, orientation and degree of myelination of white matter tracts in vivo... this technique demonstrated increased structural connectivity in synesthetes compared with controls in three brain regions: the right fusiform gyrus, which is near regions involved in word and color processing, and the left intraparietal sulcus (IPS) and frontal cortex, both of which are part of a network of regions involved in binding and consciousness
Figure - The outer cortical surface with relevant brain regions indicated.
The color-selective hV4 is indicated in red, and the visual word form area is indicated in green. Cross-activation between these regions, mediated by increased anatomical connectivity, correlates with the generation of the additional experiences of grapheme-color synesthesia, and the degree of connectivity determines their strength. The posterior IPS, thought to be involved in binding, is in blue. Additional anatomical connectivity in this region may be critical for synesthetic binding, which must operate on the colors generated by the cross-activation between grapheme regions and hV4. These regions have been projected to the left hemisphere for simplicity.