Most of us perceive the sensory attributes of stimuli fairly accurately, but in some rare cases, stimuli in one modality can provoke an additional sensation in a different modality. For example, a patient has been described who reports that “2 is red but 5 is green.” When Ramachandran and Hubbard (2001) showed this person a page like the one in Figure 1a, with the instruction to quickly point to all the 2s, the person was able to complete the task much faster than most other people can. Because the shapes of 2 and 5 are so similar, it takes a while for most people without synesthesia to pick out each 2 among all those 5s. But to the subject with synesthesia, the stimuli looked more like Figure 1b, which made the 2s much easier to pick out. As much as 2–4% of the population displays some form of synesthesia, which may be especially associated with creativity (Cytowic and Eagleman, 2009) and with enhanced neural connectivity (Rouw and Scholte, 2007). You can test whether you have synesthesia online at http://www.synesthete.org.
Cytowic, R. E., and Eagleman, D. M. (2009). Wednesday is indigo blue: Discovering the brain of synaesthesia. Cambridge, MA: MIT Press.
Ramachandran, V. S., and Hubbard, E. M. (2001). Psychophysical investigations into the neural basis of synthaesthesia. Proceedings of the Royal Society of London. Series B: Biological Sciences 268: 979–983.
Rouw, R., and Scholter, H. S. (2007). Increased structural connectivity in grapheme-color synesthesia. Nature Neuroscience 10: 792–797.