Synesthesia is a cross-firing of the senses. For example, a person may perceive particular digits to have specific colors, sounds, shapes or even personalities. One of the more common forms of synesthesia is called grapheme, or color synesthesia, in which the synthesthete sees each letter or number as an exact shade that can be picked out of a color chart. However, people who have grapheme can also identify the physical color of the letter, that is, the color of the ink. How do synthesthetes see in two colors at once? Bruno Laeng , Kenneth Hugdahl and Karsten Specht from the Universities of Oslo and Bergen found that synthesthetes use the same parts of the brain to process the two kinds of color.
The researchers put two such volunteers in an fMRI to see which parts of their brains were responsible for identifying the different colors. In some cases, they gave the subjects letters or numbers that were physically colored (via the wavelength of the ink) the same shade that that person perceived the letter or number to be. In other cases, the ink color was completely different from the perceived color. In all trials, the same part of the brain was activated, namely the color processing part of the brain. In other words, the information about the color produced by the ink’s wavelength and the ‘imagined’ synesthetic color were both processed by the brain in the same way.
This indicates that synthesthetes feel as if they are really seeing both colors equally; not something the rest of us can easily understand.