People with synesthesia often report perceiving letters as appearing in different colors. But how do their brains accomplish this feat?
The Nobel prize-winning physicist Richard Feynman once wrote in his autobiographical book (What do you care what other people think?): "When I see equations, I see letters in colors -- I don't know why […] And I wonder what the hell it must look like to the students." This neurological phenomenon is known to psychologists as synesthesia, and Feynman's experience of "seeing" the letters in color was a specific form known today as "grapheme-color" synesthesia. What is perhaps most puzzling about this condition is that people actually claim to see two colors simultaneously when reading letters or numbers: the real color of the ink (e.g. black) and an additional -- synesthetic -- color.
Now a new study, published in the March 2011 issue of Elsevier's Cortex, has revealed the patterns of brain activity that allow some people to experience the sensation of "seeing" two colors at the same time.
A group of researchers in Norway used functional magnetic resonance imaging (fMRI) to investigate the brain activity patterns of two grapheme-color synesthetes, as they looked at letters written in different colors, presented on a screen while inside an MRI scanner. The participants had previously been asked to indicate the synesthetic colors that they associated with given letters and were then presented with single letters whose physical color sometimes corresponded to the synesthetic color and other times was clearly different.
Prof. Bruno Laeng from the University of Oslo, along with colleagues Kenneth Hugdahl and Karsten Specht from the University of Bergen, had reasoned that increasing the similarity between the physical and synesthetic colors should affect the level of activity seen in areas of the brain known to be important for color processing, and their results confirmed this expectation, revealing that the strength of the observed brain activity was correlated with the similarity of the colors.
The authors concluded that the same brain areas that support the conscious experience of color also support the experience of synesthetic colors, allowing the two to be "seen" at the same time. This supports the view that the phenomenon of color synesthesia is perceptual in nature.
- Bruno Laeng, Kenneth Hugdahl, Karsten Specht. The neural correlate of colour distances revealed with competing synaesthetic and real colours. Cortex, 2011; 47 (3): 320 DOI: 10.1016/j.cortex.2009.09.004
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