The Brain of a Synesthete

“When I see equation, I see the letters in colors – I don’t know why. As I’m talking, I see vague pictures [of mathematical functions] with light tan j’s, slightly violet-bluish n’s, and dark brown x’s flying around.” So writes the late physicist Richard Feynman in his book, What Do You Care What Other People Think? If Feynman consistently saw j’s as light tan and n’s as violet-bluish, then he had synesthesia, a condition in which stimulation of one sense leads to an involuntary experience in another. So, when a “grapheme-color synesthete” hears a number or letter, he or she automatically sees that number or letter in a specific color, a color that remains associated with that particular letter or number throughout life. There are many forms of synesthesia involving different combinations of the senses; for example, one person might see a shape when they taste a certain food.

What causes these odd perceptual associations? Perhaps, there is an increase in connectivity between different regions in a synesthetic brain. This idea was tested with brain imaging as reported in a recent paper in the Journal of Neuroscience. In this study, 12 grapheme-color synesthetes and 12 non-synesthetes were put into a fMRI machine and instructed to close their eyes but remain alert and awake. So, the synesthetes were not having any synesthetic experiences while in the scanner. Instead, the investigators were asking whether the brain, even in its resting state, showed a different pattern of connectivity in the synesthetes than in the non-synesthetes.

Indeed, a different pattern and strength of connectivity was found. The data suggests that the visual areas of the brain, in particular a color region (V4), were more connected to the auditory areas in synesthetes. This makes sense if hearing a number evokes a color image of the number. In addition, the visual areas in synesthetes were more connected to the right frontoparietal region, a brain region that may be important for the binding or the strong association of the color sensation with the letter or number. What's more, the strength of the connectivity between the visual, auditory, and right frontoparietal regions paralleled the strength of the synesthetic experience for the different subjects. Even in the resting brain then, connectivity is different between people with synesthesia and those without.

We cannot ever know exactly what it’s like to be in someone else’s head. Like our dreams, our waking mental life is personal and private. Yet, this study suggests that, with sophisticated brain imaging, one can predict, at least to a small extent, how another person thinks and imagines.