Lonely people might feel the number one is blue, but that's no metaphor to grapheme-color synesthetes, who really do see numbers and letters as different colors from the rest of us. A new study confirms that synesthetes actually see the colors they claim to see and provides clues to the brain areas involved in the condition.
In synesthesia, one sensory experience triggers another; music might trigger various colors, for example. Grapheme-color synesthesia is a slightly different condition, because it only involves vision. When looking at a black letter "A", for example, a grapheme-color synesthete may see the letter as red or green. Scientists initially questioned the veracity of the condition, but recent visual studies have indicated that synesthetes really do see their colors, although responses varied widely among subjects. To figure out what is going on inside their heads, Edward Hubbard, a cognitive neuroscientist now at the French National Institute of Health (INSERM) in Paris, turned to behavioral tests and functional magnetic resonance imaging (fMRI).
In the first experiment, Hubbard and colleagues at the University of California, San Diego and the Salk Institute in La Jolla, California, asked grapheme-synesthetes and non-synesthetes to identify a shape like a triangle of black fives surrounded by black twos. The task was taxing for non-synesthetes because the numbers look so similar, but it was a snap for synesthetes, indicating that they really do see fives and twos in different colors. Some synesthetes did better than others at the task, however, and also outperformed their comrades on a similar trial involving peripheral vision, indicating that not all grapheme-color synesthetes are created equal.
When the team examined the volunteers with fMRI while they viewed letters and numbers, synesthetes showed increased blood flow between a brain region involved in color-processing (known as V4) and brain regions that process numbers and letters. In addition, fMRI revealed that the synesthetes who did better on the visual tasks showed much more activity in the V4 area than poorer-performing synesthetes. So not only is there a neurological basis for this type of synesthesia, but differences in brain activity explain the variation seen among these synesthetes, says Hubbard, whose team publishes its findings today in Neuron.
"It's a huge contribution to correlate the behavioral measures to patterns of activation," says Michael Dixon, a cognitive neuroscientist at the University of Waterloo in Ontario, Canada. "The study signals a way of changing the thinking on synesthesia."