In people with the neurological condition synesthesia, one sense triggers another. There's nothing wrong with their eyes or ears, but they might hear colors, or for that matter, taste shapes or feel sounds. Although researchers thought this phenomenon only happened when someone directly perceives something in the outside world, a new study of a synesthete doing arithmetic shows that the mere concept of a number is enough to color her world.
Synesthesia was first documented by Charles Darwin's cousin, Sir Francis Galton, in 1880. The condition is still poorly understood. In fact, it's not even clear how common it is; estimates of prevalence range from 1 in 25,000 to 1 in 2000. (Not every synesthete is aware of the condition. Researchers came across another person with synesthesia in the course of the study who hadn't known he had the condition: "Doesn't everybody see digits in colors?") But there is a scientific consensus that sensory neurons are somehow tangled in the brain. For people with color-digit synesthesia, this means that a black numeral appears with anything from a dull wash of color to swirling hues called photisms.
But are things just as mixed up in the mind's eye? To find out, a team led by experimental psychologist Mike Dixon of the University of Waterloo, Canada, tested the claims of one synesthete, code named 'C,' that she could solve arithmetic problems in color. Researchers gave her a variant of a classic psychology experiment called the Stroop test. In the standard version, someone reading the names of colors aloud is slower if the words are printed in incongruent colors, for example, if the word "red" appears in green ink. The researchers presented C with numerals in either a congruent color--for her, the number 7 looks yellow--or an incongruent color, such as a blue 7. When the numbers and colors matched, she named the colors in two-thirds the time it took to name mismatched colors.
For the conceptual test, the researcher gave C arithmetic problems, such as 5 + 2. Then they immediately showed her a color patch. When the color matched the correct answer, C was faster to name the color than control subjects were. When the patch color didn't match the arithmetic answer, C took about 100 milliseconds longer than controls, the researchers report in the 27 July issue of Nature.
According to synesthesia researcher Jason Mattingley at the University of Melbourne, C's result clearly shows that people with synesthesia aren't just seeing with crossed senses; in some cases, they're thinking with crossed senses, too. Psychologist Daniel Smilek, a study co-author, hopes that such studies might help explain why synesthetes' senses mingle.