2 is orange but 'two' is blue

The world of synesthesia

For nearly 300 years, people who claimed to hear colors, feel sounds or taste shapes risked being dismissed as having overactive imaginations. Last year, however, several scientific studies of this rare condition produced compelling evidence that it is a genuine perceptual phenomenon. Now, a Vanderbilt study of the most common form of synesthesia – the perception that numbers, letters and words have distinct colors – has confirmed these earlier findings and provided additional evidence that synesthetic experiences originate during the central stages of visual processing in the brain.

The Vanderbilt study, reported in the March 19th issue of the journal Proceedings of the National Academy of Sciences, consisted of administering a battery of perceptual tests to an adult male known as WO who experiences synesthesia. The tests go far beyond the tests of individuals with synesthesia that have previously been reported. Little is known about synesthesia’s causes or its prevalence. Estimates range from one in 2,000 to one in 25,000 and there is also some evidence that the condition is more common in women than in men. Nevertheless, a number of famous people – including the poets Baudelaire and Rimbaud, the painters Kandinsky and Klee, the composers Lizst and Scriabin – have been linked to synesthesia. In an attempt to describe what synesthesia is like, novelist and synesthete Vladimir Nabokov wrote that he saw the letter “c” as light blue; associated “a” with the look of “weathered wood,” and got a feeling like “a sooty rag being ripped” from the letter “r.”

The synesthete who served as the subject of the new study is a middle-aged man who reports that he has had the condition since early childhood. “WO sees letters, numbers and individual words printed in black-and-white in vivid color. If the characters are printed in different colored ink, he can see that color as well,” says Thomas J. Palmeri, assistant professor of psychology, who headed up the study. His collaborators are Centennial Professor of Psychology Randolph Blake, Assistant Professor of Psychology René Marois, graduate student Marci A. Flanery and Professor of Pathology and Psychiatry William Whetsell, Jr.

After determining that WO had normal eyesight and color vision, the researchers began by documenting the consistency of his synesthetic color associations. To do so, they drew up a list of 100 common words of one syllable. In two sessions, separated by more than a month, they asked him to name the synesthetic color associated with the words in the list. They found that his color associations were remarkably consistent. In the two sessions, he associated the same color with 97 of the words. His only mismatches occurred in confusing beige, off-white and light brown. He also consistently matched the synesthetic colors of 12 words with colors from a Pantone palette containing more than 1,000 shades. “These associations are highly reliable. WO says that the colors have stayed the same all of his life and our observations lend credence to his claim,” says Blake.

Another group of tests were designed to explore the range of conditions that invoke synesthetic colors. In one test, the researchers created large characters made out of small characters, such as a large four made out of a large number of small twos. They found that, when WO’s attention was directed to the large four, it took on the dark green color that he associates with four. But when his attention was directed to the small twos, the color suddenly switched to orange. The researchers also created figures in which different parts of the letters or numbers were presented separately to WO’s right and left eyes. So his brain had to fuse the right and left views together before he could see the entire figures. He reported having no trouble seeing the colored figures.

“This strongly suggests that his synesthetic associations take place at a central level of vision processing after information from the two eyes has been combined,” Palmeri notes. Next the researchers administered a series of tests to determine the extent to which WO’s synesthetic colors act like real colors. There are standard perceptual tests where color is a disadvantage and another set of tests where it is an advantage. The researchers adapted these tests for WO’s synesthesia.

The Stroop test is a classic example of how color can interfere with a simple identification task. When subjects are asked to name the color of ink in which color words are printed, they can do so faster when the ink color and color name are same (“red” printed in red ink) than they can when the two are different (“green” printed in red ink). The researchers adapted this test by printing words in colors that either agreed or conflicted with their synesthetic color and then recording how long it took WO to identify them. For example, WO sees the word “moose” as pink, so they printed it in pink letters and he sees the word “charge” in green, so they printed it in blue. They found that it took him longer to name words where the synesthetic color conflicted with the ink color than it did when they were the same. A standard test where color confers a real advantage is a search test where the target is a different color than the distracters surrounding it. Subjects report that the colored target “pops out” of the page. As a result, the time it takes them to pick out the target is about same regardless of the number of distracters in the scene. WO sees “2” as orange and “5” as green but he sees both “6” and “8” as nearly the same shade of dark blue. Using a special font so that the “2” and “5” are mirror images, the researchers made up two sets of computer displays. One set had single white “2s” hidden among varying numbers of white “5s.” The other set had single white “8” hidden among varying numbers of white “6s.”

Non-synesthetes who took these tests found the 2-among-5 search slightly harder than the 8-among-6 search. But WO found the 2-among-5 search much easier and was able to find the target significantly faster than non-synesthetes because the figures appeared as different colors, while his performance on the 8-among-6 search, which look the same to him, was comparable to that of the non-synesthetes.

“Although there are some subtle differences, these tests showed that synesthetic colors act in a way that is very like real colors,” Palmeri summarizes.

These findings support a model of synesthesia recently advanced by Vilayanur Ramachandran and Edward Hubbard from the University of California, San Diego that is based on an idea that has been around for decades: that synesthesia is caused by a subtle cross-wiring in the brain. Specific regions in the brain process information about different aspects of the visual scene, such as color, shape and motion. Recent brain mapping studies have found that a primary color area is adjacent to an area that handles numbers and letters. Another color area lies next to a primary auditory area. If the neurons in these regions were more densely wired or strongly connected than normal, it could explain why some people see words, numbers and sounds in color. For more news about Vanderbilt research, visit Exploration, Vanderbilt’s online research magazine at http://exploration.vanderbilt.edu.