All of you are probably familiar with color opponency, but just in case, I'll give you a quick refresher. I'll even start with the history. In the 19th century, there were two competing theories of color vision. The first was the Young-Helmholtz theory (sometimes called the trichromatic theory), which argued that there were three types of photoreceptors: one for red, one for green, and one for blue. The second was Ewald Hering's color opponency theory, which argued that there were three color pairs: black-white, red-green, and blue-yellow. Each color in the pair canceled out, or inhibited the other (if you're interested in this sort of thing, you should definitely read up on Hering's theory -- tthe whole thing was a result of a brilliantly simple deduction). Until about the 1950s, when technology allowed scientists to look at the functioning of individual nerve cells, these two theories were thought to be fundamentally at odds with each other, but it turns out that they're both basically right. In essence, color vision works like this: there are three different types of cones (red, blue, and green) on the retina, consistent with the Young-Helmholtz theory, which send their signals to two types of bipolar or ganglion cells, which process red and yellow or blue and green, consistent with Hering's theory. The color opponency that occurs as a result of this configuration is responsible for certain types of color blindness (e.g., red-green color blindness), as well as for color after effects .
Over the last few years, researchers have become more and more fascinated with synaesthesia, and particularly with one of its rarer forms, grapheme-color synaesthesia. For grapheme-color synaesthetes , the graphical representation of letters and numbers produce color experiences, with different letters and numbers being associated with different colors. Evidence from both behavioral and neuroimaging studies is building that this sort of synaesthetic experience is both automatic (not under the control of the synaesthete), and that they occur in the same areas of the visual cortex in which ordinary color processing takes place. Which raises the question, do grapheme-color syneaesthetic experiences display color opponency like ordinary color vision?
The answer appears to be yes. In a study published in this month's issue of Psychological Science, Nikolic et al.1 gave six grapheme-color synaesthetes a version of the Stroop task. In the classic version of the Stroop task, people are presented with words in colored fonts, and asked to name the color of the font. Some of the words are color words, and in some of those cases, the named color differs from the color of the font. When this is the case, interference occurs, and people are significantly slower at naming the color of the font. In Nikolic et al.'s version of the Stroop task, the synesthetes were presented with letters or numbers that were associated with particular color experiences. There were three conditions: the "congruent condition," in which the color of the letter or number was the same as the associated synaesthetic color; the "incongruent opponent" condition, in which the color of the letter or number was the opponent (e.g., red for green) of the synaesthetic color; and the "incongruent independent" condition, in which the color of the letter or number was from a different opponency pair (e.g., if the synaesthetic color was green, then the color of the figure might be blue). In each condition, the participants were asked to name the color of the figure, and their response times were measured.
The figure above, from Nikolic et al.'s Figure 2 (p. 483), shows the results. The graph on the left represents the average response times for the 6 synaesthetes, and the graph on the right represents the average response times for 12 non-synaesthetic participants to the same stimuli. As you can see, the non-synaesthetes responded to each of the stimuli at about the same speed. The synaesthetes, however, showed significant differences in response times for each condition. In the graph, the fastest condition (labeled "C") is the "consistent condition," in which the figure's color and the synaesthetic color it produced were the same. Notice that reactions to this consistent were faster than the baseline condition (labeled "B") in which synaesthetic participants responded to symbols that didn't elicit synaesthetic color experiences, and faster than the response times of the non-synaesthetic participants as well. This suggest that the consistency between the presented and synaesthetic color actually produced a benefit. On the other hand, the two inconsistent conditions were significantly slower than all the others, and the "inconsistent opponent" condition (labeled "O"), in which the figure color and synaesthetic color were opponents, produced response times that were significantly slower than even the "inconsistent independent" condition (labeled "I").
These results suggest that color-opponency is operating in the perception of synaesthetic colors. Nikolik et. al. argue that this result indicates the involvement of "early visual areas in the generation of synaesthetic color experiences." Recall that color opponency occurs right after the trichromatic processing of color in the retina. So for synaesthetes to show color opponency in their synaesthetic color experiences, they have to be processing that color information in at least neighboring areas of the visual system. Which is pretty cool, when you think about it.
1Nikolic, D., Lichti, P., Singer, W. (2007). Color opponency in synaesthetic experiences. Psychological Science, 18(6), 481-486.
Thanks for pointing and explaining this very interesting research.
I can't pick out what is more weird, the incongruent condition results, or the congruent condition one. Why would synaesthetes recognize color faster? Has anybody compared times of recognizing colors vs. recognizing graphemes (in non-synaesthetes I mean)? If we recognize graphemes faster than colors, I guess that might (kind of) provide way to explain congruent condition results.
Is the conclusion of the research described in the post at odds with Ramachandran's explanations of synaesthesia?
Did anyone test for eventual color after-effects in case of synaesthetes?
Tanasije, if I remember correctly, the essence of Ramachnadran's theory of synaesthesia is that it's the result of cross-wirings. I suppose such cross-wirings could take place in the early visual system (though if it's too early, it might be tough).
There's actually a some evidence that synaesthetes experience afterimages and other visual illusions associated with opponency and the center-surround structure of cells in the early visual system. Randolph Blake and his colleagues have found that synaesthetes experience the watercolor effect with synaesthetic colors (the paper's here), and the McCollough effect (paper's here).