Color synesthesia is a condition in which sensory or cognitive inducers elicit atypical binding of these inducers to concurrent color experiences.
Marks of Color Synesthesia
Synesthesia is a condition in which stimulation in one sensory or cognitive stream involuntarily, or automatically, leads to associated internal or external (illusory or hallucinatory) experiences in a second unstimulated sensory or cognitive system [1, 2, 3, 4, 5, 6, 7, 8, 9]. Although most cases of synesthesia are developmental and run in families, acquired cases have also been reported following traumatic brain injury, demyelination, ischemia, tumors, post-traumatic total ocular blindness, and neuropathology involving the optic nerve and/or chiasm [10, 11, 12].
Color synesthesia is a special kind of synesthesia that comprises cases of synesthesia in which a noncolored sensory or cognitive stimulus involuntarily leads to internal or external color experiences. The prevalence of color synesthesia is unknown. Estimates range from 1 in 200 to 1 in 250,000 [13, 14]. Some speculate that color synesthesia may be present in more than 4 % of the population .
One of the best-known forms of color synesthesia is grapheme-color synesthesia, in which numbers or letters are seen as colored. But lots of other forms of color synesthesia have been identified, including week-color synesthesia, sound-color synesthesia, taste-color synesthesia, fear-color synesthesia, etc.  For lack of space, this entry shall focus primarily on grapheme-color synesthesia.
Because of the automatic nature of synesthesia and its test-retest reliability, color synesthesia is not to be confused with memory associations or stereotypical colors of objects. For example, there is no evidence that color synesthetes simply remember the colors of entities or images they were exposed to earlier in their lives or associate stimuli with their stereotypical colors .
Synesthetic color experience is unique for each synesthete. For example, the letter A may trigger the color red in one grapheme-color synesthete but trigger the color blue in another. In fact, each grapheme has been found to trigger each of the 11 Berlin and Kay colors in different synesthetes (red, pink, orange, yellow, green, blue, purple, blown, black, white, gray). Despite the uniqueness of synesthetic color experience, synesthetic colors sometimes fall into certain clusters. For example, grapheme-color synesthetes tend to associate A with red, E with yellow or white, I with black or white, and O with white [17, 18].
Low-Level Versus High-Level Perception
An open question about color synesthesia is whether it is a form of low-level or high-level perception. According to Ramachandran and Hubbard , synesthesia is a form of low-level perception, a “sensory” phenomenon. As they put it:
Work in our laboratory has shown that synesthesia is a genuine sensory phenomenon. The subject is not just “imagining the color” nor is the effect simply a memory association (e.g., from having played with colored refrigerator magnets in childhood) [19, p. 51].
Visual search paradigms are supposed to be indicators of whether synesthetic experience requires focal attention. If synesthetic experience does not require focal attention, then digits with unique synesthetic colors should capture attention, which would lead to highly efficient identification of inducing digits. If, on the other hand, synesthetic experience requires focal attention, then synesthetic colors do not capture attention, and the identification process should be inefficient . Perceptual features must be processed early enough in the visual system for them to attract attention and lead to pop-out and segregation [23, 24]. So the appearance that synesthetic experience can lead to pop-out and segregation indicates that synesthesia is a low-level perceptual phenomenon [7, 19, 20].
While a significant number of grapheme-color synesthetes are more efficient in visual search paradigms than controls, this does not clearly show that attention is not required for synesthetic experience, however. In one subject PM, it was shown that quick identification of graphemes occurred only when the graphemes that elicit synesthetic experience were close to the initial focus of attention . Smilek et al.  used a variation on the standard visual search paradigm to test subject J’s search efficiency. J was shown an array of black graphemes on a colored background, some of which induced synesthetic experience. The colored background was either congruent or incongruent with the synesthetic color of the target. The researchers found that J was more efficient in her search when the background was incongruent than when it was congruent. This indicates that the synesthetic colors attracted attention only when they were clearly distinct from the background.
Edquist et al.  carried out a group study involving 14 grapheme-color synesthetes and 14 controls. Each subject performed a visual search task in which a target digit differed from the distractor digits in terms of its synesthetic color or its display color. Both synesthetes and controls identified the target digit efficiently when the target had a unique display color, but the two groups were equally inefficient when the target had a unique synesthetic color. The researchers concluded that for most grapheme-color synesthetes, graphemes elicit synesthetic color only once the subject attends to them. This indicates that synesthetic colors cannot themselves attract attention because they are not processed early enough in the visual system.
The fact that the very same grapheme can elicit different color experiences in synesthetes depending on the context in which it occurs suggests that synesthetes need to interpret what they visually experience before they experience synesthetic colors. Though Ramachandran and Hubbard  argue that grapheme-color synesthesia is a form of low-level perception (a “sensory phenomenon”), they grant that linguistic context can affect synesthetic experience. They presented the sentence “Finished files are the result of years of scientific study combined with the experienced number of years’ to a subject and asked her to count the number of “f’s” in it. Most normal subjects count only three “f’s” because they disregard the high-frequency word “of.” Though the synesthete eventually spotted six “f’s” she initially responded the way normal subjects do.
Ramachandran and Hubbard  suggest that these contextual effects can be explained by top-down factors. Whether this is right, however, will depend on whether color experience processed in early visual areas is indeed affected by top-down factors. If it is not, then top-down influences cannot explain the contextual effects. A better explanation of contextual influence then may be that interpretation of low-level perceptual information is required for synesthetic experience.
Another explanation of the disagreement about whether color synesthesia gives rise to pop-out effects may bear on the fact that few studies of pop-out effects have properly distinguished between projector synesthesia and associator synesthesia as well as what Ramachandran calls “higher synesthesia” and “lower synesthesia.” Lower grapheme-color synesthesia is synesthesia (either projector or associator) that arises in response to sensory stimuli, whereas higher grapheme-color synesthesia is synesthesia (either projector or associator) that arises in response to thoughts of graphemes. It is possible that the majority of synesthetes are higher synesthetes and that only lower synesthetes experience pop-out effects.
The precise neural mechanism underlying color synesthesia is unknown. One hypothesis, the so-called local cross-activation hypothesis, proposed by Hubbard and Ramachandran, holds that grapheme-color synesthesia arises due to cross-activation between color areas in the visual cortex and the adjacent visual word form area [7, 20, 28]. This suggestion is inspired by the observation that local crossover phenomena can explain other illusory and hallucinatory experiences, such as phantom limb sensations.
A second hypothesis is that color synesthesia may be due to disinhibited feedback from an area of the brain that binds information from different senses [3, 29, 30]. The main piece of evidence cited in favor of this hypothesis comes from an analogous case in which a patient PH reported seeing visual movement in response to tactile stimuli following acquired blindness . As PH was blind, he could not have received the information via standard visual pathways. It is plausible that the misperception was a result of disinhibited feedback from brain regions that receives information from other senses.
The fact that synesthetic experiences can arise when subjects are under the influence of psychedelics provides some further evidence for the disinhibited feedback hypothesis . The synesthetic effect of psychedelic substances may be due to an inhibition of feedback from areas of information binding. It is unknown, however, whether drug-induced synesthesia and congenital synesthesia have the same underlying mechanism.
A third hypothesis is that color synesthesia arises as a result of aberrant reentrant processing [21, 32]. The hypothesis is similar to the disinhibited feedback hypothesis but suggests specifically that high-level information reenters color areas in visual cortex and that it is this form of reentrant information processing that leads to the experience of synesthetic colors. This model would explain why visual context and meaning typically influence which synesthetic colors a grapheme gives rise to [32, 33].
It is plausible that different forms of color synesthesia proceed via different mechanisms. Cases of color synesthesia have been reported in which the visual cortex is not involved in generating synesthetic colors [12, 34]. None of the three aforementioned hypotheses, despite their plausibility in run-of-the mill cases, can explain more unusual cases of color synesthesia.
Cognitive Advantages of Color Synesthesia
If pop-out effects require attention to the synesthetic graphemes, grapheme-color synesthesia is unlikely to give subjects much of a cognitive advantage in visual search tests. However, there may nonetheless be cognitive advantages associated with color synesthesia. For example, some case studies suggest that grapheme-color synesthetes may have greater recall ability for digits and written names when compared to non-synesthetes [35, 36].
In rare cases color synesthesia has been associated with extreme mathematical skills. Subject DT, for example, sees numbers as three-dimensional colored, textured forms . His synesthesia gives him the ability to multiply high digits very rapidly. He reports that the product of multiplying two numbers is the number that corresponds to the shape that fits between the shapes corresponding to the multiplied numbers. Subject DT’s color synesthesia also gives rise to extreme mnemonic skills. DT currently holds the European record in reciting the decimal points of the number pi. An fMRI study comparing DT to controls while attempting to locate patterns in number sequences indicated that DT’s synesthetic color experiences occur as a result of information processing in nonvisual brain regions, including temporal, parietal, and frontal areas .
These two unusual case studies suggest that at least some forms of color synesthesia can give rise to cognitive advantages in the area of mathematics. As the visual cortex does not appear to be directly involved in generating the synesthetic images in either subject, the two cases also suggest that at least some forms of color synesthesia are best characterized as forms of high-level perception that proceeds via a nonstandard mechanism.
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