Synesthesia is the mental experience of perceiving one type of sense data in terms of another. For example, people might see specific colors when they hear specific sounds (sound-to-color synesthesia); associate colors with particular words, letters, or numbers (grapheme-color synesthesia); or experience specific tastes when they hear certain words or sounds (lexical-gustatory synesthesia). As many as 35 different types of synesthesia have been identified, and individual people might experience just one of them or more than one.
The rate of synesthesia in the general population (i.e., people who aren't specifically trying to create the experience with drugs or other methods, but just have it happen to them naturally) is about 1 in 2,000 people, so it's rare but not highly unusual. Famous people who have reported spontaneous synesthesia include the novelist Vladimir Nabokov, the composer Olivier Messiaen, and the physicist Richard Feynman. Nabokov described his synesthesia of color and language in this way:
The long a of the English alphabet ... has for me the tint of weathered wood, but a French a evokes polished ebony. ... I see q as browner than k, while s is not the light blue of c, but a curious mixture of azure and mother-of-pearl.
The French symbolist poets used synesthesia as a metaphor for truth, attempting to identify "correspondences" between vision, words, and other forms of expression. Arthur Rimbaud might have had a true experience of synesthesia based on his poem "Vowels" which equates vowel sounds with different colors, although his friend and fellow poet Paul Verlaine didn't believe it. Their compatriot Baudelaire used synesthesia-like imagery to make a famous point about poets' vocation, but he seems not to have had the actual sensory experience.
The most statistically common manifestation of synesthesia is sound-color synesthesia, where people see colors when hearing certain sounds. Most people say that they perceive these colors "in the mind's eye," rather than as something in their physical environment. But a minority of people say that they do see the colors hovering in the air, usually within arm's reach.
In a very small set of cases, people with synesthesia report that the experience is actually distracting or overwhelming. But for most people the experience of synesthesia is viewed as positive or enriching, and people who have this "extra sense" usually say that they would not want to give it up. Here are some examples:
Synesthetes beat nonsynesthetes in some perceptual tasks. Michael Banissy found that synesthetes who see synesthetic colors can discriminate between colors better than nonsynesthetes; synesthetes who feel synesthetic touches can discriminate between touches better than nonsynesthetes. Julia Simner and her colleagues studied synesthesia with sequence-space synesthesia -- in which sequences such as numbers, letters, days of the week, and months of the year are seen as specific visual forms at specific locations in space -- and found that they are better than nonsynesthetes at mentally rotating a 3D object to see if it matches another object. ...
For all we know, [synesthete] Michael Watson's idiosyncratic [mental] interface was richer and more adaptive than our own. We do know that it was an aid to Watson in cooking. As Richard Cytowic observed: "He never followed a recipe, but liked to create a dish with an 'interesting shape.' Sugar made things taste 'rounder,' while citrus added 'points' to the food." Watson's interface was no less dynamic than ours: "The shape changes with each moment, just as flavor does. ... French cooking is my favorite precisely because it makes the shapes change in fabulous ways" (Donald Hoffman, "The Case Against Reality," p. 140).
An important feature of true synesthesia is that for a given person, the associations between one sense and another are always the same over time. So, for instance, a piece of music doesn't just evoke a psychedelic swirl of colors. It instead evokes a specific color, or a set of colors in a specific sequence. If you play the same piece of music again years later for the same person, it will evoke the same sequence of colors. The person doesn't have to remember what they said previously, because they experience the colors as a fixed characteristic of the music itself. In other words, they are not reacting to the music with colors on an emotional level; they are instead perceiving the colors as if those colors were integral to the music. True synesthesia has its own typical psychophysics, with low-pitched sounds usually producing an experience of large and rounded shapes, but high-pitched sounds producing smaller and more angular shapes. Loudness and brightness are also often connected, with soft sounds evoking dim colors and loud sounds producing bright ones. The brightness of a color perception in response to music depends on its loudness and pitch together, and modulates in response to both characteristics. People without synesthesia approximate this experience when they describe music in a major key as "bright" and music in a minor key as "dark," although they generally are using the words metaphorically rather than to indicate a true visual perception. If you have had some unusual perceptions and want to know whether they qualify as synesthesia, you can take an online test here to find out.
There is probably a genetic component to synesthesia, as the experience tends to run in families. However, different members of the same family might experience different types of synesthesia. Furthermore, the specific associations are often different -- e.g., a parent might see the sound of the letter A as red, while her child sees it as blue. Specific genetic markers involved in synesthesia have been tentatively identified on chromosomes 2q, 16, 5q, 6p, and 12p. In one study, different chromosomal patterns were associated with 5 types of synesthesia: colored music, colored sequences (alphabet, letters, days of the week, etc.), colors triggered by touch or emotions, sequences displayed as visual stimuli, and colors triggered by nonvisual stimuli (e.g., tastes). This finding opens the possibility that one could eventually create a genetic test for synesthesia, and even be able to predict what type of synesthetic perceptions a particular person is likely to have.
Some of the linkages in synesthesia might be due to particular types of perception being located next to one another in the brain. For instance, color perception and written letters cause neurons to fire that are next to one another in the fusiform gyrus of the cortex, so one hypothesis is that there might be some cross-over of activity from one set of neurons to the other. If the relevant neurons are in the cortex, then synesthesia would be a sort of Narrative-system short-circuit where people are expressing themselves using other senses rather than primarily perceiving in a different sensory mode. That is, of course, not how most people with synesthesia see it.
Another theory suggests that synesthesia actually involves the suppression of higher-level (Narrative) processing while emotional or sensory processing (Intuitive) is enhanced.
Or perhaps cross-sensory perceptions are actually relatively common, but people without synesthesia simply have more active inhibitory mechanisms in the prefrontal cortex that prohibit those perceptions from coming to conscious awareness. This idea is reminiscent of William Blake's saying that "if the doors of perception were cleansed, everything would appear as it is, infinite" -- in other words, that our views of reality are limited in ways that reality itself is not. This third theory is supported in part by the finding that children are more likely to report synesthesia-like experiences, for example making associations between musical pitch and brightness even before they can articulate the concept of "pitch." And even infants seem to associate space (up or down) with tone (ascending or descending notes) in studies that examined the selective gaze of children as young as three months.
Taken as a whole, scientific studies suggest that synesthesia is not merely a Baudelaire-like matter of using one concept as a metaphor for another, but an experiential reality triggered by something within the Intuitive mind. The strength and persistency of specific associations also supports this interpretation -- people with true synesthesia are not likely to change or forget their perceptions over time. Of course, people do have to express themselves in language, and some of the best-known examples of synesthesia have been among people like Nabokov who could eloquently express their experiences. But many more people may be capable of this broadened perceptual experience, and it likely occurs at the Intuitive level among many people who don't have Nabokov's gift for expressing it through their Narrative mind.
Fascinating
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