Synesthesia can occur between nearly any two senses or perceptual modes. Given the large number of forms of synesthesia, researchers have adopted a convention of indicating the type of synesthesia by using the following notation x → y, where x is the "inducer" or trigger experience, and y is the "concurrent" or additional experience. For example, perceiving letters and numbers (collectively called graphemes) as colored would be indicated as grapheme → color synesthesia. Similarly, when synesthetes see colors and movement as a result of hearing musical tones, it would be indicated as tone → (color, movement) synesthesia.
While nearly every logically possible combination of experiences can occur, several types are more common than others.Grapheme → color synesthesia
How someone with synesthesia might perceive certain letters and numbers.
Another example of real synaesthesia for letters and numbers.
In one of the most common forms of synesthesia, grapheme → color synesthesia, individual letters of the alphabet and numbers (collectively referred to as graphemes), are "shaded" or "tinged" with a color. While synesthetes do not, in general, report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., A is likely to be red).Sound → color synesthesia
In sound → color synesthesia, individuals experience colors in response to tones or other aspects of sounds. Simon Baron-Cohen and his colleagues break this type of synesthesia into two categories, which they call "narrow band" and "broad band" sound → color synesthesia. In narrow band sound → color synesthesia (often called music → color synesthesia), musical stimuli (e.g., timbre or key) will elicit specific color experiences, such that a particular note will always elicit red, or harps will always elicit the experience of seeing a golden color. In broadband sound → color synesthesia, on the other hand, a variety of environmental sounds, like an alarm clock or a door closing, may also elicit visual experiences.
Color changes in response to different aspects of sound stimuli may involve more than just the hue of the color. Any dimension of color experience (see HSL color space) can vary. Brightness (the amount of white in a color; as brightness is removed from red, for example, it fades into a brown and finally to black), saturation (the intensity of the color; fire engine red and medium blue are highly saturated, while grays, white, and black are all unsaturated), and hue may all be affected to varying degrees. Additionally, music → color synesthetes, unlike grapheme → color synesthetes, often report that the colors move, or stream into and out of their field of view.
Like grapheme → color synesthesia, there is rarely agreement among music → color synesthetes that a given tone will be a certain color. However, when larger samples are studied, consistent trends can be found, such that higher pitched notes are experienced as being more brightly colored. The presence of similar patterns of pitch-brightness matching in non-synesthetic subjects suggests that this form of synesthesia shares mechanisms with non-synesthetes.Number form synesthesia
A number form is a mental map of numbers, which automatically and involuntarily appears whenever someone who experiences number-forms thinks of numbers. Number forms were first documented and named by Francis Galton in "The Visions of Sane Persons". Later research has identified them as a type of synesthesia. In particular, it has been suggested that number-forms are a result of "cross-activation" between regions of the parietal lobe that are involved in numerical cognition and spatial cognition. In addition to its interest as a form of synesthesia, researchers in numerical cognition have begun to explore this form of synesthesia for the insights that it may provide into the neural mechanisms of numerical-spatial associations present unconsciously in everyone.Personification
Ordinal-linguistic personification (OLP, or personification for short) is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities. Although this form of synesthesia was documented as early as the 1890s modern research has, until recently, paid little attention to this form.
For some people in addition to numbers and other ordinal sequences, objects are sometimes imbued with a sense of personality, sometimes referred to as a type of animism. This type of synesthesia is harder to distinguish from non-synesthetic associations. However, recent research has begun to show that this form of synesthesia co-varies with other forms of synesthesia, and is consistent and automatic, as required to be counted as a form of synesthesia.Lexical → gustatory synesthesia
In a rare form of synesthesia, lexical → gustatory synesthesia, individual words and phonemes of spoken language evoke the sensations of taste in the mouth.
Jamie Ward and Julia Simner have extensively studied this form of synesthesia, and have found that the synesthetic associations are constrained by early food experiences. For example, James Wannerton has no synesthetic experiences of coffee or curry, even though he consumes them regularly as an adult. Conversely, he tastes certain breakfast cereals and candies that are no longer sold.
Additionally, these early food experiences are often paired with tastes based on the phonemes in the name of the word (e.g., /I/, /n/ and /s/ trigger James Wannerton’s taste of mince) although others have less obvious roots (e.g., /f/ triggers sherbet). To show that phonemes, rather than graphemes are the critical triggers of tastes, Ward and Simner showed that, for James Wannerton, the taste of egg is associated to the phoneme /k/, whether spelled with a "c" (e.g., accept), "k" (e.g., York), "ck" (e.g., chuck) or "x" (e.g., fax). Another source of tastes comes from semantic influences, so that food names tend to taste of the food they match, and the word "blue" tastes "inky."