Synaesthesia is the general name for a related set of various cognitive states having in common that stimuli to one sense, such as smell, are involuntarily simultaneously perceived as if by one or more other senses, such as sight or / and hearing (see Cytowic 1989; Baron-Cohen & Harrison 1993). For example, the sounds of musical instruments might make you see certain colors, each color specific and consistent with the particular instrument playing; a piano, for example, might produces a sky-blue cloud, and a tenor saxophone produce an image of electric purple neon lights. One highly documented case of synaesthesia involved Michael O. Watson, who felt at or within his right hand different flavors — the flavor of spearmint, for example, felt like cool smooth glass columns (see Cytowic 1989, 1993).
Synaesthesia is additive; that is, it adds to the initial (primary) sensory perception, rather than replacing one perceptual mode for another. For example, with synaesthetically colored musical instruments, you both hear and “see” the sounds; the visual images do not replace the audial sensations. Both sensory perceptions may thus become affected and altered in the ways they function and integrate with other senses. Synaesthesia is generally “one-way”; that is, for example, for a given synaesthete, tastes may produce synaesthetic sounds, but sounds will not produce synaesthetic. However, there have been a few rare cases of «bi-directional» synaesthesia, in which, for example, music induces (synaesthetic) colors and seeing colors induces (synaesthetic) sounds — the correspondences, however, are not the same in both directions!
Synaesthesia may be divided into two general, somewhat overlapping types. The first, “synaesthesia proper”, is as described above, in which stimuli to a sensory input will also trigger sensations in one or more other sensory modes. With the second form of synaesthesia, certain sets of things which our individual cultures teach us to put together and categorize in some specific way — like letters, numbers, or people’s names — also get some kind of sensory addition, such as a smell, color or flavor. The most common forms of cognitive synaesthesia involve such things as colored written letter characters (graphemes), numbers, time units, and musical notes or keys. For example, the synaesthete might see, about a foot or two before her (the majority of synaesthetes – about 70% — are female), different colors for different spoken vowel and consonant sounds, or perceive numbers and letters, whether conceptualized or before her in print, as colored.
Let us now explore a little bit of the history of synesthesia in music: