Synaesthesia is a condition in which stimuli of one type evoke sensations in another sensory modality. For example, hearing particular sounds might evoke strong sensations of colour or (more rarely) words might evoke strong tastes in the mouth.
In The Hidden Sense, social scientist Cretien van Crampen investigates synaesthesia from an artisitic and scientific perspective. He interviews a number of synaesthetes, and finds that none of them considers their condition to be an impairment. He also describes the profound influence that synaesthesia has had on artists such as Kandinsky and van Gogh and writers and poets such as Edgar Allan Poe and Baudelaire.
First described in the late 1880s, synaesthesia has often been dismissed as a purely subjective phenomenon. But the condition has garnered a great deal of interest in recent years, and there is now some evidence for the neural basis of synaesthesia. As van Crampen explains in the introduction to his book:
When synaesthetes insisted that letters have colors, researchers attributed it to their strong imagination...In other cases, in was felt to be a learned association...Another frequently heard explanation for synaesthesia is that the colors of letters are not perceptions but are rather a type of associative metaphor. The word "sea" would thus be associated with a blue color because the word evokes an image of the sea for the inner eye. However, the synaesthete may tell you that the word "sea" has red, yellow, and purple colors...
Brain scans of synaesthetes [now]...provide proof of the neurological existence of synaesthesia...In one test, a synaesthetic person was blindfolded and placed in a recording tnnel of the brain-scanning apparatus and wore headphones that produced spoken words at regular intervals...activity in the areas of the brai nresponsible for hearing and color vision occur simultaneously when a blindfolded synaesthete hears a word. Under the same conditions, the brains of non-synaesthetes generated activity only in the areas known to be responsible for hearing.
French composer Oliver Messiaen had it. According to Simon Rattle, conductor Andre Previn once asked the composer about a rehearsal performance of Tarangulila. Messiaen confusingly replied, "Just play it a little more orangey-green." :)
There are times I wonder if I have a mild case of it or if its just an active imagination, trained from color-music associations from having seen Fantasia at age 4.
Charades, a 'Freudian slip'?
I'm not sure about Kandinski, but really, the evidence that van Gogh, Poe and especially Baudelaire were synesthetes or even "influenced by synesthesia" is zero and this myth should be put to rest. Baudelaire's poem "correspondances" is about a deeper theory of art he wrote about in other places. It has nothing to do with synesthesia per se. If he was a synesthete, we would know for sure, as we have plenty of (auto)biographical material on him. For van Gogh and Poe, i can't even say where the idea that they were synesthetes might come from. I didn't read van Campen's book though, so I would be curious to know what exactly he has to say about these artists.