Synaesthesia (also spelled synesthesia); from the Greek (syn-) “union”, and (aesthesis) “sensation”; is the neurological mixing of the senses. A synaesthete may, for example, hear colors, see sounds, and taste tactile sensations. Although this may happen in a person who has autism, it is by no means exclusive to autistics. Synaesthesia is a common effect of some hallucinogenic drugs such as LSD or mescaline.
Synaesthetes often experience correspondences between the shades of color, tones of sounds, and intensities of tastes that provoke alternate sensations. For instance, a synaesthete may see a more intense red as the pitch of a sound gets higher, or a smoother surface might make one taste a sweeter taste. These experiences are involuntary, are not metaphorical, are not merely associations, and are consistent throughout life, although some young synaesthetes seem to lose their ability by or during adulthood. Depressant drugs tend to increase the depth of the perception.
Synaesthesia can even occur when one of the senses no longer functions properly, e.g., a person who can see colours when words are spoken can still see the colours if he becomes blind in later life.
Two of the most common forms of synaesthesia are seeing sound or seeing letters and numbers in color.
Richard Cytowic wrote a pop-psych book about this condition entitled The Man Who Tasted Shapes.
Some researchers and theorists have suggested that synaesthesia may have played a part in early humans' development of writing and written literacies.
Alternate spellings exist (synaesthesis), and many of those who experience the phenomenon identify as "synaesthetes".