Synaesthesia (also spelled synesthesia) 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 autists. 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 not metaphorical or merely associations; rather, they are involuntary and are consistent throughout life, although some young synaesthetes seem to lose their ability by or during adulthood.

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.

In synaesthesia's most common form (Grapheme-color synaenesthesia), individual letters of the alphabet, as well as numbers, are "shaded" or "tinged" with a color. The alphabet color pattern is different for every indivi...

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A medical condition where one type of stimulation evokes the sensation of another