A blending of sensory impressions
"The root of the word 'synaesthesia' focuses on the blending of sensory impression," explains Gregor Volberg from the University of Regensburg. In the psychology of perception, Volberg's specialist area, this term is used to denote the event "when a stimulus triggers more than the regular, expected response, i.e. an expanded, irregular reaction." It concerns responses that essentially should have nothing at all to do with the original information, "for example, when an acoustic stimulus calls forth a visual perception" - a kind of bonus perceptive ability.
Others see letters in certain colours. Experts call this type of visualisation grapheme-colour synaesthesia, the most common form of the condition. But precisely how many different forms there are is as yet unknown. Volberg estimates that there are dozens of them, including more exotic ones such as lexical-gustatory or gustatory-auditory synaesthesia, in which certain words or sounds evoke tastes. Many of those affected experience more than just one extraordinary perceptual faculty. Like Leonore Egbert. She also has grapheme synaesthesia and sees sounds in colours. In addition to this, specific motions, for example a car passing by or the branches of trees swaying in the wind, evoke discrete acoustic impressions. And then there is, of course, her perception of touch.