Synaesthesia is a relatively rare condition that gives people extraordinary perceptual experiences from everyday normal sensory input. For example, someone with synaesthesia might be able to taste or hear colours.
Scientific studies have identified many different types of synaesthesia such as spatial associations for numbers, days, and months, or even colours experiences for different swimming styles. Although not all of these experiences have been scientifically validated, there exist about 60 different reported forms of synaesthesia including grapheme-colour synaesthesia, where a letter printed in black triggers a highly specific and consistent colour experience.
What is common to all the different forms of synaesthesia is that the experiences are involuntarily and automatically triggered by something, a so-called inducing stimulus or what is simply called an “inducer”.
Although idiosyncratic, synaesthetic experiences are consistent over time within the same individual. So while “A” may elicit a red colour experience for one synaesthete and a blue colour for another, it will always elicit the same colour experience for a specific individual. For most synaesthetic individuals, synaesthetic experiences have perceptual qualities. For instance, grapheme-colour synaesthesia entails the subjective experience of seeing colours.