to distinguish those synesthetes whose perceptions have a definite sense of spatial quality from those whose perceptions do not. Prevalence Estimates of prevalence of synesthesia have ranged widely, from 1 in 4 to 1 in 25,000–100,000. However, most studies have relied on synesthetes reporting themselves, introducing self-referral bias. In what is cited as the most accurate prevalence study so far, self-referral bias was avoided by studying 500 people recruited from the communities of Edinburgh and Glasgow Universities; it showed a prevalence of 4.4%, with 9 different variations of synesthesia. This study also concluded that one common form of synesthesia grapheme–color synesthesia (colored letters and numbers) is found in more than one percent of the population, and this latter prevalence of graphemes–color synesthesia has since been independently verified in a sample of nearly 3,000 people in the University of Edinburgh. The most common forms of synesthesia are those that trigger colors, and the most prevalent of all is day–color. Also relatively common is grapheme–color synesthesia. We can think of "prevalence" both in terms of how common is synesthesia (or different forms of synesthesia) within the population, or how common are different forms of synesthesia within synesthetes. So within synesthetes, forms of synesthesia that trigger color also appear to be the most common forms of synesthesia with a prevalence rate of 86% within synesthetes. In another study, music–color is also prevalent at 18–41%. Some of the rarest are reported to be auditory–tactile, mirror-touch, and lexical–gustatory. There is research to suggest that the likelihood of having synesthesia is greater in people with autism spectrum condition. History The interest in colored hearing dates back to Greek antiquity when philosophers asked if the color (chroia, what we now call timbre) of music was a quantifiable quality. Isaac Newton proposed that musical tones and color tones shared common frequencies, as did Goethe in his book Theory of Colours. There is a long history of building color organs such as the clavier à lumières on which to perform colored music in concert halls. The first medical description of "colored hearing" is in an 1812 thesis by the German physician Georg Tobias Ludwig Sachs. The "father of psychophysics," Gustav Fechner, reported the first empirical survey of colored letter photisms among 73 synesthetes in 1876, followed in the 1880s by Francis Galton. Carl Jung refers to "color hearing" in his Symbols of Transformation in 1912. In the early 1920s,