A synesthete is a person who experiences vivid colors whenever they see, hear or even just think about ordinary letters and digits.

Although synesthetes will consistently see the same colors associated with the same letters or digits, few of the experiences have appeared to be shared with other synesthetes.

Now in a study of 70 synesthetes, and a re-analysis of 19 more in previously published data, psychologists Julia Simner of the University of Edinburgh and Jamie Ward of the University of Sussex found synesthetes do, in fact, share certain grapheme-color combinations. For example, the letter 'A' is frequently associated with seeing the color red.

They also found the particular pairings are determined by how frequently graphemes and the color terms are used in language: common letters -- such as "A" -- pair with common color terms like red. Uncommon letters -- such as "V" -- pair with uncommon color terms like purple. That, the scientists say, shows perceptual synaesthetic experiences are influenced by environmental learning.

The research appears in the journal Psychological Science.