"The number one, for example, is a brilliant and bright white, like someone shining a flashlight into my eyes. In his memoir Born on a Blue Day, writer Daniel Tammet describes how synesthesia affects his world. Synesthesia is thought to be more common in people with autism (Alex has been diagnosed with pervasive developmental disorder-not otherwise specified, a form of autism with milder symptoms), and in creative people, including artists, musicians, and poets. For instance, in the same 2013 study in Frontiers in Psychology, a research subject said she tasted potato chips when she heard the word "woman." Other people with synesthesia may perceive numbers or letters as certain colors regardless of what color they happen to be printed in (grapheme-color synesthesia), or they may "taste" certain words (lexical-gustatory synesthesia). Alex says he has always heard music in colors, also known as sound-to-color synesthesia, or chromesthesia, an ability invisible to those around him. About 4 percent of the population has synesthesia, according to a 2013 study in Frontiers in Psychology. In reality, synesthesia is a fairly rare neurologic phenomenon in which the brain processes data in several senses at once-one sense activates another unrelated sense, and they are experienced simultaneously.