Is Synesthesia A Brain Disorder?

In a provocative review paper just published, French neuroscientists Jean-Michel Hupé and Michel Dojat question the assumption that synesthesia is a neurological disorder. In synesthesia, certain sensory stimuli involuntarily trigger other sensations. For example, in one common form of synesthesia, known as 'grapheme-color', certain letters are perceived as allied with, certain colors. In other cases, musical notes are associated with colors, or smells. The cause of synesthesia is obsc