History of synesthesia research
Synesthesia is a neurological condition in which two or more bodily senses are coupled. For example, in a form of synaesthesia known as grapheme-color synaesthesia, letters or numbers may be perceived as inherently colored. Historically, the most commonly described fom of synesthesia (or synesthesia-like mappings) has been between sound and vision.
The earliest mentions of synesthesia are debated. Although it is well established that Pythagoras referred to the music of the spheres whether or not this early mention refers to synesthesia or rather to a metaphorical/mystical interpretation of mathematical harmonies is unclear.
John Locke in An Essay Concerning Human Understanding reports:
A studious blind man, who had mightily beat his head about visible objects, and made use of the explication of his books and friends, to understand those names of light and colours which often came in his way, bragged one day, That he now understood what scarlet signified. Upon which, his friend demanding what scarlet was? The blind man answered, It was like the sound of a trumpet.
— Locke, An Essay Concerning Human Understanding Chapter IV, section 11
Again, whether this is an actual instance of synesthesia, or simply reflects metaphorical speech is debated.
Numerous other philosophers and scientists, including Leibniz, Isaac Newton and Erasmus Darwin may have referred to synesthesia, or at least synesthesia-like mappings between colors and musical notes. Indeed, the father of psychophysics, Gustav Fechner reported a case of colored vowels in 1876.
These early investigations aroused little interest, and the phenomenon was first brought to the attention of the scientific community in the 1880s by Francis Galton (Galton 1880a; Galton 1880b; Galton 1883). Following these initial observations, research into synesthesia proceeded briskly, with researchers from England, Germany, France and the United States all investigating the phenomenon. These early research years corresponded with the founding of psychology as a scientific field (see history of psychology). By 1926, Mahling cites 533 published papers dealing with colored hearing (or hearing → color synesthesia) alone (Marks 1975).
Due to the difficulties in assessing and measuring subjective internal experiences, and the rise of behaviorism in psychology, which banished any mention of internal experiences, the study of synesthesia gradually waned during the 1930s. Marks (1975) lists 44 papers discussing colored hearing from 1900 to 1940, while in the following 35 years from 1940 to 1975, only 12 papers were published on this topic.
In the 1980s, as the cognitive revolution had begun to make discussion of internal states and even the study of consciousness respectable again, scientists began to once again examine this fascinating phenomenon. Led by Larry Marks (Marks 1975) and Richard Cytowic (Cytowic 2002 first ed. 1989 Cytowic 2003 first ed. 1993) in the United States, and by Simon Baron-Cohen and Jeffrey Gray (see Baron-Cohen & Harrison 1997 in England, research into synesthesia began by exploring the reality, consistency and frequency of synesthetic experiences. In the late 1990s, researchers began to turn their attention towards grapheme-color synesthesia, one of the most common (Day 2005; Rich, Bradshaw & Mattingley 2005) and easily studied forms of synesthesia. In 2006, the journal Cortex published a special issue on synesthesia, composed of 26 articles. Synesthesia has been the topic of several recent scientific books and novels and a recent short film has even included characters who experience synesthesia (for more information, see the main synesthesia page).
Mirroring these developments in the professional community, synesthetes and synesthesia researchers have come together to found several societies dedicated to research and education about synesthesia, its consequences and uses. In 1995, the American Synesthesia Assocation was founded, and has been having annual meetings since 2001. In England, the UK Synaesthesia Association arose out of a similar desire to bring together synesthetes and the people who study them, and has held two conferences (2005, 2006). Similarly, Sean A. Day has administered the "synesthesia list" a e-mail list for synesthetes and researchers around the world.