What is synesthesia?

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A phenomenon wherein one sensory stimulus—a word or a musical note, for example—automatically induces a second, unstimulated sensory perception, typically a color.
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Causes and Symptoms

The term “synesthesia” derives from the Greek syn- (union/together) and aisthesis (sensation/perception). Regarded as an intriguing but perhaps bogus curiosity, synesthesia was once disparaged as a subjective experience unworthy of scientific interest. Since the 1980s, however, neuroimaging techniques have established the synesthetic experience as a genuine sensorineural phenomenon and a legitimate subject for scientific research. Described as rare by some and as common by others, synesthesia gives rise to widely divergent estimates of its prevalence: from one twenty thousand people to one in twenty-three.

Various combinations of synesthetic experiences have been documented, but color is the most common concurrent perception. Of the many different types, none has been more intensively studied than grapheme-color synesthesia; specific characters written in black print induce the experience of a specific color. The induced color experiences, termed “photisms,” are vivid and consistent; a specific grapheme always induces the same color in the same synesthete. A spoken word or musical note can act as the stimulus, and days of the week or months of the year also trigger colors—for some, Friday will always be chartreuse. Rarer synesthetic experiences involve touch or taste. Mirror-touch synesthesia activates a tactile sensation in the synesthete’s body when someone within sight is touched. In tactile-emotion synesthesia, textures induce distinct emotions. Some synesthetes experience a specific taste on hearing a word or piece of music.

Synesthesia has a strong genetic component, although the mode of inheritance is unclear. Familial aggregation in synesthesia was first noted by Sir Francis Galton in 1880. One study found a greater than 40 percent prevalence of synesthesia among first-degree relatives of synesthetes. A report of monozygotic (identical) male twins discordant for grapheme-color synesthesia casts doubt on what was initially believed to be X-linked dominant inheritance; documented male-to-male transmission also argues against it. Genetic linkage studies suggest a complex pattern of inheritance, perhaps with multiple gene loci. Different types commonly coexist in the same family, and heterogeneity is characteristic. Within one type, the same word or letter evokes different colors in different individuals, and the subjective experience differs as well.

Diagnosis and Detection

Recognition begins with a self-report. Various psychological tests can distinguish between synesthetes and nonsynesthetes; neuroimaging is generally limited to research. Some diagnostic criteria have been generally accepted: synesthetic sensory experience is involuntary, automatic, and durable—synesthetes typically report their sensory associations as consistent over years.

Considerable evidence supports the neural basis of synesthesia. Neurologic injury can evoke synesthetic associations, and drugs such as mescaline can induce them. Brain imaging, however, identifies synesthesia as a neural phenomenon. The pattern of cerebral blood flow during a synesthetic experience, first recorded in the 1980s, was described as abnormal. Subsequent studies have used positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to show differences in brain structure and activity between synesthetes and nonsynesthetes.

Grapheme-color synesthesia has been the main focus of fMRI studies. According to posited models, increased connectivity between contiguous brain areas in the cerebral cortex facilitates cross-activation. The fusiform gyrus, involved both in color and grapheme processing, has shown increased activation in addition to increased cortical thickness, volume, and surface area. Increased white matter volume and connectivity has been observed in brains of some synesthetes compared with those of nonsynesthetic counterparts.

Can developmental mechanisms explain synesthesia? Some investigators suggest that incoming sensory information in infants is normally jumbled together, and pruning of comingled neural and synaptic connections comes with development. According to this hypothesis, sensorineural connections in synesthetic adults somehow escaped the pruning process, perhaps via mutation.

Perspective and Prospects

Synesthesia has a long and distinguished history. Around 1710, an English ophthalmologist reported a blind patient who experienced color visions that were induced by sound. Jonathan Swift’s title character in Gulliver’s Travels (1726) encountered a group of blind apprentices taught by their blind master to mix colors by touch and smell. Swift’s source was believed to be the Royal Society scientists of his day, notably Robert Boyle. Francis Galton established synesthesia as a scientific entity in 1880.

The idea that the various senses are fused could be found in literature much before it resonated in neurology. Mary Shelley described the creature in Frankenstein (1818) as having a difficult time separating his various sensations because he simultaneously saw, felt, heard, and smelled. Later in the nineteenth century, psychologist William James expressed his view that incoming information from different senses is fused in a child before it is later untangled.

The renaissance of scientific research into synesthesia is prompted by the recognition that synesthesia can be a window into the nature of perception. Among the remaining questions is its true prevalence, which is of more than theoretical interest. Some evidence suggests that synesthetic cross-modal mechanisms are universal, even if below the level of consciousness in most adults. Human sensory experiences and their interconnections will undoubtedly continue to intrigue scientists and artists alike well into the future.

Bibliography:

American Synesthesia Association. http://synesthesia.info.

Beeli, Gian, et al. “Synaesthesia: When Coloured Sounds Taste Sweet.” Nature 434 (March 3, 2005): 38.

Mass, Wendy. A Mango-Shaped Space. Boston: Little, Brown, 2003.

Novich, Scott, Sherry Cheng, and David M. Eagleman. "Is Synaesthesia One Condition or Many? A Large-Scale Analysis Reveals Subgroups." Journal of Neuropsychology 5, no. 2 (September 2011): 353–71.

Ramachandran, Vilayanur, and Edward Hubbard. “Hearing Colors, Tasting Shapes.” Scientific American 288, no. 5 (May, 2003): 42–49.

Sollberger, Michael. "Rethinking Synesthesia." Philosophical Psychology 26, no. 2 (April 2013): 171–87.

Van Campen, Cretien. The Hidden Sense: Synesthesia in Art and Science. Cambridge, Mass.: MIT Press, 2007.

Witthoft, Nathan, and Jonathan Winawer. "Learning, Memory, and Synesthesia." Psychological Science 24, no. 3 (March 2013): 258–65.

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