Synesthesia occurs when the stimulation of one sensory or cognitive pathway produces an automatic experience in another. People whose minds function in this way are known as synesthetes. Evidence indicates that such individuals may need to process the meaning of an initial stimulus (the inducer) before experiencing the involuntary response (the concurrent). To reflect its conceptual elements, the phenomenon is sometimes called ideasthesia.
One of the most common types of synesthesia is also the easiest to study. People with grapheme color synesthesia perceive numbers and/or letters as being tinged or shaded with particular colors. The resulting connections between symbols and hues are involuntary, consistent, and memorable.
While every synesthete’s overall perception is unique, researchers have observed that some associations occur more frequently than others. For example, the letter A is most often associated with red. As a general rule, letters that appear regularly in a synesthete’s primary language are linked to relatively common colors. Conversely, an English speaker might associate Z or Q with an obscure color, such as ultramarine or puce.
While early exposure to such objects as colored, letter-shaped fridge magnets may lead to synesthesia in some people, this sort of experience is not typical. Described by filmmaker Stephanie Morgenstern as “sense-fusion,” synesthesia can seem very matter of fact to those who experience it. “Numbers seemed naturally to have colors,” Morgenstern has said of her synesthesia.
Synesthetes may not literally see a certain letter as its related color. They may instead more subtly perceive its association with that hue, as though it possessed an aura. This distinction means that synesthetes can observe the actual color of a printed symbol while simultaneously sensing its synesthetic hue.
Few synesthetes seem to view their condition as a disorder or disability. In fact, the phenomenon helps some remember information such as telephone numbers or long names. Like color blindness, synesthesia is best understood as an unusual way of perceiving the world. Only rarely have people indicated that synesthesia causes them to experience unpleasant sensory overload.
While grapheme color synesthesia appears fairly often in the general population (in an estimated one of every 90 people), it is hardly the only type. More than 60 varieties of the phenomenon have been reported, though very few of those have been studied scientifically. Individuals with swimming-style synesthesia associate each style with a specific color. They need not swim to perceive synesthetic hues; the mere mention of, say, the backstroke can produce a sense of color in the mind’s eye.
Another type, sound color synesthesia, is often referred to as chromesthesia. People who experience it may associate colors with music, general atmospheric sounds, or some combination of the two. Many chromesthetes connect louder tones with brighter colors and higher tones with lighter colors. The famed composers Liszt and Rimsky-Korsakov, while not confirmed chromesthetes, supposedly argued about the colors of certain musical keys.
Many cases of synesthesia may be genetically inherited. However, the condition can also be brought on by drug use, stroke, a seizure, or even blindness. Recent studies of synesthesia have focused on what the phenomenon can reveal about humans’ cognitive and perceptual processes. A small percentage of synesthetes perceive concurrent colors as being projected in space, whereas most sense them internally. Synesthesia’s intensity varies greatly among individuals. Some synesthetes realize early on how different their sensory perception is from the norm. Others remain unaware of their atypical nature for a significant portion of their lives.
Synesthesia’s history as a recognized occurrence goes back to ancient Greece. Though scholars of the day did not understand “colored hearing” as profoundly as we currently do, they were curious about it. Many centuries later, Isaac Newton wondered whether synesthetically linked musical tones and colors shared the same frequencies. (They do not.) The first medical description of synesthesia was published in Germany, in the year 1812, as part of a thesis.
The initial scientific survey of synesthesia came decades later, in 1871. By the 1930s, the rise of behaviorism led to a general disregard for humans’ subjective experience in mainstream psychology. This development virtually eliminated synesthesia as a scholarly focus until the 1980s. At that time, cognitive approaches gained greater prominence and subjective experience returned to scholars’ attention. By the late 1990s, grapheme color had become the primary topic of synesthesia research.
Two main theories have arisen to explain how synesthesia works in the brain. One possibility is that cross-activation between the brain’s grapheme- and color-recognition areas causes synesthetic perception. Indeed, fMRI scans have revealed that grapheme color synesthetes’ color-recognition area can be activated by words. Such cross-activation might occur because synapses that developed early in a synesthete’s life have not been “pruned” as thoroughly as those of a non-synesthete. Alternatively, it is possible that synesthesia occurs when feedback associated with sensory perception is not inhibited normally. If this is true, then late stages of sensory processing may affect earlier ones, causing the “sense-fusion” synesthetes experience.
The Internet age has facilitated communication among synesthetes worldwide, leading to both unofficial communities and organizations such as the American Synesthesia Association. Well-known synesthetes include scientists Richard Feynman and Nikola Tesla, author Vladimir Nabokov, artist Wassily Kandinsky, and musicians Olivier Messiaen and Billy Joel. Even today, synesthesia is hardly a household term, but the phenomenon continues to fascinate both scholars and laypeople who investigate it.