Every day, we experience the world around us without a second thought, and we might broadly assume that those around us perceive their environments in much the same way. Although the mechanisms of perception are usually universally relatable, there is an intriguing phenomenon that prompts us to think twice about the act of perception itself: synesthesia. 

Synesthesia refers to a phenomenon where, when one sense is stimulated, an unrelated sense is experienced involuntarily; the word itself has Greek roots, meaning something along the lines of “combined senses.” For example, in a well-known form known as grapheme-color synesthesia, letters or numbers, or both, are perceived as inherently colored–“shaded” or “tinted.” Another common form of synesthesia is chromesthesia, where some individuals may “see” a color upon hearing sounds or specific musical notes or keys. Many more types of synesthesia are hypothesized to exist, such as “cross-links” between sound and tactile sensation (auditory-tactile) or between words and tastes (lexical-gustatory). Partly because synesthesia is a highly subjective experience, estimates of its prevalence based on self-referral vary widely. One study that claims to avoid this bias shows a prevalence of 4.4%.

Although this condition may sound a little concerning, synesthesia doesn’t appear harmful; research suggests that it may even be helpful in certain contexts. Some individuals with grapheme-color synesthesia have extraordinary memory compared to non-synesthetes, and a review found that grapheme-color synesthetes may have an especially enhanced ability at visual memory. It has also been hypothesized that some people with chromesthesia may have perfect pitch due to their ability to perceive sound through two senses simultaneously; in fact, many famous composers, such as Franz Liszt and Nikolai Rimsky-Korsakov seem to have experienced chromesthesia. 

What might have caused these fascinating phenomena? “Nature,” or “nurture”? Or perhaps a little bit of both? One theory suggests that “semantic representation,” or the meaning of the synesthesia-inducing stimulus, plays a role in associating concepts with perception; these associations may form through “concurrent” stimuli in childhood and be altered throughout the synesthete’s lifetime. One particular case supports this strongly–the associations between colors and letters formed by one grapheme-color synesthete were traced back to the colored alphabet fridge magnets she had during childhood. 
 
There is also tentative evidence that a genetic factor could influence synesthesia as well–some studies seem to show a higher prevalence of synesthesia among first-degree relatives of synesthetes, and one study, originally intending to search for genes related to the perception of pain, seems to have stumbled upon a mammalian gene–α2δ3–related to synesthesia; mutant mice which had this gene removed displayed additional activity in visual, auditory and olfactory regions of the brain when exposed to painful heat, leading to the speculation that the loss of the α2δ3 gene may lead to pain being “seen, heard, and smelled.” 

All in all, synesthesia is just another phenomenon that demonstrates the beauty of diversity—how every one of us perceives the exact same world but experiences it in sometimes vastly different ways.
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P.S. The author is a grapheme-color synesthete!