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Senses Working Overtime: Exploring Synesthesia

Skye Harmony's picture

Perhaps 1 in 25,000 people (1) is affected by synesthesia, a condition that causes people to perceive a blend of sensory experiences triggered by only one sensory input. Theoretically, any pairing of two or more senses is possible, but sight and sound are the most commonly experienced senses for both triggers and synesthetic responses, and taste and smell are rare. Most of the pairings are unidirectional. Colored hearing, in which people see colors when they hear certain sounds, is one of the most common forms of synesthesia. (1) Examples of the many other kinds of synesthesia include seeing colors when reading written text, tasting distinct flavors when hearing sounds, and seeing a detailed 3-D image of numbers and dates in space. A case of audiomotor synesthesia has also been reported: hearing words prompted a boy to assume particular body postures (1).

Synesthetes were long considered crazy and their condition was not recognized as scientifically valid or diagnosable; however, in the past few decades researchers have brought legitimacy to synesthesia. They have found that all synesthetic responses are automatic and voluntary, so people are powerless to control them. The specific responses are arbitrary, that is, not meaningfully related to the trigger, and they remain constant over time, usually throughout the person’s entire life. The consistency between test-retest scenarios, even when decades have elapsed in between, shows that synesthetes certainly aren’t making anything up.

According to researcher Colin Blakemore, synesthesia occurs in blind people, but only those who have had vision at some point in their lives. When non-synesthetes go blind, they can fairly quickly gain the ability to see colors when stimulated by touch or when thinking about numbers. Some experience color when feeling Braille: each Braille character has a particular color, and they see the shape of the character in front of them in that color “like little LED dots” (2). This suggests that not only can the visual cortex expand its duties once it is no longer responsible for sight, but synesthesia can result from rapid rewiring within the brain, even in adulthood.

Synesthesia seems to have a genetic basis; the tendency to have synesthesia, but not the particular type, runs in families. It occurs more often in females, with a female to male ratio of up to 8:1 (depending on the study). Most synesthetes are otherwise “normal,” though they often have enhanced memory; they are good at recalling speech, writing, and the spatial location of objects. They might have slightly less than average math skills and might suffer from right-left confusion. They also seem to have many “unusual experiences” (déjà vu, precognitive dreams, etc), though there aren’t figures available to compare this to the incidence in the general population. (1)

PET scans and other imaging techniques have proven helpful in figuring out what’s going on in the brains of synesthetes. ABC News consultant Randolph Blake reports that synesthetes’ brains show activity in multiple sensory areas when exposed to a single sensory trigger. For example, when black letters are flashed to grapheme-color synesthetes, the color areas of their brains are active (3). However, according to neurologist Richard Cytowic, functional pictures of the brain show a decrease in cortical metabolism during synesthesia; this is almost unheard of, as any sort of brain activity is normally accompanied by increased metabolism of the neocortex (1). Cytowic has determined that “synesthesia depends only on the left-brain hemisphere and is accompanied by large metabolic shifts away from the neocortex that result in relatively enhanced limbic expression” (1). Specifically, Cytowic has identified the hippocampus as “an important and probably obligate node in whatever neural structures generate the synesthetic experience” (1). This is further backed up by the fact that during seizure discharges of the hippocampus, non-synesthetic people may have synesthetic responses such as experiencing flashing lights, whining noise, heat, and taste (1).

So what actually causes synesthesia? Daphne Maurer proposed the Neonatal Synesthesia Hypothesis (NS) in 1993. It builds upon the Cross-Modal Transfer (CMT) hypothesis, which states that infants are capable of recognizing objects in more than one modality even after they have been presented with information from only one modality, and which has been backed up by scientific evidence. NS states that synesthesia is a normal part of infant development, and only after humans are a few months old do they begin to have modular sensory perception. There’s already supporting evidence of NS in other species: young hamsters and kittens have temporary connections between different sensory regions of the brain. Preliminary research in humans has also supported NS; infants up to age 2 months show “wide-spread cortical responses” to visual and auditory stimuli. (4) If the NS theory holds true, adult synesthesia could be explained as the brain’s failure to complete the process of sensory modularization. This would explain why synesthesia is genetically passed down but the type of synesthesia is not: the inherited condition would simply be the tendency to lack modularity in the sensory system. It remains to be explained, however, why some people have synesthesia as children but not as adults.

I saw a show about Daniel Tammet, who is both a high-functioning autistic savant and a synesthete (5). He began experiencing synesthesia after being diagnosed with epilepsy as a child. He sees each number (up to 10,000!) as a distinct shape with its own color and texture and claims that this helps him perform seemingly superhuman mathematical feats, such as reciting Pi to over 22,500 digits. He says he accomplished this not by memorizing Pi, but just by thinking about it and seeing the digits in front of him. There is another famous synesthete known as S who also experienced this kind of passive memory, as if the stuff remembered itself with no effort on S’s part (1). I find this amazing, and it suggests that synesthesia may sometimes be linked to other faculties, particularly those that seem beyond the capacity of a normal human brain. Could synesthesia somehow help people become human calculators? Cytowic (1) writes that synesthetes have a high incidence of autism as well as dyslexia and ADD, and suggests further research into possible links between the conditions.

Synesthesia isn’t a case of getting output with no input; it’s a case of getting unexpected outputs, and more outputs than usual. We know that synesthesia involves cross-wiring between different regions of the brain, but it is unclear whether synesthetes actually have more neurons than other people or if they just have more connections between neurons in different areas. Synesthesia is proof that we still don’t know what goes on inside the “box” of the nervous system. We can determine which “boxes” are involved, but neurologists need to further research what’s actually happening in the bundles of connecting white matter.

It is incredibly difficult for most people to imagine what it would be like to have synesthesia; we can’t conceptualize tasting sound or seeing colors along with letters or music. This goes back to what we talked about in class: everyone has a unique perception of the world and we can’t understand other people’s perceptions because we have never experienced them. Most synesthetes do not think they have a problem, and many of them enjoy the way they perceive the world. Synesthesia is a thought-provoking phenomenon because unlike many neurological abnormalities it does not typically cause suffering and does not need a cure- it’s just an opportunity to guess at how the brain might work and try to get it less wrong.



1. Cytowic, Richard E. Synesthesia: Phenomenology and Neuropsychology. Psyche 2.10 (1995). <>

2. Blakemore, Colin. Interview. The Science Show. 10 Nov 2003. <>

3. “Seeing Life in Colors: Crosswired Senses.” ABC News 15 Aug 2006. <>

4. Baron-Cohen, Simon. “Is There a Normal Phase of Synaesthesia in Development?” Psyche 2.27 (1996). <>

5. Daniel Tammet- The Incredible Brain. <>



Skye Harmony's picture

more evidence for NS?

I just read a post by Simone Emily from March 3 in which she described a study on a sighted woman who was blindfolded and asked to learn braille. Her brain's vision center was activated while she was reading braille, suggesting that the vision center can help process tactile information when not preoccupied with processing vision. Simone wondered if this observation could be related to synesthesia.

This seems to me like support for the Neonatal Synesthesia Hypothesis. If the brain's sensory regions are capable of acquiring such flexibility in such a short amount of time (I'm not sure how long the experiment went on, but I think a few months), it shows that modularization is not a permanent necessity. It is plausible that the brain's sensory regions all used to interconnect, and thus it is easy for them to rebuild or reuse those connections.

Paul Grobstein's picture

more on synesthesia

"synesthesia could be explained as the brain’s failure to complete the process of sensory modularization"

Now that's an interesting idea, which in turn raises the question of what benefit is there to be gained from "sensory modularization"? See "Is this a Banana" and comments following.