Synesthesia: A Fascinating Sensory Condition

Below is a copy of a paper I wrote on synesthesia, a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. In other words, some pretty cool shit, and maybe something you yourself experience. I will probably make another blog post summarizing the paper in an informal way, but for anyone looking for a deep and academic psychology read, I hope you enjoy.

What if the letters of the words you are currently reading were colored like a rainbow? What if reading the words of this sentence caused you to taste a favorite food, such as bacon or french toast? What if the next time you tried to complete a math problem, you became distracted by the fact that a number such as 98 shouldn’t exist because 9 is fastidiously introverted while 8 is a talkative party animal, and the two would never be seen next to each other? What if the next time you listened to Purple Haze, you actually witnessed something purple in front of your eyes as the notes hit your ears? If you are most of the world’s population, these ideas may sound fantastical, absurd, or simply impossible. If you are an individual with synesthesia, however, there is a chance you have already experienced some of the phenomena listed above, and likely experience such astounding stimulation on a daily basis in some form or another.

There are many different types of synesthesia, and the condition can be acquired in multiple ways. Synesthesia is still not fully understood in terms of the mechanisms that bring about its symptoms or its underlying cause(s), but theories exist which are backed by solid evidence. The study of this condition has also shed light on synesthetic symptoms found in non-synesthetic individuals, raising interesting questions and ideas as to how arbitrary the associations made by synesthetes truly are, and whether their insights can shed light onto the mechanisms guiding humanity’s various perceptions of the world.

Although a consistent scientific definition is hard to find, synesthesia can be described as an experience of sensory and/or cognitive connections made between seemingly unrelated sensations, thoughts, or images (Brogaard, 2013) (Simner, 2012). This process can be operationalized with the idea of an inducer and a concurrent experience, where the inducer (such as hearing a word) triggers a concurrent sensation or thought (perceiving the word verbally as well as colorfully). As described earlier, this can lead to experiences such as tasting a color or word, seeing musical notes in one’s visual field, assigning personalities to letters and numbers, as well as many other combinations of cross-sensory thoughts or sensations. One of the defining characteristics of synesthesia is the experience of these thoughts or sensations as automatic, meaning they occur without conscious effort on the part of the synesthetic individual, and cannot be suppressed (Brogaard, 2013).

Reflective of the current knowledge of synesthesia overall, the prevalence of this condition is disputed. Studies have estimated the prevalence of synesthesia to be anywhere between 0.0008% to 4.4% of the general population. (Baron-Cohen, Burt, Smith-Laittan, Harrison, Bolton, 1996.) (Simner, Mulvenna, Sagiv, Tsakanikos, Witherby, Fraser, Scott, Ward, 2006.) (Rich, Bradshaw, & Mattingley, 2005). Likewise, Baron-Cohen et. al found conflicting evidence of a gender difference in the prevalence of the condition, citing a much stronger incidence among females, while Simner et. al and Rich et. al reported no significant difference between genders. Differences in reporting, which consisted of either objective tests or self-reports, may account for these differences, though there were still conflicts among studies which used the same method of reporting (Baron-Cohen, et. al, 1996) (Simner, et. al, 2006). (Rich, et. al, 2005).

When reviewing the literature, one can see that a large reason for the conflicting evidence between researchers over the prevalence of synesthesia is due to the many different types of the condition that exist. Synesthesia is a broad term, as one synesthete can have symptoms almost entirely unrelated to another’s. Because there are many different types of synesthesia, it can be difficult to properly assess how many synesthetes there are in a population, as researchers often only test for one type of this diverse condition, either out of a lack of resources or interest in other types. Data exists for several types of synesthesia, each with their own unique set of symptoms and prevalence within the overall population.

Grapheme-color synesthesia is the most commonly studied form of synesthesia, and has been estimated to make up anywhere from less than 1 percent (Baron-Cohen et. al, 1996), 1 percent (Simner, et. al, 2006), or 2-4 percent (Simner, Ward, Lanz, Jansari, Noonan, Glover, & Oakley, 2014) of the general population. Among individuals with synesthesia, Rich et. al’s study has estimated this type to make up 86% of the synesthetic population, though this was over a small sample size, and conflicts with previous evidence of a much smaller percentage found among the synesthetic population (Baron-Cohen, et. al, 1996) (Simner, et. al, 2006) (Chun & Hupé, 2013).

The inducers for an individual with this type of synesthesia are often numbers, letters, or words such as the days of the week or the name of the months, which are concurrent with the experience of an associated color (Rich, et. al, 2005). For instance, a grapheme-color synesthete would perceive the letters of this sentence with a specific color for each letter, such as “T” eliciting the color teal, “O” eliciting orange, and so on. These associations are consistent over time, as a letter or number’s color does not change regardless of when they are presented or what situation they are presented in (Rich, et. al, 2005) (Simner, et. al, 2014). Each individual’s association for the color relating to these “graphemes” can be unique, though Rich et. al found evidence of a consistent association between certain colors and graphemes among those
surveyed. For example, the letter “R” was associated with the color red in 36% of the sample, “Y” was associated with yellow for 45% of the sample, and “D” was associated with brown for 47% of the sample. This association was also found in a weaker form among non-synesthetic individuals, which raises interesting questions related to synesthetic predispositions in the general population, which is addressed later in this paper.

“Ticker-tape” synesthesia is the experience of an automatic visualization of words as they are thought of or spoken, either in the visual field or in one’s mind (Bastiampillai, Dhillon, & Chui, 2014) (Chun & Hupé, 2013). It is one of the least studied forms of synesthesia, as a quick look through the literature produced only two studies addressing this phenomena. According to Chun & Hupe, this form of synesthesia was estimated to occur in about 7% of the population.
With the aid of interviews, Chun & Hupe’s study reported various differences in the manifestation of this type, including seeing words as either static or moving, with some individuals visualizing words in their head, and others seeing words leaving the mouths of other individuals as they spoke to them. Among other interesting symptoms found within the study, one subject reported that “during a verbal fluency task, ideas ‘stacked up’ visually behind her head before streaming through her mouth as she said them aloud. When too many ideas were being held there, some would disappear before she could say them and thus disappeared from memory.” This example highlights a potential for synesthetes to shed light on cognitive processes in unique ways, in this case giving a visual representation for a process that seems to be tied to working memory.

Another form of synesthesia takes the experience of listening to sounds or playing music to visually vivid places, and is loosely defined as “sound-to-color” synesthesia. In a study consisting of 63 synesthetes, individuals who experienced the sight of colors concurrent to hearing music (which is also referred to as chromaesthesia) were prevalent at 41%, while individuals who experienced color concurrent to sounds in general were found at a prevalence of 33% (Niccolai, Jennes, Stoerig, & Van Leeuwen, 2012). This data suggests sound-to-color synesthesia is a relatively common form of synesthesia as a whole.

Chromaesthesia in particular is often associated with absolute pitch, a better distinguishing of variances in musical notes than typical individuals, and a better ability to distinguish variances in similar colors than others (Bernard, 1986). According to a quote from Duke Ellington, the famous jazz musician appears to have experienced chromaesthesia, and gives a vivid example of its symptoms: "I hear a note by one of the fellows in the band and it’s one color. I hear the same note played by someone else and it’s a different color. When I hear sustained musical tones, I see just about the same colors that you do, but I see them in textures. If Harry Carney is playing, D is dark blue burlap. If Johnny Hodges is playing, G becomes light blue satin" (George, 1981). The fact that Ellington perceives the same note as a different color when played by different individuals points to a potential multi-modal manifestation of his synesthesia, which is discussed later in the paper, and appears to be quite common among synesthetes.

An arguably more cognitively complex form of synesthesia than what has been discussed so far is “ordinal-linguistic personification”. This subtype of synesthesia causes individuals with the condition to assign personalities and/or genders to sequential linguistic units such as letters, numbers, days of the week, months, and so on (Simner & Holenstein, 2007). For instance, viewing the number 7 could cause a concurrent experience of ascribing a laid back personality style to it. In Chun & Hupe’s 2013 study, this form of synesthesia was the most common type found, with an estimate for its prevalence within the general population sitting around 12%. Interestingly, there seem to be general rules for the personalities assigned to each linguistic unit (Simner, Gärtner, & Taylor, 2011). An example found in Simner’s study was the assignment of personalities high in agreeableness and low in neuroticism to letters which appear in higher frequencies (such as the letter A) than others (such as X). This evidence points to a potential generalizability in the rules governing people’s preferences across different situations, as this seems to be a particularly complex synesthetic manifestation of the mere-exposure effect (a psychological phenomenon by which people tend to develop a preference for things merely because they are familiar with them).

Another fascinating type of synesthesia related to numbers is “number-form” synesthesia, which is the conscious experience of numbers in spatially-defined locations and configurations either in the mind’s eye or external environment (Sagiv, Simner, Collins, Butterworth, & Ward, 2006) (Gertner, Henik, & Kadosh, 2009). An example would be an individual viewing the numbers 1-10 as if they were occurring in a bumped number line form, potentially with the order of each number in a different place than its sequence would indicate (6 being located to the right of 8). This ability has been found to be both helpful and harmful to individuals who possess it, as some individuals display a better ability of mentally rotating and visualizing objects in both a mental and external space than the typical population (Havlik, Carmichael, & Simner, 2015), while others have impairments in understanding the semantic meaning of numbers when their
position in space in a numerical task is incongruent with their own mental configuration (Gertner, et. al, 2009).

Taking the concept of empathy to an extreme, a rather unique and interesting type of synesthesia is the “mirror-touch” variety. This phenomenon occurs when a synesthete witnesses or imagines other individuals or objects being touched, and experiences tactile stimulation on their own body in response (Chun & Hupé, 2013). Prevalence of mirror-touch synesthesia seems to be fairly common among synesthetes (Banissy, Kadosh, Maus, Walsh, & Ward, 2009), and in Chun & Hupe’s study the incidence for this type was estimated to be around 10% of the general population.

The concurrent sensations felt in a mirror-touch synesthete’s body after witnessing or imagining someone being touched can range from feelings of pain, pleasure both sexual and general, temperature, and possibly any other type of sensation felt in the human experience (Chun & Hupé, 2013). Additionally, individuals with this form of synesthesia may also experience tactile stimulation when objects are involved, as 20% of Chun & Hupe’s sample reported a tactile response when viewing a lamp being touched. The intensity of these sensations are often tied to the intensity of the action being observed (Chun & Hupé, 2013) (Banissy, Kadosh, Maus, Walsh, & Ward, 2009). There also seems to be an emotional component to this form of synesthesia. In Chun and Hupe’s study, many respondents reported feeling more enhanced tactile stimulation in response to observing individuals with whom they were more familiar, with some respondents even reporting that these were the only individuals with whom they felt any stimulation in response to. Some respondents did not report this, and were able to experience stimulation in response to fictional characters and cartoons. This and previous examples from other forms of synesthesia exemplify the uniqueness of each synesthete’s experience to another’s.

Interestingly, the vast majority of synesthetes appear to have two forms of synesthesia, as well as multi-modal synesthesia, which occurs when an inducer and its concurrent sensation belong to 2 different sensory modalities (Niccolai, et. al, 2012). Niccolai et. al found that 83% of their subjects had more than one form of synesthesia, while 92% of their sample experienced multi-modal symptoms.

Examples of multi-modal synesthesia are found throughout the literature, and raise interesting questions as to the neural underpinnings for each type of the phenomenon (Chun & Hupé, 2006) (Simner, 2006). For example, Chun & Hupé found several subjects who did not exhibit enough symptoms to be considered mirror-touch synesthetes, though still underwent a mirror-touch like experience when they viewed their personal belongings being touched by others, and did not have this experience after viewing another person or object being touched. This example points to the role of emotions in the experience of synesthetic symptoms. Another fascinating example of the impact one form of synesthesia seems to have on other, seemingly unrelated sensory modalities, comes from a study by Bankieris and Simner, in which grapheme-color synesthetes were able to correctly identify the meaning of foreign words at a rate higher than control subjects and above chance levels (Bankieris & Simner, 2015).

The symptoms and experiences of those with synesthesia raise interesting questions as to how and why these sensations occur in the brain, and just as with many other aspects of synesthesia, there is controversy and debate in their explanation. Neurologically, there appears to be some form of “cross-binding” occurring within the parts of the brain associated with sensory perception, in particular, both grey and white matter pathways in the parietal cortex (Brang, Williams, & Ramachandran, 2012) (Simner, 2012). Hyper-connectivity in white matter pathways throughout the brain are also found in synesthetes (Simner, 2012).

The idea of “cross-binding” has led many researchers to explain this occurrence from a developmental standpoint. The vast majority of synesthesia cases are developmental in nature, meaning the condition was acquired naturally over an individual’s childhood (Simner, et. al, 2006) (Baron-Cohen, et. al, 1996). This has led to the popular theory of “neonatal synesthesia,” in which every child is born with their senses undifferentiated until they are naturally differentiated over time, except in the case of those with synesthesia (Ronga, Bazzanella, Rossi, & Iannetti, 2012) (Simner, 2012). This theory is supported by the fact that there seems to be a genetic basis for synesthesia, as it has been found to run in families (Baron-Cohen, et. al, 1996).

While this theory is intuitive and seems to explain synesthesia from a developmental standpoint, there are issues with it. Ronga et. al point out that the concurrent associations made by synesthetes seem to be too heavily reliant on a process of learning than a purely natural and automatic one caused by a lack of neuron differentiation. Ronga e. al’s argument is supported by the cognitive complexity of many forms of synesthesia, and the fact that each synesthete’s experience tends to differ from that of another to a larger degree than would be expected by such a process. The researchers argue that the key to understanding the development of synesthesia is in the mechanisms in which the learned associations occur.

Berit Brogaard, in her 2013 study, seems to have found strong evidence for such a mechanism (Brogaard, 2013). Brogaard notes that there are others ways of acquiring synesthesia separate from a developmental standpoint. Synesthesia can be experienced temporarily by individuals taking hallucinogenic drugs, and acquired suddenly and permanently by individuals with severe head trauma. Citing a general apathy toward explaining forms of synesthesia which occur by acquired and drug-induced means, Brogaard argues that an excess of serotonin is the common factor in each form of synesthesia.

Serotonin is the neurotransmitter that floods the brain during the use of hallucinogenic drugs such as LSD, as well as after a traumatic brain injury in response to cell death. Serotonin would thus explain the sudden and acute symptoms of synesthesia occurring in adults who did not previously acquire the condition through developmental means.
In regards to the developmental form of synesthesia, Brogaard argues that serotonin is at work as well, and uses examples of SSRI’s and the presence of an excess of serotonin in individuals within the autism-spectrum as evidence.

Though there is only correlational evidence that synesthesia and autism-spectrum disorders are related, individuals within the autism-spectrum have been found to have a prevalence of synesthetes over three times higher
than the general population (Brogaard, 2013). Excess serotonin in the brain during child development is known to be a strong factor in the development of autism-spectrum disorder (Brogaard, 2013). If serotonin is involved in developmental synesthesia as the evidence suggests, it could be implicated in the unusual structure binding found in the brains of synesthetes, as a prolonged release of serotonin affects learning and the formation of neurons (Brogaard, 2013).

Another piece of evidence in support of serotonin as a key mechanism causing synesthesia lies in the presence of synesthetic abilities in the general population (Bankieris & Simner, 2015) (Brang, Williams, & Ramachandran, 2012). For instance, it has been found that non-synesthetes assigned similar colors to letters as synesthetes in an association task, and both synesthetes and non-synesthetes have been found to associate sounds with higher pitch to brighter colors (Bankieris & Simner, 2015) (Simner, et. al 2014). These studies suggest that both synesthetes and the general population share the same general underlying neurological structure to elicit cross-sensory experiences, with some type of mechanism causing an enhancement of these experiences in those with synesthesia.

In addition, in Bankieris & Simner’s 2015 study, English speaking synesthetes and non-synesthetes scored above chance on a task of identifying the meaning of Italian words without previous knowledge of the language, while synesthetes scored significantly higher than the control group. This suggests a presence of cross-sensory abilities in non-synesthetic individuals, as well as a potential universality in the assignment of meaning to the formation of
words. Studies such as Bankieris and Simner’s highlight the ability of synesthesia as a tool to discover underlying learning processes and cognitive tendencies in large and diverse populations, and potentially local ones as well.

As more is learned about the abilities of synesthetes, a question may arise: is possessing these abilities an advantage or disadvantage? The answer is both. It seems reasonable that an individual with cross-sensory experiences would be more inclined to pursue activities related to evoking the senses, and may contain an advantage in doing so over typical individuals. Evidence for this is the higher prevalence of synesthetes enrolled in art universities compared to non-synesthetes (Rothen & Meier, 2010). Synesthetes also seem to display a slight advantage in general associative memory than typical individuals (Pfeifer, Rothen, Ward, Chan, & Sigala, 2014).

In contrast, the presence of synesthesia can be distracting and an impediment to certain functioning. For example, a man with ticker-tape synesthesia hearing multiple voices at once could have his visualized words jumbled, and would have great trouble distinguishing which person was saying what (Bastiampillai, Dhillon & Chui, 2014). In cases of mirror-touch synesthesia, sharing the pain of those around you would be a potentially debilitating condition. Some mirror-touch synesthetes only experience a sensation of pain when viewing other people or objects being touched, regardless of the nature of the contact (Chun & Hupé). Number-form synesthesia can be a disadvantage when one’s conceptualization of numbers in space is not adapted to a mathematical task (Gertner, et. al, 2009). Depending on one’s symptoms, synesthesia can be a blessing, a curse, or inconsequential.

Synesthesia is a fascinating psychological phenomenon. It is a unique condition, with much to be learned from the biological methods which lead to its manifestation and the cognitive and learning processes which bring about its symptoms. Research into synesthesia has yielded intriguing information for those who do not possess the condition as well, and further investigation may shed even more light on the interrelations between different aspects of the human mind.