Introduction
Synesthesia (literally “together are the senses”) is a fascinating neurological phenomenon wherein the stimulation of a sensory or cognitive pathway leads to the involuntary or automatic sensation of one or more other sensory pathways (Johnson et al 2013). One of the many forms of synesthesia, Grapheme-Color Synesthesia, an individual may see colors that are strictly and rigidly intertwined with alpha-numeric symbols; a particular number is always a certain color, regardless of location or font. Grapheme-Color Synesthesia is only one of many forms of Synesthesia but is the focus of this research paper due to the prevalence and relevance of this form of Synesthesia (Asher & Carmichael 2013).
Several theories exist to explain the unusual phenomena that is synesthesia. Synesthesia has been shown to run in families and run on the X chromosome due to the 6:1 female to male ratio of individuals with synesthesia (Asher & Carmichael 2013). The most common and highly accepted theory is that of “Hyperconnectivity” in which a defective gene causes neurons to unsuccessfully prune in the immature brain and therefore create connections between certain parts of the brain (Rouw 2013). Synesthetic children often realize their condition at a young age, noting that their unique perceptions are not common amongst the population (Mauer et al 2013). Many synesthetes benefit from their condition, be it in the form of Spatial Sequence Synesthesia or Chromesthesia (Asher & Carmichael 2013). The majority of the benefits that come from synesthesia greatly enhance the abilities of many artists and their creative output. The test used to diagnose visual synesthesia is called the Revised Test of Genuineness; it uses a randomized visual and/or audio test to establish evidence of synesthetic induction (Lynall and Blakemore 2013) (Asher & Carmichael 2013).
I find Synesthesia to be profoundly interesting. I first heard about it on an episode of House M.D. It is an incredibly unusual and peculiar condition that is unlike any other condition in the realm of medicine. The reason I picked this for research is because the condition itself begs the question of what we consider to be real; the difference between perception and sensation. Synesthetes are receiving the same sensory data as those not affected, though their perceptions are vastly different; in fact, other sensory pathways are perceived due to a crossing of biological perception mechanisms.
Anatomy & Physiology
Hyperconnectivity
It has been thought by scientists for the last century that cross-wiring (within the brain) is the basis for Synesthesia (Ramachandran & Hubbard 2001). The main theory is that the areas of the brain that control color and graphemes are “crossed.” This model states that the grapheme areas (Posterior Temporal Grapheme Area [PTGA]) and the color processing area of the brain (V4) are inherently connected, causing synesthesia. It is not mere coincidence that the most common form of synesthesia, Grapheme-Color, involves two areas that control grapheme (PTGA) and color (V4) are proximate to one another (Rouw 2013).
These inherent connections between these two areas result in a powerful co-activation that occurs in these areas during the synesthetic process. Hence, it is believed that a defective gene causes an abundance of cross connections and/or defective pruning (Ramachandran & Hubbard 2001). A deficient amount of pruning (the process by which redundant neurons die off) during infancy could be the cause for the cross-wiring between these areas (Rouw 2013). These connections that are established would carry over into childhood and through the rest of the lifespan (which explains why the “rules” of G-CS are rigid and unchanging) (Rouw 2013). Thus, a sensory activation of numbers or letters could cause an automatic stimulation of the neurons concerned with color. In the immature brain, the process of pruning rids the brain of excess neurons to make way for the essential neurons (Asher & Carmichael 2013).
This might also explain why a synesthete usually will experience more than just one form of synesthesia. The defective gene that causes a deficiency of pruning in one form of the brain likely will occur in another region of the brain, thus causing another form of synesthesia (Rouw 2013). Although a single gene may be the cause of synesthesia, the gene could end up being expressed in an erratic and random manner to varying degrees and in different loci (Asher & Carmichael 2013).
This could be dependent upon different transcription factors that occur during DNA to RNA transcription. Interestingly, accounts have been made for the fact that patients have become synesthetic (or lost their synesthesia) due to injuries. In one such case, an artist was driving along the highway and his vehicle was struck by another car and he became color blind (Rouw 2013). Prior to the incident, the man had Chromesthesia (a form of Synesthesia when one can see colors associated with musical tones), however, the color blindness caused his synesthesia to become nullified. The interesting point to be made here is that the man also had an acute case of alexia (the inability to see words), indicating that a particular region of the brain was affected due to the accident, causing the color blindness and alexia. It is also reasonable to conclude that this area of the brain was the same area that causes Grapheme-Color Synesthesia (since the artist could no longer see color or identify graphemes) (Rouw 2013).
Disinhibited Feedback
There is another theory that has gained much traction in recent years called Disinhibited-Feedback theory. This model assumes that there are no issues with interconnections and that synesthetic induction occurs from normal neural connections. Connections feed afferently into areas of the brain that receive signals from various pathways, called pathway convergence (similar to the notion of neural convergence) (Grossenbacher & Lovelace 2001). With non-synesthetes, top-down signals follow these feedback mechanisms and may be amply inhibited so that synesthetic induction does not occur. However, with synesthetes, the sensory information that enters a particular convergence area via the inducer pathway could propagate down the concurrent pathway causing disinhibition of the feedback signals (Grossenbacher & Lovelace 2001). Afferent signals in the inducer pathway (stimulus) activates neurons in particular areas of the brain wherein the inducer and concurrent pathways (other sense that is stimulated) converge and that very feedback signal from the first area propagates down the concurrent pathway to trigger the concurrent perception, creating synesthetic induction (Jancke 2013). In this model, synesthesia is the result of activity that is formed from within the inducer pathway during the synesthetic perception, this can be triggered in horizontal connections (between the pathways) or because of the pathways which converge. This pattern continues along the entire pathway until it reaches the end of the pathway which is the thought or perception that is present [Figure 1]. The disinhibited-feedback theory is a bit different from some of the other cortically based Synesthesia theories that exist in scientific text. Grossenbacher & Lovelace (2001) suggested that infants are born with congenital dense connections from one cortical sensory area to another (Jancke 2013).
Limbic Mediation
A variation of disinhibitive feedback, the Limbic Mediation hypothesis, was initially proposed by R. Cytowic & F. Wood (1982) and postulates that synesthesia results as a mediation between the Hippocampus and the Limbic System; this is an area of the brain where many different sensory signals meet, possibly even more connective fibers leading from the Limbic System to the the Neocortex (Jancke 2013). This strong connection may be the reason why synesthetes in general tend to be more clued into cross-modal connections. This is a working hypothesis, not a theory, so it still has to undergo thorough examination by other researchers. The hypothesis does not account for what the possible pathway activations are or what accounts for the myriad forms of synesthesia (Jancke 2013).
X Marks The Spot
The first group of researchers, Ramachandran & Hubbard 2001, studied Grapheme-Color Synesthesia, the most famous type (Ramachadran & Hubbard 2001). The second group of researchers, Baron-Cohen et al (1996), studied Chromesthesia, which is far less common (Ramachandran & Hubbard 2001) . Even though its prevalence seems to be variable, scientists overwhelmingly agree that synesthesia is in fact inherited through genes on the X chromosome. Baron-Cohen et al (1996) conducted a scientific survey in which they determined the familiarity of synesthesia. The data collected indicated that synesthesia is far more common in females with a ratio of 6:1 (Ramachandran & Hubbard 2001). More than 1/3 of the participants of the study stated that they had other members of their family who were also synesthetes. The combined data recovered by the researchers strongly suggest the theory of linkage along the X-Chromosome as a dominant trait (Asher & Carmichael 2013). In any case, the inheritance factors are complicated and complex due to the varying types of synesthesia. Although, it is believed that one gene causes the defunct pruning or excessive cross activation, determining that the genes expression is sporadic, leading to different areas being affected, thus different types of synesthesia arise (Lovelace 2013).
Clinical Considerations
Signs
Synesthetes that experience Grapheme-Color Synesthesia perceive numbers and letters inherently as a particular color (Ramachandran & Hubbard, 2001). For example, a girl said to her Father that the letter “P” may appear as red. However, when a line is added at the base of the loop in the letter “P”, it becomes an “R”, thus becoming a different color (Green for example). The Synesthete would quite literally see the letter change color (Duffy, 2013). Her father was perplexed by her observation. This is a particular case because the daughter was 16 years old; most individuals with Synesthesia convey their perceptions at a very young age, not realizing that it is something that not everyone perceives. The signs of Synesthesia are usually first reported by Synesthetes when they are very young, around toddler age (Maurer et al 2013). Many children who are Synesthetic do not understand that their perceptions are unusual. In many cases, a parent may discover his or her child’s synesthesia through the use of fridge magnets. For example, a parent may be helping his or her child to learn about the alphabet in the form of letter magnets. The child might state that the letter “A” is red or the letter “J” is purple. It is obvious to the parent that the “A” and “J” are not the colors that the child perceives.
Symptoms
There are many forms of Synesthesia, but this research paper focuses mainly on Grapheme-Color Synesthesia. With this form, individuals seem to perceive certain letters or numbers as an inherently specific color. The letter “A” may seem gold, or the letter “B” may seem chartreuse. There is no reason for why some letters and numbers are certain colors, it is completely arbitrary and random (Ramachandran & Hubbard 2001). The only exception to this rule is that when a word is seen, the color of the word is the same as that of the first letter. So, if the letter “B” is seen as chartreuse then in the word “Bee,” it would be seen as chartreuse, even if the letter “E” is seen as pink. What is particularly interesting is the fact that many Synesthetes report seeing colors associated with numbers and letters but never seeing them in the “real world,” only in their perceptions of alpha-numerical characters (Ramachandran & Hubbard 2001).
Diagnosis
Asher et al (2006) created a test for genuine synesthesia called the Revised Test of Genuineness for Visual Synesthetes (TOG-R) (Lynall & Blakemore 2013). This test hosts a few criteria for identifying visual and/or audio synesthesia that make the test more verifiable than that of previous models: an accurate scoring system and a method for phenotyping different aspects of synesthesia. The test is administered demonstrated 51 days, months, nouns and verbs (Grapheme-Color) and 43 instrumental sounds (Chromesthesia) as well as other natural and unnatural sounds and they were all presented on notecards (or CDs for the latter) (Lynall & Blakemore 2013). The TOG-R’s scoring algorithm was far more precise than that of the original test. Patients choose from a slew of sounds associated with certain colors. The chart used has several different shades of several different colors. The proximity of the choice of color (on the chart) to the previous choice earns the patient a certain amount of points based on similarity. The amount needed is based on a mathematical algorithm that determines if the individual is synesthetic or not. This method of testing was verified by the Asher et al (2006) during an experiment where they used the same test methods however they used 25 synesthetes and 25 non-synesthetes (the control group). The control group had a week in between testing intervals compared to the four week hiatus for the synesthetes. The synesthetes were 80–100% consistent (with correlation between colors and sounds) when the non-synesthetes were 20% consistent despite the one week in between testings.
Treatments
Synesthesia is not a debilitating condition, therefore it does not have any treatments to correct it. In fact, having synesthesia has been shown to increase memory or artistic ability for individuals with the condition. The aim of this section of my research will focus on two forms of synesthesia that can enhance the lives of those with the condition: Chromesthesia and Spatial Sequence Synesthesia (Simner 2013).
Chromesthesia
Chromesthesia, for example, is a form of synesthesia wherein a synesthete perceives sound and colors as inherently congruent. For example, an individual may see particular colors that are triggered by everyday sounds such as a door slamming shut, a pen clicking or the sound of a footstep (Mulvenna 2013). Many Chromesthetes perceive colors that are associated with musical notes. As with Grapheme-Color Synesthesia, the connections made between the two are arbitrary. Many profoundly talented musicians have/had chromesthesia: Duke Ellington, Billy Joel and Stevie Wonder (Mulvenna 2013) (Cytowic 2013). Chromesthetes with musical chromesthesia use this to their advantage as a way to reach “perfect pitch.” This is utilized to be able to more easily identify the correct notes/pitch.
Spatial-Sequence Synesthesia
The next best example of synesthesia being beneficial to one’s life is Spatial-Sequence Synesthesia. With this form of synesthesia, SS-Synesthetes perceive numerical patterns either in the “mind’s eye” or in the external environment (Jonas & Jarick, 2013). For example, an SS-Synesthete might perceive the numbers on a calendar to be in the form of a spiral staircase or as an isosceles triangle. Some perceive individual numbers to be at a particular distance in space. Hence, the number “1” may be further away than that of “2.”
However, there are many cases of SS-Synesthetes who see numbers, dates and events arranged as a timeline (Hubbard 2013). This particular form of SS-Synesthesia is of particular interest. Psychologist Judy Simner of the University of Edinburgh in Scotland performed an experiment wherein 10 SS-Synesthetes and 10 non-synesthetes (control group) were tested to find out if their condition gave them an advantage through unconventional mnemonic skills and patterns (Hubbard 2013). The test required the participants to recall 120 public events that occurred between the years 1950 and 2008. A couple of the dates inquired were the year Nelson Mandela was released from prison (1990) or the year My Fair Lady won the Oscar for Best Picture (1965). The results showed that, on average, non-synesthetes named the incorrect date by 8 years; SS-Synesthetes were closer on average by 4 years. The SS-Synesthetes also were able to recall various events or experiences from their own lives by twice as much as those of the non-synesthetes. The results showed that the SS-Synesthetes were not exceptionally better than the non-synesthetes; they did, however, show a marginal advantage (Hubbard 2013). However, a connection to Hyperthymestic Syndrome in which a person can recall any event from their lifetime because they visualize history as a timeline.
Opinion
I find the model of Chromesthesia to be more beneficial to an individual with this form of Synesthesia. The ability to see a perfect note or key is a staggering advantage over other musical artists. Many singers or musicians never learn to reach perfect pitch/key, the ones that do learn perfect pitch take years of practice and dedication to the art. Think of how much rigorous practice, determination and discipline is required to reach perfection in its truest sense; Chromesthetes can do this naturally. The experiment for Spatial Sequence Synesthesia showed that SS-Synesthetes were only marginally better at recollecting memory than that of non-synesthetes; showing a lesser advantage than that of Chromesthetes. However, I feel that SS-Synesthetes could be trained or practiced to increase their synesthetic abilities. Perhaps even learn a way to use it to obtain a perfect mental calendar for each day and future date. It would be quite an advantage for an individual to be able to keep a complete mental calendar to organize their lives and obligations.
It is estimated that Synesthesia occurs in 1 in 200 people, while other studies place the ratio at 1 in 2,000 individuals (Johnson et al 2013). The disparity between the estimates likely is due to the variability and differentiation of criteria that constitute true synesthesia.
Conclusion
Synesthesia is a fascinating phenomena that continues to intrigue scientists by its distinct symptoms, a blending of the senses. Synesthesia is an interesting model to inspect when pondering the difference between sensation and perception. Hyper connectivity between sensory areas of the brain which cause the sensory pathways to overlap. Another theory, disinhibitive feedback, suggests that concurrent pathways run parallel to the inducer pathway (stimulus) and these pathways cross and cause feedback which are exhibited as synesthetic induction. The theory Limbic Mediation proposes the notion of a strong connection between the amygdala and the neocortex. This, however, is a work in progress as it lacks explanation of certain aspects of synesthesia.
Synesthetes start to notice the symptoms of the condition from a very early age, however they don’t realize that it is abnormal. Most synesthetes feel that their synesthetic experiences are beneficial and actually show advantages to memory or artistic ability. Spatial-Sequence Synesthetes are better at remembering timelines, history, names and dates because they can visualize chronological timelines. Chromesthetes, who see colors associated with sounds are more proficient at reaching perfect pitch because they can visualize their notes and pitch. For me, I believe that Chromesthetes and SS-Synesthetes both exhibit some abilities above those of non-synesthetes, to varying degrees.
Grapheme-Color Synesthesia is the most highly researched form of synesthesia, likely due to the fact that it is the most prevalent. This is the gold standard of Synesthetic research. I chose this topic for its inherent and profound curiosity and mystery that surrounds its symptoms.
Synesthesia is a complex and stimulating phenomenon that is caused by a myriad of factors. The syndrome presents in a multitude of forms and it can be used as an interesting dissection of perception and what it means to sense the world around us. Synesthetes all over the world provide an exclusive perspective for perceiving the world. They consume the same sensory information that non-Synesthetes do, but their perception is positively disparate.
All references are from the following text:
Oxford Handbook of Synesthesia. (2013). United Kingdom: OUP Oxford.
