By Josephine Doucette & Bellajeet Sahota
Art by Taylor Yingshi
An Overview of Synesthesia
Remember that scene in Ratatouille, when Remy the rat tastes a piece of cheese for the first time ? As the cheese dances atop his taste buds, it evokes yellow shapes that twirl and jump to music across his visual field. When he then takes a bite of a strawberry, new red swirls appear with their own unique soundtrack . Remy is able to see and hear the flavors he tastes, but this unique mixing of the senses does not just exist in the fantastical world of Pixar. Approximately one in every 2000 people has synesthesia, a neurological condition in which one sensory experience—like seeing color—is triggered by a stimulus that typically does not activate this sense, such as the taste of cheese . This definition of synesthesia is the most commonly used. However, it is tentative, as there are a multitude of ways that synesthetes, or people with synesthesia, experience this unique sensory phenomenon, making it characteristically hard to define . In interviews with 572 self-reported synesthetes, 35 different types of synesthesia were counted . While it has proven difficult to construct a comprehensive definition for synesthesia, identifying the commonalities between its numerous subtypes may be the only way to develop a thorough neurocognitive theory of synesthesia.
Among the vast variety of experiences caused by synesthesia, the most straightforward involve distinct sensory domains like those related to the five senses. For example, in sound-color synesthesia, visual and auditory domains interact: auditory stimuli like music or other sounds trigger color sensations . In the case study of a synesthete musician conducted by Mills et al., scientists investigated how the participant described different genres of music . The synesthete reported that heavy metal music takes the form of “big blocks of dark colors,” while country music elicits unpleasant color combinations like orange and green. For this particular synesthete, the sounds of different instruments also have different color associations: violin music is typically auburn, while slide guitar music resembles “puddles of colored water” . Many of today's popular musicians, like Lorde, see colors during their creative process . When writing her song “Tennis Court,” Lorde details how the chords started as a “really dated” tan color, but, as she continued writing, the song became full of vibrant greens. Speaking about how she uses her synesthesia, Lorde said it is “about getting the actual [song] to sound like what [she’s] been seeing” . Much like Lorde, another artist, TK, uses his color-taste synesthesia—in which the senses of sight and taste are intertwined—to refine his visual artwork . For this synesthete, greens taste bitter, reds taste sweet, and yellows taste sour. If the painting tastes too sweet, he knows there is too much red; if it is too sour, there is too much yellow . The colors of a chord or the flavors of a painting may never even cross the mind of a non-synesthete, but for synesthete artists, these unique sensory phenomena are indispensable to their work.
Arguably the most common and studied type of synesthesia is grapheme-color synesthesia, in which letters and numbers, also called graphemes, evoke specific colors . A grapheme-color synesthete looking at the page of a book would see a host of colors that are specific to each letter: the letter “a” may be a deep burgundy, while the letter “k” is sky blue. Unlike sound-color and color-taste synesthesia, this type does not involve two completely different senses. Rather, it combines the two distinct visual realms of shape and color perception .
Though varied in their manifestation, all synesthesias involve a specific trigger, called an inducer, and a resulting experience, called a synesthetic concurrent . For grapheme-color synesthetes, the inducer is the sight of a grapheme, while the synesthetic concurrent is the distinctive color elicited by that grapheme . The precise nature of inducers and concurrents varies across individuals, but for a synesthetic experience to be considered genuine, it must be specific, consistent, and automatic . Grapheme-color synesthetes see a specific color associated with a distinct letter; they do not just see colors at random. For example, the letter “e” could be orange and “a” could be red to one synesthete. In order for the experience to be specific, the colors cannot be interchangeable. This color association also remains consistent: the same letter induces the same color over the course of the synesthete’s life. Following the previous example, the letter “a” would always be red to this synesthete, no matter what . Finally, the idea of automatic associations is an important aspect of synesthesia . For grapheme-color synesthetes, the perception of color tied to certain letters is automatic and cannot be repressed. If this type of synesthete is in the presence of the stimulus (in this case, the letter), they must see the associated color . It is akin to how one cannot help but pull one’s hand away from a hot surface .
The Synesthete Brain
Due to the variability of synesthetic experiences, researchers have not come to a clear consensus on the neural mechanisms behind this perceptual phenomenon . However, there are two main proposals: the cross-activation theory and the disinhibited feedback
theory . The cross-activation theory suggests that synesthetic experiences arise from an abnormally high number of neural pathways that physically connect different brain regions . This hyperconnectivity most likely results from a lack of neural pruning, a process that eliminates unnecessary neural connections to conserve energy as the brain develops [12,14]. In grapheme-color synesthesia, the inducer activates both the brain region used for letter recognition, the anterior fusiform gyrus, and the region used for color processing, area V4 in the occipital lobe . Under the cross-activation theory, this simultaneous activation results from hyperconnectivity between the two regions, which are both located in the greater fusiform gyrus . One stimulus, such as the sight of letters, can activate both regions because the physical neural pathway branches out into two directions like a wishbone . The cross-activation theory allows for a straightforward explanation of synesthesia by suggesting an anatomical connection between the brain regions that process the input from the inducer and produce the concurrent output .
A neuroimaging technique known as fractional anisotropy (FA) has uncovered evidence for anatomical hyperconnectivity between different brain regions, which is consistent with the cross-activation theory. FA determines the density of white matter by measuring the diffusion, or movement, of water molecules through the brain . The level of diffusion indicates the number of axons in a given brain region: without obstacles, water molecules can diffuse freely in any direction, but in the presence of white matter, water diffuses only along the axons. Thus, a higher FA value correlates with a larger quantity of axons, which suggests higher connectivity . In a study conducted by Rouw et al., grapheme-color synesthetes displayed higher FA values in the fusiform gyrus—which houses both brain regions involved in grapheme-color synesthesia—signifying a heightened amount of neural connections in this area compared to the control group . Though compelling, FA does not map precise axonal pathways, so it remains unclear if these abnormally dense connections span between the specific areas of the fusiform gyrus involved in grapheme-color synesthesia (the anterior fusiform gyrus and area V4) . Since FA has only been used by researchers to study specifically grapheme-color synesthesia, there still remains a question of how other types of synesthesia would present using this method.
While the cross-activation theory postulates that synesthesia is caused by the existence of atypical neural connections between two brain regions, the disinhibited feedback theory hypothesizes that an abnormal amount of activation of neural pathways underlies synesthesia . In other words, the cross-activation theory suggests atypical anatomy in the brains of synesthetes whereas the disinhibited feedback theory proposes atypical activation of pathways that exist in both synesthetes and non-synesthetes alike . When we receive sensory input, the stimulus travels to the brain’s association cortex on the surface of the cerebrum before bouncing off to a specialized region that helps us understand what this sensory information signifies [19,20]. For example, when we see the letter “a,” the association cortex sends this visual input to the anterior fusiform gyrus, which processes letter recognition [17,20]. For non-synesthetes, pathways to any other regions are inhibited, so they experience only the shape and form of the letter . However, for synesthetes, the disinhibited feedback theory hypothesizes that when the association cortex is stimulated by the inducer, feedback travels down to two regions: one that produces the typical response to the stimulus and another that produces the atypical concurrent sensation . When a grapheme-color synesthete sees the letter “a,” the association cortex sends feedback to both the anterior fusiform gyrus and area V4 . The pathways to these regions are both disinhibited, so they both become activated by the same visual stimulus, causing grapheme-color synesthetes to experience color when viewing graphemes . The disinhibited feedback theory suggests that both synesthetes and non-synesthetes have the same potential for neural activation that would result in synesthetic perception . However, their variance in sensory experience results from a difference in inhibition of the neural pathways to sensory brain regions .
The disinhibited feedback theory has been tested in sound-color synesthetes using functional magnetic resonance imaging (fMRI) . This technique measures the levels of oxygenated blood flow across different brain regions, which is a proxy for brain activity . Oxygenated blood has different magnetic properties than deoxygenated blood, and fMRI tracks these discrepancies to visualize the routes of oxygenated blood. As brain regions become more active, they require more oxygen, which results in greater oxygenated blood flow to that area . A study by Neufeld et al. investigated the workings of sound-color synesthesia in which areas A1—the primary auditory cortex—and V1—the primary visual cortex—are both activated, causing the perception of color in response to sound . To test this, the investigators used fMRI to measure levels of activation between the two involved brain regions and a “pathway convergence site.” This is a part of the association cortex where complex sensory information is processed and then funneled to relevant brain areas. Their results demonstrated increased activity between the left inferior parietal cortex (IPC) and the A1 and V1 areas . The IPC is considered a “multimodal association area” within the association cortex because it receives input from both visual and auditory stimuli .
The increased communication of the A1 and V1 areas with the IPC suggests that these two areas are not connected to each other . Instead, it seems that they are activated in tandem by pathways that stem from a common association area. Since the neural pathways from association areas may be disinhibited in the brains of synesthetes, the data showing the communication between the two sensory regions and the IPC supports the disinhibited feedback theory. Importantly, the IPC is connected to both visual and auditory regions in all people, so the heightened activation of these areas in synesthetes does not derive from an anatomical difference . Despite not having been tested on enough types of synesthesia to prove that it is the primary neural mechanism, these results provide compelling evidence for the disinhibited feedback theory.
The Difficulty in Defining Synesthesia
As shown by the extensive range in types of synesthesia, as well as the current lack of consensus as to which neural theory is correct, the implications of the data from many synesthesia studies remain up for debate. The science behind the more basic types of synesthesia discussed previously is elusive enough; however, something that complicates our understanding of synesthesia even further is the fact that the inducing stimulus does not necessarily have to be sensory in nature . The stimulus can instead be ideological by having roots in the semantic knowledge of a concept. In other words, some types of synesthesia are considered “ideasthesias” because their inducers are ideas or mental representations instead of sensory experiences .
The existence of ideasthesia is evident in many grapheme-color synesthetes who also experience lexical-color synesthesia, a form of synesthesia in which entire words, not just letters, elicit a specific color . In most cases, the color of the first letter dictates the color of the rest of the word. For example, if a synesthete sees the letter “c” as lime green, the words cat and cap would be lime green as well. However, for some synesthetes, the color they associate with certain words is entirely different from that of the first letter. According to one lexical-color synesthete, “banana should be dark blue . . . but it’s yellow” . In this case, the synesthete’s understanding of the meaning of the word banana is the inducer for the concurrent color association . This goes beyond associations that are simply sensory; the meaning of the word, instead of the physical sight of the letters, induces the specific synesthetic association . This sort of semantic-dependent synesthesia compounds the challenge of mapping out synesthetic experiences in the brain, given that semantic knowledge is not limited to one region but can be found in several widespread areas in the brain .
With this under consideration, the true definition of synesthesia becomes even more challenging to pin down because it expands beyond the traditional interpretation that links sensory stimuli to sensory experience. We propose a more comprehensive definition of synesthesia: a condition resulting in atypical sensory experience as a result of a sensory or ideological stimulus. This definition still maintains that synesthetic experiences must be specific, consistent, and automatic, but it does not restrict inducers to sensory inputs. With this more expansive definition, we can incorporate novel types of synesthetic sensory perception into this domain, including tasting words, seeing time manifested in shapes, and even experiencing colors when viewing specific swimming styles.
Novel Types of Synesthesia
In one novel type of synesthesia known as lexical-gustatory synesthesia, words do not trigger colors; instead, they trigger flavors . For example, one synesthete reports an “overwhelming notion of orange jelly when he hears the word shoulder” . In this type of synesthesia, the flavor of a word can be induced by merely the thought or idea of the word, even without actually hearing it . This phenomenon is called a “tip-of-tongue state.” Researchers have studied tip-of-tongue states in lexical-gustatory synesthetes by showing pictures of uncommon things whose names are difficult to recall (for example, platypus and castanets) and asking for the associated taste. In one study, 89 out of 550 synesthete participants reported a specific taste before they were able to name the object. One of these 89 participants reported tasting tuna while “castanets was on the tip of her tongue” and reported the same association a year later during a surprise retest . For this synesthete, her mental representation of the word castanets was enough to elicit a synesthetic experience. In other words, it was not the auditory experience of hearing this word that induced the associated taste; instead, the meaning of the word was the inducer.
Straying even further from the traditional sensory definition of synesthesia, time-space synesthesia results in an association of units of time with specific locations in space . For most time-space synesthetes, their synesthesia manifests in a unique visualization of a calendar in which different months appear as colors or shapes in distinct spaces relative to the body . In the account of one time-space synesthete, the months are organized in a ring around their body, each month as a different hue . These calendars differ across synesthetes: months may be different colors, take up a different proportion of the ring, or appear as a different shape entirely. These associations can be projected in front of the synesthete or in their mind’s eye, and they also can be stationary or move with the person as they turn . Time-space synesthesia differs greatly from other synesthesias because, unlike letters and words, there is no existing sensory input for units of time . Instead, units of time exist solely as concepts within the mind. Nonetheless, reports from time-space synesthetes confirm that these phenomena are consistent and involuntary, establishing time-space synesthesia as a legitimate and completely idea-based type of synesthesia .
In another remarkable and extremely uncommon type of synesthesia called swimming style-color synesthesia, synesthetes perceive color when viewing different swimming styles . This type further challenges the typical sensory definition of synesthesia because it demonstrates the possibility of both semantic and motor associations. Swimming style-color synesthesia has been confirmed in two grapheme-color synesthetes who are also both semi-professional swimmers . To confirm that their color associations were truly synesthetic, both subjects were shown black-and-white photos of different swimming styles and asked to choose from more than 5,500 color shades to find the closest match to their concurrent color . Because of the importance of consistency in synesthetic experiences, they were retested after a few weeks and their color consistency was compared to that of a non-synesthete control group. The synesthetes showed significantly greater consistency in their test answers than the control group, thus confirming the authenticity of their synesthetic associations .
Along with lexical-color, lexical-gustatory, and time-space synesthesia, swimming style-color synesthesia demonstrates that mental representations of some concepts can sufficiently elicit a synesthetic experience. The two confirmed swimming style-color synesthetes experienced genuine synesthetic associations when merely looking at a photo of a swimming stroke without physically swimming . However, swimming style-color synesthesia differs from these other unconventional types in that the mental representation involves motor imagery, or imagination of motor actions . Thus, swimming style-color synesthesia demonstrates that synesthetic inducers can stray very far from traditional sensory input and involve not only semantics but also input in the form of motor imagery.
What We Can Learn
Synesthesia is various in its types and mysterious in its physical mechanisms. In trying to define synesthesia, we may overlook some of the complexities of a synesthete’s experiences. The study of synesthesia, instead, should revolve around the unconventional and unique interactions that people have with the world through relations of different sensory modalities. Through synesthesia, we see that the human experience is manifold in greater ways than we may have thought before. Synesthesia helps us see that some people literally experience the sensory world differently. Though synesthetic science may be in its beginning stages, synesthesia’s byproducts, like art and music, are all around us. One does not have to have synesthesia to appreciate Lorde’s music or Remy’s visions of cheese and strawberries. Many people may not relate to Remy’s perception of color and sound in response to different tastes; however, through the lens of synesthetic experiences, we can better appreciate the nuances of our senses.
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