Different Forms of Condition of Synesthesia and Its Features

Synesthesia

Abstract: Synesthesia is a condition in which a multiple sensory response is triggered by a particular stimulus, causing the synesthete, as people with the condition are called, to generate automatic and consistent associations. Although synesthesia was originally dismissed as an illegitimate condition, growing studies and the advancement of technology provide support that synesthesia is a real neurological phenomenon. Furthermore, there is evidence showing that synesthesia has a genetic factor, and is even a beneficial trait, causing increased memory capability and eliciting creativity in some individuals. The cases of Daniel Tammet, an autistic savant with synesthesia, and Stephen Schwartz, a musical composer, help support this idea.

“Taste the rainbow”, a popular slogan used by Skittles, is not as absurd as it may sound. In fact, those with synesthesia, appropriately named synesthetes, can have this ability. Synesthesia, as its Greek roots imply, refers to a “blending of the senses” (Palmeri, 2006). A stimulus perceived by one sense is simultaneously perceived by another sense. Even so, for a given stimulus, the number of senses prompted can be more than two. To illustrate, a visual image may elicit a sound; a sound may elicit a taste. Synesthetes experience this phenomenon consistently, meaning that the same image can repeatedly elicit a specific sound. Although this illustration suggests a form of learning known as acquired association, the multiple sensory responses to a stimulus characteristic of synesthesia are involuntary. Usually synesthetes don’t recognize that they have synesthesia, as they believe their perceptions are real. Thus, synesthetes are often diagnosed by other people, who notice the extraordinary perceptions of the synesthetes.

For one to be diagnosed with synesthesia, specific clinical criteria must be met. Synesthetes' experiences of intertwined senses have to be involuntary and consistent. Various tests have been developed for diagnosing different types of synesthesia. Many tests measure how quickly and accurately subjects respond to a stimulus based on what perceptions they claim to have. Diagnosing grapheme-color synesthesia, for example, subjects are first given a "color consistency test" where they can select a color that matches what they experience when different symbols, words, or numbers are presented. After this initial step, subjects would then take a timed test that shows a grapheme and a color, and they would each have to respond as quickly as possible to whether or not that color matches their original perceptions. People who genuinely experience synesthesia respond to this test far more accurately than non-synesthetes, showing that a synesthete's perception of words being associated with different colors are involuntary and automatic, and are not memorized or learned through habituation. (Cytowic, 1995).

Since the first recorded cases of synesthesia, people have questioned the validity of the experiences of synesthetes. Indeed, synesthetes often face stigma from skeptics, who say that synesthetes simply have “overactive imaginations” (Palmeri, 2006). Understandably, rejection of synesthesia as a legitimate condition stems from the scarcity of exhaustive research during those times. In 2001, hoping to provide evidence for the legitimacy of synesthesia, Ramachandran and Hubbard published a paper concluding that synesthesia, specifically grapheme-colour synesthesia, in which numbers and letters are also registered as colors, was more a sensory effect than a cognitive effect (Ramachandran, 2001). They were able to conclude this based on five experiments with two synesthetes. In one such test, subjects were prompted to find an embedded shape among the 5’s and 2’s (look at image #1). The mirror images of the 5’s and 2’s serve to distinguish synesthetes from non-synesthetes since synesthetes, with their distinct colors for distinct numbers, were expected to detect the image more readily than non-synesthetes. In fact, after measuring the performances of both synesthetes and non-synesthetes, the two scientists found that synesthetes “were significantly better at detecting the embedded shape than non-synaesthetic subjects” (Ramachandran, 2001). As further evidence, functional magnetic resonance imaging scans (fMRI scans) of synesthetes and of non-synesthetes show differences in brain activity between the two, indicating that synesthesia is in fact a real phenomenon (Kiederra, 2005).

There are several types of synesthesia, the most common and well-researched being grapheme-color synesthesia, where symbols or words are perceived to have different colors. Other less common types of synesthesia include lexical-gustatory synesthesia, where words and colors have certain tastes associated with them, and ordinal-linguistic synesthesia, where different personalities are given to sounds, symbols, tastes, or days of the week. With grapheme-color synesthesia, there has been strong evidence supporting a neural basis for the condition. By using neuroimaging methods, studies show that “cross-wiring” between certain regions of the brain cause an individual to experience different senses simultaneously. For a grapheme-color synesthete, a region of the visual cortex and the color area of the fusiform gyrus have interconnected neural connections, which results in experiencing colors when seeing to words and numbers (Ramachandran, 2001). An excess of neural pathways may be linked with with problems occurring during the neurodevelopmental process, specifically involving synaptic pruning. Synaptic pruning is a regulatory process that occurs from infancy until late childhood where extra or unnecessary neural connections between regions of the brain are removed. This process plays a key role in early brain development, and in synesthetes, it is hypothesized that pruning does not occur as normal or is inhibited, explaining the abundance of synaptic connections. (Synesthesia: Opening the Doors of Perception, 2010). Additionally, infants have many interconnected regions of the brain because of incomplete pruning. At this age, senses are not very distinguishable yet, when one is exposed to a stimulus, like a face, the visual cortex may be activated along with the auditory cortex or other parts of the brain, similarly to synesthesia. Another explanation for the cause of synesthesia describes a disinhibition of already existing neural connections. While the cross-activation hypothesis describes that a person with synesthesia has more connections than a non-synesthete, disinhibited feedback suggests that a synesthete doesn’t have abnormal pathways but only a disinhibition of normally existing ones (Grossenbacher and Lovelace, 2001). This explanation would also mean that a non-synesthete has neural connections linking multiple senses, but they are inhibited through processing areas of the brain.

Synesthesia has been studied and found to run in families, with 40% of synesthetes reporting a close relative with the condition (Brang and Ramachandran, 2001), yet the individual perceptions experienced vary from person to person. Additionally, even the types of synesthesia vary within a family of synesthetes, suggesting that the genetic basis for it is more general rather than specific (Ward, 2011). A genetic factor would also help support the cross activation hypothesis that synesthesia happens due to a disruption of regular synaptic pruning. A mutation of a gene that initiates this process may lead to these extra neural connections, but no clear gene has yet been found to explain this (Ward, 2011). Researchers studying the heritability of synesthesia have previously thought that it might have been an X-linked chromosomal disorder, due to a high prevalence of female synesthetes, however this bias is smaller than suggested earlier because of more recent testing done with larger populations of synesthetes (Simner et al., 2006). Other studies demonstrate that multiple genes on different chromosomes (2, 5, 6, and 12 instead of X) may be responsible for synesthesia (Asher et. al., 2009).

Genuine synesthesia is separate from learning associations, however there are different cases of acquired synesthesia. Losing a sense, such as sight, has resulted in reorganization of neural connections between different sensory areas. As a result of synaptic plasticity, connections are rewired and further strengthened. Acquired auditory-visual synesthesia is often reported in people who became blind, but only after an extended period of time (generally after one year). (Ward, 2011) Phantom limb syndrome is another form of acquired synesthesia that is a result of plasticity and reorganized pathways. Visual cues or other forms of sensory reception would cause a patient (with an amputation) to experience sensations of pain where the limb used to be (Ramachandran, 1996). Acquired synesthesia may occur following damage to the spinal cord or brain, but these cases are generally rare. In one case, an 45 year old man who suffered a posterolateral thalamic hemorrhage acquired both auditory and visual synesthesia (Schweizer et. al., 2013). He developed synesthesia nine months after suffering a hemorrhagic stroke in his brain’s thalamus. He acquired lexical-gustatory (color-to-taste) and ordinal linguistic (sound-to-emotion) synesthesia. For example, eating raspberries makes him think of a shade of blue, and seeing blue creates a taste of raspberries. High-pitched instruments, and especially the “James Bond” theme song, gives him a feeling of elation. He is also able to turn his synesthesia off and on. Finally, there are types of temporary synesthesia that can be induced by epileptic seizures and the use of antiserotonergic hallucinogens. However, these should be distinguished as pseudo-synesthetic experiences as there are fundamental differences between true synesthesia and these examples. This is mainly because these perceptions are “inconsistent, not automatic and highly dependent on the current state of the subject” (Sinke et. al., 2012).

Synesthesia is generally not considered to be a debilitating condition, so most who are affected to not require any treatment. In fact, grapheme-color synesthetes tend to perform better on memory recalling tasks because of already existing associations between symbols and colors (Carriere et. al., 2009). For example, many synesthetes explain that seeing colors with different letters helped them to memorize names, phone numbers, or dates. One extreme case relating to this is that of Daniel Tammet, a highly functioning autistic savant (a person with extreme prodigious capabilities) who accredits memorizing over 22,500 digits of Pi to his synesthesia. When he recites numbers, he visualizes landscapes with different colored shapes that represent each digit. Tammet also associates numbers with emotions and other symbols and therefore likely experiences multiple types of the condition. His autobiography, Born on a Blue Day, describes his life with Asperger’s and synesthesia. Synesthesia has been reported in autism spectrum disorders, and savantism in some cases may result from a combination of both conditions (Asher et. al., 2009).

Other forms of synesthesia, such as auditory-color types, are often regarded to be advantageous for creative activities, especially for musical composition. One case of this type of synesthesia is that of the famous composer Stephen Schwartz. He composed the Disney Pocahontas song, "Colors of the Wind," Pippin, Godspell, and Wicked. He also has three Grammy awards, three Academy awards, and six nominations. Schwartz has a rare form of synesthesia called chromesthesia, where those diagnosed sounds are associated with colors. In his case, flat notes tends to yield warmer colors and sharp notes yield colder colors. For example, D-flat major is a deep orange, whereas A is blueish-green. He may also have a form of color-to-emotion synesthesia because he describes using the wrong note in a song as being the wrong color and not feeling right. And although it often described as a positive, this would explain a possible negative effect of synesthesia: some perceptions may be distracting, or “not feel right” when they are different from what the synesthete experiences (Perry, 2013).