Synesthesia is a remarkable instance in which one property of a stimulus evokes a second experience not usually associated with the first. There are many variants of synesthesia, including synesthetic experiences of color, taste, touch, and sound. In the last few decades, research has highlighted the classification and mechanisms underlying this unique phenomenon. The following article explores the types of mechanisms that contribute to synesthetic experiences, as well as the broader cognitive and perceptual traits associated with synesthesia. Furthermore, it discusses the different types of synesthesia and their applications.
Synesthesia was first discovered by Georg Tobias Ludwig Sachs, a German physician who reported colored vowels as part of his Ph.D. dissertation. Since then, the father of psychophysics, Gustav Fechner, brought attention to this phenomenon through his empirical survey of colored letter photisms. Today, synesthesia is commonly known as an anomalous blending of the senses in which the stimulation of one modality simultaneously produces sensation in a different modality. Synesthetes can hear colors, feel sounds, and taste shapes. This peculiar phenomenon is as rare as it is strange: the estimated occurrence of synesthesia is rarer than one in every 20,000 people . More than being able to correlate seemingly unrelated things, many synesthetes report having an unusually good memory for things such as phone numbers, security codes, and polysyllabic anatomical terminology because digits, letters, and syllables take on such a unique panoply of colors.
Several researchers have distinguished synesthesia from other psychological phenomena using visual imagery and certain forms of imagistic memory. The primary characteristics used to diagnose synesthesia are automaticity, reliability, and consistency . Firstly, synesthetic associations are automatic in nature – they are produced outside the intentional control of the individual and cannot be directly inhibited . The automaticity of synesthesia helps to distinguish it from paradigm cases of mental imagery. While hearing a certain sound may lead one to imagine certain scenes or colors; for example, such visual imagery is typically under a significant degree of intentional control. An individual can typically start or stop imagining something at will. However, while synesthetic responses possess qualities similar to those exhibited by mental imagery, they are automatic and cannot be controlled thus distinguishing them from other psychological phenomena.
Secondly, synesthetes are typically able to reliably experience synesthetic responses when presented with triggering stimuli, meaning that whenever synesthetes come into perceptual contact with the triggering stimulus, their responses will be induced . These responses are not transient or inconsistent, though they can be when they are induced neuropharmacologically through the use of psychoactive, hallucinogenic substances . Furthermore, synesthetes often experience this phenomenon from early childhood, which helps distinguish the condition from ordinary associations grounded in memory. Many synesthetes often associate vivid mnemonic imagery with specific smells or sounds; for example, and these memories arise automatically.
Finally, although there may be slight variations among synesthetes, the majority of the synesthetic associations within an individual appear to be relatively consistent over time . For example, specific auditory tones may tend to elicit the same types of color-specific responses. To test this consistency, neuroscientists typically administer a series of tests over a period of time. The rationale is that if an individual has synesthesia, their consistency of responses in the retesting phase will be much higher compared to those without synesthesia . This was seen in Baron and Cohen’s study, which reported that 92.3% of reported synesthetes gave consistent responses when they were retested a year later without warning .
Types of Synesthesia
Out of all variants of synesthesia, grapheme-color synesthesia is the most prevalent. Grapheme-color synesthetes tend to associate written letters and numerals with colors. For example, this type of synesthete may always associate the number seven with the color red. While there are some similarities among the associations made by all grapheme-color synesthetes, it is highly unlikely that two people with this form of synesthesia will experience the same associations. In this form of synesthesia, cross-wiring occurs between the brain’s color and number area, which are both located in the fusiform gyrus . The neural substrate of synesthesia has been thoroughly studied in grapheme-color synesthesia using both psychophysical tests and functional imaging: several groups have demonstrated that simple achromatic graphemes activate both grapheme regions, as well as color area V4, a region of the visual cortex that shows a stronger response to colors than to grayscale stimuli, in the brains of synesthetes, which is consistent with the view that synesthetic colors are sensory in nature . Predicting this finding of “cross-activation” between grapheme and color regions, researchers Ramachandran and Hubbard proposed that synesthesia results from an excess of neural connections between associated modalities, possibly due to decreased neural pruning between adjacent regions that are interconnected in the fetus . Consistent with this suggestion, a number of studies have demonstrated anatomical differences in the inferior temporal lobe near regions related to grapheme and color processing in synesthetes, including increased fractional anisotropy and gray matter volume. This can sometimes make grapheme-color synesthesia an aid to the synesthete’s memory and learning.
The second variant of synesthesia is ordinal linguistic personification, a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities or genders. These synesthetes tend to perceive ordered sequences such as numbers, letters, days, and months with inherently distinct personality traits or gender. As with many other types of synesthesia, the associations are roughly constant for the synesthete but are not necessarily the same among synesthetes who exhibit the condition . While ordinal linguistic personification has been noted to co-occur with other forms of synesthesia, it has been shown to co-occur with greater frequency in synesthetes who display grapheme-color synesthesia . An example of this type of synesthesia is when an individual perceives the letter “A” as female and the letter “T” as male, or the numeral “5” as sneaky.
The third variant of synesthesia is chromesthesia, also known as sound-to-color synesthesia. In chromesthesia, sounds heard by the synesthete are associated or perceived as particular colors. While the synesthete hears the sound just like everyone else, they simultaneously and naturally experience a color that remains more or less constant with that specific sound. Similar to the pairs found in other types of synesthesia, the pairings in chromesthesia are consistent for each synesthete but are not automatically the same for another synesthete with chromesthesia. What researchers have discovered, however, is that synesthetes tend to associate high pitched sounds with light, bright colors . Low pitched sounds, on the other hand, are more likely to be matched to darker colors. There is also some evidence to suggest this phenomenon can be found (to a lesser extent) among non-synesthetes. Subgroups among chromesthesia synesthetes include those for whom the condition is triggered by all types of sounds and those for whom musical notes only generate their sound-to-color associations. Furthermore, some of these synesthetes report that the colors are only evoked by people's voices. A note-worthy group related to chromesthesia synesthetes are people who have color-to-sound synesthesia when colors are perceived as sounds. In instances where an individual is identified as having both sound-to-color and color to sound synesthesia, the pairings often remain the same in both directions.
Another variant of this phenomenon is seen in spatial sequence synesthesia, or visuo-spatial synesthesia, a common condition in which ordinal sequences such as months, numbers or the letters of the alphabet are perceived to occupy spatial locations in the mind's eye or peripersonal or extrapersonal space . Basically, a synesthete who experiences this phenomenon may see the spatial arrangement with their “mind's eye" or in the actual space around them. For example, these synesthetes may think of the letter “A” as farther away in space than the letters “B” or “C.” They might also perceive the time on a clock as located in the specific points in the space around them. Furthermore, these individuals have been shown to have a superior ability to recall events, as well as the ability to see into the past or travel through time . There have also been studies that indicate a heightened ability among persons with spatial sequence synesthesia to recall the events which have occurred in their own lives. This condition, known as hyperthymesia, is often linked to autism and savant syndrome .
The fifth variant of synesthesia is mirror-touch synesthesia, a rare condition that causes individuals to experience a similar sensation in the same part or opposite part of the body that another person feels. The mirror-touch sensation can be activated by both real-life and in-person situations, as well as by watching someone on a screen. While most other forms of synesthesia are developmental, mirror-touch synesthesia can also be acquired after sensory loss following an amputation . This was seen in Dr. Ramachandran’s mirror box experiment, where amputees reported that the pain in their “phantom” limb was eased when they simply viewed their arm moving in the mirror box. A few other examples include a mother sensing a stethoscope on her back when the doctor places a stethoscope on her child’s back or feeling the pain of an athlete simply by watching them get injured on television. Understandably, some studies of mirror-touch synesthesia have linked it to synesthetes having a heightened sense of empathy for the pain being suffered by others . The condition has proven to be helpful to synesthetes working in certain professions, such as doctors and massage therapists.
The rarest out of all the variants is auditory-tactile synesthesia, also known as hearing-touch synesthesia. In this instance, sounds heard by the synesthete produces a tactile sensation on certain areas inside and outside of the body, which is caused by the cross-wiring of the auditory and somatosensory cortices in the brain . There are a plethora of sensations that can be experienced, which tend to vary among synesthetes . For example, a sound that feels like a tingling sensation to one synesthete may be perceived by another as the pressure we normally associate with something pressing against our skin. Since the primary stimulus that produces a particular sensation varies among auditory-tactile synesthetes, the same trigger may produce a variety of responses among different individuals. There are occasions when synesthetes describe the perceived tactile sensation as pleasant but on other occasions, sounds can produce sensations that are distracting, uncomfortable or outright painful.
Another rare form of synesthesia is number form synesthesia, where the synesthete involuntarily sees a mental map of any group of numbers they think about. This phenomenon is caused when the synesthete's brain takes place between close-lying regions within the parietal lobe which governs spatial and numerical cognition . The arrangements in number form synesthesia differ from the conventional number line we are all taught in school and could be idiosyncratic and unchanged throughout the synesthete's lifetime . The synesthete may have a particular form for months of the year and another for dates. Furthermore, the number of forms that are perceived are not related to colors and do not necessarily possess any form of symmetry. The number of forms may be complex or simple and may involve curved or straight lines or a combination of both. Because of this, number form synesthetes process information more efficiently when it is presented in a manner that matches their brain’s number form and may have difficulty learning conventional mathematics .
The eighth variant is lexical-gustatory synesthesia, a rare form of synesthesia in which spoken and written language causes individuals to experience an automatic and highly consistent taste/smell. Like other forms of synesthesia, lexical-gustatory synesthesia is also known to develop during early childhood . Synesthetes with lexical-gustatory synesthesia tend to make linkages between word and foods they were exposed to as children. The experience of the evoked sensations is often linked to the sound of the word, but some researchers think it may be connected to the meaning of the word, as well . However, not all words and sounds induce the same intensity or complexity of taste in the synesthete. There are certain words and sounds which produce no taste response at all.
Finally, the last and newest form of synesthesia is misophonia, a phenomenon in which certain sounds trigger emotional or physiological responses that some might perceive as unreasonable given the circumstance. Apart from being very rare, misophonia is also one of the most troubling examples of synesthesia. It is a condition in which the synesthete experiences negative and aggressive emotional reactions to sound . It has been described as a hatred of sound, with the most common triggers being human-related sounds, such as breathing, chewing and lip-licking . The sounds that trigger this phenomenon are unavoidable, habitual sounds, which makes it even harder for synesthetes to live around people. It is crucial that synesthetes with misophonia receive cognitive behavioral therapy with the inclusion of background noise to mask the ‘annoying’ sounds.
Heredity in Synesthesia
While a proven genetic basis for synesthesia remains elusive, the phenomenon tends to run in families, as more than 40% of synesthetes report a first-degree relative with the condition . Pedigree analyses have shown high transmissibility from parent to offspring, and each type of synesthesia is caused by a unique gene or set of genes . In other words, even though genetic undertones impose a predisposition to synesthesia, it does not determine how the gene is expressed or what type of synesthesia the individual will experience. Indeed, individuals with one type of synesthesia are much more likely to have another as well, an observation that was made by researchers Ramachandran and Hubbard as support for the idea that the defective pruning gene or genes confer a general propensity to linking unrelated sensations or even concepts. Furthermore, while individual synesthetes often display multiple forms of the phenomenon, large-scale factor analyses suggest that some variants co-occur with greater frequency within a single individual, suggesting that some forms are more highly related, which is suggestive of a common origin .
However, non-clustering forms still co-occur with greater frequency than predicted by prevalence rates in the general population, significantly impeding theories of single genetic markers and the notion of independence among different forms of the condition. Further research in examining the prevalence of synesthesia found a significant gender gap with a 6:1 ratio of female synesthetes to males, leading to the suggestion that synesthesia is an X-linked condition . However, prevalence studies using random sampling have shown an even distribution of synesthesia among the genders, suggesting that the discrepancy was based on methodological flaws and self-report biases in earlier studies. In light of these conflicting results, research into the genetics underlying synesthesia remains in a nascent stage and will require much larger sample sizes and variants of the condition to understand the underlying factors for transmission.
In recent years, research has shown synesthesia to be a highly heritable phenomenon with numerous benefits to cognitive processing and memory. Synesthesia seems to occupy that mysterious boundary zone between elementary sensations on the one hand and higher-level abstractions on the other, intriguing scientists and providing an experimental lever for investigating high-level mental processes. To appropriately understand this condition with relation to normal cognition, future researchers must focus on both technically and intellectually diverse contributions from all areas of biology. Experiencing this unique phenomenon can give individuals a notable approach to life, which can work as both a blessing and a curse. Synesthetes often possess enhanced memory and creativity, making them excellent artists, musicians, and writers. However, they can also struggle with traditional schooling or experience ridicule from their family or peers. Thus, it is crucial for us as a society to not only encourage further research on the topic but also to promote awareness and acceptance of synesthesia.
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