What is synesthesia

Synesthesia is an extraordinary way of perceiving the world, involving experiences of connections between seemingly unrelated sensations, images or thoughts ¹. The word “synesthesia” comes directly from the Greek (syn-) “union”, and (a­sthesis) “sensation”, thus meaning something akin to “a union of the senses”. Synesthesia is a perceptual condition of mixed sensations: a stimulus in one sensory modality (e.g., hearing) involuntarily elicits a sensation/experience in another modality (e.g. vision). Likewise, perception of a form (e.g., a letter) may induce an unusual perception in the same modality (e.g. a color) ². People with synesthesia experience unusual sensations (e.g., of color, of taste) when doing things that wouldn’t usually trigger those sensations for non-synesthetic people. In some cases, this means that a synesthete might experience a sensation in one of the 5 senses (hearing, vision, taste, touch, smell) that is triggered by a different sense (e.g., sounds trigger tastes, smells trigger colors etc.). For example, seeing the number 7 may lead to an experience of navy blue, hearing the word “bliss” may flood the mouth with the flavor of bread soaked in tomato soup and hearing the key of C minor may elicit a bright purple spiral radiating from the center of the visual field. The trigger of the experience is called “the inducer,” whereas the additional experience to which it gives rise is called “the concurrent” ³. In visual synesthesia, the concurrent may be projected out into space and experienced as located in the visual scene outside the subject’s mind, or it may be merely imagistically or semantically associated with the inducer ⁴.

Synesthetic associations are supplementary, arbitrary, idiosyncratic, and usually have an involuntary feel (they are not evoked at will and are not chosen) ⁵. They usually cannot be suppressed, so they are also often described as automatic (in a weak sense) or inevitable ⁶. Examples of types of synesthesia that fulfill this definition include not only canonical varieties like colored hearing, but also colored letters and numbers (graphemes), sequence-space (including number-lines) ⁷, and personifications of numbers and letters ⁸. These three most frequent types are found in 1–10% of the population ⁹.

The three major characteristics of synesthesia are: synesthesia experiences are often reflect common associations (e.g., seeing light colours when hearing a high pitched tone), associations are persistent, and for visuospatial forms synesthetes, associations have a definite position in space ¹⁰. Various theories have been proposed to explain why synesthetes are able to see what they see or hear they hear. The most prevalent has been the activation of neural areas associated with the specialized sensations. For example, for grapheme-color synesthesia, it has been shown that there is an activation of the V4 color area along with the grapheme area (right inferior temporal gyrus) in the synesthete brain ¹¹. Till now, cases of synesthesia are still fairly rare, and often come in many different variants combining various modalities. It is an interesting paradigm to further understand how different modalities in the brain can interact via neural connections, and how your behavior and perception can be changed due to your brains. Furthermore, increase in research in synesthesia due to the advent of technology can further the usage of concepts of synesthesia in sensory prosthetics. This is called intentional synesthesia technology, and it aims to apply synesthetic concepts to replace an impaired sense with a functional one to better the quality of life for those that are missing one sense or another.

Is synesthesia real?

The present consensus is “that synesthesia is neither imagination nor is it metaphorical thinking, instead it has a neural basis” ¹². The subjective experience of synesthetic colors could activate brain regions normally responsible for the perception of real colors differently from when non-synesthetes just imagine or remember colors. Such neural activation could be due to extra neuronal connections, in particular from neighboring regions (cross-activation theory), or from a difference in neuronal transmission (disinhibited feedback theory). Several studies also suggested more distributed neuronal differences resulting in different brain network properties in synesthetes. In his thorough review on synesthesia, Ward ⁶ states that “candidate neural mechanisms of synesthesia all have something in common insofar as they are believed to reflect differences in connectivity relative to the neurotypical brain. Moreover, these differences are typically assumed to lie at the cortical level, reflecting the complex nature of the inducer/concurrent” ⁶. This echoes a former statement by Blake et al. ¹³ on grapheme-color synesthesia: “Virtually all neural models of synesthesia propose that it arises from an atypical pattern of connectivity between form processing and color processing centers of the brain,” with the difference in connectivity being structural or functional. In either case, the experience of synesthesia should induce neural activity in the regions normally involved in the experience of the concurrent (the induced association). That is, color centers should be activated when grapheme-color synesthetes read achromatic letters or words (the inducers), imagine them or listen to them.

How many different types of synesthesia are there?

It is traditionally believed that there are five senses: vision, hearing, touch, taste and smell. If one were to calculate the number of permutations (vision-hearing, vision-touch, hearing-touch, etc.) then one would get 20 potential types of synesthesia. However, it is misleading to describe synesthesia in terms of pairwise associations between these senses. The basic senses can be broken down in to yet more dimensions. For instance, in the ‘vision’ domain some synaesthetes might experience color, whereas others might experience shapes, and yet others might experience movement. Another difficulty with this formulation is that synaesthesia can be triggered by things which are not strictly sensory, such as numbers, letters, words, and names. Thus, the number of potentially different forms of synesthesia is likely to be very big indeed ¹⁴.

What is the most common type of synesthesia?

Recent University of Sussex research suggests that experiencing days and months in color is probably the most common type of synesthesia that involves one of the traditional five senses. Experiencing days, months, number and the alphabet in a spatial form (e.g. in lines, circles, spirals) is even more common (maybe 20 percent of the population). Most research on synesthesia has concentrated on colours for letters and numbers. These synesthetes typically experience colors whenever they see letters, hear letter names and even think about letters (e.g. A=red, B=blue, C=yellow). Words also tend to have colors that are derived from one or more of their constituent letters. Most people have a ‘feeling’ of colour but they don’t actually see the ink or paper as being colored (although some synesthetes do). Synesthetic colors generally co-exist with ‘real’ colors and they do not over ride each other. Color is by far the most common experience of synesthetes, although it can be triggered by a whole host of things and not just letters.

Is it possible for a synesthete to have more than one type of synesthesia?

Yes, around half of all synesthetes have more than one type of synesthesia, such as taste, sound and touch all producing colours. The fact that different types of synesthesia co-occur in the same individuals suggests that there is a common cause for all types of synesthesia, rather than every type of synesthesia having its own independent cause.

Can synesthesia occur for taste and smell?

Yes, and this is something that scientists are actively researching in their labs.

Does synesthesia run in families?

Yes. Around 40 percent of synesthetes know of at least one other family member who has it. The relative is not always immediate family. For example, you could have a synesthetic cousin, but have non-synesthetic brothers and sisters. It can also skip generations. For example, it could be that no living relative has it but that, say, your great-grandparents did have it. Synesthetes often choose not to discuss their synesthesia with their family in case they are dismissive of it. It is possible for a synesthete to ‘come out’ late in life, only to find that their brother or sister or parent has been privately experiencing something very similar. The type of synesthesia isn’t strongly heritable. So someone with taste synesthesia often has a family member with the, more common, types of color synesthesia.

Types of synesthesia

So far, over 80 types of synesthetic phenomena have been described ¹⁵, ¹⁶. The apparently most common form (with a 64.4% prevalence among synesthetes) is grapheme–color synesthesia, in which achromatic letters or digits automatically trigger an idiosyncratic color perceptual experience (e.g., the letter ‘m’ induces blue color percepts) ¹⁷. The second most prevalent form is time unit (e.g., Monday, January)–color synesthesia (22.4%), followed by musical sound–color synesthesia (18.50%) ¹⁸. Inducers and concurrents also include smells, tastes, temperatures, personalities and emotions, and can be multiple during a single synesthetic experience. Thus, percepts induced by grapheme–color synesthesia are occasionally accompanied by shape, texture, movement features, and even nonvisual percepts such as smells and tastes, particularly emotions ¹⁹. Synesthetic colors generated in grapheme–color synesthesia are determined by systematic rules rather than randomly occurring, and based on the psycholinguistic mechanisms of language processing. The same occurs with both Latin characters and Chinese ideograms ²⁰.

An additional type of colored synesthetic experience was recently described and termed ‘swimming-style color synesthesia’. It is characterized by the generation of specific colored percepts upon conceptual representation of swimming in a particular style (i.e., breast, backstroke, crawl, and butterfly) ²¹. This phenomenon could be triggered by either presenting a picture of a swimming individual or asking the tested individual to think about a given swimming style. It was speculated that this synesthetic type was caused by overactivity in the mirror neuron system responding to the specific representation ²².

Synesthetic experiences are labeled ‘lower’ when triggered by elementary perceptual processes (e.g., texture) or ‘higher’ when involving a higher cognitive process (e.g., semantic, computing) ²³. Synesthetes who experience the atypical percepts in an internal space (‘in the mind’s eye’, as they sometimes describe it) have been categorized into ‘associators’, whereas those for whom the additional, atypical percept appears to be ‘out there’, overlaying the actual, external surrounding, are designated as ‘projectors’ ²⁴.

According to Grossenbacher and Lovelace ³, there are three different types of synesthesia:

1. Developmental, or genuine, synesthesia
2. Acquired synesthesia
3. Drug-induced synesthesia
4. Learned synesthesia

Developmental synesthesia

Developmental synesthesia is the most common type and is a form of the condition that has persisted since birth or early childhood and that remains relatively stable and systematic over time: each inducer has a highly specific concurrent. ²⁵. Developmental synesthesia also tends to run in families ²⁶. For the most common forms of developmental synesthesia, the Synesthesia Battery, an automated online test (https://www.synesthete.org/pretest.php), allows for rigorous testing of both the tightness of the synesthetic association and its stability and systematicity over time ²⁷.

Developmental synesthesia appears to be the most frequent type of this condition, with a 4.4% estimated prevalence rate ²⁸. It can run in families and demonstrate Mendelian transmission ²⁹. Different forms of synesthesia can be observed in the same person or in the same family ³⁰. The condition is occasionally associated with autism spectrum disorders, like Asperger syndrome ³¹.

The following criteria have been proposed to help establishing a diagnosis of developmental synesthesia: induced percepts should be elicited by a specific stimulus, they should be automatically generated, and typically have percept-like qualities ³². Usually, pairings of inducers and concurrents are specific (i.e., a particular stimulus consistently triggers the same synesthetic percept). They tend to be stable over time in a given individual, although this has recently been challenged by the finding that synesthetic ability can disappear over time ³³.

Acquired synesthesia

Acquired synesthesia is a form of the condition that emerges after brain injury or disease or artificial technologies like sensory substitution ³⁴. Acquired synesthesia has been reported following stroke ³⁵, traumatic brain injury ³⁶, neuropathology involving the optic nerve and/or chiasm ³⁷, seizures ³⁸, migraine ³⁹, post-hypnotic suggestion ⁴⁰ and sensory substitution ³⁴. One case of injury induced synesthesia is reported by Ro, et al. ⁴¹ in which a woman with lesions in the ventrolateral nucleus of the thalamus developed auditory-tactile synesthesia after the lesions, such that hearing a tone produced a touch sensation. It was suggested that the synesthesia may have developed due to neuroplasticity following the lesion resulting in rewiring of the brain, and subsequently greater connections between the auditory area and the sommatosensory area. This rewiring may have occurred due to the fact that pathways travelling through the ventrolateral nucleus no longer transmit signals, leaving the brain to have to find new ways to transmit information. This explanation of acquired synesthesia would support the cross-activation theory, as new connections being formed between sensory modalities are responsible for causing the synesthetic sensations. The disinhibited feedback model would not be able to explain synesthesia due to this plasticity. However, several differences between injury induced synesthesia and regular synesthesia have also been pointed out, such that injury induced synesthesia seems not to be automatic, as patient who acquired synesthesia after injury only appeared to experience the sensations approximately 80% of the time, whereas regular synesthetes who were born with synesthesia experience these associations all the time ⁴².

Audio-visual synesthesia has been reported to be the most common acquired type ³⁷. Like developmental synesthesia, acquired synesthesia tends to be automatic and systematic over time, though in some cases it only persists for a limited time period ³⁷. Experientally, acquired synesthesia may be indistinguishable from developmental synesthesia, though it is sometimes less inducer-specific, that is, the same concurrent may have several different inducers ⁴³. Cases have also been reported in which the acquired experience is simpler than the developmental counterpart, often similar to light flashes (phosphenes) or pure color experiences ³⁷.

Drug-induced synesthesia

Drug-induced synesthesia is a blending of sensory or cognitive streams that is experienced during exposure to a hallucinogen (psilocybin, methamphetamine, lysergic acid diethylamide (LSD), mescaline, peyote) ⁴⁴. Unlike the developmental and acquired varieties, the drug-induced form is usually limited to the most intense phases of intoxication, though in some cases it continues for weeks or months after exposure to the drug ⁴⁵. Experientally, drug-induced synesthesia can vary from simple color experiences to complex, surrealistic landscapes consisting of, for example, oddly shaped objects with multicolored contours or images with ornamental or kaleidoscopic compositions ⁴⁶. These cases of drug induced synesthesia would lend support to the disinhibited feedback theory, as it suggests a regular individual with no anatomically distinct brain is able to acquire synesthesia by simply taking a drug. The cross-activation theory would not predict drug induced synesthesia to be possible, as these individuals do not have the increase in connection between modalities suggested to be responsible for causing synesthesia. However, scientists have called into question whether drug induced synesthesia is the same phenomenon as actual synesthesia. For example, one criteria for actual synesthesia is the fact that associations must remain the same over time (e.g., an A will always be red, and will not turn into a blue A later on in life). However, drug induced synesthesia appears to produce different associations each time, thereby calling into question how genuine it is and whether these cases are valid ⁴².

Learned synesthesia

One study published recently have attempted to observe if normal individuals can learn synesthesia by reading a book filled with colored letters (e.g., apple) over a two to four week period ⁴⁷. Participants read over 100,000 words during this time span. They measured participants accuracy and reaction time to a modified Stroop task, such that participants were required to name the colour of the letter that is presented. In prior experiments, it has been shown that synesthetes are more accurate and faster to respond when the colour of the letter was congruent to their perceptual experience (e.g., a red A) versus when it is incongruent with their experience (e.g., a blue A) ⁴⁷.

Participants performed this modified Stroop task before and after reading the coloured books. Prior to reading, there were no observable differences between congruent letters and incongruent letters in terms of response time and accuracy. However after reading, participants were both faster and more accurate when the letters were congruent with the letter colours in the book, and slower and less accurate when the letters were incongruent with the letter colors in the book. If participants were not experiencing colours after reading, there should be no difference between congruent and incongruent colours as colours should not be associated with a particular letter. Participants’ subjective experiences were also recorded, and it was found those who answered highly on whether or not they experienced colour as they saw the letters also showed the strongest Stroop effect. However it is not certain whether participants are performing better on congruent Stroop tasks because they are experiencing synesthesia, or whether it is merely a strong learned association due to prolonged reading of the books ⁴⁸.

What causes synesthesia

Synesthia is a specialized phenomenon in which activation in one sensory modality leads to the activation of another sensory modality. Multiple combinations of different modalities lead to a diverse range of synesthesic possibilities. The first reported case of synesthesia was over 200 years ago, but the mechanisms as to how it occurs is still under debate today. Several opposing theories have been proposed to try and explain the mechanisms as to how these individuals can experience these unique sensations. The two most prominent theories are the cross activation theory ⁴⁹ and the disinhibited feedback theory ⁵⁰ and each have different pieces of evidence supporting their claims. Recently due to the advent of new technology, researchers have also been able to discover genes that are thought to contribute to how synesthesia occurs ⁵¹. However for those that are not fortunate enough to have the genes involved with synesthesia, acquired forms of synesthesia such as learned synesthesia as well as other forms of drug induced and injury induced synesthesia have been reported, which further adds to the debate over the mechanisms of synesthesia.

Assumptions have been made on the mechanisms underlying synesthesia, including hyperconnectivity between cortical areas ⁵², reduced level of feed-back from inhibitory cerebral structures ⁵³, learned association in early life ⁵⁴, and a normal perceptual mechanism incompletely suppressed in synesthetes ³¹. Neurocognitive models have been elaborated ⁵⁵.

In recent years, brain-imaging studies brought further evidence that synesthetes connect more inside and between sensory regions and less with remote areas, especially the frontal cortex. These individuals exhibit increased intranetwork connectivity in medial visual, auditory and intraparietal networks, and internetworks connectivity between the medial and lateral visual networks, the right frontoparietal network and between the lateral visual and auditory networks. In contrast, nonsynesthetes have more intranetwork connections within frontoparietal network ⁵⁶. When presented with inducers, synesthetes exhibit a clustering pattern of activated brain areas uniting more visual regions, whereas nonsynesthetes activate particularly frontal and parietal regions ⁵⁷.

Involvement of the bottom-up and top-down mechanisms has further been considered ⁵⁸. The bottom-up model stipulates that the concurrent representation is prompted by the inducer representation via over represented and overactive horizontal connections, whereas the top-down model proposes that the inducer stimulates the concurrent percept via an input from a convergent, higher order integrator ⁵³.

Using dynamic causal modeling, Van Leeuwen et al. ⁵⁹ have shown that projectors exhibited effective connectivity patterns involving a bottom-up mechanism, whereas associators used a top-down mechanism. However, a recent electroencephalographic (EEG) study found evidence favoring the top-down disinhibited feedback model as the core of the synesthetic phenomenon ⁶⁰. Reduction of long-range couplings in the theta frequency band could facilitate the top-down feedback. An fMRI study demonstrated that, in contrast with projectors, associators’ synesthetic experience was related to areas linked to memory processes, including hippocampus and parahippocampal gyrus ⁶¹.

It was suggested that congenital alterations in thalamic circuitry might be responsible for atypical cortical morphology and connections, found with different synesthetic phenotypes ⁶². Cytoarchitectonic maturation of the primary sensory areas and the development of their specific connections are highly dependent on the thalamic input ⁶³. Enucleation in prenatal macaque drastically alters the equivalents of V1 and V2 visual cortices, and induces rich noncanonical connections with somatosensory, auditory, and frontal areas ⁶⁴, resembling transient fetal connections ⁶⁵. Thus, the visual cortex ends up treating other types of information. Likewise, congenitally blind humans exhibit occipital cortex activation following auditory or somatosensory stimulation ⁶⁶. It is therefore conceivable that in developmental synesthesia, congenitally anomalous sensory input leads to abnormal synaptic pruning and differences in brain connectivity. In grapheme–color synesthetes, low white matter densities in pulvinar, medial and lateral ventral posterior nuclei, and low fractional anisotropy in medial dorsal and ventral anterior nuclei suggest a constitutional disconnection and hypoconnection between thalamus and cerebral cortex ⁶⁷. The concerned white matter tracts project to the left prefrontal cortex and bilateral temporal and posterior parietal cortex, regions that in synesthetes are distinct both in structure and function. Secondary synesthesia after thalamic stroke also support the involvement of thalamic output in synesthetic phenomena ⁶⁸.

Synesthesia symptoms

Grapheme-Color Synesthesia

This is one of the most common types of synesthesia. A person who experiences this may associate/see individual letters or numbers with a specific color. Usually, two people won’t report the same color for letters and numbers. However, studies have shown that many synesthetes will see some letters the same way (for example, ‘A‘ is likely to be red).

Sound-to-Color Synesthesia

When sound triggers the visualization of colored, generic shapes, sound-to-color synesthesia is at play. For certain people, the stimuli are limited, and only a few types of sounds will trigger a perception. However, there are cases wherein many different sounds trigger color visualizations. Usually, the perceived colors appear in generic shapes – squares, circles, etc.

Number-Form Synesthesia

A number form is essentially a mental map that consists of numbers. When a person with number-form synesthesia thinks about numbers, a number map is involuntarily visualized. It is sometimes suggested that the number forms are a product of “cross-activation” between regions in the parietal lobe – a part of the brain that is involved in numerical and spatial cognition.

Personification

This type is known as ordinal-linguistic personification (OLP). An individual who experiences this will associate ordered sequences with various personalities. Ordered sequences may include numbers, letters, months and so on. For example, a person with ordinal-linguistic personification may look at the letter ‘A’ and think in his mind that ‘A’ is a rude letter.

In addition to thinking that certain ordinal sequences have a personality, a synesthete may also imbue a personality within an object. While occurrences have been reported early on, this is a type of synesthesia that has only gained attention from researchers in recent years.

Lexical-Gustatory Synesthesia

Lexical-gustatory synesthesia is one of the more rare synesthesia types. Synesthetes who experience this kind of synesthesia evoke different kinds of tastes when they hear certain words or phonemes. According to research, associations between the words and what a synesthete is able taste are constrained by tastes he or she has experienced early in life. So, if an individual hasn’t had mashed potatoes or bacon, they won’t be tasting those flavors as a result of this variation.

Synesthesia test

Not sure if you’re a synesthete? Test yourself for synesthesia with this quick preview online questionnaire here (https://www.synesthete.org/pretest.php)

If you think you are a synesthete, register here (https://sussex.onlinesurveys.ac.uk/syn) to begin with the full questionnaire.

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