What Is Synesthesia

  • Anna Bourouliti PhD Neuroscience, D.U.Th., Democritus University of Thrace, Greece
  • Maariya Rachid Daud Doctor of Philosophy - PhD, Bioprocessing and Chemical Engineering, The University of Manchester

Synesthesia is a condition that challenges our common view of perception. In other words, synesthesia refers to expressing associations between irrelevant things. For instance, a person with synesthesia might have associated a certain number with a specific colour. For example, a person who has synesthesia may associate the number with the colour blue. Every time that person thinks of the number seven, it is blue. That being just one example of synesthesia, the following paragraphs explore the various synesthesia types and the main mechanisms responsible for their manifestation.


Synesthesia involves the association of two counterparts, the inductor and the concurrent. The inductor usually refers to a stimulus, either a sensory one (e.g., sound) or a cognitive one (e.g., a word). The concurrent is the sensation evoked by the inductor. Concurrent perception is not a conscious process but rather an automatic response to a certain stimulus. An important characteristic of the relationship between the inductor and the concurrent is that it is stable, meaning that a specific stimulus (e.g., the sound of the bell) always evokes a specific sensation (e.g., the colour red). Moreover, for a synesthete, the nature of the concurrent is usually consistent.1 However, consistency in cases of synesthesia is debatable due to testing procedures.2 Interestingly, the majority of synesthesia cases are characterized by a one-way link between stimuli and sensation so that the sensation itself is unable to evoke the perception of the stimulus. In any case, though, synesthesia does not interfere with normal perception but rather adds a “layer” to it. It should be noted that synesthetes are unaware of their perceptive capability being an unusual characteristic and may think of it as an integral part of human cognitive abilities.1

Genetic factors in synesthesia

The observation that family members exhibited synesthesia characteristics indicated that the condition is of a genetic origin. Analysis of genetic trees regarding the heritability of synesthesia led to the conclusion that synesthesia is a condition linked to the X chromosome, which may be inherited from the mother to her offspring or the father to his daughter but not his son. However, research has failed to support this notion. It appears that synesthesia might be polygenic, and one of the genes that are responsible for the condition is also responsible for brain development and plasticity.1

Neuronal mechanisms in synesthesia

It is estimated that synesthesia is the result of differences in neuronal communication compared to normal situations. Neuronal communication may be altered either by different wiring of neurons, especially during development, or in the absence of inhibitory mechanisms that would prevent making associations observed in synesthesia. Miscommunication between neurons may also be a result of abnormalities in neurotransmitter release or neurotransmitter uptake from neuronal cells. The latter is supported in cases where synesthesia is exhibited under medical treatment targeting psychiatric disorders. It should also be noted that brain regions known to be implicated in the perception of stimuli or sensations reported as pairs in synesthesia are structurally close to one another. This could be an indication of altered communication between neighbouring brain regions that have possibly occurred during development. Last but not least, it is likely that more than one of the above theories are true.3

Types of synesthesia

Synesthesia affects approximately 2 to 4% of humans4 and cases of synesthesia have been reported since more than two hundred years back.5 The first definition of synesthesia was worded in 1937 by Vernon and referred to imaginary modes evoked by stimuli.6

There are more than sixty different forms of synesthesia, including colour, sound, touch, and taste.1 Either the inductor or the concurrent can be any one of the following:

  •     Visual 
  •     Auditory 
  •     Tactile 
  •     Gustatory 
  •     Olfactory 
  •     Kinaesthetic 
  •     Thermal 
  •     Algesic6

Pairs of any different experiences from the list above can potentially make a synesthetic pair of a stimulus and its evoked sensation, or in other words, constitute a type of synesthesia.

Grapheme/Lexeme-visual synesthesia

In grapheme/lexeme-visual synesthesia, an achromatic symbol (e.g., letter, number - grapheme) or word (lexeme) is paired with a specific colour.1 For example, a synesthete may have associated the word dog with the colour green or the letter “O” with pink. Interestingly, the shape of the letter “O” resembles that of the number 0. However, even if they are seemingly identical, a synesthete may have associated each symbol with a different colour (e.g., pink and yellow, the letter and the number, respectively). In this case, even a typo might not be able to bypass the synesthete’s connection of letters/numbers and colours. More specifically, if the word “OD0R” contains the number symbol in the place of the third letter, the synesthete might still perceive it as pink because of the context. On the other hand, if the context does not indicate a specific letter or number matching the symbol, then in a row full of “Os” and zeros, a synesthete might be able to tell them apart much faster than another person because of their different colours. It should be noted that the aforementioned example is a hypothetical case based on knowledge about the nature of synesthesia. Real examples used for testing synesthesia cases are mentioned in detail in the “Synesthesia assessments” section.

Mirror-touch synesthesia

In mirror-touch synesthesia, synesthetes experience tactile sensations when they witness them performed on someone else. This may be due to the inability to perceive oneself by synesthetes.7 If a synesthete receives tactile stimuli by touch while witnessing another person being touched at the same time but at different places, then the synesthete will probably answer wrong when asked where they themselves were touched.1

Sound-vision synesthesia

An auditory stimulus may evoke tactile, gustatory, or visual sensations. For example, listening to certain music may evoke visual colours. A case report of a musician with brain injury states that synesthesia was acquired after the injury, rendering the musician capable of seeing the notation, naming the chords of music, and exhibiting increased creativity.8 Even before this case report was published, there was a hypothesis that synesthesia is linked with creativity. Indeed, it seems that synesthesia is often observed in artists, but further research is necessary in order to draw conclusions on the matter.4

Synesthesia assessments

The TOG assessment

The standard test used to determine if a person has synesthesia is known as the “test of genuineness” (TOG) assessment. The assessment is also referred to as the test-retest procedure. The approach requires a first test/report of the synesthete’s relevant ability, and then, in a year or more, a second test of the reported synesthetic characteristics is performed, but this time without warning. The TOG assessment relies on the consistency and stability of synesthesia, which requires that the evoked sensation will be the same in a year’s time. After performing the second test, a person is considered a synesthete if their responses are indeed in accordance with the first report. However, failing matching responses during the second test does not necessarily mean that the tested subject is not a synesthete but rather that if the case is one of synesthesia, then the manifestation of the condition is not stable or consistent with a specific type. One of the limitations of the TOG assessment that challenges its reliability is that it does not investigate the aetiology of synesthesia. For instance, a reasonable question regarding synesthesia, which the TOG assessment cannot address, is whether the associations between stimuli and sensations occur due to memories formed during a person’s lifetime. However, it appears that synesthesia often has an early onset during childhood. Consequently, it seems unlikely that synesthesia is of a mnemonic nature, at least in certain cases.2

The stroop assessment

The Stroop assessment is a task that measures the time it takes for a grapheme-visual synesthete to answer a certain question and compares it to a non-synesthete time to answer the same question. The question is in regard to the colour of a certain word or letter. If a person without synesthesia is presented with the letter “K” in a blue colour, then answering what colour the letter “K” is is pretty easy and consequently quick. However, it is not as easy for a synesthete to perceive an achromatic letter K in a different colour, let’s say green. The synesthete has to inhibit their seemingly natural response “green” and communicate the right color, which is blue. As a result, the time for inhibition of the wrong response would result in their total time to answer the question being slightly longer than that of people without synesthesia.2

The visual search assessments

There are two slightly different assessments that rely on visual sensations, and more specifically, colours, evoked by achromatic numbers. The tasks take advantage of the similar shape of the numbers 2 and 5 when written in certain fonts. The first task requires subjects to detect the space that contains many numbers two in a wider area of five. Synesthetes usually perform better than other people, meaning they are able to detect the numbers two faster. Interestingly though, their ability to perform faster is not due to detecting an area of a different color than the rest. Synesthetes have to actually pay attention to the numbers in order to distinguish twos from fives. The other visual task includes just one number two among many fives and calls for synesthetes to detect the number two in a single, direct glimpse of the whole set of fives. Again, synesthetes are able to perform better than non-synesthetes.2


Could a song have a flavour? Could a number be on a colour? For a synesthete, the answer is yes. Synesthesia refers to associations between otherwise irrelevant stimuli and/or sensations. A certain stimulus, like a song, which is an auditory stimulus, triggers a sensation, such as taste. Unfortunately, the genetic factors, as well as the neuronal mechanisms responsible for synesthesia, are still vague. Furthermore, assessments of synesthesia are few and have limitations, such as the type of synesthesia tested. Consequently, further research towards understanding synesthesia and the development of proper assessment tools is essential.


  1. Bragança GFF, Fonseca JGM, Caramelli P. Synesthesia and music perception. Dementia & neuropsychologia. 2015;9(1):16-23. https://pubmed.ncbi.nlm.nih.gov/29213937/
  2. Mylopoulos MI, Ro T. Synesthesia: a colourful word with a touching sound? Frontiers in psychology. 2013;4:763. https://pubmed.ncbi.nlm.nih.gov/24155733/
  3. Hubbard EM. Neurophysiology of synesthesia. Current psychiatry reports. 2007;9(3):193-9. https://pubmed.ncbi.nlm.nih.gov/17521514/
  4. Brang D, Ramachandran VS Survival of the synesthesia gene: why do people hear colours and taste words? PLoS biology. 2011;9(11):e1001205. https://pubmed.ncbi.nlm.nih.gov/22131906/ 
  5. Banissy MJ, Jonas C, Cohen Kadosh R. Synesthesia: an introduction. Frontiers in psychology. 2014;5:1414. https://pubmed.ncbi.nlm.nih.gov/25566110/
  6. Simpson L, Mc KP. Types of synaesthesia. The Journal of Mental Science. 1955;101(422):141-7. https://pubmed.ncbi.nlm.nih.gov/14368215/
  7. Ward J, Banissy MJ. Explaining mirror-touch synesthesia. Cognitive neuroscience. 2015;6(2-3):118-33. https://pubmed.ncbi.nlm.nih.gov/25893437/
  8. Abou-Khalil R, Acosta LMY. A case report of acquired synesthesia and heightened creativity in a musician after traumatic brain injury. Neurocase. 2023:1-4. https://pubmed.ncbi.nlm.nih.gov/37149895/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Anna Bourouliti

PhD Neuroscience, D.U.Th., Democritus University of Thrace, Greece

Back when I was a curious little creature, I was fascinated by science and aspired to work in a laboratory. To satisfy my thirst for scientific knowledge, I pursued studies in Molecular Biology and Genetics, entered the field of Health Sciences, and eventually fulfilled my dream of conducting research. This journey began with my undergraduate studies and progressed to obtaining an MSc and later, a PhD degree in Neurosciences. I have now left hands-on experiments behind, and I currently work as a medical writer, monitoring advancements in health sciences from a close perspective.

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