Synesthesia

What it is

A way of perceiving in which one thing, such as a letter, a word, or a sound, automatically triggers an extra experience, such as a color, a taste, or a shape. Cleveland Clinic says it is not a disease or a medical condition.

Common experiences

What people experience depends on the type. Examples include colors for letters or numbers, colors for days of the week, colors for sounds, and feeling in your own body what you see happen to someone else (Cleveland Clinic). Cleveland Clinic also says many people find synesthesia useful for learning and remembering information.

What helps

Cleveland Clinic says developmental and acquired synesthesia do not need treatment. Because many people find it useful for learning and remembering, letting a person use their own associations, such as color-coded notes, is a reasonable support. Sensory sensitivity is also more common in people with synesthesia (Ward et al., 2017), so see the Sensory processing differences guide.

Across life

Colored days was the most common form in one prevalence study, and about 1 percent of people had grapheme-color synesthesia, where letters or numbers have colors (Simner et al., 2006). In children, the colors for letters and digits become more fixed with age. In one study, 34 percent were fixed at ages 6 to 7 and 71 percent at ages 10 to 11 (Simner & Bain, 2013).

Evaluation and treatment

Cleveland Clinic says synesthesia is not a disease or a medical condition. Researchers use standardized tools such as the Synesthesia Battery to study it (Eagleman et al., 2007). If cross-sensory experiences start suddenly in someone who never had them, see a doctor, because synesthesia can be acquired through brain damage and can be a symptom of certain brain-related conditions (Cleveland Clinic).

Myth vs fact

Myth: people with synesthesia are imagining it or faking it. Fact: researchers developed tests that demonstrate the reality of the condition, its automatic and involuntary nature, and its measurable perceptual consequences (Eagleman et al., 2007), and one prevalence study used objective tests to establish that reports were genuine (Simner et al., 2006).

Myth: it is a disease. Fact: Cleveland Clinic says synesthesia is not a disease or a medical condition.

Full chapter

Definitions and terminology

Synesthesia (spelled synaesthesia in the UK and in the titles of many journal articles) is a way of perceiving in which one thing automatically triggers an extra experience. The neuroscientist David Eagleman defines it as 'unusual perceptual or cognitive pairings', for example words that trigger tastes, letters that trigger the sensation of texture, or music that induces the sensation of shapes and color (Eagleman, 2012). His research page describes it as a perceptual condition in which information between the senses is blended (Eagleman research page). A critical review of the brain-imaging research offers a different definition: a 'subjective phenomenon of additional experiences that sometimes, but not always, involves mixing sensory modalities' (Hupé & Dojat, 2015). Researchers do not all define it the same way. Simner (2012) argued that a definition has to be broad enough to cover 61 different variants, and presented evidence that might argue against three claims often made about it: that it is a 'merging of the senses', that it gives rise to consistent associations over time, and that its associations are spatially extended. Ward (2013) notes that synesthesia has been known about for 200 years. In this guide, the trigger is what sets the experience off (for example a letter), and the extra experience is what the person senses in response (for example a color).

Types of synesthesia

Cleveland Clinic describes several types. In grapheme-color synesthesia, letters, numbers, and symbols are linked with specific colors. In day-color synesthesia, colors are linked with days of the week, which Cleveland Clinic calls one of the more common forms. In sound-color synesthesia, hearing certain sounds brings colors. In auditory-tactile synesthesia, sounds cause touch-based sensations such as temperature changes, pressure, or pain. In hearing-motion synesthesia, sounds are linked with seeing things move, such as a 'whoosh'. In mirror-touch synesthesia, seeing something happen to someone else produces a physical feeling in your own body, and it has its own section below. Cleveland Clinic describes time-space synesthesia only as visualizing things in a very specific way. Researchers also study sequence-space synesthesia, in which sequences like days of the week or months elicit visuospatial representations (Lunke & Meier, 2018). Other pairings appear in the research literature, such as words that trigger tastes or letters that trigger the sensation of texture (Eagleman, 2012). Some people have more than one type: one group in a 2018 study had both grapheme-color and sound-color synesthesia (Lunke & Meier, 2018). Counts of the types differ. Eagleman (2012) cites an estimate of up to 150 reported forms, and Simner (2012) refers to 61 variants, so any list of types, including this one, is a sample and not the full range.

How common it is

Estimates depend on who is counted and how. Cleveland Clinic says experts estimate that at least 4 percent of people worldwide experience synesthesia, and that some forms, especially time-space, may affect as many as 1 in 8 people. Simner and colleagues (2006) reported a prevalence 88 times higher than had previously been assumed. The earlier assumption was 0.05 percent of births, so the new figure works out to about 4.4 percent. The study did not rely on people referring themselves, and it used objective tests to establish whether reports were genuine. Colored days was the most common variant, and grapheme-color synesthesia was prevalent at about 1 percent. The researchers found no strong difference between males and females. They concluded that earlier reports of more females than males likely arose from, or were exaggerated by, sex differences in self-disclosure. Figures differ between studies, and the definition of synesthesia is itself debated (Simner, 2012).

Developmental and acquired synesthesia, and what causes it

Developmental synesthesia is the kind that develops in childhood. Acquired synesthesia begins later in life. Cleveland Clinic says synesthesia runs in families and involves more connections between different brain areas. Ward (2013) describes the causes as 'genetic differences' in developmental synesthesia and 'plasticity following sensory loss' in acquired synesthesia. Cleveland Clinic adds that synesthesia can be acquired through brain damage, and that hallucinogens can sometimes cause it. It also says that synesthesia is not a disease or a medical condition, but that it can be a symptom of certain brain-related conditions. The exact causes are still being studied, and the next section describes a disagreement about the brain evidence.

What brain research shows

Researchers disagree about what brain imaging shows. One study used diffusion tensor imaging, a type of MRI that measures how water moves along white matter, to compare 18 people with grapheme-color synesthesia with matched controls. It found greater anisotropic diffusion in the synesthetes, which the authors described as more coherent white matter, and concluded that increased structural connectivity is associated with grapheme-color synesthesia (Rouw & Scholte, 2007). A 2015 critical review reached a much more cautious conclusion. Hupé and Dojat (2015) wrote that most differences claimed for synesthetes are unsupported, due mainly to low statistical power, statistical errors, and methodological limitations. Their review casts some doubts on whether any neural correlate of the synesthetic experience has been established yet. They suggest that, rather than being a neurological condition (a structural or functional brain anomaly), synesthesia could be reconsidered as a special kind of childhood memory, whose signature in the brain may be out of reach of present brain imaging techniques. Both papers are published scientific positions, and this guide does not present a single brain explanation as settled. In the sources used here, the disagreement is about the brain evidence and how to define synesthesia, and it is not about whether people have these experiences.

Projectors and associators

Researchers distinguish two ways of experiencing the extra sensation. Projectors experience it in external space, and associators experience it 'in the mind's eye' (Dixon et al., 2004). Dixon and colleagues also found that the two groups differ in their patterns of Stroop interference, a standard way of measuring how one feature of a stimulus interferes with another. Rouw and Scholte (2007) reported greater connectivity in the inferior temporal region for projectors than for associators. Eagleman (2012) also describes this distinction. In everyday life, the point is that two people with grapheme-color synesthesia can describe their experience very differently. It helps to ask a person what their own experience is like instead of assuming.

Synesthesia in childhood

Simner and Bain (2013) followed children with grapheme-color synesthesia over several years. They retested 80 children at ages 10 to 11 who had first been tested at ages 6 to 7 in earlier work by the same group, where over 600 children had been randomly sampled. They reported that associations such as A = red 'mature over time from relatively chaotic pairings into a system of fixed consistent associations'. The share of letters and digits with fixed colors was 34 percent at ages 6 to 7, 48 percent at ages 7 to 8, and 71 percent at ages 10 to 11. On average, the 10- to 11-year-old synesthetes had fixed colors for 25.7 of 36 letters and digits. Not every child followed the same path. Three children previously classified as synesthetes no longer met the requirements at the latest test, and the researchers also report cases where synesthesia had died out at an older age or was slower to develop than in other children. Because consistency is the basis of many research tests, these findings mean that a young child's answers may be less consistent than an older child's.

Is synesthesia a diagnosis?

Cleveland Clinic states that synesthesia is not a disease or a medical condition. Some research papers use the word condition in a descriptive sense. Eagleman and colleagues call synesthesia an unusual condition in which stimulation of one modality evokes sensation or experience in another (Eagleman et al., 2007), and Baron-Cohen and colleagues call it a neurodevelopmental condition (Baron-Cohen et al., 2013). Hupé and Dojat (2015) suggest it could be reconsidered as a special kind of childhood memory rather than a neurological condition. The DSM-5-TR has no diagnosis for synesthesia. Cleveland Clinic notes that synesthesia can be a symptom of certain brain-related conditions, which is one reason a sudden new onset should be checked by a doctor (see Support and treatment). Whether you think of synesthesia as a form of neurodivergence is your choice. Neuro-Common includes it because it is a well-documented difference in how people perceive and experience the world, and because research links it with sensory sensitivity and with autism.

How it is identified

Synesthesia is private, because only the person who has it can perceive the extra experience. Eagleman and colleagues write that this difficulty in verifying and quantifying private perceptual experiences is why synesthesia 'slipped out of the scientific spotlight for decades'. They describe the later introduction of tests that demonstrate the reality of the condition, its automatic and involuntary nature, and its measurable perceptual consequences (Eagleman et al., 2007). Their Synesthesia Battery is a unified collection of tests with a questionnaire and several online programs, a standard scoring system, and results available immediately to the people who take them, and it was built to support research (Eagleman et al., 2007). The research page of the Eagleman group describes the Battery as free and open to the public. These tests have limits. The Test of Genuineness-Revised proved unsuitable for synesthetes with autism, and the authors called for more feasible validation methods (Baron-Cohen et al., 2013). Simner (2012) presented evidence that might argue against the claim that synesthesia gives rise to consistent associations over time, and children's associations become consistent gradually (see Synesthesia in childhood). Because Cleveland Clinic says synesthesia is not a disease or a medical condition, the sources used here describe research testing and not a medical diagnosis.

Mirror-touch synesthesia

In mirror-touch synesthesia, seeing something happen to someone else produces a physical feeling in your own body (Cleveland Clinic). In research terms, observing touch to another person induces a tactile sensation on the observer's own body (Banissy et al., 2009). The same researchers reported that it is 'just as common, if not more common than' grapheme-color and other frequently studied varieties of synesthesia. A 2016 study tested whether it goes along with heightened empathy. It compared 46 mirror-touch synesthetes, 36 grapheme-color synesthetes, and 46 controls, and its title states the finding: mirror-touch synesthesia is not associated with heightened empathy (Baron-Cohen et al., 2016). Some mirror-touch experiences in that study were triggered by viewing objects being touched. The same study found that 30 percent of the 135 mirror-touch synesthetes the researchers recruited also reported an autism spectrum condition, and that mirror-touch synesthetes had higher autistic trait scores than controls.

Autism and sensory sensitivity

Several studies link synesthesia with autism. In an online study, 164 autistic adults and 97 comparison adults completed a synesthesia questionnaire. Synesthesia was reported by 18.9 percent of the autistic group (31 of 164), almost three times the rate in the comparison group (7.22 percent, 7 of 97) (Baron-Cohen et al., 2013). A 2017 study gave the Glasgow Sensory Questionnaire to synesthetes, autistic people, and controls. Both the synesthete group and the autistic group showed increased sensory sensitivity compared with controls, with a large effect size, and a similar pattern of over- and under-sensitivity across multiple senses. A Pattern Glare Test also corroborated visual sensitivity. Scores on the Autism-Spectrum Quotient were elevated in the synesthetes, and its attention-to-detail subscale placed them in the autistic range. The authors concluded that atypical sensory sensitivity is an important shared feature between autism and synesthesia (Ward et al., 2017). These are findings about groups. They do not mean that everyone with synesthesia is autistic, or that every autistic person has synesthesia. The Autism guide and the Sensory processing differences guide cover these topics in more depth.

Wellbeing and challenges

Cleveland Clinic says many people find synesthesia useful for learning and remembering information. It also says that in severe cases synesthesia can be strong enough to affect your ability to concentrate or focus, and that this is rare. Sensory sensitivity is more common in people with synesthesia than in controls (Ward et al., 2017), so some people may find ordinary sensory environments more demanding than others do, and the Sensory processing differences guide covers strategies. If your experiences are distressing, or are making it hard to concentrate, talk with a doctor or mental health professional, who can look at the whole picture instead of assuming that synesthesia is the only factor. If you are in crisis or thinking about suicide, the Crisis help page lists where to turn.

Memory, creativity, and strengths

Cleveland Clinic reports better memory abilities relating to a person's form of synesthesia, and says many people find it useful for learning and remembering. A 2018 study compared 97 synesthetes of four types with matched controls: 27 with grapheme-color, 21 with sound-color, 25 with both grapheme-color and sound-color, and 24 with sequence-space synesthesia. It reported a general memory advantage, plus advantages that depended on type. There was a benefit for color stimuli in grapheme-color synesthetes and a benefit for music stimuli in the group with both grapheme-color and sound-color synesthesia (Lunke & Meier, 2018). Ward's (2013) review covers synesthesia's influence on perception, imagery, memory, art and creativity, and numeracy. A 2026 study found synesthesia far more prevalent in musicians than in non-musicians, with an odds ratio of about 4, across grapheme-color, sequence-space, and sound-color types. For sound-color synesthesia, the estimated prevalence was 0.3 to 1.3 percent in non-musicians and 1.3 to 7.3 percent in musicians (Williamson et al., 2026). These are group averages, not guarantees, and not everyone with synesthesia experiences a benefit.

Support and treatment

Cleveland Clinic says that developmental and acquired synesthesia do not need treatment, and that for drug-induced synesthesia treatment can vary depending on the drug. Most support is therefore about fit and understanding. Reading about the type you have can help you describe it to other people. You can use your associations where they help, for example color-coded notes. Where sensitivity is the problem, reduce sensory load (see the Sensory processing differences guide). If synesthesia is making it hard to concentrate or is distressing you, talk with a doctor or mental health professional. If cross-sensory experiences begin suddenly in someone who has never had them, tell a doctor, who can look for a medical cause, because Cleveland Clinic notes that synesthesia can be acquired through brain damage and can be a symptom of certain brain-related conditions. If the change comes with sudden confusion, trouble speaking, or difficulty understanding speech, the CDC lists these among the signs of stroke and says to call 9-1-1 right away if you or someone else has any of the signs. Outside the United States, call your local emergency number.

School and work

Because synesthesia does not need treatment (Cleveland Clinic), support at school and work is mostly about fit. Students and employees can be allowed to use their own associations when they help, such as color-coded notes, since many people find synesthesia useful for learning and remembering (Cleveland Clinic). A person who says that letters or numbers have colors should not be marked down or corrected for saying so. Young children's associations are still settling (Simner & Bain, 2013), so a child's answers about colors may vary. If sensory sensitivity (Ward et al., 2017) or trouble concentrating (Cleveland Clinic) gets in the way, ask about adjustments to the sensory environment, as described in the Sensory processing differences guide. A person who wants formal adjustments can base the request on the effect on their day and not on the label. The Resources page lists where to find information on rights.

Identity and being believed

Only the person who has synesthesia can perceive the extra experience. Eagleman and colleagues note that this made it hard for science to verify, and that tests were later introduced that demonstrate the reality of the condition (Eagleman et al., 2007). Simner and colleagues (2006) used objective tests to establish whether reports were genuine, in a sample that did not rely on people referring themselves. Hupé and Dojat (2015) question the brain evidence and its explanation, but their abstract begins by describing additional sensations experienced by some people, and they define synesthesia as the subjective phenomenon of additional experiences. In the sources used here, their questions concern how to explain the experiences and not whether people have them. Cleveland Clinic is clear that synesthesia is not a disease. It is also not only caused by drugs, because it runs in families and hallucinogens are only one way it can be acquired (Cleveland Clinic). Who talks about synesthesia is not the same as who has it. Earlier reports of more females than males likely arose from, or were exaggerated by, sex differences in self-disclosure (Simner et al., 2006). Your own experience is real to you whether or not you ever take a test. If you want to explore it, the Eagleman research page describes the Synesthesia Battery as free and open to the public. The Resources page lists support organizations and know-your-rights information.

For families, teachers, and colleagues

Listen to what the person describes and ask how it feels. Do not ask them to prove it or perform it. Do not treat it as an illness, because Cleveland Clinic says it is not a disease or a medical condition. Teachers can let students use their associations when they help, expect young children's associations to still be settling (Simner & Bain, 2013), and ask about sensory needs, because sensory sensitivity is more common in people with synesthesia (Ward et al., 2017). Do not assume that a person with synesthesia is autistic, or that an autistic person has synesthesia, because the research describes group patterns (Baron-Cohen et al., 2013; Ward et al., 2017). Colleagues can treat what a person tells them as information the person chose to share, and ask what helps before suggesting changes. Encourage a visit to a doctor if the experiences start suddenly, or if concentration problems or distress develop (Cleveland Clinic). If the person is in crisis, the Crisis help page lists where to turn.

Sources

1. American Psychiatric Association. (2022). Diagnostic and Statistical Manual of Mental Disorders (5th ed., text rev.).
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13. Eagleman, D. M., Kagan, A. D., Nelson, S. S., Sagaram, D., & Sarma, A. K. (2007). A standardized test battery for the study of synesthesia. Journal of Neuroscience Methods, 159(1), 139-145. https://doi.org/10.1016/j.jneumeth.2006.07.012
14. Baron-Cohen, S., Johnson, D., Asher, J., et al. (2013). Is synaesthesia more common in autism? Molecular Autism, 4, 40. https://doi.org/10.1186/2040-2392-4-40
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16. Baron-Cohen, S., Robson, E., Lai, M.-C., & Allison, C. (2016). Mirror-touch synaesthesia is not associated with heightened empathy, and can occur with autism. PLOS ONE, 11(8), e0160543. https://doi.org/10.1371/journal.pone.0160543
17. Ward, J., Hoadley, C., Hughes, J. E. A., Smith, P., Allison, C., Baron-Cohen, S., & Simner, J. (2017). Atypical sensory sensitivity as a shared feature between synaesthesia and autism. Scientific Reports, 7, 41155. https://doi.org/10.1038/srep41155
18. Williamson, L., Bailey, S., & Ward, J. (2026). Increased prevalence of synaesthesia in musicians. Perception, 55(4), 330-343. https://doi.org/10.1177/03010066251390106
19. Centers for Disease Control and Prevention. Signs and symptoms of stroke. https://www.cdc.gov/stroke/signs-symptoms/index.html (retrieved October 3, 2026).

Citations are listed so you can verify claims and read further. Guidance is revised over time, so check for newer editions.