Synesthesia is a condition in which a sensory experience produces a reaction that involves a different sense. But synesthesia may be more multifaceted than that, with a variety of psychological and neurological factors that may contribute to—or come as a result of—this phenomenon. 

Whether you have experienced synesthesia before or are simply curious to learn more, this article aims to provide a closer look at what synesthesia is, how it manifests, theorized mechanisms, and more. 

What is synesthesia?

Synesthesia is a phenomenon in which an experience of one sense also produces a reaction involving a different, unrelated sense simultaneously.

Synesthesia is relatively rare, with scientists estimating that it affects approximately 4% of the population. Synesthesia is not considered a mental illness and is generally harmless.

Everyday examples of synesthesia

Some everyday examples of synesthetic perceptions include:

• Hearing colors
• Tasting words
• Feeling sounds
• Feeling sensations seen happening to someone else, called mirror touch

What synesthesia is not

To gain a better understanding of what synesthesia is, it can help to clarify what it is not. 

Synesthesia is not a hallucination, nor is it a product of one’s imagination. Unlike non-synesthetes, who, for example, may naturally associate a particular color with a certain number, synesthetic perception is involuntary, automatic, and cannot be turned off. 

Synesthesia is not a neurological condition, and it doesn’t require treatment. In rare cases, new-onset synesthesia may require medical evaluation to determine whether it is an underlying or unrelated neurological condition, but most of the time, it’s a stable, lifelong phenomenon.

How does synesthesia manifest?

A wide range of synesthesia types have been identified, with some estimates speculating that there may be more than twenty different kinds that involve various combinations of sensory information. Below, we’ll outline some common forms of synesthesia.

• Grapheme-color synesthesia: Grapheme-color synesthesia causes synesthetic associations between specific letters and numbers (or “graphemes”) and different colors. For example, the letter X might be associated with synesthetic colors like red or yellow.
• Spatial sequence synesthesia: Individuals with spatial sequence synesthesia perceive items in a sequence or ordered list as part of the physical space around them. For example, one might perceive the letters of the alphabet in a line in front of them. Depending on the person, numbers might take the form of mental imagery, or they might be physically visible.
• Auditory-tactile synesthesia: This describes a synesthetic experience in which a specific sound induces a particular sensation in the body (for example, a sense of cold, warmth, or tingling in response to the sound of a door closing or rain falling). 
• Mirror-touch synesthesia: Mirror-touch synesthetes perceive physical sensations that are happening to others as though they were happening to them. For instance, witnessing someone else’s face being touched might induce the same sensation in the face of the observer. 
• Chromesthesia: Sometimes referred to as “colored hearing” or “sound color synesthesia,” this form of visual synesthesia creates associations between certain sounds and specific color sensations. For example, hearing music from a harp might evoke a perception of the color blue.
• Number form synesthesia: This is one of the visual forms of synesthesia. It causes the synesthete to experience numbers physically in the space around them. 
• Lexical gustatory synesthesia: For those with this form of synesthesia, specific words induce the perception of specific tastes. For example, the word “hotel” might cause the listener to taste the flavor of apples. 
• Ordinal linguistic personification: Individuals with this form of synesthesia associate different items in a sequence with different personalities and genders. 
• Hearing-motion synesthesia: A type of synesthesia where moving or flashing, silent objects trigger sounds, such as tapping or beeping.
• Time-space synesthesia: Individuals with this type of synesthesia perceive units of time, such as days, months, or years, as having abstract concepts, such as specific shapes, 3D forms, or physical locations, forming a sort of mental map around their body.
• Rare and unusual forms: A number of rare forms of synesthesia also exist, such as swimming style synesthesia, which involves seeing colors when watching a specific swimming stroke. This is an unusual form that has minimal research associated with it, and only two rare cases, studied in 2011.

Consistency and automaticity of experiences

Two key characteristics of synesthesia are that the sensory enhancements it produces are stable over time and occur automatically. For example, people with synesthesia may always make the same color associations when they see a particular letter or feel the same tactile sensations when they hear a particular instrument, without making any effort or having any intention to do so. Automatic and consistent connections may be the most clinically significant markers of true synesthesia.

Developmental vs acquired synesthesia

According to the Cleveland Clinic, experts don’t fully understand what happens in a person’s mind to cause synesthesia, but they have identified three main types: developmental synesthesia, acquired synesthesia, and drug-induced synesthesia. 

Developmental synesthesia and early childhood

People with developmental synesthesia are typically considered neurodivergent because their brains work in a different way than those of people who are neurotypical. While experts don’t know exactly why this happens, they do have some theories.

• Brain development. Some experts believe that everyone has some type of synesthesia in very early childhood and that it may be part of the brain’s natural development, with some research showing that it may result from residual neural connections that remain in place in the brains of synesthetes. 
• Brain structure. Research has indicated that people with synesthesia have more connections between certain brain areas, which may explain why multiple areas are activated from one type of sensory input.
• Genetics. Some research indicates that synesthesia results from genetic causes. For example, one study found that grapheme-color synesthesia is more common in identical twins than fraternal twins, suggesting a genetic component.

Acquired synesthesia and new-nonset symptoms

Acquired synesthesia can occur due to damage to the brain. Some research indicates that, due to “the brain's ability to adapt to changing environmental conditions, injuries, or experiences,” synesthesia may result. Various things can cause this, including sensory deprivation, brain tumors, and traumatic brain injury. That said, according to the Cleveland Clinic, acquired synesthesia may have some differences. Specifically, it may not be as consistent or as frequent, and the effects can be shorter-lived.

Drug-induced synesthesia can also be considered acquired. It can occur due to the use of certain drugs, particularly psychedelic drugs and hallucinogenic drugs, like LSD, peyote, and psilocybin. People with this type of synesthesia may also have a slightly different experience than those with the developmental type. Drug-induced synesthesia may be more dependent on emotional state, may only occur in certain circumstances, and may be accompanied by hallucinations.

What does the research say about synesthesia psychology?

Synesthesia is not considered a mental illness or a mental health condition.

In some instances, synesthesia develops later in life, such as in the aftermath of a brain injury or as a temporary effect of psychoactive substances (such as cannabis or hallucinogens). 

Synesthesia research is evolving, with scientists aiming to find a neurological basis for synesthetic phenomena using tools like functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). Some researchers have proposed that synesthesia may be due to increased cross-communication between different brain subregions, but studies are ongoing.

Can synesthesia impact mental health?

As previously mentioned, synesthesia is generally considered non-harmful to physical health or brain function. Some synesthetes may enjoy the altered perceptions the condition induces or the chance to experience the world beyond certain cognitive and physiological constraints. 

That said, those with synesthesia may feel isolated or lonely due to experiencing the world differently from others. Other synesthetes, such as those with mirror-touch or auditory-tactile synesthesia, may sometimes find themselves experiencing distressing physical sensations. However, various resources exist that may help many synesthetes tend to their well-being and mental health, ranging from mental health support groups to mindfulness and meditation.

Therapy may also be a helpful resource for those whose synesthesia is causing them distress. In particular, cognitive behavioral therapy (CBT) has been studied for its potential to treat chronic pain and pain-related disability. This may make it an appealing option for synesthetes experiencing unpleasant sensations. 

Some people with busy schedules may struggle to commute to a therapist’s office for in-person sessions. With online therapy, you can attend sessions with a licensed therapist from the comfort of your own home. You can also message your therapist at any time, and they will respond when they’re available, which may be helpful if you’re navigating stress from synesthesia throughout the day.

Online therapy has been studied as an effective and cost-efficient alternative to in-person therapy. One study from 2023 found that online cognitive behavioral therapy (CBT) was an effective treatment for mental health conditions such as depression, anxiety, chronic pain, and more.