Synesthesia: What to Do When One Sense Comes Through as Another

Synesthesia is a neurological phenomenon in which information meant to stimulate one sense stimulates additional senses, as well. For example, someone with synesthesia may "see" music, "hear" colors, "taste" textures, "smell" movement, and so on. Approximately 4% of the general population has some form of synesthesia.

This article covers the different types of synesthesia.

What Is Synesthesia Like?

Synesthesia is essentially the merging of multiple senses. It is not a disorder, but rather a way of perceiving the world.

Not everyone experiences synesthesia the same way since synesthesia depends on which senses are paired. Some examples of how synesthesia might feel include:

• Experiencing music as colors
• Seeing or associating colors with numbers or letters
• Tasting words or music
• Feeling tactile, physical sensations when you hear sounds
• Hearing sounds when you see noiseless movements

For the most part, people report that these sensations are experienced internally, in their mind. A rare group of people may see colors or images projected in their vision, typically a few feet away from their body.

Who Gets Synesthesia?

Synesthesia has been described as a neurodevelopmental condition. Synesthesia can be:

• Developmental: Being born with synesthesia, likely influenced by genetic factors
• Acquired: Onset of synesthesia later in life, after an injury, illness, or other changes affecting the brain
• Drug-induced: Caused by hallucinogenic drugs

Synesthesia is also associated with higher rates of certain mental health conditions and traits. This includes:

• Anxiety disorders
• Obsessive-compulsive disorder (OCD)
• Autism
• Neuroticism
• Schizophrenia

Is Synesthesia a Form of Autism?

No, synesthesia is not a form of autism. However, synesthesia is about 3 times more common among people with autism than it is in the general population.

A 2023 twin study found that there is a genetic link between autism and synesthesia.

Both synesthesia and autism are examples of neurodivergence. Autistic people with synesthesia may commonly experience tactile/physical sensations without actually being touched, brought on by observing someone else being touched, sounds, words, and more.

What Causes Synesthesia?

Scientists do not agree on the exact cause of synesthesia.

Theories on what causes synesthesia include:

Hyperconnectivity

One theory is that synesthesia is caused by an overabundance of neurons in the brain. It is believed that individuals with synesthesia don't go through the normal "pruning" process, when excess neurons are lost with age.

In adulthood, those with synesthesia have hyperconnectivity of neurons in the sensory areas of the brain. This could explain why auditory input meant to stimulate the sound perception area of the brain, may also pass through the visual perception area of the brain and lead to sound-color synesthesia.

Feed-Backward Connections

An alternate theory is that people with synesthesia have normal brain "architecture," or in other words, a normal amount of neurons, but these neurons operate in an unusual way. There is a lack of inhibition, leading to the neurons firing backward into overlapping sensory areas of the brain, leading to synesthetic experiences.

Genetic

Many scientists agree that there is a genetic component to synesthesia. Synesthesia commonly occurs in families, and twin studies have even found genetic links between synesthesia and conditions like autism and schizophrenia. A single "synesthesia gene" has not yet been identified.

Learning

Associating two senses together can be useful in early childhood for learning numbers, letters, words, music, and more. Some researchers argue that synesthesia is, in part, a learned skill leftover from childhood.

Other Synesthesia Types and Symptoms

Synesthesia symptoms vary based on the type, and a person can have more than one type.

Types of synesthesia include:

• Grapheme-color synesthesia: Letters and/or numbers (graphemes) are automatically associated with colors (either seen literally, or in the mind's eye). This is the most common type of synesthesia, and it's estimated about 1% to 2% of the population has this.
• Chromesthesia (sound-color): Sounds are associated with colors or elicit visual colors.
• Mirror-touch synesthesia: When observing physical sensations happening to someone or something else, you feel the same physical sensations.
• Lexical-gustatory synesthesia: Words are associated with certain tastes.
• Auditory-tactile: Sounds elicit physical touch sensations.
• Spatial sequence synesthesia (time-space): Ordinal sequences (such as times, days of the week, years, etc.) appear as points in space. For example, the days of the week might be in a circle, or months of the year may surround you.
• Number form: Numbers are experienced in a spatial configurations, such as around or in front of the person. Some think this is another presentation of spatial sequence synesthesia.
• Ordinal-linguistic personification: Number sequences, days of the week, months, and other ordinal sequences are associated with fixed personalities, genders, or relationships.
• Misophonia (sound-emotion): Certain sounds cause strong emotions, such as anger, annoyance, frustration, and sadness. Common triggers include chewing, lip smacking, or tapping.

This is not a complete list of all types of synesthesia. There are countless forms of synesthesia, typically described as the sensory inducer, such as sound, to the sensation, such as smell.

It is possible for someone to have more than one type of synesthesia at once. For example, someone with both grapheme-color and ordinal-linguistic synesthesia might experience the number four as both yellow and male.

Synesthesia Testing

It is challenging to test synesthesia because each person's experience is very individual. The Synesthesia Battery is one validated test for grapheme-color synesthesia.

Tests for synesthesia, including the Synesthesia Battery, can be taken online.

Consistency Test for Synesthesia

The consistency test looks for grapheme-color synesthesia. During this type of test, you are given stimuli (such as numbers or letters) and asked to share your synesthetic experience (such as an associated color). This test is done multiple times over weeks, months, or years.

A test positive for synesthesia would have consistent answers over time. For example, if a synesthetic person said the number eight is green, they would still experience it as green a year or more later. Synesthetes are consistent about 90% of the time, whereas non-synesthetes are consistent only about 20% of the time.

How Is Synesthesia Treated?

Synesthesia is not a mental illness. Treatment for synesthesia does not yet exist. In fact, many people with synesthesia view this condition as a gift, a special power, or an extra sense.

However, some people with synesthesia experience sensory overload, which can make it challenging to go in public or exist in loud, colorful, or busy places. Talk to your healthcare provider if sensory overload from synesthesia is impacting your functioning.

Long-Term Outlook for People With Synesthesia

Most people with synesthesia are able to live a normal life. In fact, for many people synesthesia is "normal" to them, and when they realize that not everyone else experiences the world the same way, this can sometimes be viewed as something that makes them special and unique.

Research also tells us that people with synesthesia have advantages in memory and creativity. Many people with synesthesia have success in creative professions as poets, artists, musicians, and more.

Summary

Synesthesia is the merging of senses. Information meant to trigger one sense may actually trigger several, for example seeing the number eight as green, tasting the word "generous" as chocolate pudding, or seeing music as fireworks. There are many types of synesthesia.