What Causes Synesthesia? Understanding Cross-Sensory Connections

Synesthesia has been the subject of scientific scrutiny since at least the 19th century, when a doctor named Georg Sachs described his experience with synesthesia in a medical dissertation. Since then, researchers have made great strides in synesthesia research, uncovering insights into synesthesia’s causes and neurological underpinnings. In this article, we’ll explore what causes synesthesia, from its roots in the human genome to the complex relationship between synesthesia, brain development, and early learning. 

Synesthesia overview

Synesthesia is a phenomenon characterized by the blending or mixing of sensory experiences. It is a condition where the stimulation of one of the senses, such as sight or sound, leads to an automatic perception in another. For example, a person with synesthesia, sometimes called a synesthete, may perceive colors when they hear music or taste words. Synesthesia may present in one of two ways: 

1. Projective synesthesia

Sensory experiences evoke vivid and involuntary mental images or perceptions.

2. Associative synesthesia

Sensory stimuli trigger consistent and involuntary associations between different sensory modalities.

There are several types of synesthesia, each involving some combination of the senses. Among the most common forms of synesthesia are:

• Chromesthesia: Also known as sound-to-color synesthesia, this involves perceiving specific colors in response to hearing different sounds or musical notes.
• Grapheme-color synesthesia: In grapheme-color synesthesia, individual letters or numbers are perceived as inherently colored.
• Number-form synesthesia: In number-form synesthesia, numbers or mathematical concepts are perceived as having specific spatial locations or arrangements.
• Spatial sequence synesthesia: Spatial sequence synesthesia involves perceiving sequences, such as numbers, months, or days of the week, as occupying specific spatial locations.
• Lexical-gustatory synesthesia: In lexical-gustatory synesthesia, words or spoken sounds evoke specific tastes or sensations in the mouth.
• Mirror-touch synesthesia: In mirror-touch synesthesia, watching someone being touched or experiencing tactile sensations can elicit a corresponding tactile sensation in the synesthete.
• Time-space synesthesia: The passage of time is perceived spatially or geometrically, with days, weeks, months, or years appearing in specific spatial arrangements.

Understanding synesthetic experiences

Researchers believe that synesthesia may occur due to cross-activation between different parts of the brain’s sensory cortex. Each part of the sensory cortex is responsible for processing a different sense; in a synesthete, unusual connections between these areas lead to the blending or mixing of the senses.

This cross-activation may happen because some brain regions that typically control or suppress irrelevant sensory information are not working as effectively, allowing different senses to influence each other more strongly. Cross-activation may be due to an imbalance between the cells in the brain that excite signals and those that suppress them.

Causes of synesthesia

Research into synesthesia has revealed some insights as to how and why synesthesia develops. However, scientists have not yet identified the exact causes of synesthesia. While there are several theories and hypotheses, the precise mechanisms underlying synesthesia remain a subject of ongoing investigation. It is believed that a combination of genetic, neurological, and environmental factors may contribute to its development.

Genetics

Synesthesia seems to have a genetic component and is often found among close relatives. However, there does not appear to be a single synesthesia gene. One study identified 37 genes that were predictors of synesthesia between family members. 

Some research suggests that sensory cross-activation may occur due to a genetic variant affecting the pruning gene.

Serotonin hyperactivity

Some research has explored the role that the neurotransmitter serotonin might play in synesthesia’s development. Elevated serotonin is often associated with consuming psychedelic drugs, which can produce synesthesia-like experiences. Serotonin is also known to spike in response to brain injury—which, in rare cases, has triggered late-onset synesthesia. 

The relationship between autism and synesthesia also supports the hypothesis that serotonin may be a causal factor. Atypical serotonin levels are often found in those with autism, another form of neurodivergence. Individuals with autism have also been found to be more likely to experience synesthesia than the general population and may be prone to other types of sensory processing issues.

Early learning and conceptual factors

In addition to the sensory cortex, synesthesia may also involve the part of the brain responsible for processing conceptual information. This may explain, in part, why certain types of synesthesia seem to blend cognitive and perceptual processes, often involving concepts like numbers, sequences, words, shapes, and even character traits.

Most people are exposed to this type of conceptual information in early childhood while the brain’s sensory functions are growing and changing. Synesthesia tends to manifest in very early childhood when this development takes place. It’s possible that this stage of early learning and brain development may play a role in the development of synesthesia.

Other causes of synesthesia

There have been rare cases where synesthesia emerges after a traumatic brain injury or stroke damages the brain’s sensory cortex. When damaged, the brain floods the affected area with serotonin and other neurotransmitters, which can lead to permanent changes. Thus, damage in the sensory cortex may trigger the development of crossed sensory pathways. One type of synesthesia, mirror-touch synesthesia, can, in rare cases, be triggered by the amputation of a limb. 

In the absence of neural damage, synesthesia is not known to develop later in life. However, users of psychedelic drugs often report synesthesia-like experiences while under the influence. As the drug wears off, these experiences tend to subside. Psychedelic drug use is not known to cause permanent or irreversible synesthesia. Similarly, temporary synesthetic experiences have been induced via sensory deprivation. 

Synesthesia as a neurodivergent trait

Synesthesia is considered a neurodivergent trait, representing a natural variation in the human brain rather than a disorder. Neurodivergent individuals have neurological differences outside the typical range, but these differences are not inherently adverse or pathological. Synesthesia can offer unique perceptual experiences and creative insights to those who have it, and many synesthetes embrace their condition as a positive aspect of their identity. 

Synesthesia is closely linked to autism, another form of neurodivergence. Autism and synesthesia may have some neurological similarities, though synesthesia is not a type of autism. However, research has found that individuals with autism have a higher prevalence of synesthesia compared to neurotypical individuals, with one study showing the rate of synesthesia in autistic adults to be almost three times higher than in the control group. 

Synesthesia and mental health

Synesthesia is not a mental illness; however, it is associated with some mental illnesses. For instance, synesthesia may be a risk factor for developing posttraumatic stress disorder or PTSD. Additionally, children with synesthesia may have higher rates of anxiety. 

There may also be instances in which synesthesia interferes with everyday functioning. For example, synesthetes may be prone to overstimulation, particularly in loud or chaotic environments. Synesthesia may also make focusing or concentrating on specific tasks or stimuli difficult. 

Those experiencing adverse mental health effects related to their sensory differences may benefit from the intervention of a mental health professional. Therapy can be helpful for learning coping and self-soothing strategies for managing challenging situations. Research has found talk therapy to be effective for managing a wide range of common mental health concerns, including those that sometimes co-occur with synesthesia, such as PTSD.

Online therapy may be worthwhile for neurodivergent individuals who find appointments overstimulating or uncomfortable. Virtual sessions through an online therapy platform like BetterHelp can be a convenient, affordable way to receive mental health services and have been shown to be just as effective—and in some cases more effective—than traditional face-to-face therapy.