Synesthesia and the Neurodivergent Brain

Synesthesia, the involuntary blending of sensory modalities, has intrigued philosophers, artists, and scientists for centuries. Today, advances in neuroimaging, genetics, and psychedelic research are revealing why some brains naturally “cross‑wire” and how we can temporarily induce similar states with plant medicines or ritual practices.

Defining Synesthesia and Its Historical Roots

Synesthesia (from the Greek syn = “together” and aisthesis = “sensation”) is a neurological condition in which stimulation of one sensory channel automatically triggers a concurrent experience in another. The hallmark features are:

Involuntary: The cross‑modal perception occurs without conscious effort.

Consistent: A given stimulus elicits the same secondary sensation over time (e.g., the letter “A” always appears red).

Automatic: No learned association is required; the brain’s wiring creates the link.

Perceptual: The experience is vivid enough to be described as a genuine sensory event or strong association.

Common “types” include grapheme‑color (letters/numbers → colors), lexical‑gustatory (words → tastes), chromesthesia (sounds → colors), and spatial‑sequence (numbers occupy specific locations in space). Estimates vary, but roughly 4–5 % of the general population report some form of synesthesia (Psychology Today), with higher prevalence among artists, musicians, and writers. For a bigger list of various types of synesthesia, you can check out the Synesthesia Tree.

There are two overall types of synesthesia (Wikipedia):

• “Projective synesthesia: seeing colors, forms, or shapes when stimulated (the widely understood version of synesthesia)”

• “Associative synesthesia: feeling a very strong and involuntary connection between the stimulus and the sense that it triggers”

The Color of Words: Early Mentions and Scientific Emergence

The study of synesthesia has roots stretching back to Classical Antiquity, where Greek philosophers such as Plato noted that "the senses are united" in the soul, hinting at cross-modal perception, though these musings remained philosophical rather than empirical.

In the 19th century, French neurologist Gustav Fechner (1857) began documenting observations that would contribute to early scientific interest in the phenomenon. By the early 20th century, case reports from Leonard Meyer and Michele L. B. described "colored hearing" in patients with temporal-lobe lesions, linking synesthetic phenomena to cortical organization.

The modern era emerged during the 1970s–1980s when psychologist Simon Baron-Cohen conducted systematic surveys, establishing that synesthesia runs in families and is more common among individuals with exceptional artistic abilities; his work sparked the first large-scale epidemiological studies and cemented synesthesia as a legitimate field of cognitive neuroscience.

Literary accounts also contributed to early awareness, for instance, even Henry David Thoreau wrote of Ralph Waldo Emerson’s daughter, Ellen Emerson, who told him that she could see the “colors of words.”

Contemporary Neuroscience

In a 2024 study by Ward and colleagues, some key findings regarding synesthesia were found. Firstly, those with synesthesia showed differences in myelin content across the cortex, with 32 regions having increased myelin and 38 regions having decreased myelin. This suggests that synesthesia involves altered stabilization of intracortical circuits.

They also found “flatter” network structure across the board, with regions that are usually major hubs being less defined and increased centrality in other regions which are usually less prominent.

Instead of relying on a few super-connected command centers to do all the heavy lifting like most people, synesthetes have a more balanced brain network where the workload is shared more equally across many different areas.

Synesthetes also showed smaller estimated total intracranial volumes (eTIV) but larger cortical surface area relative to that volume. This could suggest delayed early brain growth (smaller head size early on) followed by compensatory cortical expansion, but this theory has certainly not been proven yet.

In a review article by Schwartzman et al. from 2019, they referred to studies showing that natural synesthetes have lower thresholds for "phosphenes" (seeing flashes of light) when a magnetic pulse (TMS) is applied to their visual cortex. This suggests their visual cortex is more easily excited or "wired up" than in non-synesthetes.

They also discuss stronger electrical responses in the brain when viewing flickering checkerboards, indicating heightened sensory-perceptual processing in specific modalities, as well as potential theories on how synesthesia arises.

One prevailing theory is that synesthete brains are not merely hardwired differently, but that their brains may have undergone a specific learning process or developmental trajectory that strengthened cross-modal connections.

This theory persists and is rooted in evidence indicating that synesthesia experiences can be induced through training. This is an interesting area of exploration for future research, especially considering it touches on that age-old debate of nature versus nurture.

Synesthesia and Neurodivergence: Prevalence in Autistic & ADHD Populations

Large‑scale surveys consistently report elevated rates of synesthesia among neurodivergent individuals, with around 19% of autistic people having synesthesia (Engelbrecht and Silvertant, 2024), a big difference from the 4-5% of the general population that has synesthesia. 

Overlap in neurodivergent groups is not uncommon, for instance the autism and ADHD overlap known as AuDHD is a well documented phenomenon. It's not hard to believe that neurodivergent individuals would also experience synesthesia at higher rates.

Two complementary mechanisms help explain this overlap:

1. Sensory Integration Differences – Autistic brains often exhibit atypical sensory gating, meaning that the usual “filter” that limits extraneous input is less stringent. This creates a fertile environment for cross‑modal interactions.

2. Enhanced Local Connectivity – Neuroimaging of autistic participants reveals stronger short‑range connections within cortical columns, which can facilitate the binding of adjacent sensory maps.

For ADHD, the story could be similar but rooted in dopaminergic dysregulation that affects attentional networks. The resulting “noisy” neural environment could inadvertently promote the merging of sensory streams, manifesting as synesthetic-like experiences.

In a 2024 study by Ward and colleagues, scientists noted that “these brain-based individual differences within the neurotypical population [of those with synesthesia versus those without] can be as large as those that differentiate neurotypical from clinical brain states.”

Cognitive and Creative Implications

Research indicates that synesthetic individuals often score higher on tests of divergent thinking, a core component of creativity. When combined with neurodivergent traits, such as hyperfocus in autism or rapid idea generation in ADHD, the synesthetic brain may excel at forming novel associations.

Research consistently supports the hypothesis that synesthesia enhances creativity, a link evidenced by both the unique artistic expressions of famous synesthetes like Van Gogh and empirical data showing that heightened visual imagery—a core feature of synesthesia—predicts superior creative performance in problem-solving tasks (Nowlan, 2021).

Clinical Relevance

Understanding the synesthesia‑neurodivergence link is valuable for clinicians:

• Diagnostic Clarity: Synesthetic reports can serve as an ancillary marker when assessing sensory processing differences.

• Therapeutic Tailoring: Interventions that respect a synesthetic individual’s multimodal perception (e.g., multisensory learning tools) can improve engagement and outcomes.

• Risk Management: Some neurodivergent individuals may experience sensory overload; recognizing synesthetic amplification can guide environmental accommodations (e.g., reduced fluorescent lighting, muted soundscapes).

Psychedelics, Synesthesia, and the “Bridge” Between Worlds

Classic psychedelics—psilocybin (magic mushrooms), lysergic acid diethylamide (LSD), and N,N‑dimethyltryptamine (DMT)—act primarily on serotonin 5‑HT₂A receptors distributed across the cortex. Activation of these receptors leads to reduced thalamic filtering, increased global connectivity, elevated entropic brain activity, and most famously, enhanced neuroplasticity.

When it comes to psychedelics inducing or mimicking synesthesia, the most consistent finding is that serotonin agonists (drugs that activate serotonin receptors, specifically the 5-HT2A subtype) are the most effective.

When synesthesia is induced by these substances, the most common form is auditory-visual synesthesia (e.g., hearing music and seeing colors or shapes). While there are many similarities between congenital synesthesia and psychedelic-induced synesthesia, there are notable differences, namely the transience of the phenomena and the potential for placebo and the user's expectations to induce synesthesia-like phenomena.

Also, it’s important to note that mind-altering substances like psychedelics can enhance or inhibit existing synesthesia in natural synesthetes. For example, serotonergic psychedelics (like LSD) tend to enhance the intensity of natural synesthetic experiences, while certain antidepressants (SSRIs like fluoxetine) have been reported to inhibit or suppress them.

Collectively, these changes mimic the neurophysiological profile of natural synesthesia, albeit temporarily and pharmacologically induced (Luke and Terhune, 2021).

Shamanic Traditions as Natural “Psychedelic” Protocols

Indigenous shamanic practices have long pursued altered states that blur sensory boundaries:

• Rhythmic Drumming (≈ 4–7 Hz) entrains theta oscillations, which suppress the posterior cingulate cortex—a hub that normally maintains sensory segregation.

• Repetitive Chanting engages motor‑speech circuits that feed back into auditory and somatosensory cortices, encouraging a fluid exchange of information.

• Sensory Immersion (fire, incense, darkness) forces the brain to rely on fewer external cues, amplifying internal cross‑modal signaling.

These rituals achieve a non‑chemical modulation of the same thalamic and cortical pathways targeted by psychedelics, suggesting a convergent mechanism: loosening the brain’s sensory “gatekeepers.”

Therapeutic and Creative Applications

1. Psychedelic‑Assisted Therapy (PAT): Clinicians monitor synesthetic reports as a proxy for depth of perceptual integration. Patients who describe vivid color‑sound blends often report heightened emotional insight, which can be leveraged during integration sessions.

2. Artistic Innovation: Musicians and visual artists intentionally dose low amounts of psilocybin (micro‑dosing) to stimulate synesthetic cross‑talk, facilitating novel compositional structures and multimedia collaborations.

3. Design for Neurodiversity: Recognizing that a portion of the population experiences synesthesia could inform inclusive UX & workplace systems design—e.g., adding color‑coded audio cues in software for users who may experience sound as visual patterns during heightened states for example, or designing productivity systems to account for the unique skills of synesthetes.

Practical Ways to Explore Synesthetic Perception

Even without a lab or a psychedelic, you can safely probe your own sensory integration:

1. Focused Listening Exercise – Sit in a quiet room, play an instrumental piece, and close your eyes. Write down any colors, shapes, or textures that arise.

2. Cross‑Modal Association Game – Pick a random word (e.g., “mountain”) and assign it a taste, a scent, and a hue. Notice how quickly your brain generates these links; this mimics the automatic nature of synesthesia.

3. Sensory‑Rich Ritual – Light a scented candle, play soft drumming, and dim the lights. The combination of auditory, olfactory, and visual stimuli can temporarily lower thalamic filtering, offering a mild “synesthetic glimpse.”

These exercises are harmless, require no substances, and can deepen empathy for neurodivergent individuals who experience the world in a richly layered fashion.

Hearing the Colors of Neurodiversity

Synesthesia sits at the intersection of neuroscience, neurodiversity, and altered consciousness. It is a naturally occurring variant of brain wiring that thrives in individuals whose sensory gates are more permissive—whether due to genetic factors, neurodevelopmental differences, or the pharmacological loosening caused by classic psychedelics.

Ancient shamanic rituals achieved comparable states through rhythm, chant, and immersive environments, underscoring a universal human fascination with dissolving the borders between sight, sound, taste, touch, and smell.

Diversity is an important part of nature (Seaberg, 2015), and synesthesia is one subset of the inherent diversity found in the human brain and perception, illustrating that the human nervous system is not a monolith but a spectrum of configurations where unique wiring patterns can lead to heightened sensory integration, enhanced creativity, and distinct cognitive profiles that are valuable variations rather than pathological anomalies.

For designers, therapists, and creators, acknowledging synesthetic perception is a call to build richer, more inclusive experiences—from color‑coded podcasts to multisensory art installations. For researchers, it offers a living model of how the brain can flexibly integrate information, a principle that may one day inform treatments for a host of psychiatric and neurological conditions.

The next time you hear a song and imagine a splash of turquoise, or you notice a colleague describing letters as flavors, remember: you are witnessing the brain’s extraordinary capacity to bridge worlds that most of us keep neatly separated. Embrace that bridge, explore it responsibly, and let it inspire new ways of seeing, hearing, and feeling the world around you.

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