Chromesthesia is a form of synesthesia in which sound gives rise to involuntary experiences of color, shape, and movement. Sometimes referred to as “colored hearing,” a person with sound to color, shape, pattern, or texture synesthesia might perceive a friend’s voice as a looping blue ribbon. A mourning dove’s coo may flicker emerald at the edges; a passing siren might fracture into shards of putty gray and ochre. These perceptions are not metaphorical or imagined, but immediate and consistent, arising alongside ordinary hearing as part of a unified sensory world. For those who experience it, sound is not only heard, it is seen.
I have chromesthesia as part of my genetically inherited “deluxe set” of neurocognitive phenomena. I think of it as one of my mid-level synesthesias. Perhaps it is not as robust as my colored graphemes and mirror touch, or maybe I simply notice it less. Either way, I am less engaged with my own colored hearing and do not typically have awe-inspiring or startling moments with it.
But a few months ago, I had an occurrence with chromesthesia that struck me with intense curiosity. I was fascinated by what transpired, but could not find any peer-reviewed research to validate my experience. I have been sitting with that instance, unsure how to convey what happened.
It was cold in San Francisco the morning in question. When I arrived to my manual therapy clinic, the furnace outside my door was on full blast. I tend not to use that unit. It is old, inefficient, noisy, and does a poor job warming the common room. I keep my own office cozy with a compact modern space heater; my patients do not need heat from the outdated heating system. But one of my office mates had arrived before me and set the thermostat to 70 degrees. As I began my first session, I could hear the clacks and bangs of the ancient furnace as it moved through its cycles.
My work is tremendously physical in nature. I have been practicing clinical manual therapy for more than 30 years, and my skill set is deeply informed by my years working in an outpatient physical therapy clinic. I provide deep tissue massage and orthopedically focused treatments such as scar mobilization and myofascial release. This work is energetically demanding and, frankly, exhausting. On most days, I take a nap at some point in the afternoon so I have enough stamina to get through my afternoon.
On the day in question, I was napping on my massage table and had entered a light sleep. I know this state as one in which my chromesthesia can be more flagrant, but what happened next was completely unexpected. The furnace clicked off. I heard the servos in the thermostat as they turned, which elicited the fluffy gray particles I typically experience with that type of sound. A few minutes later, when the metal housing around the furnace cooled, I heard a loud clunk! then thunk! which is a familiar sound around the office and the reason I almost never use that furnace. I hate those banging metal clangs.
When the furnace housing contracted, at the instant I heard the clunk and thunk, I saw two forms, one bluish in color and the other a burnished gold. They coincided with the two-part sound of the cooling furnace. The clunk was blue and spheroid, and the thunk was gold, angular, and amorphous.
At the exact moment I saw these two forms, I felt my eyes rapidly adjust from the soft-gaze state of dozing to an intense focus. I literally felt the muscles in my eyes constrict, and it hurt, in the way it can be painful to come out of a darkened movie theater into bright light. I felt the rapid constriction of my iris, the pupillary light reflex snapping my pupils from a dilated, resting state into tight focus. My eyes shifted abruptly from low-light sensitivity to bright-light conditions. Rods were overwhelmed, cones not yet fully engaged. The change was immediate and slightly painful. But the light in the room had not changed at all. I was still napping, lying on my back with my eyes covered by a dark scarf.
The only difference in my environment was a sound, a clunk and thunk that triggered brightly colored forms that drove my eyes to respond as if I were truly seeing something. I have been wanting to write about this moment and lean into the reflexive movements of my visual system, but I could not find a research paper to validate this perceptual oddity. I have a solid professional understanding of human anatomy. I know what I felt. I drew what I saw as soon as I got home. But I still wanted some form of scientific authentication.
That validation came yesterday when I read “Pupil size reveals the perceptual quality and effortless nature of synesthesia” by Christoph Strauch, Casper Leenaars, and Romke Rouw. Published in eLife, an open-access scientific publishing platform focused on the life sciences and biomedicine, this paper demonstrates that synesthetic perception is not merely associative or imagined, but physiological and automatic. When synesthetes see color in response to otherwise colorless stimuli, their pupils constrict and dilate as if real light were present. In other words, the eye itself responds without effort or intention, offering measurable evidence that synesthetic experience operates at the level of perception, not metaphor.
What happened on that table was not imagination, and it was not metaphor. It was a sensory event that recruited my body, my eyes, and my nervous system in real time. The sound did not simply suggest color. It produced it with enough force to move the muscles of my iris. At the moment this synesthetic phenomena occurred, I remember saying to myself I just heard that with my eyes!
For years, I have trusted similar perceptions because they are consistent and embodied, but there is something clarifying about seeing that trust reflected in research. It gives language to an experience that resists language. It also sharpens my own understanding of what my synesthesia is: not an embellishment of reality, but an extension of it, an an opportunity to hear things with my eyes.
Very interesting – especially the idea that synesthesia can be objectively reflected in pupil responses.
What stayed with me while reading:
The bodily reaction itself doesn’t surprise me that much – it aligns well with what we know about perception and orienting responses.
What I find truly remarkable is something else:
That you don’t just describe this change, but seem to actually feel it as it happens.
The study shows that the body reacts as if the perception were real.
You seem to go one step further by being aware of that reaction itself.
For me, that’s where an additional layer of experience emerges.
Hi Avi, thanks for your comment. The moment I describe in this essay, that instance when I felt my pupils contracting was remarkable. I was so pleased to read the research by Dr. Rouw et al, and to have the information provided by the paper. My involvement with the synesthesia community and IASAS (https://iasasevents.com/) has guided me toward noticing my perceptual experiences and really paying attention to them. It’s such a strange thing to think about the brain paying attention to itself! Thanks again for reaching out. Warm wishes to you from San Francisco, California.