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Why do I hear colors? The neuroscience explanation
Growing up, I listened to a lot of classical music. While some people found the genre to be generally boring and thought that all of the pieces sounded alike, I experienced each piece not only aurally, but visually. Each piece had a distinct color pallet, with colors flowing into each other like watercolor paint as motifs changed or different instruments were introduced. I never thought that it was anything out of the ordinary. I would describe pieces that “sounded” lavender and navy blue with hints of gold as if it was common sense, only to be met by confused expressions. I didn’t understand. Doesn’t everyone “see” music? That’s when I learned about a condition called “synesthesia.”
Synesthesia is an umbrella term for conditions in which the stimulation of one of the senses, the “inducer,” is linked with the perception of a typically “unrelated” sense, the concurrent. Thus, while taste and smell are normally processed together to perceive flavors, it is much rarer to perceive words with specific flavors, which is why lexical-gustatory (LG) synesthesia is considered a neurological condition. [Ipser, Ward, and Simner, 2020] Some forms of synesthesia, such as LG, are more rare than others. The type that I previously described is called chromesthesia, or more commonly, colored hearing. Typically, this involves the visualization of associated colors in the “mind’s eye,” but some people with the condition do see colors projected out in front of them. [Carpenter, 2001.] There is also grapheme-color synesthesia, in which letters, numbers, or words have specific colors associated with them, and ordinal linguistic personification, in which “listed” items, such as letters, numbers, days of the week, or months, have distinct personalities. These conditions might explain why I feel so strongly that my name is purple as opposed to orange and why the number “3” has always been green and a boy. But what causes these interesting sensory associations?
While the mechanism of synesthesia is not very well understood, there are several pieces of evidence that suggest a neural basis for the condition: consistency of sensory associations (e.g. a song consistently induces the same color perception), potential acquisition during neurological illness, differences in cerabral blood flow, and familial cocurrence. [Schiltz, et. al, 1999] The genetic basis of synesthesia still requires more research to be well understood, studies suggest that it is heritable and that the phenomenon is heterogenous and polygenetic. [Brang and Ramachandran, 2011] This would explain why my dad and bother also have strong opinions regarding the color of songs that often result in lively discussions over how well our interpretations “make sense” to each other.
Several theories have been proposed as to the mechanism of synesthesia, most of which are backed by studies on grapheme-color synesthesia. One of the most supported theories is the cross-activation mechanism, which suggests that grapheme color synesthesia is caused by the retention of neuronal connections between the visual word form area of the brain (an inferior temporal region) and the color-processing region (hV4). This model may partially explain the genetic component of synesthesia. Another theory is the disinibited feedback mechanism, in which there is unusual feedback from the temporo-parietal-occipital junction or another “multisensory nexus,” which is often supported by evidence of drug-induced synesthetic experiences rather than congenital synesthetic experiences. The theory of “re-enterant processing” is a hybrid of both of the previously mentioned theories. Lastly, one of the newer theories regarding the mechanism of synesthesia is the “hyperbinding” model, which states that the overactivation of parietal mechanisms responsible for “binding” sensory information into a holistic representation of the world may be present in synesthetes. [Hubbard, 2007]
One study, with supporting evidence found from other literature, found that there was increased gray matter in the superior posterior parietal lobe in grapheme-color synesthetes compared to non-synesthetes. [Rouw and Scholte, 2010] Since the superior posterior parietal lobe is involved in integrating sensory information, so the increased number of neurons found in synesthetes would likely account for the increased likelihood of inter-sensory connections. They also found that projection perception of color is associated with increased activation of the visual, auditory, and motor cortexes, which are responsible for piercieving the outside world, while the mental perception of color is associated with increased activation of the hippocampus and parahippocampal gyrus, which are more involved in memory. The authors state that their findings are most consistent with a combination of the cross-activation model and hyperbinding in the superior parietal cortex. Another study used behavioral and neuroimaging techniques on grapheme-color synesthetes and found that the perception synthetic colors likely originates from increased hV4 activation, which also aligns with the cross-activation model. [Hubbard, et. al, 2005]
While qualitative records of synesthesia have been discussed for over one hundred years (at least), it’s very interesting to analyze some of the more recent quantitative methods used to analyze how synesthesia works. The use of imaging techniques, such as MRI and fMRI, creates some compelling evidence as to how synesthesia-based sensory perception occurs, however, it’s hard to fully understand how it works considering that many studies are statistically inconclusive. Additionally, while primarily studying grapheme-color synesthesia makes sense considering it’s the most common form of synesthesia, more studies with more rigorous methodology are required to better understand some of the more complex and rarer forms of synesthesia. Regardless, it’s reassuring to have more scientific evidence explaining to others why the Habanera “sounds” red to me.
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