In the 19th century, Francis Galton noted that certain people who were otherwise normal “saw” every number or letter tinged with a particular color, even though it was written in black ink. For the past two decades researchers have been studying this phenomenon, which is called synesthesia.
In an “Unsolved Mystery” article and accompanying podcast to be published November 22 in the online, open-access journal PLoS Biology, David Brang and VS Ramachandran strive to bring synesthesia into the broader fold of biology and to the scientific study of the arts through understanding its evolutionary basis.
Number-color associations made by a synesthete. Source: PLoS Biology
Ramachandran and colleagues have demonstrated that synesthesia is an authentic and repeatable phenomenon, and that it has a sensory basis rather than a high-level mental association.
For grapheme-color synesthesia (where colors and numbers evoke perceptions of colors), they suggested that this occurs through cross activation between sensory brain regions concerned with color and number. However, showing that the phenomenon is valid and caused by enhanced connectivity in the brains of synesthetes still left open the questions of how and why synesthesia evolved in the population.
“At least 60 different forms of synesthesia have been documented (i.e., different combinations among the senses),” the study reports, “reflecting the extreme heterogeneity of the condition, and one could easily assume that each type of synesthesia is caused by a unique gene or set of genes.”
As Dr. Ramachandran points out, one possible answer comes from the fact that synesthesia is purported to be 7 times more common in artists, poets and novelists than in the rest of the population.
“Synesthetes report spending more time engaged in creative activities,” he reports. Dr. Ramachandran suggests that “if the mutant gene was expressed diffusely throughout the brain (not just in color and number regions) and concepts and ideas are also represented in distinct brain regions, then a more ‘cross-wired’ brain would have a greater propensity to link seemingly unrelated ideas.” This ‘hidden agenda’ of the synesthesia gene (making some outliers in the population more creative) gives rise to one possibility of why it has survived.
Recently, research has confirmed numerous cognitive and perceptual benefits that are associated with synesthesia, any of which could be argued to produce a stronger basis for selection. As extreme examples of these benefits, two well-characterized savants have demonstrated remarkable memory abilities based on their synesthesias: Daniel Tammet used his synesthesia to memorize pi to 22,514 digits.
These demonstrations of enhanced processing of sensory information suggest a provocative evolutionary hypothesis for synesthesia: synesthetic experiences may serve as cognitive and perceptual anchors to aid in the detection, processing, and retention of critical stimuli in the world; in terms of memory benefits, these links are akin to a method of loci association.
In addition to facilitating processes in individual sensory modalities, synesthetes also show increased communication between the senses unrelated to their synesthetic experiences, suggesting that benefits from synesthesia generalize to other modalities as well, supporting their ability to process multisensory information. Furthermore, others have argued that synesthesia is the direct result of enhanced communication between the senses as a logical outgrowth of the cross-modality interactions present in all individuals. Taken collectively, these data suggest that synesthesia may be associated with enhanced primary sensory processing as well as the integration between the senses.
It is precisely because synesthesia seems to occupy that mysterious boundary zone between elementary sensations on the one hand and higher level abstractions (such as gender and personality, and even emotions; e.g., sandpaper evoking the sensation of jealousy) on the other that the phenomenon intrigues us and provides an experimental lever for investigating high-level mental processes.
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