Synesthesia is a condition that occurs in roughly 4% of the population in which perception of a physical stimulus triggers an automatic experience from two or more senses that are unrelated. [1] Synesthesia represents a relationship between an inducer (the physical stimulus, such as a written letter) and a concurrent (another sensory response, like the color yellow).
The cause of this phenomenon is not widely understood, but some theories involve it being an exaggerated form of cross-modal perception, due to the crossing of neuronal sensory networks, or a disruption in standard neuron pruning that typically occurs in young babies. [2]
What are some common forms of synesthesia?
Grapheme-color: Where written symbols (like letters and numbers) are linked with colors. Chromesthesia: Where auditory signals (like music) are linked with colors. Personification: Where graphemes appear to the synesthete with personalities (like a flirty young woman representing the number 6). Sequence-spatial: Where sequences (like sequences of numbers) are experienced in spatial arrays. Additionally, some synesthetes differ in the "quality" of their perceptions. This means that some synesthetes experience the concurrent as if it exists in reality alongside the inducer, whereas others synesthetes experience the concurrent as a response that evokes an emotional association. [3]
It is not clear what underlying differences exist between the forms of synesthesia or why synesthetes with the same form experience different concurrent responses to the same stimuli.
What does the world look like to a synesthete?
How is synesthesia diagnosed?
It is often difficult to "diagnose" synesthesia, as many synesthetes do not realize their sensory experience is unusual. Current tests for synesthesia rely primarily on memory-based assessments of associations with concurrents. [1] Neuroimaging techniques have been largely unsuccessful at targeting one area of the brain that consistently contributes to experiences of synesthesia. [4] Despite efforts to target a true cause of synesthesia, the neurobiological and psychological roots are still a mystery to researchers.
Who are some famous synesthetes?
Duke Ellington, musician
Wassily Kandinsky, artist
Vincent Van Gogh, artist
Charli XCX, musician
Vladimir Nabokov, author
Pharrell Williams, musician
Synesthetes are 8 times more likely to work in a creative industry than non-synesthetes. [5]
What is yet to be understood about synesthesia?
Scientists have still not settled on a widely agreed-upon definition for synesthesia. Although the many different types of inducer-concurrent relationships (i.e. grapheme-color) have been observed, the reason why there are different forms of synesthesia and why synesthetes with the same form have different concurrents (i.e. see different colors in response to the number 7) or have a different quality of response is not understood. Additionally, the neurobiological mechanism underlying the disorder is still an avenue of future research, despite many research groups attempting to tackle this question. And finally, the complex heritable nature of synesthesia is not well-known. While synesthesia has been found to run in families, family members often do not experience the same form or the same concurrents.
What is the ROBO3 gene?
ROBO3 - which stands for roundabout guidance receptor 3 - is a gene expressed during the early development of the central nervous system. [7] ROBO3 is involved in regulating and guiding a process called axoneogenesis, or the formation of axon networks within the brain. [8] Specifically, ROBO3 has been expressed during the development of axons called commisssurals - axons that forge relationships between the two sides of the brain. Commissural axons frequently carry information about voluntary muscle movement and sensory inputs. Human mutations in ROBO3 are known to contribute to the rare disease of horziontal gaze palsy with severe scoliosis. One key symptom of this disorder is midline crossing defect. [7] Homologs of roundabout guidance receptors like ROBO3 are present in many vertebrates, but ROBO3 is primarily conserved in mammalian organisms. [8]
How does ROBO3 connect to synesthesia?
New research has implicated ROBO3 as a gene of interest in the underlying mechanism causing synesthesia. [9] Using gene ontology analyses and neuroimaging-based hypotheses, researchers identified six genes containing rare variants among 43 families that all contained members with sound-color synesthesia. A few specific variants are believed to impact ROBO3's regulation of axon guidance. This regulatory pathway would provide support for the theory that neuronal networks that process sensory information in a synesthete's brain may become crossed.
Specifically in this project I am interested in exploring the relationship between ROBO3's regulatory guidance of commissural axons and the potential existence of intersecting neural networks carrying sensory signals. By combining these new bases of knowledge, I believe more can be learned about the intriguing condition of synesthesia.
References
Witthoft, Nathan, and Jonathan Winawer. “Learning, Memory, and Synesthesia.” Psychological Science, vol. 24, no. 3, 2013, pp. 258–265., www.jstor.org/stable/23355114.
Simner, Julia, et. al. "Synaesthesia: The prevalence of atypical cross-modal experiences." Perception, vol. 35, 2006, pp. 1024-1033., https://doi.org/10.1068/p5469.
Hupe, Jean-Michel, and Dojat, Michael. "A critical review of the neuroimaging literature on synesthesia". Frontiers in Human Neuroscience. 2015. https://doi.org/10.3389/fnhum.2015.00103
Simner, Julia. "Why are there different types of synesthete?" Frontiers in Psychology, vol 4: 558, 2013. doi: 10.3389/fpsyg.2013.0055.
Friocourt, Francois, and Chedotal, Alain. "The Robo3 receptor, a key player in the development, evolution, and function of commissural systems." Developmental Neurobiology, 2016. https://doi.org/10.1002/dneu.22478.
Tilot, Amanda K., et. al. (2018). Rare variants in axonogenesis genes connect three families with sound–color synesthesia. Retrieved from https://doi.org/10.1073/pnas.1715492115.