Do you perceive sounds as colors? Numbers as musical notes? Or, perhaps, when you eat mac and cheese, the color purple brightens your mind. Studies show that an estimated 4.4% of the population experience these unique sensory crossovers (known as synesthesia)!
While most people know of the more commonly shared examples of synesthesia (such as those listed above), it actually occurs in many more forms! For example:
Day-color synesthesia, in which people associate or see a color for each day of the week;
Mirror-touch synesthesia, in which people see something happen to someone else and physically feel it themselves (very strongly); ,
Auditory-tactile synesthesia, in which sounds cause people to feel touch-based sensations (such as pressure, pain, or temperature changes); and,
Grapheme-color synesthesia, in which letters or numbers are also perceived as colors.
As for the causes of synesthesia, before we dive into its brain-wiring, let’s look at three different types of synesthesia (classified according to how it is acquired):
Developmental synesthesia is a term for those who were born with synesthesia. People with this type of synesthesia are classified as “neurodivergent”.
Acquired synesthesia is formed as a result of brain damage; and the effects of this condition are less pronounced, and are likely to fade over time.
Drug-induced synesthesia: in some cases, psychedelics can induce symptoms of synesthesia (especially ones that are hallucinogenic). Interestingly, however, these symptoms often depend on the emotional state of the person.
Synesthesia came to the forefront of neuroscientific research in the late nineteenth century – and despite advancements in scientific knowledge, little is known about its brain mechanisms. However, multiple models have emerged to explain the different types of synesthesia; the three most prominent of these are the cross-activation model, cortical disinhibition, and the re-entrant feedback theory.
The cross-activation theory (proposed by Ramachandran and Hubbard in 2001), which first emerged as a model to explain grapheme-color synesthesia (but can be expanded to other types as well), posits that direct connections between different areas of the brain cause sensory crossovers. In the case of grapheme-color synesthesia, the visual word form area (the part of your brain which processes word shapes) sits next to hV4 (as seen in Figure 1.1), a color processing region of the brain – and connections between these two regions cause the synesthete to associate word forms with colors.
Figure 1.1
The disinhibition theory and the re-entrant theories, on the other hand, propose that synesthesia is a result of faulty inhibition. These theories suggest that when higher cortical areas of the brain (such as the parietal lobe), which are responsible for suppressing certain actions in lower cortical areas, fail to do so, non-relevant sensory pathways are activated. These non-relevant sensory pathways are those “additional” senses that synesthetes experience.
It is also possible that there is no single answer. Perhaps different types of synesthesia each are caused by their own unique neural mechanisms! Or maybe, multiple neural mechanisms work together to produce a single type of synesthesia. The possibilities are endless!
The brain is unique and incredibly complex – and the example of synesthesia demonstrates just how much.
References
A. Carmichael, D., & Simner, J. (2013, September 11). The immune hypothesis of synesthesia. Frontiersin.org; Frontiers. https://www.frontiersin.org/articles/10.3389/fnhum.2013.00563/full
Carpenter, S. (2001, March). Everyday fantasia: the World of Synesthesia. Https://Www.apa.org. https://www.apa.org/monitor/mar01/synesthesia
Hubbard, E. M., & Ramachandran, V. S. (2005). Neurocognitive Mechanisms of Synesthesia. Neuron, 48(3), 509–520. https://doi.org/10.1016/j.neuron.2005.10.012
Hupé, J.-M., & Dojat, M. (2015, March 31). A critical review of the neuroimaging literature on synesthesia. Frontiersin.org; Frontiers . https://www.frontiersin.org/articles/10.3389/fnhum.2015.00103/full
Synesthesia: Opening the Doors of Perception – Dartmouth Undergraduate Journal of Science. (n.d.). Sites.dartmouth.edu. Retrieved January 17, 2024, from https://sites.dartmouth.edu/dujs/2010/05/30/synesthesia-opening-the-doors-of-perception/#:~:text=Disinhibition%20of%20Feedback&text=This%20hypothesis%20states%20that%20in
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