One childhood dream persisted in me for a long time. I can still remember with distinction the day when the idea struck me. I was on my way home after a school day, chewing on a piece of chocolate taffy. A patch of grass, shady at all other times of the day, was lit by the sunlight of 4 o’clock.
The grass looked translucent; its green color seemed unbound by its shape and diffused into the light beam. I was enraptured and stood there for a long time, watching colors and dust dance in the divine natural light. And when I finally withdrew myself from the moment, a thought flashed through my mind— I wish I could taste what I had just seen. Just imagine a taffy that tastes like the color of grass dissolving in a light beam or a lollipop that tastes like the clear sky of August. Thinking about all the spectacles that I would like to taste, I spat out the sticky taffy that only tasted mundane.
The eleven-year-old me probably didn’t expect that there are people in this world who could actually taste what they hear, hear what they see, or see what they touch. This phenomenon where the stimulation of one sensation leads to the automatic activation of another is known as “synesthesia”. Only in recent years has synesthesia been a subject of serious research . Prior to the advent of neuroimaging techniques, the
detection and understanding of synesthesia relied largely on synesthetes’ self-report, which made the existence of synesthesia a matter of debate— are people who claim to be synesthetic just more imaginative than the average people (Banissy, Jonas, & Cohen Kadosh, 2014)? In 2002, using functional magnetic resonance imaging, researchers showed that synesthetes who see color in response to spoken words had activation in brain areas that are specialized for color perception. Even when they trained the control subjects to learn color-word associations and specifically asked the control subjects to imagine the color when hearing its associated word, researchers were still unable to induce the activation in the same color perception area that was observed in “color-hearing” synesthete (Nunn et al., 2002). This is one of the many evidence that settled the debate— synesthesia is a real thing. It is involuntary, automatic and likely not caused by mere associative learning.
So, what could account for synesthesia? Synesthesia is generally believed to be caused by over-connectivity between two sensory modalities that allows for “cross-activation” upon the stimulation of one modality (Brang & Ramachandran, 2011). To simply this statement, we could understand this over-connectivity as two brain regions “talking” more with each other than they should. At this point, it is appropriate to introduce the concept of neuroplasticity. Our brain is
a highly dynamic organ. Throughout our lifetime, our brain could change in every way imaginable—the size of brain regions could change, the number of neurons or synapses could change and the location where neurons’ axons set camp could change (Sapolsky, 2017). Among all the possible changes, the most pertinent to our current discussion is how the number of neurons or synapses could change. We are born with more neurons than what are found in the adult brain. Why? During late fetal development, neurons engage in a sort of competition, or a micro-scale natural selection if you will, where neurons that fail to migrate to the correct position and make strong synaptic connections get pruned away like extra branches being chopped off a shrub (Sapolsky, 2017). As a fetus, we have many interconnected regions that, if things go according to the manual book, would be disconnected as we develop. However, if something goes awry in the pruning process, the interconnection might be preserved, which explains why two sensations like tasting and seeing could become wired to each other (Brang & Ramachandran, 2011).
There is also a genetic component to synesthesia. It has been suggested that synesthesia is hereditary with ~40% of synesthetes reporting having a first-degree relative who has the same condition (Brang & Ramachandran, 2011). In fact, the rapid development of DNA sequencing techniques has already led researchers to fish out some genes that are associated with synesthesia by studying the entire set of genes that could code for proteins from three families that have sound-color synesthesia (Tilot et al., 2018).
Synesthesia is very complex as all stunning tricks of the brain are. Genetics and the resulting over-connectivity may be the basis that provides the necessary condition for developing synesthesia, but how synesthesia pans out for different individuals might also be influenced by experience and learning. There is a case study (Ward & Simner, 2003) about a man who could “taste” words. To this man, the word “parents” tastes like apple, “academy” tastes like chocolate, “this” tastes like “bread soaked in tomato soup” and “safety” tastes like “toast lightly buttered”.
The question is, why do these words taste the flavor that they do? For example, wouldn’t it make more sense if “academy” tastes like bitter black coffee? To approach this problem, researchers hunted for a pattern underlying the word-flavor pairs. It was found that there are phonemes (i.e. units of sounds) that were critical and determined what a word would taste like. These critical phonemes often come from the name of a food. For example, post, past, coast, most all tasted like toast for this synesthete, and the word toast itself tastes like toast. There are more examples like this: union tastes like onion and Barbara tastes like rhubarb. Importantly, this suggests that learnt verbal meanings could affect how synesthesia is manifested. But as always, there are irregularities that the researchers cannot yet account for. For instance, why don’t the words host and boast taste like toast as well? Why would the sequence of phonemes matter (why do fox and scoff taste different when they are made up of the same set of phonemes)?
There are still a lot of mysteries surrounding synesthesia that await a solution. I personally think that synesthesia is the closest that we have to a super power in the real world. I’m not born with over-connected sensory modalities and I’m way past the developmental stage where synesthetic connections are strengthened, so I only get to read a paper as they are, without evoking color sensations or romantic tastes like “lightly buttered toast”. But maybe one day my dream would come true, where there will be candies that can conjure up all sorts of sceneries like the snowcap of Mount Fuji and the clouds suffused with blazing orange at dusk.
Banissy, M. J., Jonas, C., & Cohen Kadosh, R. (2014). Synesthesia: an introduction. Frontiers in Psychology, 5, 1414. https://doi.org/10.3389/fpsyg.2014.01414
Brang, D., & Ramachandran, V. S. (2011). Survival of the synesthesia gene: why do people hear colors and taste words? PLoS Biology, 9(11), e1001205–e1001205. https://doi.org/10.1371/journal.pbio.1001205
Nunn, J. A., Gregory, L. J., Brammer, M., Williams, S. C. R., Parslow, D. M., Morgan, M. J., … Gray, J. A. (2002). Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words. Nature Neuroscience, 5, 371. Retrieved from https://doi.org/10.1038/nn818
Sapolsky, R. M. (2017). Behave: The biology of humans at our best and worst.
Tilot, A. K., Kucera, K. S., Vino, A., Asher, J. E., Baron-Cohen, S., & Fisher, S. E. (2018). Rare variants in axonogenesis genes connect three families with sound–color synesthesia. Proceedings of the National Academy of Sciences, 115(12), 3168 LP-3173. https://doi.org/10.1073/pnas.1715492115
Ward, J., & Simner, J. (2003). Lexical-gustatory synaesthesia: linguistic and conceptual factors. Cognition, 89(3), 237–261. https://doi.org/https://doi.org/10.1016/S0010-0277(03)00122-7
Feature image source: Ever Widening Circles