Seeing Sounds and Tasting Letters: The Colourful World of Synaesthesia

If synaesthesia results from a lack of neural pruning and neural pruning takes place during development, were we all born as synesthetes?

Did you know that there are people with superpowers amongst us, who can hear colours or taste sounds? Around every 1 in 2000 individuals in the world have synaesthesia, a rare condition caused by the abnormal wiring of the brain (Baron-Cohen et al., 1996). Synaesthesia is the experience of a specific modality of senses in the absence of real stimuli, when another modality of sense is being experienced (Hubbard & Ramachandran, 2005). To put it more simply, synaesthesia is a condition where an individual associates numbers, letters or even sounds with colours or can taste sounds or hear colours. This sounds like an extra-terrestrial quality, one to be seen in a science fiction movie. However, synaesthesia, although a rare condition, can be explained by science.

The most common type of synaesthesia is grapheme-colour synaesthesia (Hubbard & Ramachandran, 2005). In this form of synaesthesia, individuals report some kind of an innately provoked connection between letters/numbers and colours (Paffen et al., 2012). For instance, they perceive the number two as red, the number five as green and so on. The associated colours may change according to individual, but are stable over time (Martino & Marks, 2001). A rather ingenious test designed by Ramachandran tackled the question of whether these experiences were imaginary or real sensory experiences (Ramachandran, 2019). The Ishihara test used in colour-blindness scanning, which involves distinguishing a red number from a green background or vice versa, was re-adapted to test for synaesthesia. Instead of coloured dots, the display consisted of numbers two and five written in digital font, which are very hard to distinguish for normal participants. It was found that synesthetes, who claimed to associate numbers with colours could solve the test very quickly as they could see the “red” shape formed by twos popping out from the green background consisting of fives (Ramachandran, 2019). This test provided evidence for synaesthesia being a real sensory phenomenon and not simply a product of imagination.

There are other types of synaesthesia that occur in much lower rates, including tactile, gustatory and auditory synaesthesia. An interesting example of complex synaesthesia was observed in patient SC, who had tactile, olfactory and gustatory sensations of food upon hearing music or being exposed to any form of language (Colizoli et al., 2013). The experienced sensations represented specific dishes such as potatoes with gravy sauce, far from creating only general sensations such as sourness or sweetness. SC’s synaesthesia was unidirectional, meaning that food sensations did not elicit linguistic or musical sensations. Most observed cases of synaesthesia are similarly unidirectional, but not all of them elicit experiences of the same strength (Martino & Marks, 2001). Colour-grapheme synaesthesia may be regarded as a weak form of synaesthesia, whereas an example of strong synaesthesia is the patient Carol who could diagnose herself based on the colours that her pain evokes (Martino & Marks, 2001). Patients like Carol, who draws portraits to understand her emotional and physical well-being, and SC who makes voice portraits, can turn their synesthetic qualities into advantage through art.

Another striking example of how synaesthesia can be a great asset was demonstrated by David Temmet who could learn any language to native fluency within a week. The synesthetic savant was invited to Iceland to learn the Icelandic language, which is deemed the hardest language to learn. The results were shown on live television one week later, fascinating large audiences. Temmet could speak Icelandic language to an advanced level and answer questions from native speakers. He explained his ability with the way he experiences many senses together, allowing him to remember foreign words as colour sensations and sensory experiences. As demonstrated in this example, synaesthesia is not a deficit or a disorder, but rather a rare ability that could possibly enable superior memory and learning abilities (Watson et al., 2014).

Synaesthesia is a fascinating phenomenon, but what is the mechanism underlying this condition and why is it so rare? One explanation for synaesthesia is that it occurs due to the continuation of connections that are normally destroyed via neural pruning (the removal of connections that are not necessary) or an uninhibited communication between two sensory areas in the brain (Hubbard & Ramachandran, 2005). For example, grapheme-colour synaesthesia is explained by a connection between the colour and number centres lying in close proximity in fusiform gyrus of the brain (Ramachandran & Hubbard, 2001). Another explanation points to genetics or acquired changes in the brain, for instance through injuries, as the reason for synaesthesia (Grossenbacher & Lovelace, 2001). Moreover, some drugs such as LSD are suggested to elicit a pharmacologically induced temporary synesthetic period (Grossenbacher & Lovelace, 2001). Although there is a variety of opinions on this issue, the exact cause of synaesthesia is still not well-established.

The conflicting explanations for the mechanisms underlying synaesthesia provoke further questions. If synaesthesia results from a lack of neural pruning and neural pruning takes place during development, were we all born as synesthetes? There are opposing views on this issue. One study tested infants with displays of a ball moving up and down while either a high or a low sound pitch was simultaneously played (Walker et al., 2010). High-pitch sound and ball standing high on the display and low-pitch sound with low height on the display represented congruent auditory-visual pairing. Low pitches matched with the ball high on the display and vice versa indicated incongruent pairing. Congruent pairings were preferred by infants compared to incongruent pairings, according to looking time measures. It was concluded that infants have synesthetic qualities because they prefer congruent auditory-visual pairings. These findings were opposed by another study which not only failed to replicate these findings, but also argued that the preference for congruent pairings indicate an ability to detect the interaction of sound and visual cues rather than synaesthesia (Lewkowicz & Minar, 2014). Investigation of this issue is very complicated as infants cannot articulate their opinions and experiences. Research of synaesthesia in infants depends mainly on preferential looking tasks, the results of which should be interpreted very carefully.

The study of synaesthesia can teach us about how sensory modalities interact, the interaction of neural components comprising these modalities, and how the interaction of senses can enhance other cognitive systems, leading to better learning and memory. This interesting phenomenon may provide insights on how learning techniques involving the use of multiple senses can be used in education. The research on this area may have potential implications on the way education is shaped and the way that learning mechanisms as well as sensory mechanisms are understood; hence bearing significant importance for scientific progression.

References

Baron-Cohen, S., Burt, L., Smith-Laittan, F., Harrison, J., & Bolton, P. (1996). Synaesthesia: prevalence and familiarity. Perception, 25(9), 1073–1080.

Colizoli, O., Murre, J. M. J., & Rouw, R. (2013). A taste for words and sounds: a case of lexical-gustatory and sound-gustatory synesthesia. Frontiers in Psychology, 4. Available from: https://www.frontiersin.org/articles/10.3389/fpsyg.2013.00775/full

Da Vinci metodu ile dünyanın en zor dilini 1 haftada öğrendi. (2018). Accessed on 10th February 2020. Available from: https://www.youtube.com/watch?v=rOjelpjwkGg

Embodied Brains and Disembodied Minds TEDxUCSD. (2019). V.S. Ramachandran. Accessed on 10th February 2020. Available from: https://www.youtube.com/watch?v=3PVatC2Noyo&list=LL9FkLBeQyFhNOUR8z28dseg&index=2&t=0s

Grossenbacher, P. G., & Lovelace, C. T. (2001). Mechanisms of synesthesia: cognitive and physiological constraints. Trends in Cognitive Sciences, 5(1), 36–41. Available from: http://www.daysyn.com/GrossenbacherLovelace2001.pdf

Hubbard, E. M., & Ramachandran, V. (2005). Neurocognitive Mechanisms of Synaesthesia. Neuron, 48(3), 509–520. Available from: https://www.sciencedirect.com/science/article/pii/S0896627305008354#bib72

Lewkowicz, D. J., & Minar, N. J. (2014). Infants Are Not Sensitive to Synesthetic Cross-Modality Correspondences. Psychological Science, 25(3), 832–834. Available from: https://journals.sagepub.com/doi/full/10.1177/0956797613516011

Martino, G., & Marks, L. E. (2001). Synesthesia: Strong and Weak. Current Directions in Psychological Science, 10(2), 61–65. Available from: https://journals.sagepub.com/doi/pdf/10.1111/1467-8721.00116

Paffen, C., Smagt, M. V. D., & Nijboer, T. (2012). Color-grapheme synaesthesia affects binocular vision. Journal of Vision, 12(9), 682–682. Available from: https://www.frontiersin.org/articles/10.3389/fpsyg.2011.00314/full

Ramachandran, V. S., & Hubbard, E. M. (2001). Psychophysical investigations into the neural basis of synaesthesia. Proceedings of the Royal Society of London. Series B: Biological Sciences, 268(1470), 979–983. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1088697/

Walker, P., Bremner, J. G., Mason, U., Spring, J., Mattock, K., Slater, A., & Johnson, S. P. (2009). Preverbal Infants’ Sensitivity to Synaesthetic Cross-Modality Correspondences. Psychological Science, 21(1), 21–25. Available from: https://journals.sagepub.com/doi/full/10.1177/0956797609354734

Watson, M. R., Akins, K. A., Spiker, C., Crawford, L., & Enns, J. T. (2014). Synesthesia and learning: a critical review and novel theory. Frontiers in Human Neuroscience, 8. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3938117/