In shaman culture, the shamanists were used to build a fire, make some music, and use herbs that have psychedelic chemicals inside them. At a time like this, they were used to seeing colours as musical notes or rhythm. As some chemicals trigger these multiple sensations, they can occur on their own. Richard Feynman, Nikola Tesla, and some other scientists; a few composers, authors, and actors can see words, formulas, notes, and voices in colours even those that are colourless or black. Not just a colour-sound dilemma; Richard Boyle reported a congenitally blind man who recognizes the colours by touching the objects1.
Synesthesia is the condition of a stimulated sensory modality that causes some unusual experiences like it is said above, in a second unstimulated modality2. The first accepted and documented case is by George Sachs in 1812. He and his sister suffered from albinism and in his thesis, he described his synthesis by using colours for numbers and letters1,2.
According to the first studies of synesthesia, it occurred at a 1/200000 ratio in the population. Recent studies show that 4-5% of the population have synesthesia4. Although synesthesia is a rare condition, it has at least 54 types2. Grapheme-colour type of it is the most studied type3. The other types and their prevalence can be seen in this table.

The synesthetic version of the Stroop test is used for understanding if a person is synthetic. The original one uses colour names that wrote in congruent and incongruent colours. Syntheses that sense “B” in green, will be faster while naming the printed colours if the colours are congruent with the synesthesia. This effect is mentioned in other synesthesia types but still differs in syntheses on measures such as in functional magnetic resonance imagining (fMRI)1.
In addition, for describing synesthesia in a person, these criteria are needed4:
- It occurs as a reflex and automatically.
- It has a place in the time-space. So, it can not occur everywhere and every time.
- It is stable and happens in the same form.
- It can not be forgotten easily.
Reasons
Synesthesia can occur as idiopathic (temporary and causeless) or acquired (because of a disease such as epilepsy). The neural basis of synesthesia has two main theories about the development: cross-activation and disinhibited feedback theories2,3.
Each sensory cortical area is specialized for one sensory modality in adults. In the case of cross-activation theory, some cortical sensory areas have not been as specialized as they must become. Because of that condition, extra connections can occur between sensory cortical areas. An experiment that was made about the brains of kittens explains there are transient connections between visual, auditory, tactile, and motor cortices. Therefore, it can be said that for babies and early childhood in humans, those extra connections which are pruned later, are functional. May these extra connections between sensory cortical areas make toddlers learn better3.
Another theory, disinhibited feedback explains that weak inhibition systems between sensory cortical areas cause the development of synesthesia. This weal inhibition system is found in individuals who do not have enough social skills. In the end, the signals that can not be stopped or weakened cause synesthetic symptoms3,4.
Synesthesia can be inherited by genes, but it depends on epigenetic mechanisms and environmental conditions. Research supports the idea that synesthesia is more common in women than in men through a dominant X-linked gene. However, recent studies provide that synesthesia may not be only X-linked and can be more complex2,3,4.
Synesthetic individuals can have advantages and disadvantages in their life. They can solve mathematical problems or see the differences between colour tones easily. Also, they have high imaginative intelligence, so they can find creative solutions. Instead of the advantages, Synesthesia always does not have to be real. Thereby, the falsified reality causes problems for the individuals in wild nature4,5.

References:
- Ward, J. (2013). Synesthesia. In Annual Review of Psychology (Vol. 64, pp. 49–75). Annual Reviews Inc. https://doi.org/10.1146/annurev-psych-113011-143840
- Hubbard, E. M., & Ramachandran, V. S. (2005). Neurocognitive mechanisms of synesthesia. In Neuron (Vol. 48, Issue 3, pp. 509–520). https://doi.org/10.1016/j.neuron.2005.10.012
- Spector, F., & Maurer, D. (2013). Synesthesia: A new approach to understanding the development of perception. Psychology of Consciousness: Theory, Research, and Practice, 1(S), 108–129. https://doi.org/10.1037/2326-5523.1.s.108
- Gerçek “Kafa Karışıklığı”: Sinestezi ve Evrim – Evrim Ağacı. (n.d.). Retrieved July 5, 2022, from https://evrimagaci.org/gercek-kafa-karisikligi-sinestezi-ve-evrim-361
- Brang, D., & Ramachandran, V. S. (2011). Survival of the synesthesia gene: Why do people hear colors and taste words? PLoS Biology, 9(11). https://doi.org/10.1371/journal.pbio.1001205
Visual References:
- Table: Ward, J. (2013). Synesthesia. In Annual Review of Psychology (Vol. 64, pp. 49–75). Annual Reviews Inc. https://doi.org/10.1146/annurev-psych-113011-143840
- Müzikte Renkler Gören Sinestezik Ressam: Melissa McCracken [Alıntı Tarihi: 5/7/2022] https://www.sanatlaart.com/muzikte-renkler-goren-sinestezik-ressam-melissa-mccracken/
Inspector: Elif BÖCÜ