What is Synesthesia?

Synesthesia (sin-ess-THEE-zh-uh) is a neurological condition where the stimulation of one sense causes the automatic experience of another sense.^1,2  Synesthesia is an intriguing phenomenon that adds an extra layer to human perception of the world.

Sensing synesthesia

There are five commonly known human senses: 

1. Sight
2. Hearing
3. Smell 
4. Taste 
5. Touch. 

Usually, one type of stimulus, like sound, will activate one related sense such as hearing. However, synesthetes (people with synesthesia) might hear a sound and also see it, smell it, taste it, or physically feel it. ²

Who has synesthesia?

Synesthetes often grow up with these unusual sensations, considering them a normal part of everyday life that are not worth mentioning. Until somebody else points out the difference, many synesthetes have no idea that they have a singular experience of the world.³

It is estimated that at least 4.4% of the population experience some form of synesthesia. However, the true prevalence remains to be discovered. This is because there is limited research surrounding this condition and there are many complexities relating to its manifestation. 

Synesthesia usually develops early in childhood, though rare cases have shown it can emerge later in life. Interestingly, the occurrence of synesthesia appears to be evenly distributed among men and women. ⁴

Types of synesthesia

More than 100 variations of synesthesia have been reported, and individuals can experience multiple types. Some of the most well-known types are as follows.^3,5

Grapheme-colour synesthesia: Seeing letters and numbers as having associated colours.

Sound-colour synesthesia: Hearing sounds that trigger the seeing of colours.

Lexical-gustatory synesthesia. This involves thinking, hearing, or reading a word that leads to a specific taste in the mouth.

Mirror-touch synesthesia: Seeing someone else experience a tactile feeling (e.g., pain) which leads to feeling the same sensation. People with this type of synesthesia are known to have higher empathy.

Categories of synesthesia

Each of the listed variations can be categorized as either projective or associative. In projective synesthesia, human perception involves literally seeing colour or shapes, hearing a sound, or feeling an object when the unrelated sense is triggered. Associative synesthesia creates a strong and involuntary connection between the stimulus and an unrelated sense. 

For example, someone with sound-colour synesthesia might hear a trumpet and visually perceive an orange triangle (projector). At the same time, another person might strongly associate the sound of a trumpet with the colour orange (associator).⁶

Diagnosing synesthesia

Synesthesia is not recognized as a disease or mental disorder but rather as a difference in perception resulting from increased neural connections in the brain’s sensory areas. 

Although there is no official clinical diagnosis for synesthesia, there are tests available to determine the degree to which someone experiences it. The key characteristics defining synesthesia are its involuntary nature, stability over time, and vividness. ⁷

What causes synesthesia?

While environmental factors like drugs, sensory deprivation, or brain damage can induce synesthesia, its strong genetic component is what has been extensively studied.⁴ Synesthesia runs in families, with 40% of synesthetes reporting a close relative with the condition. 

However, the type of synesthesia can vary among family members. This shows a genetic tendency for the condition but not a predictable outcome. 

For instance, parents with one type of synesthesia can have children with a completely different type. Alternatively, in the case of the specific associations between letters and colours, the type can differ between the affected parent and the child; a parent might see the number three as blue but, the child (if they inherited the same type of synesthesia) sees the number three as yellow. Research indicates that the inheritance of synesthesia is complex and involves multiple genes that have yet to be fully identified.^3,4 

Brain-imaging studies

Brain imaging studies can observe and measure synesthesia. For example, synesthetes who experience coloured hearing show increased activity in the visual areas of their brain when they hear sounds.⁴ This provides evidence of the brain processing sensory information differently.⁴

From a structural and functional perspective, scientists propose that synesthesia arises from an abundance of neural connections between different sensory areas in the brain. During fetal development, these interconnected regions, which are usually close to each other, may not undergo the usual “neural pruning” process. This pruning is the brain’s way of removing connections that are no longer needed. 

Furthermore, researchers observed changes in white and grey matter density in those with synesthesia. These changes further support the idea that synesthesia is associated with excess neural connections in the brain.^3,4

Implications and impact

While some synesthetes report occasional challenges like sensory overload and exhaustion due to excessive stimulation, many cherish their extraordinary perceptions.⁹ Synesthetes benefit from their heightened sensory experiences as they possess remarkable memories and are known to excel in artistic endeavours. Some notable synesthetes include the author Vladimir Nabokov, artist Vincent van Gogh, and musicians Pharrell Williams and Lady Gaga.^3,10

A need to explore synesthesia 

Synesthesia continues to baffle scientists and fuel curiosity. ³ It presents an excellent opportunity to delve into the complexities of human perception and unlock the mysteries of the brain. Ongoing research aims to understand better, the genetic and functional components of synesthesia and explore how environmental factors can impact this special type of human cognition.

References

1. Herman LM. Synesthesia. Encyclopedia Britannica. Accessed June 23, 2023. https://www.britannica.com/science/synesthesia
2. Synesthesia. Cleveland Clinic. Updated May 03, 2023. Accessed June 23, 2023. https://my.clevelandclinic.org/health/symptoms/24995-synesthesia
3. Ward J, Simner J. Chapter 13 – Synesthesia: The current state of the field. Multisensory Perception. 2020; 283-300. https://doi.org/10.1016/B978-0-12-812492-5.00013-9
4. Brang D, Ramachandran VS. Survival of the Synesthesia Gene: Why Do People Hear Colours and Taste Words? PLoS Biology. 2011;9(11):e1001205. doi:https://doi.org/10.1371/journal.pbio.1001205
5. Fitzgibbon BM, Giummarra MJ, Georgiou-Karistianis N, et al. Shared pain: From empathy to synaesthesia. Neuroscience & Biobehavioral Reviews. 2010;34(4):500–512. doi:https://doi.org/10.1016/j.neubiorev.2009.10.007
6. Simner J. Why are there different types of synesthete? Frontiers in Psychology. 2013;4. doi:https://doi.org/10.3389/fpsyg.2013.00558
7. Radhakrishnan R. How Common Is Synesthesia? MedicineNet. Published March 26, 2021. Accessed June 23, 2023. https://www.medicinenet.com/how_common_is_synesthesia/article.htm
8. Ward J, Hoadley C, Hughes JEA, et al. Atypical sensory sensitivity as a shared feature between synaesthesia and autism. Scientific Reports. 2017;7(1). doi:https://doi.org/10.1038/srep41155
9. Williams H. How synaesthesia inspires artists. BBC Culture. Published October 21, 2014. Accessed June 24, 2023. https://www.bbc.com/culture/article/20140904-i-see-songs-in-colour