What colour do you associate with the letter “E”? What sound do you hear when you feel the fluffiness of cotton? These sound strange to most people as we experience the senses in distinct ways. However, for 4% of the population, this is a completely normal experience.
Synaesthesia (“joined sensation”) is the neurological phenomenon where two or more senses are coupled together. This creates two kinds of synaesthesia: projection, where you physically sense something (such as seeing a purple circle when hearing piano music), and association, where you associate the sense with another sense (“that sounds quite orange”).
The most common form of synaesthesia that is reported is grapheme-colour synaesthesia, where certain people perceive letters and numbers as different colours. However, there are various kinds of synaesthesia, such as chromesthesia, where people associate sounds with colours (previously called “coloured hearing”). In fact, almost every combination of senses have been described, with some individuals experiencing multiple senses at the same time.
The exact origin and mechanism of synaesthesia are yet to be fully explained. The most likely explanation is that in the brain of synaesthetes, the neural pathways for the various senses cross-over more than they should, causing the simultaneous activation. There are already some cross-overs between these pathways, as evidenced by various sensory illusions that the average person can enjoy. For example, a ventriloquist can fool the audience into thinking the puppet is talking as we hear speech and see the puppet’s mouth moving.
An alternative explanation is that there may be an element of ideasthesia – where concepts are paired with sensory experience. This would mean that synaesthetes are experiencing sensations due to the idea something represents, not because of the original sensory stimulus.
For example, when a synaesthete describes that the word “tree” tastes like brie cheese, it might not be the sound of the word, but rather the concept of a tree that triggers the sensation. We actually see examples of this in day to day life in the form of metaphors. We describe a wine having a round taste or a person being sweet.
There is much to learn about the phenomenon, but synaesthesia has already deepened our understanding of how we perceive the world, process it and commit it to memory through the use of associations and mnemonics.