WHAT IS SYNAESTHESIA?

Connecting senses in fascinating ways means some people see the world completely differently

- CLAIRE ASHER

Synaesthesia is a perceptual phenomenon where stimulation of one sense triggers experiences in another sense. For example, a synaesthete might see colours when music plays, or taste flavours when they speak different words. The word synaesthesia originates from the Greek words syn for ‘union’ and aesthesis for ‘sensation’, literally translating to ‘a union of the senses’.

There are over 70 types of synaesthesia, which cause associations between different types of sensory input, but what they all have in common is that the associations are involuntary, present from early childhood, and remain consistent throughout life. It is thought that synaesthesia is caused by extra connectivity between sensory regions of the brain, so stimulation of one sense cross-activates the other. In the 1990s, sound-colour synaesthetes were blindfolded and put into a functional magnetic resonance imaging (fMRI) scanner. They displayed activity in visual regions of the brain when sounds were played – a pattern not seen in non-synaesthetes. We now know that white matter, which connects different brain regions, is organised differently in synaesthetes’ brains, and they have more grey matter in regions responsible for perception and attention. We are all born with many cross-connections between brain regions but, for most of us, these are pruned during early development. One theory is that synaesthetes’ brains go through less pruning, so they experience interlinked senses throughout life.

WHEN WAS IT DISCOVERED?