The singer Olivia Rodrigo said that her song ‘drivers’ licence’ is purple, and that songs she has written include hues of ‘purple-blue’ and ‘bright red’. An American doctor named Joel Salinas witnessed a cardiac arrest during his first week of practice in medical school, and he found himself somatically experiencing the sensation of dying.

With every physical touch he witnesses between people, and with every needle he pokes into a patient, he feels these sensations in his own body. This interfered with his line of work as a neurologist, causing sensory overload upon entering the hospital where he works. Meanwhile, I have seen numbers as colours for as long as I can remember; my mum recorded a home video of a family trip to Spain in 2003, and upon watching it as a child, I found that the screen was tinged yellow by the time stamp in the corner. 

What exactly is colour synaesthesia? There are numerous different types, including but not limited to: auditory-tactile, day-colour, grapheme-colour, hearing-motion, mirror-touch, and time-space synesthesia.

These whimsical-sounding terms are all variations on an individual’s innate association of different sensory experiences which would not typically be considered related. For example, visual perception of a number evokes a colour; thinking about the number three causes you to vaguely see the colour yellow. Or, watching somebody else get poked with a needle means that you feel the poke yourself. 

…thinking about the number three causes you to vaguely see the colour yellow

A similar, perhaps more widely-recognised quirk of perception is perfect pitch, or absolute pitch – when somebody can identify musical notes and pitches without the use of references. They can name the key of a piece of music by ear, and name the pitches of non-musical sounds such as car horns.

They may be able to sing a specific note when asked. This ability is considered a unique difference in auditory perception, and is welcomed as a musical gift. The idea of “coloured hearing” existed in Greek antiquity, and Isaac Newton believed that musical tones could be comparable to colour tones. 

Synaesthesia appears to lend itself to art and creativity, having enriched the works of visual artists such as Vincent Van Gogh, musicians such as Billie Eilish and Lorde, and composers such as Ramin Djawali, creator of the musical scores for Game of Thrones, Westworld and House of the Dragon.

In literature, synaesthesia has been used to represent the Romantic ideal of ‘transcending one’s experience of the world’, such as in Vladimir Nabokov’s ‘The Gift’ – in other words, making contact with the extrasensory world, the one which deviates from the ‘norm’. 

…the neural mechanisms responsible for synaesthesia remain largely unknown

While the neural mechanisms responsible for synaesthesia remain largely unknown, perception in the context of synaesthesia can be used as an example of the diversity of the human experience. We think of blue as an inherent property of the sky, or green as belonging to grass, rather than as human constructions. Inconsistency between individuals means that one person’s blue is different from another’s, yet we may assume that everyone’s blue is the same.

In order to comprehensively investigate scientific reasoning for an unusual phenomena, first it is necessary to understand what exactly is being investigated and why: after all, Anil Seth, professor of neuroscience at the University of Sussex, argues that embracing perceptual diversity could be ‘transformational for society’. He describes individual minds as ‘personalised inner universes’; in mine, 2003 happens to be yellow. ‘There isn’t just one beautiful world out there,’ Seth says. ‘There are many.’ If that’s the case, then everyone has access to their own realm of perception, synaesthetic or not. Outside of the norm exists an unexpected way of life.

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