Can You Taste a Book By Its Cover? Understanding Synaesthesia

Written By Hannah Labor

What colour do you see when you hear the word ‘Thursday’? Is it green, red, blue or nothing at all? If I ask you to draw out a calendar, how would you draw it? As boxes of days one after the other or a continuous circle, each month a different length from the last? Or how about as you read the words of this article? Can you taste each phrase, is it sweet or is it savoury? 

If you had an answer to any of these, you could have synaesthesia. The word shares its root with “anaesthesia”, meaning “no sensation”. With the prefix of “syn”, meaning “joined together”, synaesthesia ultimately means the joining of multiple sensations, where you can experience one sensation as a result of feeling a different one. You may have even heard of this before, with many musicians and artists, such as Billie Eilish or Franz Liszt, claiming to see certain colours when hearing specific sounds, a form of chromesthesia.

Studies have found that sounds can trigger not just colours but also shapes, textures and landscapes even when hearing ones as mundane as a car horn or a flushing toilet. However, this is not the only form of synaesthesia. As so many different sensations can be combined, there are a multitude of types, with different responses elicited in different people. Spatial sequence or “number form” synaesthesia, for example, is where a person experiences numbers or units as having specific locations in space. In a calendar, some people may view the month of February as floating in the air at a fixed distance from their body. Or lexical-gustatory synaesthesia, the ability to taste words, where someone’s name could taste like Smarties, or even feel itchy when read. 

Crossing Wires and the Senses:

The most common form of synaesthesia is grapheme-colour - where numbers and letters evoke colours - and there have been multiple studies investigating its neurological basis. Research suggests it is caused by a cross-activation between grapheme regions - responsible for recognising written units like letters and numbers - and colour-processing area V4 in the visual cortex. These connections arise from a bottom-up processing stream, indicating it is sensory in nature rather than the result of higher-level associations. These cross activations result from excessive neural connections between these regions, due to reduced neural pruning - a natural process in the development of the nervous system where the brain eliminates extra neurons and synapses to increase the efficiency of neural transmissions. 

Consistent with this theory, the brains of synaesthetes have also been shown to have distinct anatomical differences in the inferior temporal lobe of the brain, a region that plays a key role in visual processing, recognising shapes, graphemes and colours. In synaesthetes, this area had a higher fractional anisotropy, meaning there are more and better organised white matter tracts. These enhanced connections could facilitate the “spilling over” of grapheme-related signals into colour-processing regions. This could help explain other forms of synaesthesia too. 

Chromosomes or Coincidence?

As well as a neural explanation for synaesthesia, there are also theories of a hereditary basis, with trends of this phenomenon running in families and a high number of transmissions from parents to offspring. However, the type of synaesthesia varies within families, indicating its genetic basis may not be specifically for how it is expressed but rather a general predisposition to synaesthesia as a whole. People with one type of synaesthesia have a much higher chance of expressing another type as well, supporting the theory that some forms are more closely linked as clusters with a common origin.

There is also the question of why the synaesthesia gene has been preserved throughout evolution and if it may have served a separate purpose. There are many potential theories for this, such as these cross-modality experiences being present in the whole population and through normal distribution, synaesthesia just represents the extremes of it. Due to deprivation of sensory input in some brain areas, other senses may have compensated instead - this is similar to how those with arm amputations, despite a loss of tactile sensation, can experience touch in the phantom limb through the visual stimulus of seeing another’s hand being touched. 

The Hidden Agenda:

The idea of synaesthesia having a “hidden agenda” is similar to how sickle cell anaemia is related to malaria resistance. Synaesthesia is suggested to relate to creativity, particularly since it is so prevalent in artists and musicians. Some ideas suggest that with any given word there is only a limited set of associations or ideas that can stem from it. But through the overlapping of sensory associations, the basis of metaphor can expand and allow for a larger scope of creativity. However, there is still no conclusive evidence for this.

Another potential benefit of synaesthesia could be improved memory. Through using synaesthetic associations, one can memorise number sequences, such as phone numbers, using number-colour relations. For example, a man named Daniel Tammet used his abilities to memorise 22,514 digits of pi! This highlights the way synaesthesia has the ability to unlock new and undiscovered potential in the way we take in and remember information. 

But do you really have it?

Many people often confuse synaesthesia with associations with memories of past experiences. After all, synaesthesia is only present in 2-4% of the population and is much rarer than people think. Unfortunately, you may believe Thursday evokes the colour green, but may have instead recalled it being written in green on your calendar in your childhood bedroom. The phenomenon of synaesthesia is separate to associations from memory. Where most would spend some time searching for a link using memories in their brains, synaesthetes would recognise the association almost instantly since it occurs so early on in their perception process.  

“Normalised” synesthesia:

For those without synaesthesia, hallucinogenic drugs could be a synthetic way to elicit similar experiences. Although still poorly understood, serotonin agonists, such as LSD, have been shown to produce this transient effect, stemming from the theory that synesthesia originates from the over-expression of the serotonin 2A receptor on chromosome 13. 

Another possible way non-synaesthetes have most likely already experienced synesthesia is through the evolution of words like “disgusting”. Across cultures, the term “disgusting” elicits a frown or a grimace to express something that tastes bad or is morally bad. One proposition was that the frontal lobes of the brain that previously differentiated taste and smell were overtaken throughout evolution when morality and novel social norms needed to be defined. Therefore, some cross-wiring occurred that allowed the close association between taste and morality. This in turn could potentially be a form of “normalised synaesthesia”. 

Synaesthesia is still not a fully understood phenomenon, with many questions left unanswered. Despite it being previously believed to be conceptual or abstract in nature, it has the potential to give us essential guidance on how the brain and its senses really work. Its impact will continue to span the music we listen to, the art we admire, the everyday words we say and all the different ways we view the world around us. 

So, after reading this article, do you think you have synaesthesia?  

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