Synaesthesia – when our senses don’t make sense

What if we could see sounds, or taste numbers? Image 'SYNAESTHESIA' by aka Tman

What if we could see sounds, or taste numbers? Image ‘SYNAESTHESIA’ by aka Tman, licensed under CC BY 2.0

During my PhD I’ve become somewhat reliant on the steady stream of Percy Pig sweets from the M&S shop that’s conveniently located downstairs, to the point where I’m considering acknowledging Percy in my thesis. My friends and I often joke that after eating too many of the extremely sugary jellies we start to ‘see sounds’… But what if you actually could see sounds? Or taste numbers? Or feel music?

This is the case for 2-4% of the population who have a condition called ‘synaesthesia’, for which a stimulus (such as music) involuntarily triggers both the expected sense (hearing) as well as an additional sense (such as seeing colour). There are at least 60 known forms of synaesthesia, which can involve each of the five senses. For example, ‘chromesthesia’ describes the association of sounds with colour, ‘lexical-gustatory synaesthesia’ describes experiencing taste while speaking, and ‘mirror-touch synaesthesia’ involves feeling a sensation that somebody else within sight is experiencing. Importantly, synaesthesia is not a disease – it has no detrimental effect on the individual’s health. In fact, the condition is thought (at least in part) to explain differences in creativity, with higher incidences amongst artists and poets. Famous synaesthetes include actor Geoffrey Rush, artist Vincent Van Gogh and the musician Pharrell Williams who says he’d be “lost” without his synaesthesia, which he calls a “gift”.

But what causes synaesthesia? As a foetus, we start with more neurons than we need, and these are carefully ‘pruned’ throughout development. Some scientists think that different, or reduced, neuronal pruning can lead to extra connections between adjacent areas of the brain – effectively ‘cross-wiring’ the senses. The most common and best-studied form of synaesthesia is ‘grapheme-colour synaesthesia’, where the individual associates certain letters or numbers with a colour. There have been several studies showing that in synaesthetes, seeing a number activates both the part of the brain that processes the physical form of the character, but also the nearby V4 area of the visual cortex that processes colour. (The image below shows how close these brain regions are – S.)

Possible neural basis for grapheme-color synesthesia. The region of the visual pathway involved in recognizing letters and numbers (graphemes) is indicated in green, while one a region involved in color processing (hV4) is indicated in red. Due to the adjacency of these regions, there is an increased probability of connections being retained, leading to cross-activation between the grapheme area and hV4. Edward M. Hubbard, Image first appeared in Ramachandran and Hubbard, 2001. Licensed under CC BY-SA 3.0

‘Possible neural basis for grapheme-color synesthesia. The region of the visual pathway involved in recognizing letters and numbers (graphemes) is indicated in green, while one a region involved in color processing (hV4) is indicated in red. Due to the adjacency of these regions, there is an increased probability of connections being retained, leading to cross-activation between the grapheme area and hV4. Edward M. Hubbard: Image first appeared in Ramachandran and Hubbard, 2001.’ Licensed under CC BY-SA 3.0

So, who experiences synaesthesia? Most scientists agree that there is a genetic element to the syndrome, which appears to run in families. Although we don’t fully understand the genetic basis of synaesthesia, linkage studies have suggested that the condition may be influenced by multiple genes. However, it is typical that synaesthesia does not affect all family members, and related synaesthetes do not always experience the same type of synaesthesia. There has even been a case of monozygotic twins where only one sibling is a synaesthete. So although your genetics may predispose you to the condition, it is not the only factor controlling its manifestation.

There is some evidence that synaesthesia can be learned. Olympia Colizoli at the University of Amsterdam showed that non-synaesthetes who had been reading books in which the letters e, t, a and s were coloured began associating those particular letters with their assigned colours. When shown cards displaying coloured letters, if the e/t/a/s was coloured a different letter from the books, participants took longer to identify what colour it was.

It has also been suggested that brain injury can cause synaesthesia. After suffering a stroke within the sensory hub of his brain known as the thalamus, a 45-year old male reported experiencing symptoms of synaesthesia, which were confirmed by an MRI measuring his brain activity. Suddenly, he hated the colours sky blue and yellow (but yellow wasn’t deemed as bad as sky blue) – and raspberries tasted blue (but a ‘good’ shade of blue). Similarly, the James Bond theme tune triggered a feeling of ecstasy. Scientists believe that these new inter-sensory connections were formed while his brain was trying to repair the damage caused by the stroke.

Perhaps unsurprisingly, synaesthesia can also occur temporarily after taking hallucinogenic drugs like LSD. In general this form of the condition is mostly audio-visual and less consistent than genuine synaesthesia – the colour of a sound might change from one moment to the next. It is thought that the drugs enhance chemical reactions within the brain by increasing the activity of the neurotransmitter serotonin and limiting the activity of the neutral inhibitor γ-aminobutyric acid (GABA).

Although the senses of synaesthetes don’t make perfect sense, you could argue that the qualities imparted by synaesthesia contributes hugely to our humanity. The ability to see the world in metaphors has had a massive impact on literature, music and art – parts of our lives that are integral to our personalities and individuality.

– Kimberley Wiggins.

Kimberley

Kimberley is a PhD student at the University of Cambridge researching aspects of inflammation in atherosclerosis. When not in the lab she loves photography and blogging her travels around the world here. Follow her on Twitter at @kimmy_w5!

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