Synesthesia is a rare neurological condition in which the senses mix and begin to feel for others. One synesthete woman says that "Katherine's name tastes like chocolate cake." Such people can call pain by name, hear colors, see music. Would you like to know what your name tastes like? Synesthesia is not uncommon. On average, it affects one person in two thousand and usually develops during childhood. And if you were unlucky enough to enlist the support of such an all-seeing eye in your day, perhaps scientists who have crept up on the molecular and genetic origins of this phenomenon will help you.
Amanda Tilot is a scientist at the Max Planck Institute for Psycholinguistics in the Netherlands and is the lead author of a new paper investigating the root causes of synesthesia, published this week in the journal PNAS. As you might guess, everything leads to interconnectedness.
"The first studies showing family trees with people who have had synesthesia for several generations date back 130 years," says Tilot. “The last 15 years or so have been really powerful and with a lot of work in the field of psychology to understand how synesthesia develops in childhood, how children make these connections and how they are presented from a psychological point of view. But in terms of genetics, scientists have learned almost nothing."
Tilot and her team decided to fill the gap. They turned to three families in which many family members had sound-color synesthesia. There were also members without synesthesia (which, of course, made them upset). Each member of the family tree donated DNA to Tilot for research and sequencing. Scientists first looked at differences within the family. What genetic variants did the synesthetes have that their brothers, sisters, uncles, and aunts lacked? Scientists have identified 37 interesting genes in total. But by studying the differences between the three families, they found that none of these genetic variants were repeated.
This was to be expected. “Previous attempts to find specific things that bring small families together have not had much luck,” says Tilot. So they tried a different tactic. "We decided to see what types of biological processes link families." If the genes were apparently family-specific, scientists suggested, then the processes that shaped those genes would have to be repeated among different synesthetes in different bloodlines.
To Tilot's delight, this assumption turned out to be correct. The new work concluded that the brains of people with sound-color synesthesia are unusually active in one of the divisions: axonogenesis.
Basically, axonogenesis is the formation of new nerve cells, axons. Axons connect to synapses and help transmit information through the brain. Everyone needs axonogenesis to live and think, without it you could not read this article. But in people with synesthesia, this process is slightly tweaked.
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While the visual cortex is in the back of the brain and speech production is in Wernicke's area, axons in people with synesthesia stretch further, especially during childhood, when sensory abilities develop rapidly. Perhaps this is why language is perceived in colors, taste, or even spatial dimensions.
Of course, Tilot's work is not a definitive verdict on synesthesia - it is just the beginning. More research is needed for families with high rates of synesthesia, as well as synesthetes who have no known relatives with this quirk. More importantly, Tilot says, scientists must continue to work to understand the full spectrum of synesthesia, which manifests itself in dozens of different ways.
So far, Tilot's research has focused on sound-color synesthesia, also known as chromaesthesia, where people associate sounds with colors. Such artists, for example, associate music with chords, instruments, and emotional experiences. Duke Ellington, Billy Joel, Pharrell Williams also talk about similar sensory perception.
But Vladimir Nabokov and his mother had grapheme-color synesthesias, when each letter had a shade or texture associated with it. There is also lexico-gustatory synesthesia, when words have a taste.
To delve deeper into sensory mystery, Tilot and her team recruit sound-color synesthetes for further research.
Ilya Khel
