This sensory merger, called synesthesia, was first scientifically documented in 1812 but was widely misunderstood for much of its history, with many experts thinking the condition was a form of mild insanity."It's not just that the number two is blue, but two is also a male number that wears a hat and is in love with the number seven," said study co-author David Brang, of the University of California, San Diego (UCSD).
"We're not sure if these personifications are [also a symptom of] synesthesia, but we think this is what derailed a lot of scientists from being interested in it. ... They thought these people were making it all up."
(Related: "Ball Lightning May Be All in Your Head.")
Over the past 30 years, though, a growing body of evidence has shown that synesthesia has a physical basis—for example, the brains of synesthetes are wired differently, and the condition is highly heritable, which indicates there is a genetic component.
In fact, the study authors think it's possible such a strange phenomenon has survived in an evolutionary sense because it offers people certain benefits to creative thinking.
"Ninety-five to ninety-nine percent of synesthetes love their synesthesia and say it enhances their lives," Brang said.
New Ways to See the Mind's Wiring
Early misunderstandings of synesthesia were due in part because the associations that synesthetes described were very precise and detailed, prompting some experts at the time to link the condition with mental disorders such as schizophrenia.
Another early "view held that synesthesia was a 'throwback' to a more evolutionarily primitive state," said study co-author Vilayanur Ramachandran, also a neuroscientist at UCSD.
Today scientists have tools that allow them to probe the brain in ways that were impossible 200—or even 10—years ago. (See "'Brainbows' Illuminate the Mind's Wiring.")
One such tool is a type of brain scan called DTI, short for diffusion tensor imaging, which lets scientists see the connections between different brain regions.
"We can see that there's increased connections in synesthetes between the associated [senses]," Brang said.
Visualizing these connections between sensory brain regions could help explain why certain forms of synesthesia exist and why the condition tends to be unidirectional—for example, numbers can evoke colors but colors don't typically evoke numbers.
Such studies could also help test an idea proposed by some scientists that all humans have the neural mechanism for synesthesia but it's suppressed for some reason.
Another positive development in synesthesia research is that scientists are relearning how to listen to their subjects, the study authors say.
"Listening to the subjective reports of people fell out of practice in the mid-20th century, but you can learn an amazing amount of information from just sitting down for 20 minutes and talking to a patient," Brang said.
"You can begin to trust what they're experiencing."
Synesthesia a Boon to Creativity?
Studies today indicate that synesthesia is about seven times more common in artists, poets, and novelists than in the rest of the population, and some scientists have hypothesized that synesthetes are better at linking unrelated ideas.
"We worked with a novelist years ago who swore that her synesthesia helped her pick metaphors," Brang said. "She said she would know what color a word should be even before she knew what the word was."
Some savants with synesthesia have been known to perform amazing feats of memorization, such as remembering the value of pi to 22,514 digits. Other synesthetes are able to distinguish between very similar colors or have a heightened sense of touch.
(Related: "Amnesia Destroys Imagination as Well as Memory, Study Finds.")
Despite recent advances, many questions about synesthesia remain, such as whether other animals experience synesthesia, how different brain chemicals affect the condition, and the exact role of genetics in determining a synesthete's cognitive and creative abilities.
Also, Brang said, "if it's so cool and such a great trait, why don't we all have it?"
The review of synesthesia research appears this week in the online journal PLoS Biology.
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