Someone with the condition known as grapheme-color synesthesia might experience the number 2 in turquoise or the letter S in magenta. Now, researchers reporting their findings online in the Cell Press journal Current Biology on November 17 have shown that those individuals also show heightened activity in a brain region responsible for vision.
The findings provide a novel way of looking at synesthesia as the product of regional hyperexcitability in the brain, the researchers say. They also provide a window into our understanding of individual differences in perception.
"Most of us tend to assume that we experience the world in the same way as everyone else, but synesthesia provides a clear example of a group that perceives the world in a fundamentally different way," says Devin Blair Terhune of the University of Oxford. "The majority of people do not have conscious experiences of color when they look at numbers, letters, and words, whereas synesthetes do. Studying these people can thus shed light on the brain mechanisms underlying conscious awareness."
Earlier studies had shown that synesthetes who experience color for numbers and letters also discriminate among colors better than those with other types of synesthesia. Those findings hinted that an overactive visual cortex might be in play.
Terhune's team, which is led by Roi Cohen Kadosh, found that average people do indeed require three times greater magnetic stimulation to their visual cortex than synesthetes do in order to experience phosphenes, transient flashes of light or other visual disturbances.
"We were surprised by the magnitude of the difference," Terhune said. "The synesthetes in our study displayed considerably greater levels of cortical excitability than our participants without synesthesia. These results point to a very large effect that may reflect a fundamental difference between the brains of those with and without synesthesia."
It's not that the enhanced excitability of the visual cortex is directly responsible for the experience of synesthesia, however. Further experiments showed that reducing the excitability of visual cortex in synesthetes actually increased their experience of colors with numbers. Meanwhile, increasing excitability in that brain region made the synesthesia more intense.
Terhune says they now suspect that the enhanced excitability of synesthetes' brains might be related to the development of the condition, but it doesn't produce the phenomenon in adults.
Ultimately, the findings might allow for treatments designed to reduce or eliminate the experience of synesthesia or to make it even more vivid, he says. The work also raises new questions in other fields that examine atypical perceptions, such as hallucinations, he says. "Might it be that the same principle is applying also there?"
Cell Press: http://www.cellpress.com
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
When you’re the size of a human, you worry about lions and tigers and bears. But if you’re …
An amateur fossil hunter has unearthed a 7ft skeleton of a carnivorous marine reptile on a beach in south Wales.
European regulators have recommended approval of the first medicine containing stem cells to treat a rare condition caused by burns to the eye.
Ecologists say birds could hear the oncoming storm from over 100 miles away
Marine scientists plumbing the deepest part of the ocean sent microphones and collection probes baited with chicken to the bottom of a trench near Guam. Now they watch, wait ... and listen.
Lead author of two retracted papers resigns her position after failing to reproduce new approach to generating stem cells
The winners of the 2014 Olympus BioScapes Digital Imaging Competition capture a rat brain, the mouthparts of a vampire moth and other small wonders
By analysing brain activity linked to hand and arm movements, a team has created a robotic arm that a paralysed woman can control with her thoughts
Adding laser tips to ordinary shoes can improve the stride and pace of people with Parkinson's disease
Technique could someday help repair injuries