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Hyperactive Visual Cortex Neurons May Cause Orange "O"s and Purple "P"s

By Valerie Ross
Nov 19, 2011 1:36 AMNov 20, 2019 4:52 AM


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The colors that letters and numbers appear to a synesthete

What's the News: For most of us, our senses stay relatively separate: that is, we hear what we hear and see what we see. People with synesthesia

, however, actually see words as colors, taste a particular flavor when they hear a familiar song, or experience other strong, automatic linkages between senses. The neurological underpinnings of the condition---how the brain connects two usually distinct senses---have remained a mystery. But researchers have now found a possible cause, they reported yesterday

: neurons in the area responsible for the second sensation, such as the color that goes with the word, may be unusually excitable. How the Heck:

  • Six people with grapheme-color synesthesia---the most common form of the condition, in which people associate letters and numbers with colors---and six non-synesthete controls participated in the study.

  • The researchers applied transcranial magnetic stimulation, a weak magnetic field that travels through the skull and changes neuronal activity, to each volunteer's primary visual cortex, a part of the brain that processes what we see. The people with synesthesia needed only a third as much stimulation as the other volunteers before they started seeing phosphenes---the official name for little flashes you see when you rub your eyes or "see stars." That's likely because the synesthetes' visual cortex neurons are already more active, the researchers suggest, so they need less of a boost to fire and make stars.

  • The scientists then asked the volunteers with synesthesia to describe their experiences while a similar type of brain stimulation---transcranial direct current stimulation---increased or decreased brain activity in their visual cortex. Changing the excitability of neurons there, the team found, impacted how strong people's synesthetic experiences were.

What's the Context:

Reference: Devin Blair Terhune et al. "Enhanced Cortical Excitability in Grapheme-Color Synesthesia and Its Modulation." Current Biology, November 17, 2011. DOI: 10.1016/j.cub.2011.10.032

Image courtesy of JotDee / Wikimedia Commons

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