"Grapheme-color" synesthesia is the most common variant of the condition. These synesthetes associate letters and numbers with particular colors; for example, a person might consistently experience the color green with she sees the letter Q, or blue for the number 4. Since grapheme-color synesthetes are also especially good at telling colors apart, researchers at Oxford guessed that these people are extra-sensitive in the visual centers of their brains.
The researchers used magnetic stimulation to tickle the visual cortices of synesthetes' and non-synesthetes' brains. Passing a certain threshold of stimulation causes people to see flashes of light called phosphenes. In this experiment, normal people needed three times as much stimulation as synesthetes before they began to see phosphenes. In other words, as predicted, the visual cortex of a person with synesthesia is "hyperexcitable"--it's more easily stimulated than the brain of a non-synesthete.
(As a control, the researchers tested how much brain stimulation it took before subjects' hands started twitching. The threshold was the same for synesthetes and non-synesthetes. But just thinking about this experiment exceeds my threshold for the willies.)
What does hyperexcitability in the brain have to do with synesthesia? The obvious hypothesis is that it immediately causes synesthesia: People with overly sensitive visual centers in their brains have experiences of color that are below the level of consciousness for normal people.
To test this, the researchers did some more brain stimulating while they had synesthetes perform a task to trigger their synesthesia. Subjects were shown a number followed by a color and asked to name the color. If the real color matched the color they automatically perceived with the number, subjects could identify the color more quickly; if the colors didn't match, subjects made more mistakes.
The researchers used two different types of stimulation on subjects' brains. One type of stimulation increased excitability, making their visual cortices even more sensitive than usual. The other type of stimulation would have the opposite effect, quieting down the hyperactivity in their brains.
If hyperexcitability were causing the experience of synesthesia, then stimulation that increased excitability should make subjects even more synesthetic, while stimulation that quieted the brain should make them more normal. But the opposite was true. When their visual cortices were less excitable, subjects experienced more powerful synesthesia than usual (as measured by their performance on the color-naming task).
So even though synesthetes have hyperexcitable brains, toning down that excitability actually makes them more synesthetic. Luckily, we don't all have to overwork our own brains trying to resolve this paradox: the authors have a hypothesis.
People born with hyperexcitable visual centers, the authors say, may develop grapheme-color synesthesia when they're very young. Because their brains are extra sensitive to visual stimuli, the symbols and colors around them get tied together abnormally in their perception.
But as those synesthetes mature, their brain areas become more specialized. The synesthesia is locked in, and the hyperexcitable visual cortex no longer drives it. Instead, all that extra noise in the brain drowns out the synesthetic effect somewhat. So when the overactive parts are quieted down, as in this study, the synesthesia comes through even more clearly than usual.
This was a small study, and even if the theory accurately describes the synesthetes involved, it might not apply equally to others. The experience of synesthesia can vary widely between people. But if the hyperexcitability theory is true, then this weird paradox might be a kind of blessing to synesthetes. As it is, synesthesia isn't considered a disorder or hindrance; it's just a colorful quirk. If the very brain feature that created their synesthesia weren't now drowning it out, though, maybe synesthetes would experience the world as an overwhelming sensory carnival.