Synesthesia is pretty interesting. In the most common form certain colors are perceived when particular associated letters or numerals are seen (even if they are not actually colored themselves). Other more unusual forms may (for example) associate colors with the performance of certain types of swimming strokes, or involve seeing two distinct colors at the same time.
When this happens, it’s as though some wires are crossed in the brain. Research just published suggests that an effect consistent with that idea and somewhat but not entirely like it is actually happening. It seems to occur in people in whom some of their perceptual neurons are too easily excitable. Consequently, higher levels in the perceptual system register more stimuli than are actually present, as the brain attempts to make sense of the inconsistent lower level data.
The research considered the commmon type of synesthesia – grapheme-color synesthesia – where colors are associated with particular letters and numbers.
Terhune and colleagues, stimulated the visual cortex of six grapheme-colour synaesthetes by applying a magnetic coil to the scalp to produce a weak magnetic field.
They found non-synaesthetes required three times greater magnetic stimulation to their visual cortex than synaesthetes in order to experience phosphenes, transient flashes of light or other visual disturbances.
Terhune says the study is the first to show that synaesthesia is linked to hyperexcitability in the area of the brain known as the visual cortex.
“One of the really interesting things about this study is the difference in the level of excitability [between synaesthetes and non-synaesthetes] is so great,” says Terhune.
But there must be more to synesthesia than hyperactive neurons.
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.
Neuroscientists haven’t gotten to the bottom of this yet.