Researchers at the University of Padova in Italy have discovered that learning can play an important role in synaesthesia and can lead to synaesthetic behaviour even when the person isn't consciously aware of it.
Synaesthesia affects as many as one in 20 people. The most common form involves 'seeing' colours when reading words and numbers.
Dr Ilaria Berteletti and colleagues tested an Italian synaesthete using a classic test, in which the participant was shown a series of numbers presented in different colours, and asked to name those colours.
A synaesthete for whom the number two is red will find it more difficult to name the ink colour of a green two than if the number is presented in red ink, and will take longer to answer. This slowing of response is generally taken as evidence that synaesthetic experiences are real and automatic.
As predicted, the participant in this study was slower to name the colours of the presented Arabic digits when they did not match the colours that he had reported 'seeing'. Surprisingly, though, the same slowing was observed when the numbers were presented as dots, such as dice patterns - even though the participant denied seeing any colours for these types of stimuli.
The implication is that the mere concept of a number, regardless of how it's presented visually, is enough to trigger synaesthesia, even when the participant isn't aware of it.
"A lifetime of synaesthetic experiences may lead to the creation of learned associations between different classes of stimuli," said co-author Dr Edward Hubbard. "Conscious awareness of these associations is not necessary for them to affect behaviour."
The study appears in Cortex.