US scientists say they can explain why some people 'see' colours when they look at numbers and letters.
There are many types of synaesthesia
As many as one in 2,000 people has an extraordinary condition in which the five senses intermingle, called synaesthesia.
Some see colours when they hear music or words. Others 'taste' words.
The study in Neuron tracked the brain activity of people with the most common form and found peaks in areas involved with perceiving shapes and colours.
The University of California San Diego team said their findings lend support to the idea that the condition is due to cross-activation between adjacent areas of the brain involved with processing different sensory information.
This cross-wiring might develop, they believe, by a failure of the "pruning" of nerve connections between the areas as the brain develops while still in the womb.
For example, a person with synaesthesia might see red when they look at an ordinary figure '5' drawn in black ink on a white background because the red colour perception area of their brain is stimulated at the same time as the number recognition area.
The researchers conducted a series of experiments on volunteers with and without synaesthesia.
When the people without synaesthesia looked at letters and numbers only the brain areas involved with processing this information light up on brain activity scans.
In comparison, the people with synaesthesia had activity in colour perception regions as well.
Colour by numbers
Furthermore, some of the people with synaesthesia appeared to be better at 'seeing' colours than the others.
Those who had stronger colour perception had more activity in their colour perception brain areas.
Researcher Vilayanur Ramachandran said processes similar to synaesthesia might also underlie our general capacity for metaphor and be critical to creativity.
"It is not an accident that the condition is eight times more common among artists than the general population."
Dr Julia Simner, who has been studying synaesthesia at the University of Edinburgh along with colleagues at University College London, said the findings were supported by similar work looking at people who see colours when they hear sounds.
"Interestingly, we've recently analysed the letter-colour combinations of a very large number of people with synaesthesia and found that there are significant trends in their preferences.
"For example, people with synaesthesia tend to want A to be red, S to be yellow and Z to be black."
She said her research also revealed that people without synaesthesia have significant preferences for the colours of letters.
"Some of these choices were fairly obvious, such as 'O' being orange, but some were quite intriguing, and showed a similarity to those of people with synaesthesia."
Her findings are currently in press to appear in the journal Cognitive Neuropsychology.
Jennifer Green from the University of Cambridge, who has also been carrying out research in this area, said: "Some describe seeing the colours induced by letters and numbers as projected externally into space, while others report experiencing them internally, or in their 'mind's eye'.
"This research lends further support to empirical evidence suggesting that these varying descriptions represent actual differences in the way synaesthesia occurs in individuals."