The synaesthete's senses are arbitrarily linked. A true synaesthete may, e.g., see the letter 'b' as pink, or think that chicken tastes spherical, or associate back pain with lavender squiggles. These associations are constant.
Synesthesia is abnormal only in being statistically rare. It is, in fact, a normal brain process that is prematurely displayed to consciousness in a minority of individuals.
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.