Synaesthesia

A "cross talk" effect in the brain in which one sensory pathway links across to another, resulting in two outputs from one input.

The commonest form of synaesthesia is the linking of colours to printed letters and numbers. The letters may appear to be coloured even though they are printed as black. Other effects are to "see" music in colours. It is estimated that about 1 in 2500 people are synaesthetes, of whom more are women than men and a high proportion are left-handed.

See also: Hearing.