How many types of synaesthesia are there ?(See Current Biology Vol 15 No 12 R399-400)
In theory, synaesthesia can occur between or within any of senses. The most commonly observed atypical activation is colour, and the most connon stimuli to cause it are days of the week or letters of the alphabet. Individuals may have several subypes of synaesthesia -- music-colour, letter-taste, day-colour, music-shape and so on - or just one. It is uni-directional, so if numbers activate colours, colours will not activate numbers.