It is hard to accurately gauge numbers, because it is not something that people are wont to talk about too frequently. Both because there are no obvious outward signs and they may not even think anything 'unusual' in it, and also possibly because if experiences are met with derision, ridicule or doubt on the first few occasions when they mention that the sound of a car horn evokes in them the taste of roast beef, or that tasting raspberry jam makes them see the colour blue then they are not likely to mention them again. Richard Feynman, the Nobel Prize winning physicist, is now thought to have been a letter-colour synesthete, but never made mention of it during his life as anything out of the ordinary, and doesn't seem to have even been aware of synesthesia as a condition.

There is a temptation to view synethetic experiences as mental aberrations, or as dodgy wiring in the brain, hallucinations that are drug induced and hence not 'real', or as linguistic metaphors where the individuals experience exactly the same as the rest of us, but somehow are just describing it differently. These views needs to be guarded against, because bizarre as Synesthesia sounds, it is not a disease, an affliction or the product of an addled or misdirected mind. True enough certain drugs like LSD can trigger synesthetic experiences; but then plenty of people don't take these drugs and yet still have these experiences. It happens to be relatively rare where peoples senses seem as though they are joined in some way, but that doesn't make it an affliction. Interestingly this 'joining' usually seems to be one way traffic. So, that a person may hear the car horn and taste roast beef, but tasting roast beef won't make them hear the car horn.

The reasons that we know that synesthesia does have a neurological basis, and is not simply a psychological phenomena is because of both subjective and objective criteria. The fact that we are viewing this sensory mixing through the opaque goggles of someone else's description could lead us to think that it doesn't really exist. However, although peoples synethetic experiences seem to be unique, in the sense that know one else (even fellow synethists), experience the same stimulus in quite the way they do; in most cases it seems to be remarkably consistent for the individual themselves. So that if they always hear their doorbell and see a blue square for example, then this is always the case. The square doesn't suddenly become a circle, or the colour turn to pink. One of the problems has been in trying to get behind the science of this; because early research concentrated on trying to find patterns and re-surfaced bemused because there didn't seem to be any. More recent research has picked up this baton, and although there still hasn't been any totally clear relationships identified, there does seem to be patterns in certain tonalities .So that certain stimulus tend to be perceived as darker colours and others as lighter colours. There are no hard and fast rules however.