Synaesthesia is when something experienced with one sense is spontaneously associated with a different sense. It can also be applied to a sensory reaction to an otherwise abstract concept. For instance, some people 'see' music in terms of colours and shapes. Others have a strong sense of colour and shape attached to single numbers or letters of the alphabet. For synaesthetes, this is not simply the application of a metaphor or the result of a fertile imagination, but a genuine and powerful involuntary experience.

The word 'synaesthesia' comes from the Greek syn (joining) and aisthesis (sensation). Examples of possible synaesthesia can be found in works of literature dating back to classical times. The phenomenon has been discussed by the scientific community for some three centuries, and although there are still psychologists who believe that synaesthesia does not exist as a spontaneous experience, others are recognising proof that synaesthesia is an inbuilt neurological condition.

As many as one in 2,000 people experience natural synaesthesia, and there are many different forms, linking different senses or perceptions. Thus a synaesthete may associate texture with taste, smell with colour, and so on. Some forms of synaesthesia are more common; around one in 5,000 associate colours with letters, as few as one in 15,000 will, for instance, associate taste and touch. A small minority of synaesthetes experience multiple synaesthesia, providing them with an almost overwhelming sensory 'identity' for different objects or concepts. The synaesthetic experience is in addition to the normal sensory stimulus, for instance, a person who experiences 'red' when a trumpet is played will hear the trumpet as well. They may actually feel that they are seeing a red colour with the eye, or may simply receive a powerful mental image. If a synaesthete associates the colour yellow with the letter 'E', then they may see the colour overlaid on the letter, or experience a flickering effect between the synaesthetic colour and the original text. This may explain why some synaesthetes develop a dislike of coloured text, for instance on the Internet.

Some Possible Explanations

Sceptics in the psychology and neurology world believe that synaesthesia is a simply an extreme form of association, and has no natural basis. They believe that, for instance, a colour to letter association may be 'imprinted' upon a child by using lettered building blocks of a certain colour or by reading and re-reading an ABC book (A = apple = red/green etc). However, when neurologists carried out brain scans on colour synaesthetes, they discovered increased blood flow in those sections of the brain dealing with the perception of colour - a phenomenon not present in non-colour synaesthetes. This implies that synaesthesia has a genuine neurological basis and that perhaps there are additional links between perception areas in the brains of those with synaesthesia. Another implication that synaesthesia is neurological in origin can be seen in that many non-synaesthetes will experience synaesthetic effect as a result of neurological change, due to migraine, epilepsy, brain injury or the effects of certain (especially hallucinogenic) drugs.

Testing for Synaesthesia

How do you know if what you have is synaesthesia, or just a powerful imagination? Genuine synaesthesia is spontaneous, specific, consistent and durable. In tests, blindfolded subjects have been asked to give colour associations for letters, numbers and words. Tested again, hours, days and even months later, colour synaesthetes give a perfect repetition of original associations, while control subjects are around 85% inconsistent. In addition, synaesthetes tend to give scrupulously detailed descriptions of colours, rather than the basic primary and secondary colour names.

Using Synaesthesia

One of the crucial effects of synaesthesia is that it improves memory and recall. The synaesthetic experience gives additional associations for names, numbers and sounds, which can provide a vivid link to the information, and the more forms of synaesthesia a person experiences, the better their memory is likely to be. Many memory improving techniques recommended in self-help books use artificial synaesthesia, encouraging the student to form vivid sensory associations with the information they wish to remember. For the genuine synaesthete, such techniques can simply form an encumbrance.

Although synaesthesia does not seem to be linked directly to artistic ability, synaesthetic tendencies have been claimed for Liszt, Rimsky-Korsakov, Messiaen, Kandinsky, Hockney, Eisenstein and Baudelaire. Vladimir Nabokov was certainly a synaesthete, for he writes vividly of his childhood explanation of the phenomenon is his autobiography.

Living with Synaesthesia - One Researcher's Perspective

I strongly associate letters, words and numbers with colour, shape and texture. For me the effective is cumulative; that is to say that the combination and juxtaposition of colours and shapes for different numbers or letters can give each sequence a unique 'signature'. Like most synaesthetes, I have experienced these sensations as long as I can remember, and they seem perfectly natural to me: I also assumed that everybody perceived the world in this way! When I discovered that others did not know what I meant by a 'pink-sounding word' I just thought it was another bit of personal weirdness, until I heard about synaesthesia.

I have always used it - whether I wished to or not - to remember dates, phone numbers, car registrations and so on. I coasted through school and university by simply remembering a framework of dates and formulae, and for English literature, I learnt all 300 lines of one set piece of poetry!

The associations can be bizarre, but they are very vivid. I sometimes find it infuriating, as I cannot always work out why a particular association is made. I recently received a new security code for my credit card, and immediately thought: 'That number is just like having a pebble in your mouth' - why???

I'm also a migraine sufferer, and a strongly visual person. I don't understand the complex neurology, but I feel sure that all these factors link together, and go towards making me who I am.