Synaesthesia refers to the way in which our senses can be connected together in unexpected ways. Ordinarily, the main human senses – vision, hearing, touch, taste and smell – operate separately. In some people, however (and at some points in our development as infants), information from one sense bleeds over into another, with bizarre results.

So, for example when a person hears a sound, they may also see a colour. Or, when a word is heard, the person tastes something in his or her mouth!

The links at the end of the Senses front page have more information on the causes of synaesthesia, but it currently appears that regions of the brain responsible for processing sensory information are linked electrically (they would ordinarily be isolated from each other), and signals leak from one region to another close by, causing cross modal associations to occur.

The example we used in the show was number-colour synaesthesia, in which the viewing of numbers (and letters) can cause the viewer to see associated colours. We chose this form of synaesthesia because it's reasonably common, and more importantly it can be tested for during our show.

The principle behind the test is to show an image with many black numbers (in this case 5's and 2's) on a white background, with the 2's marking a geometric shape. Non-synaesthetes take some time to see the shape. On the other hand, someone who sees the numbers as being coloured should (very quickly) spot the "green triangle" of 2's among the background of "red" 5's. (We can show just how fast this should be by using an image with actually coloured numbers: everyone sees the shape instantly.)

The icons below link to four different number/colour images, include the signs used for the Senses show (the first couple), as well as two new pictures.

To see each synaesthesia image, click on the number below to bring up a window with the black and white version of the numbers.