Synaesthesia is an involuntary physical experience of cross-modal association. That is to say, stimulation of one sense causes perception of another (e.g. hearing light, seeing sound, etc.)

As a genetic disorder it is a dominant gene on the X chromosome. Usually, two senses are crossed in a single direction making 20 different possible combinations of sense.

Synaesthesia is rare, effecting about 1 in every 250,000 people. It is more common with left handed people, women, and is often seen in people with autism or ADHD.

Synaesthesia can be induced by seizure, drug use (especially hallucinogens like LSD or peyote,) neuron degeneration, brain damage or spinal cord damage.