Synesthesia is the involuntary, consistent crossing of the senses. It is not a disease or a medical condition, it is a neurological phenomenon. There is still a great deal that is unknown about how many people have these experiences, why it occurs, or which senses a person will experience as joined. Each synesthete has a completely individual extended perceptual experience. Their testimonies show unique world interfaces, heightened awareness of their perception, and acceptance of many external incongruities in the physical, social, and idea realms (which often makes them prone to creative activities). It hasn't been studied yet whether synesthetes tend to enjoy the comedic incongruities of Monty Python, but this one certainly does.
(Image: screen grab of my animation of a bird's call)