TWO polar bears are perched on a block of floating ice. One says to the other: “Do you know, I keep thinking it’s Thursday…”
To some, this kind of surreal humour is side-splitting. Others are baffled by it and can’t even raise a smile. Yet despite the importance of humour to human psychology, it is only the advances in brain imaging during the past decade that have enabled neuroscientists to pin down how the brain reacts when a joke tickles us. Armed with this knowledge, they are now solving the puzzle of why some jokes are funny to some people but leave others cold.
So what is a joke, exactly? Most theories agree that one condition is essential: there must be some kind of incongruity between two elements within the joke, which can be resolved in a playful or unexpected way.
Take the following exchange from the classic British sitcom Only Fools and Horses, when an anxious “Del Boy” Trotter visits his doctor for a heart check-up. “Do you smoke, Mr Trotter?” asks the doctor. “Not right now, thank you doctor,” he responds.
The joke’s incongruity, of course, lies in the unlikely offer of a cigarette by a doctor to a patient concerned about his heart. It is only once we understand the mismatch that we get the joke. “Humour seems to be a product of humans’ ability to make rapid, intuitive judgements” about a situation, followed by “slower, deliberative assessments” which resolve incongruities, says Karli Watson of Duke University in Durham, North Carolina.
But which parts of the brain carry out these processes? To find out, Joseph Moran, then at Dartmouth College in Hanover, New Hampshire, used functional MRI to scan the brains of volunteers while they watched popular TV sitcoms. The experiments revealed a distinct pattern of neural activity that occurs in response to a funny joke, with the left posterior temporal gyrus and left inferior frontal gyrus seeing the most activity. These regions are normally linked to language comprehension and the ability to adjust the focus of our attention, which would seem to correspond to the process of incongruity-resolution at the heart of a good joke (NeuroImage, vol 21, p 1055).
Further research, conducted by Dean Mobbs, then at Stanford University in California, uncovered a second spike of activity in the brain’s limbic system – associated with dopamine release and reward processing – which may explain the pleasure felt once you “get” the joke (Neuron, vol 40, p 1041).
Examining one particular part of the limbic system – the ventral striatum – was especially revealing, as its level of activity corresponded with the perceived funniness of a joke. …