We don't really understand why some people are ticklish, let alone why our responses to being tickled vary with circumstances. So neuroscientists have turned to their old friend the lab rat for an explanation. First, of course, they needed to learn if rats are ticklish at all. A paper in Science says they are, but only when they're feeling cheerful.
Dr Shimpei Ishiyama and Professor Michael Brecht of Humboldt University, Germany, started by asking some age-old questions: “Why does tickling induce laughter? Why are tickling effects so mood-dependent? Why do body parts differ in ticklishness? Why can't we tickle ourselves? Is ticklish laughter different from humorous laughter?”
Rats might not be very helpful with the last of these, since no one has successfully got them to respond to knock-knock jokes or sketch comedy. However, discoveries showing that, when tickled, rats emit delighted squeaks at around 50 kilohertz, far beyond human hearing, inspired Ishiyama and Brecht to suspect the other questions might be open to rodent-based investigation.
The pair gently stroked and tickled rat body parts under different conditions to see how they would react. The rats certainly seemed to like it, approaching the tickling or stroking hand, rather than trying to get away, and engaging in “joy jumps”, which other researchers have associated with rat happiness. The accompanying calls were recorded and categorized.
The researchers also tracked activity in the rats' somatosensory cortex, the part of the brain responsible for processing touch sensations. Tickling-specific patterns of neuron-firing were observed.
The rats made more calls when tickled than stroked, particularly a specific sound that Ishiyama and Brecht refer to as a “combined call”. The authors see these combined calls as analogous to human laughter.
Stimulating neurons in the somatosensory cortex directly, rather than through tickling, also caused the rats to “laugh”.
However, when the rats were raised onto a high platform or exposed to bright lights, their laughter stopped, even when tickled. The authors attribute this to the fact that, as a species, rats are fearful of heights and prefer the dark. Ishiyama and Brecht think, when anxious, their subjects found tickling no laughing matter.
Ishiyama and Brecht concluded that it was only when the rats were feeling positive that they laughed upon being tickled. This is consistent with the statement of no lesser a figure than Charles Darwin, who believed “The mind must be in a pleasurable condition” before tickling evokes laughter.
Darwin might also be pleased by the authors' suggestion that ticklishness is a very old and conserved feature of social animals. If so, it would bolster our chances of answering many of the authors' initial questions about its nature.