Just like humans, rats are ticklish and can "giggle" when tickled. Scientists believe rats could thus provide them with an insight into how the human brain reacts to tickles.
Ticklishness is one of the strangest sensation experienced by the body and the way it works remains puzzling to scientists. When we are tickled, we laugh – a positive emotion – even though tickles are only a simple touch on the body's surface and even if they may become uncomfortable after a while.
To learn more about how ticklishness arise and what in the brain causes us to laugh, scientists have worked with rats, because these animals are very similar to humans in this area.
In 1999, a study had already revealed that rats emit high pitched noises that resemble chuckles when scientists tickle them. In a research paper due to be published on 11 November 2016 in the journal Science, researchers have now identified the brain areas responsible for this phenomenon.
"Ticklishness is one of the most mysterious form of social touch, it is still poorly understood. What we set out to do, is understand what happens in the brains of humans and animals during tickles. In particular, we looked at what goes on in the brain of rats – who experience similar ticklishness to humans", says study author Michael Brecht, from Humboldt University in Berlin.
Laughing at being tickled
The scientists tickled laboratory rats and their first observations confirmed what previous studies had found: tickles are rewarding for rats and elicit 'laughter'. The rats didn't turn away from the hand that was tickling them, preferring to chase it and to play with it. They also performed unsolicited jumps which suggests they were experiencing some form of joy at being tickled.
The scientists also recorded the noises made by the rats - typically these can only be heard at frequencies that the human ear cannot perceive - but here they were converted so that researchers could listen to them. The rodents appeared to make short calls at 50kHz, a frequency which in rats is associated with joy.
During the tickling sessions, the researchers also monitored neuron activity in the somatosensory cortex. This region of the brain is particularly sensitive to touch and they believed it could be the key to understanding ticklishness.
They measured the activity of neurons in this brain area and discovered they were activated when the rats were being tickled. This was correlated with greater 'laughter'. The scientists also showed that activating these neurons artificially, without tickling the rats, produced the same reaction as when they were being tickled. This confirms the important role of the somatosensory cortex in ticklishness.
Just like in humans however, the degree of joy felt at being tickled varied with the rats' mood. When the little animals were anxious, laughter and neural firing during tickling were significantly reduced. Scientists now want to conduct more research to see how these findings may be applicable to humans.