Like people, rats are ticklish. Now, by implanting electrodes in the brains of these laboratory workhorses, researchers have identified the brain region that seems to drive the trait — an insight that could illuminate the origins of ticklishness in people.
The work, published in the 11 November issue of Science, also reveals that rats’ susceptibility to tickling is affected by mood, rather like in people.
Several other animals are ticklish, including dogs and chimps, but rats seem particularly so, and are easy to handle in the lab. So neuroscientists Michael Brecht and Shimpei Ishiyama of the Bernstein Center for Computational Neuroscience in Berlin decided to use the animals to probe what is going on in the brain.
The neurons in the trunk of the somatosensory cortex fired intensely in response to belly tickling, but less to tickling on the back and hardly at all to that on the tail. The intense firing correlated with a specific chirp pattern.
Brecht and Ishiyama then investigated whether they could get the animals to make the sound simply by stimulating the trunk of the somatosensory cortex — and found that they could, concluding that the region is key to ticklishness.
When the pair tickled rats while they were on an elevated platform under bright light, a situation designed to induce anxiety, the usual chirping-response was reduced: they conclude that fear suppresses activity in the somatosensory cortex. The fact that rats are ticklish in the same spots as humans, meanwhile, suggests ticklishness may have a hard-wired neural anatomy that is shared between some animals, they say.
Another suggestion comes from Chris Frith, neuropsychologist and professor emeritus at the Wellcome Trust Centre for Neuroimaging at University College London. In 1998, he used brain scans to show that most people will not respond to self-tickling because the region of the brain called the cerebellum, which seems to predict what kind of feeling a movement will cause, cancels out the tickling sensation. People with schizophrenia, who have difficulty distinguishing sensations from the outside world and those that they trigger, can tickle themselves, though.
“The next step would be to look at self-tickling in rats,” he says. But he acknowledges that “it might be difficult to get rats to tickle themselves”.