For many of us, the mere thought of wiggling fingers approaching our rib cage or moving towards our underarms is enough to elicit squeals of laughter and giggles. However, while we all know that tickling makes us happy, Michael Brecht, a professor at Berlin’s Humboldt University, says it is “one of the most poorly understood forms of touch.”
Scientists do not know why tickling results in such joy, why certain body parts are more susceptible to the sensation than others, or why the best tickles occur at the hands of others. To solve the mystery, that even confounded Aristotle and Charles Darwin, the researcher and study co-author, Shimpei Ishiyama, solicited some help from rats.
The rodents were selected because earlier studies have shown that young rats not only feel the sensation, but, like most humans, are most ticklish on the belly and underneath their feet. However, if you are envisioning loud squeals, you could not be more wrong. While rats do enjoy a good tickle, their laughs are long 50 kHz ultrasonic calls that are inaudible to humans.
The researchers began by inserting electrodes inside the rat’s brains to monitor the neural activity in response to the tickling. They were most interested in the somatosensory cortex, the part of the brain associated with touch. As Ishiyama tickled the rats, he noticed the neurons located at the trunk of the cortex come alive instantly. For the next experiment, he played with the rodents by moving his hand around the cage. Even though they were not being touched, the neurons continued to fire. Interestingly enough, a similar response was recorded when a small amount of current was applied to the somatosensory cortex, suggesting that it is the brain’s tickling epicenter, at least when it comes to rats.
Brecht and Ishiyama, who published their findings in Science in November 2016, say that in addition to identifying the exact location from where the sensation originates, the brain’s identical response to playing and tickling indicates that tickling is a trick by the brain to encourage social interaction and bonding.
As to why it is tough to tickle ourselves? The researchers, who observed that the neurons began to fire long before the tickling began, believe that this anticipation can only take place when there is social interaction with another person. However, the team hopes to confirm their suspicion with a follow-up study, as soon as they can figure out how to get the rats to tickle themselves.
The one mystery that remains is why some animals are immune to tickling. Among them are mice, which Brecht found intriguing given the reaction of rats. Jeffrey Burdorg, a researcher at Bowling Green State University in Ohio, who has helped develop antidepressant drugs based on his study on rats laughter, believes it may be related to the playfulness and social nature of the animal. This explains why primates like apes, monkeys, and chimpanzees are very ticklish.
However, similar to humans, even the most ticklish rats did not respond when they were in a bad mood. Brecht and Ishiyama tested this by placing the rodents on a platform about 10-inches high, surrounded by bright lights. This induced anxiety in the animals leading to the suppression of their vocalizations and brain activity that initiates tickling. Brecht speculates this change in behavior is an evolutionary adaptation that subdued fun activities during a potential threat.
While conducting research on tickling may appear to be frivolous, Ishiyama says, “Neuroscientists are so obsessed with deficits such as depression and anxiety, it’s rare to find papers about positive emotions.” Besides, it is just fun to see rats rolling on the floor laughing!
Resources: sciencemag.org,nationalgeo.com, hu-berlin.de