Laboratory rats emit squeaks and squeals of pleasure when tickled, says a new study published in Science. They can even anticipate play, firing up neurons in the somatosensory cortex region of the brain.

The finding adds to the list of benefits of playing and tickling with laboratory animals. A 2015 study in Applied Animal Behaviour Science found that tickling can reduce the stress of injections in laboratory rats.

To conduct this research, scientists from Humboldt University in Berlin, Germany tracked the brain activity of laboratory rats who participated in a daily 15 minute tickle session. Electrodes tracked brain activity and a special microphone recorded ultrasonic squeaks, according to StatNews.

"Like tickling people or pets, I rapidly move my fingers pushing on the trunk of rats. For tickling the belly of the rat, I flip the rat, push it on the floor, and rapidly move my fingers on the belly," first author Shimpei Ishiyama, Ph.D., explained to ALN.

The results showed that tickling caused networks of neurons in the somatosensory cortex to fire. Interestingly, for rats used to tickling, the neurons would start firing as soon as they were placed into the play area--before any activity had even begun.

"We recorded activity of neurons in the brain region so-called the somatosensory cortex, which processes tactile information on the body. We found these neurons respond to tickling, which was not very much surprising because tickling is a tactile stimulus. However, these neurons also respond in the same way, when the rat is joyfully playing with my hand (chasing my hand). This was surprising because there is no tactile input during play," Ishiyama added. "It makes sense in that when somebody is about to tickle you, moving fingers in front of your armpit, you probably feel ticklish and start laughing, even though you are not tickled yet. This anticipatory ticklishness is likely to be sensed also in rats and at the neural level. This suggests a neural link between ticklishness and play behavior."

Conversely, when the rats were put in stressful situations, such as when a bright light was shone in their eyes, they did not react to the tickling. A similar response can be seen in humans--when people feel unsafe, they also do not react to tickling.

The researchers also found that they could stimulate the "tickling response" by stimulating the somatosensory cortex with electrical currents.

"Positive emotions such as happiness, joy, or fun are quite underrated in science, although these are indeed what we want, and are lacking in those who suffer from disorders. Our study showed a mood-dependent modulation in the somatosensory cortex. I believe it is important for future neuroscience to study positive emotions, also in terms of understanding deficits from the other direction," Dr. Ishiyama continued.

Dr. Ishiyama recommends that all researchers add tickling to their daily animal care.

"It is suggested that tickling rats, especially when they are young, reduces stress of the rats, and reduces pain sensation caused by injection. Tickling and playing with rats make them extremely easy to handle them. Also, rats seem very happy and joyful," he concluded.