The researchers were also inspired by earlier studies. “ ‘Laughing’ Rats and the Evolutionary Antecedents of Human Joy?” published in 2003 in Physiology & Behavior, reported that rats would emit ultrasonic calls when tickled. Ultrasound is too high for humans to pick up. We can hear sound waves up to a frequency of about 20 kilohertz. Rats giggle at about 50 kilohertz.

Those calls, along with the ability to record brain activity while playing with the rats, allowed a deeper investigation of rat tickling. The researchers first accustomed young rats to play and tickling, which the rats would invite. “They are very eager to be tickled,” said Dr. Brecht.

The scientists used electrodes implanted in the rats’ brains to see what was going on when they were tickled, particularly in an area called the somatosensory cortex, where physical touch is processed. The electrodes didn’t seem to limit the rats’ interest in play and tickling, or their positive calls. Dr. Brecht said they did not pick up any alarm calls from the rats.

The scientists found that tickling and play, which involved chasing a researcher’s hand, both caused the same ultrasonic calls and the same brain cells to be active. The scientists also stimulated those cells electrically, without any tickling or play, and got the same calls.

And they found that you can’t tickle rats when they are not in a good mood, something that is also true of people.