I remember in grade school, as the box of crayons expanded into the hundreds, a cool new color of pink was invented, called Tickle Me Pink – 
well, probably just the name was invented. Maybe Crayola had more insight into science than I thought.
Wohr and colleagues tickled some rats and found that tickling induces hippocampal cell proliferation in rats emitting appetitive 50-kHz ultrasonic vocalizations. Coincidentally that’s the name of this published paper.
Even rats have affect, which was measured in ultrasonic vocalizations. When they liked tickling their vocalizations were higher (50-kHz vs. 22kHz) than when they didn’t like it. The intention of the study was to see if these trait differences were linked to the decrease of hippocampal neurogenesis.
Hippocampal cell proliferation was positively associated with those rats who liked being tickled and negatively with those who didn’t. If they figure out a way to measure HCP while the subject is still alive, maybe I’ll sign up for the human clinical trials of tickle therapy.
They're concerned about their appearances, too.
