Ishiyama, S., Kaufmann, L.V., and Brecht, M. Curr. Biol. https://doi.org/10.1016/j.cub.2019.07.085 (2019)

We cannot tickle ourselves, and it’s unclear why. Research into this centuries-old question pondered by the likes of Aristotle continues on—with rats. Rats cannot tickle themselves either, but they’ll melt into ultrasonic peals of the rat-equivalent of laughter when tickled by a person.

A new study considers the relationships between tickling and sensation. The researchers noted differences in the ultrasonic vocalizations that rats produce when they are grooming vs. simply being touched by a human vs. being tickled. Rats that were then tickled as they groomed ‘laughed’ less, suggesting that self-touch inhibits the somatosensory cortex, shown previously to be excited during tickling. Chemically inhibiting this inhibition resulted in rats that ended up ‘tickling’ themselves. A final experiment, in which the rats were trained to request tickling via nose poke, revealed signs of ambivalence—nevertheless, they still asked to be tickled.