Why are we ticklish?

A new study from the Brecht lab has found how “ticklishness” is represented in the rat brain. The study has been published on 11th November 2016 in Science.

Of all physical sensations, ticklishness is perhaps the most mysterious. Why do we laugh in response to tickling? Why are certain body parts more ticklish? Why cannot we tickle ourselves? Indeed, the mystery of ticklish perception has been discussed for more than two millennia by great intellectuals including Aristotle and Charles Darwin. Despite such long-standing interest, the mechanism of ticklishness remained elusive.

Perhaps ticklishness is a trick of the brain that rewards interacting” The researchers (Shimpei Ishiyama & Michael Brecht) then went on to investigate the response of the rat’s brain to such tickling. Specifically, the investigators studied the rat’s somatosensory cortex, a large brain structure that contains an ordered representation of the body and handles stimuli on the body. In the trunk region of the somatosensory cortex, the researchers observed nerve cells that responded strongly to tickling. Interestingly, the researchers found very similar brain responses during play behaviours as during tickling, even though the rats were not touched by the scientist. Making rats anxious – which reduces ticklishness ­– also reduced the activity in these cells and suppressed the calls. Remarkably, rats emitted calls just to electric stimulation of the cells in the trunk region of the somatosensory cortex without being tickled. Taken together, these results suggest that activity in the trunk somatosensory cortex represents ticklish sensation. Professor Michael Brecht, who led the study, says: “The data much look like we identified the ticklish spot in the rat brain. I also find the similarity of brain responses to tickling and play remarkable. Perhaps ticklishness is a trick of the brain that rewards interacting and playing.”

Top: Illustration of the rat brain (black outline: somatosensory cortex, corresponding body parts are labelled; red: trunk region).
Bottom left: the researcher tickling the belly of a rat.
Bottom right: activity of trunk somatosensory cortex (thin vertical black lines) during belly tickling (beige box).
Figure: Brecht & Ishijama

 

Movie “Why are we ticklish?” on Bernstein TV, Youtube: https://www.youtube.com/watch?v=1ukqZj9xWyk

 

Original publication:
S. Ishiyama & M. Brecht, Neural correlates of ticklishness in the rat somatosensory cortex. Science (2016).
http://science.sciencemag.org/cgi/doi/10.1126/science.aah5114
This study was supported by Bernstein Center for Computational Neuroscience Berlin, Humboldt-Universität zu Berlin, SFB665 and the Deutsche Forschungsgemeinschaft - Leibniz Prize.

 

Contact:
Prof. Dr. Michael Brecht
Bernstein Center for Computational Neuroscience
Berlin Humboldt-Universität zu Berlin
Tel: +49 (0) 30 2093 6770
Email: michael.brecht@bccn-berlin.de