The father of evolution, Charles
Darwin, once wrote “of laughter from being tickled, the mind must be in a
pleasurable condition”. Now researchers have discovered the same is true for
rats in a finding that could help shed light on the “mysterious sensation” and
also on the basic wiring of the mammal brain. In a series of experiments, the
rats found themselves being repeatedly tickled by the hand of a scientist. In a
normal situation, they appeared to find this intensely enjoyable. Researchers tickled
and gently touched rats on different body parts and observed a variety of
ultrasonic vocalizations [USVs]. Tats seemed to warm up to tickling and
vocalized less before the initial interaction than during breaks between
interaction episodes. A video of one such encounter also showed the rat making “joy
jumps” during the tickling. There was also a large amount of squeaking. However
when the rats were put on a raised platform illuminated by bright lights, they
appeared much less keen. Tickling evoked USVs were significantly suppressed. They
speculated the tickle response must have evolved early in the development of
mammals. The numerous similarities between rat and human ticklishness such as
tickling evoked vocalizations and anxiogenic modulation, suggest that tickling
is a very old and conserved form of social physicality. The mechanisms involved
in tickling are poorly understood. For example, it remains a mystery why no one
can tickle themselves. The researchers found that tickling caused neurons in
the rats’ somatosensory cortex – an area of the brain associated with the sense
of touch – to fire. They were also able to get the rats to ‘laugh’ when they
artificially stimulated the same neurons. The discovery suggests this area of
the brain may also play a role in mood. The observation that the somatosensory
cortex is involved in the generation of tickling responses suggests that this
area might be more closely involved in emotional processing than previously
thought. Identification of the neural correlated of ticklishness will allow us
to frame question about tickling in neural terms and thus help us to understand
this mysterious sensation.
