Dear Malcolm,
I was surprised the first time I saw a human transform into a tickle monster. They curled up their fingers and rhythmically poked someone else’s armpits. That is something a cat would never do.
I asked my friend John Wright about it. He’s a retired psychology professor at Washington State University.
I learned that there are two kinds of tickling.
The first is the light tickle you feel when someone gently brushes your skin. Like when someone uses a feather to barely touch the inside of your arm. Scientists call that knismesis.
It turns out you can totally tickle yourself that way. But the feeling will be much more tickly if someone else does it.
Cats like me can experience that kind of tickling, too. Especially if you very gently brush our ears or toe beans. But we don’t usually like it. It feels itchy or like creepy crawly fleas.
The second kind of tickling is a harder, more rhythmic touch. It works on certain sensitive spots on the body—like the soles of your feet, armpits or neck. It might make you laugh or scream. Scientists call this tickling gargalesis.
That kind of tickling is usually done by primates. That means monkeys and apes like chimpanzees and humans. Rats are the only other animal scientists have ever tickled that way.
Gargalesis is tickling that you can’t do to yourself. Wright told me that’s because it only works if it’s unexpected.
“If you try to tickle yourself there’s no surprise,” he said. “Your brain gets you ready for the tickle and protects you from this ‘attack.’ So instead of a tickle, it becomes a touch or maybe a scratch but not a tickle. For this reason, a successful tickle takes at least two people.”
The part of your brain that receives the tickle signal is the somatosensory cortex. If you were hooked up to a machine to watch your brain while someone else tickled you, you’d see that part of your brain light up. But if you tried to tickle yourself, nothing would happen.
That’s because another part of your brain—called the cerebellum—monitors what’s going on. It receives information from parts of your brain that control movement and the sensory receptors on your skin. Your cerebellum compares what it expects to happen based on how you’re moving with the signals it’s actually getting from your skin. If they match, it turns off the tickle feeling.
Scientists call that whole process reafference. They say it helps you experience the world more accurately. It gives you better control over your movements. It’s also a way for your brain to tune out sensations caused by your own body. That lets your brain focus on warning you about things outside your body. Like a big tickly spider crawling up your arm.
Unfortunately, it also prevents you from being your own tickle monster.
Sincerely,
Dr. Universe