| Not with my own two hands

The science behind why you can’t tickle yourself

Growing up with a brother six years older than me, I was frequently the subject of sibling abuse. Often, he’d simply pin me down, his butt inches from my face, and fart repeatedly – or, with our rotary phone unhooked, threaten to call the garbage man to come take me away. (As it turns out, the garbage man doesn’t collect small children, let alone make house calls).

On the more traumatic occasions, he’d subject me to what he liked to call “tickle torture.” Holding both my puny arms in his left hand, he’d tickle me mercilessly with his right until my laughter turned to shrieks and my mother had to shoo him away with her wooden spoon.

To my surprise, my brother didn’t invent tickle torture. The ancient Romans coated the bare feet of criminals in salt and then set a goat to the task of licking it off; the victim’s feet were often licked raw before the goat deemed the job finished. Chinese courts during the Han Dynasty favoured the tickle as a method to punish royalty: tickling, when done by a human at least, seldom leaves a mark.

For as long as we have used the tickle – for pain or pleasure – we have wondered why we can’t tickle ourselves. Darwin theorized that for a tickle to be effective it had to be unpredictable – an impossibility during a self-tickle. He was almost right.

In 1998, neuroscientists at University College London set to the task of determining what the difference was, neurologically speaking, between a good tickle and a bad self-tickle. To do this, they needed to create a tickle device that could be used inside a magnetic brain scanner – a difficult task, as anything made of metal would be sucked into the machine’s giant magnet, impaling the test subject in the process. In the end, they came up with something similar to a plastic, gag store back scratcher, modified slightly so that both the test subjects and the experimenters could operate it.

With the brain scanner running, the tickle device cocked and ready to, well, tickle, subjects experienced three conditions: the experimenters tickled the subjects, the subjects tickled themselves, or the subjects simply moved the tickle device around without actually placing it against their skin.

When the experimenters tickled the test subjects, a brain area known as the sensory cortex lit up. Not surprising. But when the subjects tickled themselves, this area wasn’t nearly as active. In fact, brain activity during a self-tickle looked about the same as when subjects simply moved the tickle device around.

A closer study of the data showed that during a self-tickle, an area of the brain near the bottom rear of the head called the cerebellum was also active. The cerebellum, it seemed, was instructing the sensory cortex to cancel out the sensory signals generated by the self-tickle precisely because it was self-produced. Such a response by the cerebellum is likely a product of evolution – a mechanism allowing our brains to respond rapidly to external stimuli while ignoring self-generated, often accidental, stimuli. Without this sensory cancellation we’d be annoyingly startled every time one hand accidentally brushed against the other.

These days, I claim – mostly in front of girlfriends and my younger cousins – to be impervious to a good tickle. It’s a big lie. Everyone with an intact spinal cord is ticklish. After years of my brother’s tickle torture, I mastered what I like to call the tickle poker face – what’s the point of tickling someone if they aren’t going to laugh?

Daniel Lametti writes every other week. If you really think you can’t be tickled, he’ll accept the challenge: thesplitbrain@mcgilldaily.com.


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