Video credit: Rosa B.
Your body rebels against you and contorts in wriggles and giggles, you burst out in the most annoying, uncontrollable laughter and are suddenly at the mercy of your attacker. It’s the tickle. What the hell is happening? Why are you unable to stop the stupid grin on your face when you really want to slap the person tickling you? Why can’t you tickle yourself? How come some people seem to be magically immune?
To start with, what exactly are the smiling spasms that rise up your chest when you get tickled? Fifteen facial muscles contract and your zygomatic major muscle is stimulated (pulling your upper lip upwards) to give you that garish grin. The epiglottis (the flap of cartilage behind the tongue that folds down when you swallow to stop food and drink entering your windpipe) half closes so you gasp as air struggles to get into your lungs.
So why is this bizarre reaction stimulated when someone catches one of your ticklish spots? Our sense of touch is composed of millions of miniscule nerve endings that lie just beneath the skin. If these are lightly stimulated they send a signal to the brain, which is analysed by the somatosensory cortex and the anterior cingulated cortex. The somatosensory cortex is the section of the brain that processes the touch sensations- pain, temperature, proprioception (proprius, meaning ‘one’s own’ and perception- the sense of your body’s position) and pressure. The signal also passes through the anterior cingulated cortex, which processes sensory information by sending nerve connections, or axons, to regions involved in language and emotion. Combined, these seem to produce the reflex action of laughter.
How bizarre- that someone running their fingers around your feet, knees or armpits should elicit involuntary laughter. Studies using robotic ticklers showed that the response was the same whether the subject was tickled by a human or a robot- so it is not even just a social reaction. What would be the evolutionary purpose of such a reaction? A lot of tickling studies used young subjects, as it seems that children are the most susceptible. This leads to the theory that perhaps it evolved as a response to play- children tickling each other may help to hone reflexes and self defence skills, as well as peer-bonding. As the most common ticklish spots are the most vulnerable in combat (such as the neck and ribs), this activity teaches children to protect these parts during tickle fights.
So why can you not tickle yourself? This is because the brain makes a sensory prediction using an internal forward model of the motor system, predicting the consequences (laughter) of the motor command (to tickle oneself), and cancels the sensory consequence. This may be part of sensory attenuation, where the brain filters out certain sensory perceptions (such as background noise) to concentrate on more important information.
Finally, how come some people seem to be immune? To begin with, some people have a more sensitive sense of touch, or have particularly sensitive nerve endings in certain parts of their bodies (the feet, for example, have a high concentration of very sensitive nerve receptors called Meissner’s corpuscles). Another fact relating to tickling is that it seems to be stronger in children (perhaps due to the evolutionary play-fight advantage discussed earlier), and some people appear to grow out of it earlier than others. However, the mind is incredibly powerful in its ability to override many sensory perceptions and physical reactions- for example, the placebo effect of feeling better after taking a pill that is believed to be medication. With a certain level of concentration or disassociation, it seems that people can think themselves out of being ticklish.
However, laughter triggers the release of endorphins, releases tension and cheers you up. Out of all the possible tortures, it is not the biggest one to worry about, so if you can’t think your way out of it, give in to the laughter and try to get your own back.
Originally published in the Student, March 2013.