Ticklishness may sound like a trivial subject to waste time on, but like many of those seemingly simple, “everybody knows the answer” type of questions, the more you dig, the more fascinating the story becomes. You may have noticed that I didn’t ask “are you ticklish?”, but rather why are you ticklish. This is because almost everybody is ticklish, although we may differ in degree.
So, what could be the reason for ticklishness? There must be a reason; natural selection eliminates useless or deleterious traits. The short answer is that it may have to do with something as profound as self-awareness.
What happens in the brain?
When we get tickled, the touch sensation is transmitted to an area in the brain that is called the somatosensory cortex. As the name implies, this area gets inputs from all our senses: sight, hearing, smell, taste, touch, and proprioception. The last one, proprioception, is the sense that lets us know where in space our body is and what our limbs and muscles are doing. There will be more about this fascinating “sixth sense” in a later article.
All these sensory inputs are then sent on to a structure in the brainstem, the cerebellum, that acts as a clearinghouse. It integrates all those incoming sensory messages and sends back messages to the motor cortex and to the prefrontal cortex, the “executive branch” of the brain.
The messages coming into the motor cortex are then translated into instructions for the appropriate muscles to react. The fact that the cerebellum is located in the brainstem, the most ancient part of the brain where all the vegetative functions are located, tells us that, just like breathing and the blood circulation, this function is essential for life itself. And that’s easy to understand: If a rat doesn’t get the message that the heat it feels on his skin and the flame he sees in his eyes together spell fire, he is going to be toast in no time. And if a hunter-gatherer couldn’t plant his feet where he needed to go, he would either starve or become lunch himself—either way the outcome would be the same.
What does all this have to do with being ticklish?
You can already guess that it has something to do with the somatosensory cortex; after all, tickling requires touching. But why should the tickle and the reaction to it be routed through the cerebellum? Because, the cerebellum prepares us for incoming stimuli, including touch, and filters out of our consciousness the ones that are insignificant. For instance, we are unaware of the constant stimuli created by our clothes as they rub against the skin unless the degree of rubbing is irritating enough to intrude on our consciousness.
We know a lot of things about ticklishness. For instance, we know that you can’t tickle yourself unless, strangely enough, you are schizophrenic. Why? Even Aristotle, 2,500 years ago, wondered about this curious finding. Had he known about the brain what we know today, he undoubtedly would have had a ready answer. It’s this—once you made the decision to tickle yourself in a certain spot, your cerebellum gets the message in real time and basically shuts down the neural sensation from that area. So, one of the hallmarks of tickling is surprise. If I know ahead of time that I am going to get tickled on the abdomen, my cerebellum would inhibit the tickling sensation and my abdominal muscles would reflexively tense up. All physicians who conducts abdominal exams know to lay an open hand, not a finger, on the abdomen and to do so in a deliberate manner so as not to surprise the patient.
Here is another thing about being ticklish. It requires that you be in the right mood. If you are in a foul mood, no amount of tickling will cheer you up; in fact, the sensation will be perceived as unpleasant and not at all “funny.” What this implies is that the tickle sensation is also controlled by regions in the brain that determine pleasure and reward.
What is ticklishness good for?
This is the hard question. A recent paper in Science magazine showed that rats are ticklish. If you tickle their abdomen they will jump with joy, emit sounds of laughter (more like squeaks, at too high a frequency for the human ear), and will follow the tickling hand as if asking for more. We know that they are enjoying themselves because their dopamine-mediated reward system gets activated.
We also laugh, and probably enjoy a squirt of brain dopamine, just like the rats. But we also laugh in response to a joke. Is it the same laughter? No, we laugh in response to tickling for a different reason than we laugh at a joke. Apparently, different neural pathways are involved. Laughing at a joke requires higher functions of cognition; tickling evokes a much more basic response, on the level of rats, sorry to say.
The tickling response is purely mechanical, so to speak. No higher functions of cognition are needed. Scientists have identified the areas in the somatosensory cortex that control ticklishness and have been able to evoke squeaks of joy by inserting electrodes that stimulate those areas.
Adults who were tickled as babies are almost always ticklish. Adults who were not, mostly hate it! What it tells us is that the circuits that mediate the tickle response are established in infancy.
But we still haven’t answered the question asked at the beginning of the article: What is ticklishness good for? Some theorize that it has something to do with social interaction. Others point out that the areas most prone to ticklishness are anatomically unprotected by bone (e.g. the abdomen). When you get many different answers to the same question, you know that we don’t really know.