Why does tickling make you laugh?

[UPBEAT MUSIC] MOLLY BLOOM: You're listening to Brains On from NPR News and Southern California Public Radio, where we're serious about being curious. I'm Molly Bloom. Judging from your emails, a lot of you are ticklish, and you have a lot of questions about it, like this one from Caden and Henry from Clarendon Hills, Illinois.

SPEAKER 1: My question is, why can't we tickle ourselves?

SPEAKER 2: Yeah!

MOLLY BLOOM: The answer to that question is found in the brain. So we talked to Martha Flanders.

MARTHA FLANDERS: Well, that was a great question. It gets at the most interesting thing that the central nervous system does. This is what makes studying neuroscience just so fascinating.

MOLLY BLOOM: She's a Professor of Neuroscience at the University of Minnesota Medical School.

MARTHA FLANDERS: I study the somatosensory system, which means how we get information from receptors in the skin to the brain.

MOLLY BLOOM: Your skin is covered with these receptors. And it's their job to tell your brain and spinal cord what's happening on your skin. Some of those inputs could be the feeling of a mosquito landing on your arm, or the heat from a fire, or the shape of whatever you're holding. But your brain has a different way of processing the movements that you do to yourself.

MARTHA FLANDERS: And your brain keeps track of the kind of sensory input you should expect from your own movement.

MOLLY BLOOM: But in many ways, it's more important for your brain to keep track of the things that you can't control, like people, animals, or objects that could brush up against your skin.

MARTHA FLANDERS: It's really essential because to do the complicated things that our brains have to do, we have to have some sort of sense all the time of what's going on out there.

MOLLY BLOOM: So when you do something to yourself, your brain dulls the feeling created by it.

MARTHA FLANDERS: The simplest way to do it is just subtract out the part of the sensory input that you expect. And then what's left over gives you information about what is novel, what wasn't expected, what isn't due to my own actions.

When you tickle yourself, your nervous system has canceled out the sensory input that was produced by you because your nervous system expected that pattern of sensory input because you produced it. So if you tickle yourself, you cancel out some of-- I mean, you can still feel it. But you cancel out some of that.

But if someone else tickles you, you've had no way to anticipate that. There's no subtraction due to your self movement. So it really heightens that unanticipated signal. And tickling is such an interesting signal that you would have no way to see that coming.

MOLLY BLOOM: There have been experiments that show it feels like a stronger push if someone else did it to you than if you did the exact same push on your own skin.

MARTHA FLANDERS: I mean, you're used to combing your hair. Men are used to shaving their face. And they hardly feel that at all. But if somebody came along with a paintbrush and started wiggling your whiskers, then you would feel that for sure.

MOLLY BLOOM: So try it. Try tickling yourself in your armpit or behind your knee. anything? Probably not. And that's a good thing since it helps you to better be able to keep track of the world around you. Thanks, brain.

SPEAKER 3: Thanks, brain.

MOLLY BLOOM: Coming up, why tickling makes us laugh. But first, we want to tickle your ears with the mystery sound.

[SOUND EFFECTS MEDLEY]

SPEAKER 4: (WHISPERING) Mystery sound.

MOLLY BLOOM: Here it is.

[BUZZING SOUND]

Any guesses? We'll be back with the answer in just a bit.

[ENERGETIC MUSIC]

Do you have a mystery sound to share with us or a question or a drawing? You can send them to us at brainson@m-- --as in Minnesota-- pr.org. You can also send us actual letters through the mail. You can find our mailing address at our website, brainson.org. While you're there, you can listen to past episodes and subscribe to our newsletter.

And speaking of letters, it's time for the Brains Honor Roll. These are the awesome kids who keep this show going by sharing their questions, mystery sounds, drawings, and high fives with us. Here's the most recent group to be added to the list.

Kavian and Farah from Berkeley, California; Ellen from Lawrence, Kansas; Larkin from Roberts Creek, British Columbia; Alina from Chula Vista, California; Sophia from Asheville, North Carolina; Itsy from Oakland, California; Gordon from Brooklyn; Heather from Philadelphia; Phillip from Berkeley, California; Kyra and Kelton from Duluth, Minnesota; Sam and Jude from Christchurch, Virginia; Emma from Alexandria, Virginia; Elias from Albany; Mansi from Stillwater, Minnesota; Isha from Paramus, New Jersey; Sophia from Ann Arbor, Michigan; Cyrus and Samson from Minneapolis; Sawyer from Hershey, Pennsylvania; Tess from Seattle; Thalia from Brooklyn; Sam from Lexington, Massachusetts; Maggie from New Market, Maryland; Kaya from St. Mary's County, Maryland; Harrison from Lakewood Ranch, Florida; Alexis, Isla, Paisley, and Joey from Wainwright, Alberta; Zane from Los Angeles; and Collin from Provo, Utah.

SPEAKER 5: (SINGING ROBOTICALLY) Brains Honor Roll [INAUDIBLE]

MOLLY BLOOM: Back to the mystery sound. Let's hear it one more time.

[BUZZING SOUND]

Here's a hint. This is the electric version. I don't have an electric one, but I do this twice a day. Ready for the answer? Cece from Wayne, New Jersey, sent us this mystery sound. Here she is with the big reveal.

CECE: This is the sound of me brushing my teeth.

[BUZZING SOUND]

SPEAKER 6: Ba ba ba ba ba ba ba ba ba ba ba Brains on.

MOLLY BLOOM: Now onto our next tickling question. Why does tickling make you laugh? The answer is surprisingly complicated. I spoke with Paleoanthropologist Kieran McNulty in front of an audience of Brains On fans at the Minnesota State Fair to find out. I asked him why some parts of our bodies are more ticklish than others.

KIERAN MCNULTY: We've got all of these receptors in our body. And some parts have a lot more receptors than others. And that explains very light tickling, tickling you feel, for example, when a Minnesota mosquito lands on your skin.

The more interesting and complicated question is heavy tickling, when someone comes up from behind and grabs our waist or our armpit. Why do we laugh? And there are definitely parts of the body that you're much more ticklish when you're touched there than other places, places like around the waist and the armpit, the neck.

There's lots of ideas as to why this might be. But probably the best explanation is that these are parts of our body that are pretty vulnerable. And you can do this experiment, even you in the audience, if you're listening at home.

If you find a place where you're very ticklish-- think about under your armpit-- touch it. And then very gently-- don't ever press too hard, but start pressing a little harder. As you press a little harder, what you'll find is it starts to hurt. You've got areas of pain right under those ticklish areas. And so part of being ticklish is actually a response to that pain, to jerk away.

Now, for a human, those of us that walk on two legs, we're also very ticklish on the bottom of our feet because we're extremely vulnerable. And if something happens to our feet, then it's very hard to get around.

MOLLY BLOOM: Why is it that when we tickle those spots, we laugh instead of just saying, don't touch me?

KIERAN MCNULTY: This is where the story gets very complicated. Lots of animals have this sort of flinch response because they're vulnerable. But when you take animals like primates, animals that are very social-- primates live in big groups. Not only do they communicate with each other through vocal gestures, but through grooming and touching each other.

Chimpanzees, gorillas, orangutans, when they're little babies, will roll around and wrestle and tickle each other and actually laugh in response-- or their equivalent of laughing-- to the tickling. And what seems to have happened and what's so exciting about talking about evolution and how things change through science and through time is that you take something that was already there, this flinch response, and you can adapt it to something new. And it becomes a way to communicate nonvocally, or without using words, at least.

And so this is a very important mechanism for establishing bonds. One of the most important bonds is between mothers and offspring. The tickling laugh is one of the first laughs. It's the first laugh that a baby ever experiences. And this is a way to form bonds between mother and offspring. And those bonds last for the entire life of the organism.

MOLLY BLOOM: So babies laugh when they're tickled. So it's a natural response that's sort of baked into us?

KIERAN MCNULTY: It seems to be a natural response. And it's not something-- and people have done experiments to try to understand, is this something they're learning from watching mom and dad? When mom and dad come up to tickle you, and they're laughing and smiling, is that something we learn? But they've done experiments that show it seems to be something that we're born with, probably from that primate heritage.

MOLLY BLOOM: When you're trying to answer a question like this as a scientist, how do you go about starting that process?

KIERAN MCNULTY: So tickling and laughing, because tickling has been studied for more than 2,000 years, believe it or not, some of the early experiments were very simple. And so trying to get at questions of whether this is learned behavior or not, scientists would wear masks and tickle young children.

And with a mask on, the children can't see, is this person laughing? Is this person scary? Is this person angry? All they know is they're being tickled. Those children still respond in a playful and laughing way.

Another experiment people have done is try tickling with a robot arm rather than tickling with a person. And the person being tickled is blindfolded, so they don't know the difference. The response is identical. So it doesn't seem to be something that we're learning or cuing from someone else, but actually something that's innate to us.

MOLLY BLOOM: And why do you think this is a question that scientists have been so interested in?

KIERAN MCNULTY: I don't know the answer to that. It really is, when you start to dig under the surface, it's a fascinating question. And I think part of the complication is that we're actually, when we're being tickled, we're experiencing, in some part, both pleasure and pain.

And if you ask most adults in the audience or anywhere, walk around the fair, they will tell you they don't like to be tickled. And there is this undercurrent of pain there that we're not often aware of. But subconsciously, we don't enjoy it. Kids, on the other hand, seem to love it. So it's something that clearly changes. And it's a very complex set of inputs. And I think that's made it interesting to a lot of people.

MOLLY BLOOM: And can you train yourself to not be ticklish anymore?

KIERAN MCNULTY: Yes. I think, again, if you look just at kids versus adults, we learn to react less and less the more familiar we are with people and with situations. Just like we can't tickle ourselves, it's harder for us to be surprised tickled when we're very familiar with people. So we can train our brains to do almost anything. And I think part of that is learning to be less ticklish.

MOLLY BLOOM: Well, great. Well, thank you so much, Kieran, for being here and answering all of our tickling questions.

KIERAN MCNULTY: I'm happy to help.

MOLLY BLOOM: That's it for this episode of Brains On. Thanks to audio engineers Corey Schreppel and Michael Osborne for making us sound good at the fair. And thanks to Jeff Jones for helping us organize it.

We love meeting everybody who came out to see us at the fair. If you're in Minnesota and want to come check out the Brains On crew in person, you're in luck. We're hosting two very special shows coming up. There's one in October called Cats Versus Dogs and another in November called Gross Out.

For more information about showtimes and tickets, head to the Fitzgerald Theater website. It's fitzgeraldtheater.org. We'll be back in a few weeks with more answers to your questions. Thanks for listening.

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