Today for lunch, I decided to figure out what it means to be ticklish. I’ve always been curious about it. I’m not super ticklish, but I can remember more than one time laughing so hard from tickling that it hurt. Over the last few days, I have been thinking about laughing and thought about writing about the biology/anatomy of laughing but it just became too big of a story for the amount of time I had today. So tickling it is.
First of all, there’s two kinds of tickling: knismesis, which is the weird ticklishness you get when someone lightly strokes your skin, or a bug is crawling on your skin, and gargalesis, which is the full-on crazy ticklishness most people have in their armpits and soles of the feet. Gargalesis is also my new favorite word. Although the sensations are similar, the mechanisms behind them seem to be very different.
Knismesis is theorized to be a serve as a protective function of the skin, alerting the organism that something foreign is on the body. The sensation is mediated by neurons that mediate pain and neurons that mediate touch, so in cases where one type of nerve modality is damaged or removed, ticklishness can still remain although in a weaker sense. In general, this type of tickling does not result in laughter. You can induce the knismesis response in yourself if you lightly rub the roof of your mouth with your finger. Careful when you do it though, it’s kind of intense. This type of ticklishness has also been well-documented in animals, and widely studied in cats.
Gargalesis is similar to knismesis in that the same neurons seem to be activated in response to the “heavy tickling” stimulus as were activated by a “light feather” stimulus. However, it is different from knismesis in that it is found only in specific sites of the body. The most common sites are the armpits, soles of the feet, sides of the torso and ribs, neck and knee. Gargalesis is also different from knismesis in that it is generally not possible to induce the response on yourself. I think that’s pretty obvious to most people. When you wash your armpits in the shower you don’t laugh hysterically. At least I don’t. However, the issue is more complicated than that. In studies where subjects used a joystick to operate a tickling robot on themselves, researchers found that the subjects did not laugh in response to tickling. So, even though the tickling stimulus was coming from somewhere else, simply anticipating the stimulus is enough to dampen the response. The cerebellum seems to be the source of this cancellation of self-produced gargalesis. Conversely, if subjects were blindfolded and the tickling robot was operated by someone else, they would respond with laughter as though they were being authentically tickled by a person (so it’s not just that the robot sucks as a tickler). I tried in vain to find an image of the tickling robot used in these studies but I’m not sure it exists on the web, so until I find it, I’m going to assume it looks like this:
So what’s the use of gargalesis? There are several popular theories but no one knows for sure why. One theory suggests that it is a defense mechanism, since most of the commonly ticklish areas are areas of particular vulnerability to injury during hand-to-hand combat. The reflexive pulling-away that most people get during tickling would be a boon to survival. Another theory suggests that maybe the ticklish spots developed in utero and were used as a means to reflexively orient the fetus’ body correctly in the womb. Yet another theory posits that tickling is in fact rooted in the need for social connection and rough play during early development. The laughing and smiling reflex to tickling serves to encourage the tickler to continue. Most people’s first experiences with tickling are with their mothers, maybe ticklishness is a way to enhance the mother-child bond. Of course at the end of the day, these are only theories and no one really knows for sure why it exists.
It is interesting that gargalesis is much less common amongst animals, and has only been well-documented in humans and a few primates. Recent work though has shown that rats can be tickled and even laugh an ultrasonic laugh (~50Hz) that can only be heard with special equipment. Apparently the rats go totally bananas, rolling around and playing, in fact actively begging to be tickled. Bearing that need for an ultrasonic listening device in mind, who knows what other animals are laughing, tickling each other, and developing complex relationships without our awareness. Certainly bonding is no unique thing to humans, so that would tend to fit the bonding theory of tickling’s origins.
After all of this exploration, I hope that this enlightens your view of what it means to be human and I haven’t ruined for you the simple pleasure of getting tickled. 🙂
Blakemore SJ, Walpert DM, Frith CD (1998). “Central cancellation of selfproduced
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Panksepp J, Burgdorf J (2003). ““Laughing” rats and the evolutionary antecedents of human joy?“. Physiol. Behav. 79 (3): 533–47.
Selden ST (2004). “Tickle”. J. Am. Acad. Dermatol. 50 (1): 93–7.