Today I found out why it is nearly impossible to tickle yourself.

It’s very hard to tickle yourself because your brain anticipates things going on around you in order to help speed up response times. More technically, the cerebellum monitors body movements and can also distinguish between expected sensations and unexpected ones, generally resulting in diminishing or completely discarding expected sensations, while paying much more attention to unexpected ones.

So your brain is actively anticipating touch sensations. When it is doing this, it is also actively discarding sensations that it deems unimportant, like when you are typing and it significantly dulls the touch sensation in your fingertips so that you don’t really notice it unless you consciously think about it. This same type of thing happens when you try to tickle yourself.

Researchers at University College London tested this by scanning the brains of subjects while the palms of their hands were touched by themselves and by experimenters. The brain scans revealed that when the touch was externally produced, the somatosensory cortex (involved in processing touch) and anterior cingulate (involved in processing pleasure) parts of the brain reacted much more strongly than when the touch was produced by the subjects themselves. In these latter cases, the brain was using information it has on hand such as motor movements of the finger and arms to anticipate the touch.

Results from a different study showed that the same internal anticipated response applies when subjects manipulated a robot, which then in turn manipulated another robot to touch the subject’s palms. This was only true, however, when the associated touch from the second robot happened right away. When this happened, the cerebellum sends information on the sensation to expect to the somatosensory cortex. With this information, some yet unknown cortical mechanism is triggered that inhibits the tickling sensation from activating.

If the subsequent robotic touch is time delayed, even delayed by as little as a 1/5 of a second, the subjects felt stronger touch sensations, similar to when the touch was not self produced.

So in short, you can’t usually tickle yourself because there is no element of surprise. Your brain is using the various internal sensory data it has available to anticipate exactly what is going to happen based on your movements and visual data. When the anticipated reaction and the actual reaction line up, your brain diminishes or even sometimes completely discards the sensation as a result of that action. On the other hand, when someone else is tickling you, there are unexpected sensations on the skin and these then can result in the tickling sensation being activated.

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Bonus Facts:

• Researchers at the Karolinska Institute in Stockholm, Sweden, studied fMRI scans between people who were actually being tickled and those who were about to be tickled and were anticipating it and discovered that the brain reacts the same to both.  Specifically, the somatosensory cortex and the secondary somatosensory cortex both lit up at around the same levels.  So even though, in the latter case, the subjects weren’t being touched, as far as their brains were concerned, they were being touched.
• It turns out, the panic response when a tarantula is crawling on your leg or the like is exactly what is happening when you are getting tickled.  The body’s response to being tickled is panic and anxiety.  It is thought that this is a defense mechanism for exactly the type of thing listed above where an external touch, such as a poisonous insect crawling on you or the like, might be occurring.  The body needs to react quickly to this unanticipated touch and without time for much conscious thought, so produces the panic reaction.
• Interestingly, the panic reaction that results from tickling doesn’t feel like tickling when the person tickling you isn’t someone you want tickling you.  In this case, it more closely resembles actual panic reactions, rather than having associated laughter.
• A recent survey of college students indicated that on average only 32% of people report enjoying being tickled.  Of the rest, 36% report disliking being tickled and 32% reporting being neutral on tickling.  In the same study, they found people who reported disliking being tickled smiled more often during tickling than those who indicated they enjoyed being tickled.  This is in line with other research that seems to indicate we smile and laugh during tickling due to nervousness, anxiety, and embarrassment.
• The Romans used to use continuous tickling of the feet as a form of torture.
• The word “tickle” comes from the Middle English word “tikelen”, meaning “to touch lightly”.
• Being tickled by a very light touch across your skin is called “knismesis”.  This type of tickle doesn’t usually produce laughter, but does produce the same type of panic response as the alternative form of tickling, called “gargalesis”.  Gargalesis is the form of tickling produced by repeatedly having pressure put on ticklish areas.
• Contrary to popular belief, most laughter is not associated with humor, but rather stems from non-humor related social interactions.  This was discovered from a study covering over 2,000 cases of naturally occurring laughter, almost none of which stemmed from jokes or other such humor devices.  Most cases were simple, short “ha ha’s” during somewhat normal conversations.  These short laughs almost never interrupted speech, but rather occurred during breaks, providing social cues to those around.  It is thought that laughter serves a similar function to yawning, namely providing “social glue” that helps bond people subconsciously.
• Apes don’t laugh the way we do, but they do produce a panting sound in the same type of situations that humans would laugh in (being tickled; during play; etc).  Likewise, rats will often produce a high-pitch sound when being tickled and during play.
• This predictive system used by the brain for anticipated sensations is called a “forward model”, where the brain’s motor system makes predictions about the consequences of some movement or action and interprets the resulting anticipated sensations as self produced or externally produced.  It then adjusts the level of the sensations felt accordingly.
• This forward-predicting that your brain is constantly doing is also why you get startled and might even jump when you think you are alone and someone sneaks up behind you and taps you on the shoulder or says “boo!”  This is the same type of bodily panic reaction as happens as a result of being tickled.