The curious thing about a tickle

September 20th, 2022

Written by: Barnes Jannuzi

You can be tickled, and you can tickle others, but you can’t tickle yourself. Researchers have demonstrated that self-produced tickles are less tickling than the same action performed by someone else2,4. You probably didn’t need a journal article to tell you that, because you likely learned it as a child. But why can’t you tickle yourself? The reason is that you are both the tickler and the tickled. More specifically, your brain is responsible for both the movement of your body and the perception of touch, allowing it to predict and ignore the feeling of being self-tickled. Your inability to tickle yourself comes down to how your brain processes sensory information and plans/executes movements.

Let’s begin with what is going on in the brain during a real-world example: you are the evil tickler of your poor younger sibling. Like any voluntary movement, regions of your brain develop a “motor plan” and send that plan down your spinal cord to the muscles in your arm and hand. You can think of this like a brain-GPS navigation system, directing your muscles on how to get your body to where you want it. These signals are precisely timed and work together to carry out the complex task of moving your arm forward, angling your wrist, and wiggling your fingers in just the right way to tickle your sibling. This seemingly simple task is truly remarkable in how accurately your nervous system has to function to pull it off correctly, but equally impressive is a process going on behind the scenes in your brain called sensorimotor prediction1.

At the same time as the motor plan is shooting down your spine, a copy of that plan is being sent to other regions of your brain such as the cerebellum, a region thought to play an important role in motor coordination3. The cerebellum also receives sensory updates from your body about where your hand actually is. Then the cerebellum can compare the real position with the motor plan to make sure that your hand physically reached your sibling’s foot, executed the tickling movement, and can correct the actions if needed. In other words, “Did your body follow the brain-GPS instructions? Or is there some correction that you need to make because you missed your turn?” Unsurprisingly, this circuit is not only working when you want to tickle someone, but around the clock, giving your brain an amazing ability to accurately maneuver your body, and correct for any errors by monitoring the sensory information sent from your body up to your brain. 

Now that we have discussed the movement of your body, let’s talk about the second half of your self-tickling conundrum: the perception of touch. There are many sensors in your nervous system, each specialized to detect something about the outside world. The enormous amount of sensory information bombarding your brain is potentially overwhelming- but not all sensory information is equally important. In fact, most sensory signals from your body are ignored by your brain. In what scientists call “sensory gating” (which you can read more about in this previous post), your brain filters out irrelevant sensory information to allow you to focus on what is relevant. 

As an example of sensory gating, as you read this article, pause for a moment and shift how you are seated. Now do it again, only this time, try to pay closer attention to the feeling of your clothes shifting/pressing against your body. Even when you are sitting perfectly still, your body is still being touched and senses the clothes you are wearing, the glasses on the ridge of your nose, the airpods in your ears, the hairband on your head. But when it comes to your normal activities, you barely notice all of these senses because you often don’t need to. By ignoring these irrelevant senses, your brain is prioritizing new and unexpected information to help you understand and interact with your world. 

To summarize why you can’t tickle yourself: Your brain is too smart to be fooled. Your brain not only plans out your movements, but also predicts what you will feel as a result of those movements, allowing it to ignore these predicted sensations and focus on the new and unexpected. Understanding how your brain accomplishes simple functions allows you to not only build a fundamental understanding of yourself, but may also inform medical interventions to help those with sensory-motor deficiencies who are having trouble navigating with their brain-GPS. While this self-tickling conundrum may not be on the top ten of life’s mysteries, nor humanities problems, it serves as an example of how the simplest and silliest questions may be a window into the remarkable functions brains do to help us perceive the world around us… often without us paying attention to it. 

References

  1. Wolpert, D. M., Ghahramani, Z., & Jordan, M. I. (1995). An Internal Model for Sensorimotor Integration. Science, 269(5232), 1880–1882. http://www.jstor.org/stable/2889276
  2. Blakemore, SJ., Wolpert, D. & Frith, C. Central cancellation of self-produced tickle sensation. Nat Neurosci 1, 635–640 (1998). https://doi.org/10.1038/2870
  3. Blakemore, SJ., Sirigu, A. Action prediction in the cerebellum and in the parietal lobe. Exp Brain Res 153, 239–245 (2003). https://doi.org/10.1007/s00221-003-1597-z
  4. Blakemore SJ, Wolpert D, Frith C. Why can’t you tickle yourself? Neuroreport. 2000 Aug 3;11(11):R11-6. doi: 10.1097/00001756-200008030-00002. PMID: 10943682.

Cover image by Ketut Subiyanto on Pexels

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