As I said before, with so many science outlets out there, it’s hard to find something new and interesting to cover that hasn’t been covered already. Admittedly, sometimes some new paper comes out that is so funny or interesting that I too fall in line with the rest of them and cover it. But, most of the time, I try to bring you something that you won’t find it reported by other science journalists. So, I’m sacrificing the novelty for originality by choosing something from my absolutely huge article folder (about 20 000 papers).
And here is the gem for today, titled enticingly “Why can’t you tickle yourself?”. Blakemore, Wolpert & Frith (2000) review several papers on the subject, including some of their own, and arrive to the conclusion that the reason you can’t tickle yourself is because you expect it. Let me explain: when you do a movement that results in a sensation, you have a pretty accurate expectation of how that’s going to feel. This expectation then dampens the sensation, a process probably evolved to let you focus on more relevant things in the environment that on what you’re doing to yourself (don’t let your mind go all dirty now, ok?).
Mechanistically speaking, it goes like this: when you move your arm to tickle your foot, a copy of the motor command you gave to the arm (the authors call this “efference copy”) goes to a ‘predictor’ region of the brain (the authors believe this is the cerebellum) that generates an expectation (See Fig. 1). Once the movement has been completed, the actual sensation is compared to the expected one. If there is a discrepancy, you get tickled, if not, not so much. But, you might say, even when someone else is going to tickle me I have a pretty good idea what to expect, so where’s the discrepancy? Why do I still get tickled when I expect it? Because you can’t fool your brain that easily. The brain then says; “Alright, alright, we expect tickling. But do tell me this, where is that motor command? Hm? I didn’t get any!” So here is your discrepancy: when someone tickles you, there is the sensation, but no motor command, signals 1 and 2 from the diagram are missing.
Likewise, when someone tickles you with your own hand, there is an attenuation of sensation, but is not completely disappeared, because there is some registration in the brain regarding the movement of your own arm, even if it was not a motor command initiated by you. So you get tickled just a little bit. The brain is no fool: is aware of who had done what and with whose hands (your dirty mind thought that, I didn’t say it!) .
This mechanism of comparing sensation with movement of self and others appears to be impaired in schizophrenia. So when these patients say that “I hear some voices and I can’t shut them up” or ” My hand moved of its own accord, I had no control over it”, it may be that they are not aware of initiating those movements, the self-monitoring mechanism is all wacky. Supporting this hypothesis, the authors conducted an fMRI experiment (Reference 2) where they showed that that the somatosensory and the anterior cingulate cortices show reduced activation when attempting to self-tickle as opposed to being tickled by the experimenter (please, stop that line of thinking…). Correspondingly, the behavioral portion of the experiment showed that the schizophrenics can tickle themselves. Go figure!
Reference 1: Blakemore SJ, Wolpert D, & Frith C (3 Aug 2000). Why can’t you tickle yourself? Neuroreport, 11(11):R11-6. PMID: 10943682. ARTICLE | FULLTEXT
Reference 2: Blakemore SJ, Smith J, Steel R, Johnstone CE, & Frith CD (Sep 2000, Epub 17 October 2000). The perception of self-produced sensory stimuli in patients with auditory hallucinations and passivity experiences: evidence for a breakdown in self-monitoring. Psychological Medicine, 30(5):1131-1139. PMID: 12027049. ARTICLE
By Neuronicus, 7 August 2016
The Science Portal 