Schizophrenics Can Tickle Themselves
No, it’s not because they have “split personalities.”
In a previous post on contagious yawns I mentioned self-tickle as another interesting and fun pop-neuropsychiatric topic. At some point most of us have probably wondered what the heck is up with tickling? (not to mention $%#*#@ magnets, how do they work?) The question of why we can’t tickle ourselves came up a few weeks ago in a conversation at work, and while my colleagues smiled and shrugged and moved on, I thought “somebody must have studied this, they’ve studied pretty much everything else I can think of.” Indeed, a quick PubMed search led me to a paper simply entitled “Why Can’t You Tickle Yourself?“.
Asking “why” is of course tricky, and one could easily come up with evolutionary explanations of how distinguishing self-touch from external touch would be beneficial in a world where we slept on the floor of caves with poisonous creepy-crawlies all around. This paper actually looks more at “how” the phenomenon of self-non ticklishness comes to be.
The basic idea is that as the primary motor cortex is signaling down the corticospinal tract to get our muscles to move, these signals are interrupted and modulated by the cerebellum, which provides a kind of buffering. When working properly, the cerebellum allows movements to be smooth, an appropriate speed, and to start and stop in the right places. It appears that the cerebellum also sends a signal to secondary sensory cortex and the anterior cingulate gyrus that indicates what movement is planned. In English it would be something like “ok, we’re swinging the leg forward to walk now, please ignore the sensation of pants rubbing against the knee.” This gating of sensation is well-described: it’s how we tune out frequent unimportant stimuli, and you can easily see why it’s an important ability to have.
It turns out that if the cerebellum alerts these areas to a movement that will contact another body part, the input from the appropriate body part is gated as well. This makes the sensations of self-tickling just predictable enough so that they aren’t, well, ticklish. The investigators used a sort of tickle-robot that the subjects could use to tickle themselves. When the robot moved in concert with the controls moved by the subject, it wasn’t very ticklish. But if the robot’s response were delayed a few hundred milliseconds, or if the direction of movement was skewed, it became more ticklish. The less the robot movements correlated with the actual movements of the subjects at the controls, the more ticklish the stimulus.
So now we know that we can be tickled by robots and that we can’t tickle ourselves because we can’t surprise ourselves. The study goes further, however. The authors suspected that psychotic symptoms such as auditory hallucinations or the feeling of being controlled by an external agent were related to a disruption in this feedback system. Both of these symptoms are often characterized as the inability to recognize one’s thoughts and actions as one’s own, leading to the logical belief that they originate externally.
So what happened when psychotic subjects tried to tickle themselves? (For some reason, the psychotic subjects didn’t get to use the robot. Perhaps because the investigators didn’t want to write in the grant proposal “we’d like to build a tickling robot and watch psychotic people tickle themselves with it.”) The subjects with psychotic symptoms found that when they tickled their own hands, it was no less ticklish than when the experimenters did it. This is a little ambiguous: it means that either schizophrenics aren’t ticklish or that they can tickle themselves. I choose to believe the latter.
It’s medical dogma that some dread illnesses have persisted because they confer some advantage to those who carry them: sickle-cell trait protects against malaria, cystic fibrosis protects against typhoid fever. Being able to tickle oneself is small comfort for having schizophrenia, but at least it’s something.