|From Van Doorn et al, 2014.|
You wouldn’t believe the amount of ink spilled by neuroscientists and psychologists attempting to explain the simple fact that we can’t tickle ourselves. A popular, long-standing theory posits that the self-tickle failure occurs because of the way that the brain cancels out sensations caused by its own movements. To do this, so the theory states, the brain uses the motor command underlying a given action to make a prediction of the likely sensory consequences of that action. When incoming sensory information matches the prediction, it’s recognised as self-generated and cancelled.
If this explanation is true, then any situations that confuse the brain’s ability to predict the sensory consequences of its own actions should scupper the sensory cancellation process, thereby making self-tickling a possibility. George Van Doorn and his colleagues have put this principle to the test in dramatic fashion. They measured the potential for self-tickling in 23 participants who underwent a body-swap illusion (open access article here).
The experimental set-up involved each participant sitting opposite the experimenter. The participant wore a pair of goggles that displayed a video feed from a camera that was either placed forward-facing on the participant’s own head (giving them a conventional first-person perspective), or was positioned forward-facing on the experimenter’s head, thus giving the participant a view from the experimenter’s perspective and provoking a body-swap illusion.
During both of these camera arrangements, the participant and experimenter each held one end of a wooden rod with foam at each end. The participant either moved the rod rhythmically with their right hand, causing the foam to rub against their own left palm (potentially causing self-tickling), and the experimenter’s left palm. Or, the experimenter was the one who moved the rod, causing the foam to rub against’s participant’s left palm (i.e. potential for tickling by another person) and his own left palm.
During the body-swap illusion, the participants said they felt the sensation of the foam, not where their real hand was located, but at the position of the experimenter’s hand. Given the illusion, they perceived this to be their own hand, even though it looked like someone else’s. Crucially, even in this strange situation, the participants were still unable to tickle themselves if they were the ones moving the rod (they felt the foam, but it didn’t tickle). They felt much more of tickling sensation when it was the experimenter who moved the rod.
The classic theory for why we can’t tickle ourselves is unable to explain why tickling is still not possible even in such extreme illusory contexts when the brain’s ability to predict the sensory outcomes of its actions is thrown into disarray. Moreover, self-tickling was still not experienced even in variations of the experimental setup, in which the body-swap illusion was combined with the “rubber hand illusion” and the movement of the foam was felt in a baseball bat viewed from the experimenter’s perspective!
Van Doorn and his colleagues said their “remarkable” findings are consistent with an alternative neuroscience theory that’s gaining currency. This “active inference” theory states that self-generated movements cause non-specific suppression of sensory input, regardless of whether predictions of the consequences of one’s own movement are accurate or not.
The researchers concluded: “We asked ‘can you tickle yourself if you swap bodies with someone else?’ The short answer is ‘no’.”
Van Doorn G, Hohwy J, and Symmons M (2014). Can you tickle yourself if you swap bodies with someone else? Consciousness and cognition, 23, 1-11 PMID: 24270589