As the more tactilely-sensitive among us will know, there’s a particular type of abdominal-aching laughter that can only be generated by tickling. Such crippling fits of uncontrolled giggling can be brought on by the brush of a feather or the mischievous digits of a lover or parent, yet the majority of us find it impossible to tickle ourselves to the same degree. However, a new study indicates that people with schizophrenia-like symptoms may be more capable of doing so, thanks to the disruption of certain cognitive processes that distinguish between one’s own actions and those of other people.

Appearing in the journal Consciousness and Cognition, the study builds on previous work indicating that self-produced somatosensory stimulation is perceived with greater intensity by people with schizophrenia. Current theories as to why this is the case center around a “forward model” of cognition, whereby the brain is able to predict the sensory outcomes of one’s own actions.

These outcomes are then processed with less intensity than externally generated sensations, thereby ensuring that greater salience is placed on unexpected stimuli than from those produced by our own activities. This leads to a number of adaptive advantages, ensuring we don’t get a fright each time we touch a part of our body like we do when someone unexpectedly grabs us. As such, we remain alert to external cues in our environment.

However, this process is thought to be impaired in people with schizophrenia, which is why those who suffer from the condition often confuse their own volition with that of an external agent. For instance, internal thoughts may be perceived as the voice of someone else giving an instruction. A common characteristic of severe schizophrenia is, therefore, the sensation of lacking control over one’s own life, and a feeling that one is being manipulated by other people or forces.

^{"Passivity experience" denotes a sense of being controlled by external agents. ostill/Shutterstock}

This inability to distinguish self-generated impulses from those of others led researchers to hypothesize that people with schizophrenic-like symptoms would be more likely to be able to tickle themselves than those lacking these characteristics. To test this, they recruited non-schizophrenic volunteers who were then analyzed in order to determine their “schizotypal traits”.

Schizotypy refers to a collection of personality characteristics that resemble certain aspects of schizophrenia, and can be detected to varying degrees in non-sufferers. Among these is a sense of what psychologists call “passivity experience,” which involves “feeling as if you were a robot or zombie without a will of your own,” the researchers write.

After completing a questionnaire designed to reveal the severity of their schizotypal traits, participants were then tickled by the researchers using a feather, before being asked to tickle themselves with the same feather.

Post-tickling, participants rated the intensity of the experience, indicating how ticklish they found it. Results showed that people with stronger schizotypal traits found the self-tickling task more ticklish than those with low levels of schizotypal traits. However, the two groups were equally ticklish when tickled by the researchers.

Based on these results, the study authors conclude that those with more schizophrenic-like characteristics “had less efficient predictive mechanisms and were less able to predict the sensory consequences of their own actions.” This, in turn, may be indicative of a wider deficiency at “differentiating between self-produced and externally-produced sensations.”