The surprising reason why most people can’t tickle themselves

Mike Blake/Reuters

Go ahead, try tickling yourself on your inner forearm or neck. If you're like most people, you'll find it doesn't work.

The sensation would make you shiver or giggle with ticklishness if someone else did it, but when you do it yourself, it no longer has any tickle power.

The inability of most people to tickle themselves has been documented by psychologists for a while, and it's thought be due to the fact that the brain creates predictions of the sensory consequences of our own actions, and then cancels them out.

Although it might be fun to be able to tickle ourselves, on balance it's probably a good thing that our brains work this way most of the time – it helps us pay more rapid attention to other people's actions rather than our own, and it contributes to our sense of self.

At the turn of the century, neuroscientist Sarah Jayne-Blakemore and her colleagues showed that some patients diagnosed with schizophrenia can tickle themselves.

This fitted with other features of their illness – for example, the patients who could self-tickle also experienced hallucinations and the feeling that other people were controlling their actions. One theory is that the apparent failure of these patients' brains to adequately cancel out the sensory consequences of their own actions could be contributing both to their symptoms and to their self-tickling ability.

Now a team of psychologists in France has extended these findings, showing for the first time that psychologically healthy people who score highly in schizophrenia-like personality traits – for example, they have vivid imaginations and are prone to mild paranoia – are also capable of tickling themselves. The findings are in the journal Consciousness and Cognition.

The researchers, led by Anne-Laure Lemaitre, first identified from an initial pool of 397 students, 27 students who scored very highly on the Schizotypal Personality Questionnaire, and 27 students who scored very low on the same measure. The questionnaire includes questions on things like unusual beliefs and strange perceptual experiences. None of the students had ever been diagnosed with a psychiatric condition. They also completed a questionnaire about their experience of feelings of passivity, measured with items like "feeling you were a robot or zombie without a will of your own".

Next, the students took part in different tickle tasks involving a brush. The participants either tried to tickle their own forearm with the brush, or the brush was controlled by the researcher. In each case, the participants rated how ticklish the brush movements felt on their arm.

Overall, the students who scored high in schizotypic traits were no more ticklish than the low schizotypic students. Crucially, however, the highly schizotypic students found the self-tickling condition more ticklish than did the low schizotypic students, and they also found the self-tickling condition just as ticklish as when the researcher applied the tickling, whereas the low schizotypic students found the self-tickling condition less ticklish than when the researcher did it.

A limitation of the study that jumps out immediately is that there was no control group with intermediate levels of schizotypic traits, meaning that if you were to interpret these results in isolation, it's possible the low schizotypic students were unusually non-ticklish in the self-tickle condition, rather than the high schizotypic students being unusually self-ticklish.

But of course, it makes sense to interpret these new findings in light of past research, including the Blakemore finding of self-tickling ability in patients with schizophrenia, and another paper from 2010 that showed high scorers on schizotypy were poor at controlling their own strength to match the force produced by a machine (another indication of diminished self-monitoring).

Moreover, in the current research, the more self-tickling sensations that the high schizotypic students reported, the more they tended to agree with items related to suspicion and unusual perceptual experiences on the Schizotypy questionnaire, such as "I'm sure I'm being talked about behind my back" and "I often hear a voice speaking my thoughts aloud", and the higher they scored on the passivity scale.

These findings don't mean that if you can tickle yourself you are likely to develop schizophrenia. However, they are consistent with the idea that the same brain processes (involved in movement control and monitoring) that may contribute to the symptoms experienced by patients with schizophrenia, may also contribute to schizophrenia-like beliefs and experiences in healthy people.

The researchers said their results show that "non-clinical subjects with schizophrenia-like symptoms have an abnormal subjective experience of willed actions". They added: "When considering a continuum ranging from the absence of a disorder to the full-blown symptoms of schizophrenia, our data provide a basis for understanding the illusions of control experienced by schizophrenic patients."

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