Can you tickle yourself? (Seriously, …)

Sebastian’s note #5: Individuals with schizotypal traits are more able to tickle themselves, presumably because they are less able to generate neural predictions of the sensory consequences of their own actions

Photo by Gabe Pierce on Unsplash

Source: Lemaitre, A.L., Luyat, M. and Lafargue, G., 2016. Individuals with pronounced schizotypal traits are particularly successful in tickling themselves. Consciousness and Cognition, 41, pp.64–71.

Efference copy

• An efference is a movement-producing signal generated by the motor system.
• A forward model uses a copy of the signal (efference copy) to predict sensory feedback from the motor command.
• The sensory discrepance is the difference between the predicted sensory feedback and the actual feedback.
• Pynn & DeSouza (2013): The use of such a system is, presumably, efficient, because it prevents the central nervous system from wasting resources processing irrelevant sensory stimuli.
• Blakemore, Wolpert & Frith (2000, 2002): It allows us to distingush the consequences of our own actions from external, potentially significant events, providing a sense of agency: ‘‘I’m the one who is causing or generating an action”

Passivity experience

Impairment of the predictive process might reduce the attenuation of the sensory consequences of one’s own actions and and prompt people to attribute the event to an external cause.

Passivity experience (PE) is the belief that one’s thoughts or actions are controlled by an external agent. PE is seen as a hallmark of schizophrenia.

Schizotypal traits

• Schizotypal traits are schizophrenia-like traits.
• Researchers regard schizotypal traits as a spectrum ranging from normal characteristics and mild forms to the emergence of schizophrenia.
• Individuals scoring high on psychological scales measuring schizotypal traits have been found to have trouble predicting the sensory consequences of their actions.
• The authors hypothesize that they should, thus, be able to more effectively tickle themselves than individuals with low scores.

Can individuals with schizotypal traits tickle themselves?

Extreme group design

• Initially, 397 students completed a 7-items excerpt from the Schizotypy Personality Questionnaire (SPQ).
• Eighty participants with extreme (low or high) scores then complete the fullversion of the SPQ.
• 27 individuals with scores in the upper quartile of the schizotypal distribution (according to the SPQ French normative data) and 27 individuals in the lower quartile participated in the main study.
• No participants reported a history of neurological or psychiatric disease.
• A mask was placed over the eyes to prevent vision to influence somaesthesia (the perception of one’s own body).

Three conditions

Participants placed their non-dominant forearms flat on the table and performed three trials each under three conditions:

1. Self-tickling (ST): Participants used their dominant hand to move a paintbrush on the non-dominant hand back and forth (to “tick” / “tack” sounds of a metronome).
2. Predictable, externally-produced tickling (ET_P): same tactile stimulation as in the first condition, but applied by the experimentesr.
3. Unpredictable, externally-produced tickling (ET_UN): In the first two conditions, both participant and experimenter could hear the sound produced by metronome. In this condition, only the experimenter could hear the those sounds.

Each trial lasted 6 metronome beats.

Tickling rates and the ISA

• Participants rated their perceived sensation of tickling at the end of each trial on a scale of 0 to 10 (“extremely tickly”).
• Based on these ratings, an index of somatosensory attenuation (ISA) was computed.
• Let ET be the average rating of sensation for externally produced tickling (ET_P and ET_UN). And let ST be the rating for self-produced tickling.
• Then the ISA is defined as follows: ISA = difference between ET and ST / sum of ET and ST
• If ST is zero (no perceived sensation of self-tickling), then ISA is 1. In this case, there is complete attenuation, i.e., no ability to self-tickle.
  If ST is equal to ET, there is no difference in the perceived sensation of self-tickling and externally-produced tickling. In that case, the ISA is 0, i.e., no attenuation.

Results

• The two groups did not differ in terms of the mean tickling ratings. Individuals with high SPQ scores are not more or less ticklish per se.
• It should be noted that tickling sensation did not differ significantly between the ET_P and ET_UN conditions.
• Crucially, in the self-tickling condition, there was a significant difference w.r.t. to ratings of perceived tickling sensation between the high-SPQ and low-SPQ groups: 3.53 vs. 2.14.
• Attenuation as measured by the ISA was almost four times lower in the high-SPQ group: 0.09 vs 0.33.
• Moreover, the more effective participants were in tickling themselves, the more passivity-like experiences they reported.

Conclusion

• Schizotypal traits are associated with a better ability to tickle oneself.
• Overall, the results suggests that individuals with schizotypal traits are less able to generate neural predictions matching the sensory consequences of ther own acts.
• The authors note that the results are in line with hypothesis that false beliefs in an external control by supernatural right are the result of disorder in perception rather than thinking.
• This would explain why delusions of control in schizophrenia participants are resistant to rational counter-arguments.

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