Apelian, C., De Vignemont, F., Terhune, D. B. (2023)
Consciousness and Cognition, 108 PMID: 36706563 DOI: 10.1016/j.concog.2023.103473
How do we perceive our body? How can we “trick” our brain? Which method is more effective to cause hand lengthening illusion: imagery or hypnosis?
This article combined the perceptual lengthened finger illusion task for body image and the line reaching task for body schema techniques with hypnosis and imagination. Eighty-two (82) people participated. In the first part of the experiment the French version of SWASH (Sussex-Waterloo Group Scale of Hypnotizability) was used to measure hypnotizability. Imagery capability or competency level wasn’t registered. After a one day minimum, came the second part with the baseline finger lengthening task (body image) and the line reaching task (body schema). In this section the hand and the wrist were hidden. After that came the two conditions (imaginary or hypnosis) version of these tasks.
In the hypnotic condition the participants got suggestions about the finger lengthening “the finger is growing like a branch and extends until it is 10 cm longer” (Apelian et al., 2023, p. 5) The imagery task was the same sentence with the instruction to imagine.
This study also used Likert scale about imagery (deliberate or spontaneous which is associated with fluency and the origin of imagery according to Apelian et al., 2023) and also response expectancies.
According to the results, the finger elongating effect was significant in both conditions, but in hypnosis the outcome was larger. As explained by Apelian (2023) the two conditions results were strongly correlated because of the similarity in the background process. These effects are not associated with deliberate imagery in both conditions, but spontaneous imagery and hypnotic suggestibility strengthened it. The response expectancy had an opposite upshot.
In both conditions response expectancy was the same. This is a main point because according to Apelian (2023) it measured before the manipulations and not after that. So, the expectancy points didn’t reflect the type of the condition and didn’t give an opportunity to the real comparison (before versus after experiment) and didn’t tell anything about the dynamic of the expectancy process during the sessions which is important in suggestibility (also miss the chance to observe and detect the interaction and synchrony of the hypnotherapist and the suspect). In addition to using the hypnotizability scale before the two conditions could make a mental connection in the participants between imagination and hypnotizability and make a priming effect. There’s a possibility that the strong correlation of the results of the two conditions is because of this priming.
Surprisingly, this study showed factors that do not have influence on this process: the type and style of introduction of manipulation for the participants, the presence of an induction phase, and the use of instructions or suggestions. Interestingly the absence of induction has no effect, maybe future research could explain this phenomenon.