It seems to be the stuff of pure fantasy: a hand made of rubber feels as if it belongs to the owner's body. Although it is hardly conceivable, it is an illusion which is in fact well-known in the field of psychology -- and one that can be produced in skilful experimental setups. Psychologists from Germany and the Netherlands have now shown for the first time how test persons can also integrate their own smartphones into their bodily selves. This means that whether an object is felt to belong to the owner's own body does not only depend on whether it has a form similar to that of a human hand. The extent to which the object is used also appears to play an important role. The results have been published (online first) in the latest issue of the specialist journal Psychological Research.
"The research question behind this is how flexible the brain is, and whether the daily use of modern technical devices can, in the long term, lead to their integration into the owner's own body scheme," as PD Dr. Roman Liepelt from the Institute of Psychology at Münster University explains. The researchers from Münster, Leiden (Netherlands) and Regensburg adapted the experimental setup used for the "rubber-hand illusion," with the test person laying their left hand on a table. The hand is screened off, so that the person cannot see it. Next to this non-visible hand the researchers placed a rubber hand, the test person's smartphone (iPhone), a computer mouse or a piece of wood shaped like a smartphone. The real, not visible hand and the visible artificial object were then stroked with a brush for a few minutes, synchronously or asynchronously (as a control condition). The fact that the test person feels their own hand being stroked, and simultaneously sees the object being touched synchronously, produces the sensation that the object is part of their own body -- because both pieces of information merge into one single percept.
The researchers used a questionnaire to measure what the test person felt. They also asked about another central aspect of the rubber-hand illusion -- the feeling that the non-visible real hand actually moves towards the rubber hand ("proprioceptive drift"). In the case of synchronous stimulation, the test persons felt more strongly that all the objects belonged to their own body. In other words, the subjective sensation of the "rubber-hand effect" could also be produced with a smartphone, a computer mouse and a smartphone-shaped piece of wood. However, the apparent spatial movement of the real, non-visible hand towards the visible object only occurred in the case of the rubber hand and the smartphone -- not with the computer mouse and the piece of wood. In other words, only the person's own smartphone produced a complete illusion similar to that of the rubber hand.
The results indicate that our bodily self is much more flexible and plastic as it was previously assumed," says Roman Liepelt, "because in our experiment a smartphone, which has absolutely no similarity to a human hand, produced an illusion similarly strong to that of an artificial hand. What presumably plays a major role is the fact that the test persons had already had extensive experience in their lives in using a smartphone." What is decisive, he adds, is the so-called multi-sensory integration. "We look at our smartphone daily and in doing so we feel intuitively how to use it," says Bernhard Hommel from the University of Leiden. "A combination of feeling, seeing and past experience probably ensures that we integrate certain objects in our body scheme." The extent of the flexibility of our bodily self is something that will have to be analysed in further studies, say the researchers.
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