Explaining away the body: experiences of supernaturally caused touch and touch on non-hand objects within the rubber hand illusion

Abstract

Background: In rubber hand illusions and full body illusions, touch sensations are projected to non-body objects such as rubber hands, dolls or virtual bodies. The robustness, limits and further perceptual consequences of such illusions are not yet fully explored or understood. A number of experiments are reported that test the limits of a variant of the rubber hand illusion.

Methodology/principal findings: A variant of the rubber hand illusion is explored, in which the real and foreign hands are aligned in personal space. The presence of the illusion is ascertained with participants' scores and temperature changes of the real arm. This generates a basic illusion of touch projected to a foreign arm. Participants are presented with further, unusual visuotactile stimuli subsequent to onset of the basic illusion. Such further visuotactile stimulation is found to generate very unusual experiences of supernatural touch and touch on a non-hand object. The finding of touch on a non-hand object conflicts with prior findings, and to resolve this conflict a further hypothesis is successfully tested: that without prior onset of the basic illusion this unusual experience does not occur.

Conclusions/significance: A rubber hand illusion is found that can arise when the real and the foreign arm are aligned in personal space. This illusion persists through periods of no tactile stimulation and is strong enough to allow very unusual experiences of touch felt on a cardboard box and experiences of touch produced at a distance, as if by supernatural causation. These findings suggest that one's visual body image is explained away during experience of the illusion and they may be of further importance to understanding the role of experience in delusion formation. The findings of touch on non-hand objects may help reconcile conflicting results in this area of research. In addition, new evidence is provided that relates to the recently discovered psychologically induced temperature changes that occur during the illusion.

Figure 1. Experiment set-up viewed from above, showing relative locations of camera, experimenters and participant.

A. Experimental set up for experiment 1. B. Experimental set up for experiment 2 and 3. The Participant is seated to the right, wearing goggles. Experimenter A is seated opposite the participant. Experimenter B is seated on the left in A. A rubber hand is used in B.

Figure 2. Temperature sensor placement on participant's hand.

Figure 3. Examples of experimental conditions: participant's visual perspective.

All concurrent with synchronous or asynchronous touch on participant's real, unseen arm. A) Moving, visible finger seen to touch visible foreign arm (experiment 1, condition 1 and 2). B) Moving visible finger seen to touch visible foreign rubber arm (experiment 2). C) Moving visible finger elevated off visible foreign rubber arm (experiment 2, condition 1). D) Moving visible finger seen to touch white box (experiment 2 and 3).

Figure 4. Temperature differences between synchronous and asynchronous touch.

Condition 1 and condition 2 temperature differences between synchronous and asynchronous touch taken as the mean of the last 30 seconds of temperature data for condition 1 and the mean of the last 60 seconds of temperature data for condition 2. Positive temperature differences indicate higher temperatures in the asynchronous tapping condition.

Figure 5. Box plots for condition 1, experiment 1.

Box plots of scores for condition 1, experiment 1, initial and 30 s for synchronous and asynchronous tapping.

Figure 6. Box plots for condition 2, experiment 1.

Box plots of scores for condition 2, experiment 1, initial, 10s, 30 s and 60 s for synchronous and asynchronous tapping.

Figure 7. Box plots for experiment 2.

Box plots of scores for experiment 2, for the elevated touch and touch on a box for synchronous and asynchronous tapping.