If I were you: perceptual illusion of body swapping

Abstract

The concept of an individual swapping his or her body with that of another person has captured the imagination of writers and artists for decades. Although this topic has not been the subject of investigation in science, it exemplifies the fundamental question of why we have an ongoing experience of being located inside our bodies. Here we report a perceptual illusion of body-swapping that addresses directly this issue. Manipulation of the visual perspective, in combination with the receipt of correlated multisensory information from the body was sufficient to trigger the illusion that another person's body or an artificial body was one's own. This effect was so strong that people could experience being in another person's body when facing their own body and shaking hands with it. Our results are of fundamental importance because they identify the perceptual processes that produce the feeling of ownership of one's body.

Figure 1. Set-up.

Experimental set-up to induce illusory ownership of an artificial body (left panel). The participant could see the mannequin's body from the perspective of the mannequin's head (right panel).

Figure 2. Questionnaire evidence for perceiving a mannequin's body as one's own.

The questionnaire consisted of the seven statements (S1–S7). S1–S3 referred to the illusion and S4–S7 served as controls. Participants indicated their responses on a seven-step scale ranging from ‘agree strongly’ (+3) to ‘disagree strongly’ (−3). The high rating scores on the illusion statements that were observed only in the synchronous condition revealed that the participants experienced the illusion. The bars represent mean values and the error bars indicate standard errors. For details see Results and Methods.

Figure 3. Physiological evidence for perceiving a mannequin's body as one's own.

The mean skin conductance responses (SCRs) for 10 participants when the illusory body was “threatened” with either a knife or a spoon. The SCR is significantly greater in the illusion condition than in either of the control conditions (p = .009 and p = .028, two-tailed t-test). The response does not differ significantly between the two control conditions (p = .484, two-tailed t-test). Error bars denote standard errors.

Figure 4. Generalisation of body ownership from the stimulated body part to the rest of the body.

The mean skin conductance responses (SCRs) for 13 participants when the abdomen of the mannequin was “threatened” with a knife after a period of synchronous or asynchronous visuo-tactile stimulation of the hands or the abdomen (the four conditions on the x axis). The SCR is significantly greater in the synchronous (illusion) conditions than in the control ones regardless of whether the hands or the abdomen were stimulated (p = .011 and p = .033, two tailed Wilcoxon Signed Ranks Test). Mean values and standard errors are displayed.

Figure 5. Only an object that looks like a human body can be owned.

The figure displays the skin conductance responses (SCRs) from 12 participants when the mannequin and a rectangular object (a green box of the same size) were threatened in turn. The threat-evoked SCR was significantly greater when the mannequin was threatened in the synchronous condition than when the rectangular object was under threat after the same synchronous stimulation (p = .008, two-tailed t-test). A significant effect of synchronous vs. asynchronous stimulation was observed only when the mannequin was threatened (p = .04, two tailed t-test). Mean values and standard errors are displayed.

Figure 6. Experimental set-up to induce the ‘body swap illusion’.

Figure 7. Objective evidence that people can experience swapping body with other people.

Mean skin conductance responses (SCR) for twenty participants when either the experimenter's or the participant's hands were threatened during the illusion and the three control conditions (error bars represent standard errors). A significantly higher SRC was registered when the ‘new’ body (the experimenter's hand) was threatened with the knife in the illusion condition than when it was threatened in the control condition (p = 0.002, paired two-tailed Wilcoxon Signed Ranks Test) or when the physical ‘old’ body was threatened in the illusion condition (p = .037, paired two-tailed Wilcoxon Signed Ranks Test). The interaction between the main factors (Hand x Condition) was significant (p = .001, Two Way Repeated Measures ANOVA on standardized variables).

Figure 8. Questionnaire results for both sexes.

The questionnaire consisted of the seven statements (S1–S7). There was no significant difference between the responses of female and male participants in the synchronous condition (p = .613, F(1,223) = .257, ANOVA). For details, see the Results and Methods.

Figure 9. Illustration of the procedure to simulate cutting the mannequin with either a knife or a spoon in Experiment #2.