Abstract
Although the illusion that the mirror image of a hand or limb could be recognized as a part of one’s body behind the mirror, the effect of adding tactile stimulation to this illusion remains unknown. We, therefore, examined how the timing of tactile stimulation affects the induction of body ownership on the mirror image. Twenty-one healthy, right-handed participants (mean age = 23.0 ± 1.0 years, no medical history of neurological and/or psychiatric disorders) were enrolled and a crossover design was adopted in this study. Participants’ right and left hands were placed on the front and back sides of the mirror, respectively, then they were asked to keep looking at their right hand in the mirror. All participants experienced two experiments; one was with tactile stimulation that was synchronized with the movement of a mirror image (synchronous condition), and the other one was with tactile stimulation that was not synchronized (asynchronous condition). The qualitative degree of body ownership for the mirrored hand was evaluated by a questionnaire. Proprioceptive drift (PD), an illusory shift of the felt position of the real hand toward the mirrored hand was used for quantitative evaluation of body ownership and measured at “baseline,” “immediately after stimulation,” “2 min after stimulation,” and “4 min after stimulation.” The results of the questionnaire revealed that some items of body ownership rating were higher in the synchronous condition than in the asynchronous condition (p < 0.05). We found that PD occurred from immediately after to 4 min after stimulation in both conditions (p < 0.01) and there was no difference in the results between the conditions. From the dissociation of these results, we interpreted that body ownership could be elicited by different mechanisms depending on the task demand. Our results may contribute to the understanding of the multisensory integration mechanism of visual and tactile stimulation during mirror illusion induction.
Highlights
In the context of self-consciousness, agency denotes the feeling of causing and controlling one’s action while body ownership refers to the sense of one’s own body—i.e., that one’s body belongs to oneself—which occurs with or without agency (Gallagher, 2000)
The Wilcoxon signed-rank test showed that the body ownership ratings of three items except Q4 were significantly higher in the synchronous condition than in the asynchronous condition (Q1, Z = −2.668, p = 0.008; Q2, Z = −3.443, p = 0.001; Q3, Z = −2.841, p = 0.004; Q4, Z = −1.912, p = 0.06) (Figure 2)
We investigated for the first time whether passive tactile stimulation could induce a body ownership illusion in the mirror image and whether this illusion was persistent
Summary
In the context of self-consciousness, agency denotes the feeling of causing and controlling one’s action while body ownership refers to the sense of one’s own body—i.e., that one’s body belongs to oneself—which occurs with or without agency (Gallagher, 2000). The RHI can only be induced when the rubber hand movement is visually synchronized with that of the concealed real hand (Kalckert and Ehrsson, 2012; Riemer et al, 2013). Previous studies have found that concomitant tactile stimuli to the rubber and participants’ hands or synchronizing the movements of participants’ hand with those of the rubber hand are important for inducing RHI, and that if the stimulation or moving timing differs (referred to as the asynchronous condition), the RHI will either be mild or not occur at all (Botvinick and Cohen, 1998; Tsakiris and Haggard, 2005; Kalckert and Ehrsson, 2014; Shimada et al, 2014). Whether a relationship between quantitative and qualitative results exists remains controversial (Longo et al, 2008; Holle et al, 2011; Rohde et al, 2011; Abdulkarim and Ehrsson, 2016)
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