![]() Each participant experienced three conditions in a randomized order in a single session, and two sessions (six trials in total) were performed for individual participants. Under all conditions of the screening experiment, the fake and actual hands were aligned in parallel, as shown in Figure 3a. The participants gazed directly at the fake hand under conditions 1 and 3, and the fake hand was seen through a mirror under condition 2. Although the fake hand remained still on the desk, the participants moved both their hands. Condition 3 was prepared as a control, under which the fake and actual hands were separated. The comparison of these two conditions may be sufficient however, to investigate whether our experimental setup created the illusion, we introduced a baseline condition, condition 3. Under conditions 1 and 2, the participants held and moved the acrylic rods, and the fake and actual right hands were moved synchronously. Three conditions were applied in this experiment, as shown in Table 1. The magnitudes of the illusion with and without a mirror were compared. We investigated whether the experience of body ownership would be evoked when the fake hand was gazed at through a mirror. We focus on whether the RHI experience is evoked using a mirror when spatial incongruency exists rather than specifying a particular aspect of the mirror images (e.g., lower reliability or weak signal level), which may moderate the spatial-consistency requirements, or discussing underlying neural bases. The decrease in visual reliability reduces the weight of the visual cues during sensory integration, which may cause the visible spatial incongruency to become less significant. Third, the decrease in the reliability of visual cues may facilitate the bodily illusion experienced under a spatially incongruent condition. Second, using a mirror lowers the light intensity of a seen hand and facilitates the inverse effect of multisensory integration. First, as stated above, the perceptual tolerance to mirrored images may lead to a spatially incongruent seen hand being accepted as a part of the body. Three explanations are considered for the above hypothesis. In one study that supports this possibility, the body ownership illusion was evoked irrespective of the egocentric or allocentric fake hand images in a mirror. The present study pursues this hypothetical proposition. This tolerance of mirrored images or spatial misallocation may robustly lead to an RHI experience in which incongruency between fake and actual hands does not disturb the illusory experience of body ownership. Further, the locations of bodily sensations are biased or recalibrated when seen through a mirror. Moreover, people do not accurately predict the mirror reflection and spatial relationships between the mirror and themselves. People tend to not recognize unnatural manipulation in mirrored images such as optically incorrect tilting, compression, expansion, and left–right flipping of mirrored images. Despite the reversed orientation and distant focal points, humans recognize a mirror image as a copy of the real environment. Under these settings, the RHI is experienced. In contrast, in the present study, the participants experienced RHI while gazing at the fake hand through a mirror. In experiments on the RHI paradigm, participants usually are asked to gaze at the fake hand, but visual cues are not entirely necessary to elicit an illusion. ![]() These findings suggest that using a mirror masks subtle spatial incongruency or degrades the contribution of visual cues for spatial recognition and facilitates multisensory integration for bodily illusions. The participants experienced RHI even when the actual and rubber hands were incongruent in terms of orientation. We then examined whether using a mirror image for RHI allows disagreement in orientation (45 ∘) between the rubber and actual hands (main experiment). Subjective evaluations using an RHI questionnaire demonstrated that embodiment of the rubber hand was evoked in the presence or absence of a mirror. First, we tested whether illusory ownership of a fake hand seen in a mirror could be induced in our experimental environment (screening experiment). The present study performed two experiments to reveal how self-body recognition of a fake hand via a mirror affects RHI. Considering that humans recognize mirror images as copies of the real world despite misinterpreting optical reflections, spatial disagreement may be accepted in rubber hand illusion (RHI) settings when a mirror is used to show a fake hand.
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