1. Introduction
The rubber hand illusion (RHI) is a phenomenon in which body ownership of a fake hand is illusorily experienced when a visible fake hand and occluded veridical hand are exposed to spatially and temporarily congruent visuotactile stimuli. Since Botvinick and Cohen [
1] first reported this phenomenon, the RHI paradigm has been used in studies on multisensory integration and body ownership. In this paradigm, congruency among multiple sensory cues is important [
2,
3]. Smaller spatial and temporal incongruency between actual and fake hands lead to more intense illusions of body ownership. For instance, when fake and actual hands are spatially incongruent (e.g., when they have different postures), the body ownership illusion is weakened [
4,
5,
6,
7,
8,
9,
10,
11]. According to Constantini and Haggard, a hand angle mismatch of
between the veridical and fake hands distracts from the body ownership illusion [
6]. Nevertheless, this angle value varies among studies [
9,
12].
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 [
2]. In contrast, in the present study, the participants experienced RHI while gazing at the fake hand through a mirror. Under these settings, the RHI is experienced [
13,
14,
15]. Despite the reversed orientation and distant focal points, humans recognize a mirror image as a copy of the real environment [
16,
17]. People tend to not recognize unnatural manipulation in mirrored images such as optically incorrect tilting, compression, expansion, and left–right flipping of mirrored images [
16]. Moreover, people do not accurately predict the mirror reflection and spatial relationships between the mirror and themselves [
16,
18,
19]. Further, the locations of bodily sensations are biased or recalibrated when seen through a mirror [
20,
21]. 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. The present study pursues this hypothetical proposition. 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 [
14].
Three explanations are considered for the above hypothesis. 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. Second, using a mirror lowers the light intensity of a seen hand and facilitates the inverse effect of multisensory integration [
22,
23,
24]. Third, the decrease in the reliability of visual cues may facilitate the bodily illusion experienced under a spatially incongruent condition. The decrease in visual reliability reduces the weight of the visual cues during sensory integration [
25,
26,
27], which may cause the visible spatial incongruency to become less significant. 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 [
28,
29,
30].
In general, experimental RHI settings require careful adjustment between a seen fake hand and an unseen veridical hand such that they are spatially consistent without critical disagreements. However, such adjustment may deter the application of RHI in commercial virtual reality or augmented reality environments [
31,
32,
33]. If the mirror moderates the requirement of small spatial contradiction, then the application of RHI may be extended.
In addition to using mirrors, our study features voluntary hand movement and self-generated tactile stimuli. In our experiments, the participants tapped a desktop and experienced a resultant tactile stimulus in their fingers. The voluntary motion or conjunction of the voluntary motion and self-generated tactile stimuli elicit RHI with a subjective illusion intensity that equals or exceeds the passive condition [
34,
35,
36,
37,
38]. Voluntary motions play a significant role in forming the sense of body ownership and may facilitate the occurrence of illusion [
34,
36,
39,
40,
41]. This study addresses the effects of a mirror on agency by considering active hand motion.
In a screening experiment, we tested whether the body ownership illusion was elicited when the participants gazed at the mirror image of a fake hand. Thus far, only one study [
15] has investigated mirror-RHI involving self-generated hand motions and tactile stimuli. Therefore, this follow-up experiment is required to investigate the reproducibility of the findings of [
15] in another experimental environment. The main experiment investigated whether the incongruency of the fake and actual hand postures (angles) was accepted in the RHI setting involving a mirror. The previous studies on mirror RHI did not address situations where actual and fake hands were angled.
This study was conducted with the approval of the Institutional Review Board, School of Engineering, Nagoya University (♯17-12).
4. Discussion
We investigated the effect of the mirror images when there was spatial incongruency between the fake and veridical hands. Here, the results of the two experiments are briefly summarized and compared with previous studies, and reasons underlying the effect of the mirror are discussed.
In the screening experiment, illusory body ownership and agency were experienced by the participants in our experimental environment when they viewed the fake hand in a mirror. No significant differences were confirmed from the magnitude of PD, whereas conditions 2 (congruent hand posture, mirrored view, and synchronous) and 5 (incongruent hand posture, mirrored view, and synchronous) tended to exhibit greater PDs. With an increase in the number of participants or repetition of measurements, the statistical judgment may change for these two conditions. Although the experimental environments are different, these results are supported by Jenkinson and Preston [
15], who investigated mirror-RHI involving active hand movements. Thus, the reproducibility of these experiments is promising. The significant difference in PD values between direct and mirror-viewed conditions were not confirmed, but the difference in the subjective ownership reports was observed between these conditions. As mentioned before, PD does not always match the subjective reports [
48]. Other related studies have reported body ownership through a mirror to the same extent [
13] or weaker [
14] compared to the condition without a mirror, whereas these studies investigated passive RHI.
In the main experiment, ownership over the fake hand was observed even when the fake and actual hands were spatially incongruent in terms of the hand angle. In our experiment, the postural incongruency of the hand was observed to potentially disturb the ownership illusion, as suggested by the post hoc comparison of the ownership scores between conditions 1 (direct-view and congruent, score = 1.33 ± 0.27) and 4 (direct-view and incongruent, score = 0.07 ± 0.49) (
,
,
). Body ownership, which should have been weakened by postural incongruency, was strengthened using a mirror. The subjective ownership scores for conditions 2 (congruent, mirror-view, and synchronous, score = 2.07 ± 0.25) and 5 (incongruent, mirror-view, and synchronous, score = 1.67 ± 0.16) did not exhibit a significant difference (
,
,
), suggesting that the ownership illusion was induced without respect to the hand angles when the mirror was used (Note that these conditions across the screening and main experiments were not planned to be tested originally.). In contrast, the presence of a mirror did not affect agency in spatially congruent and incongruent conditions. Postural incongruency tends to weaken body ownership; however, it does not affect agency in active RHI [
37,
38,
45,
49]. This is known as the dissociation between ownership and agency. Nonetheless, they can interact with each other, which has been demonstrated in the RHI condition of a mirror [
15]. The correlation coefficient between the subjective scores for agency and ownership suggests their interaction. In the screening experiment, ownership and agency did not exhibit any meaningful correlation coefficients under conditions 1 (direct view,
,
) and 2 (mirror view,
,
). In these conditions, the veridical and the observed hands were aligned; herein, the sense of agency could have been easily evoked while being unaffected by ownership. In contrast, in the main experiment wherein the hand postures were incongruent, the two types of subjective scores were slightly (but not significantly) correlated under conditions 4 (direct view,
,
,
) and 5 (mirror view,
,
,
with the Bonferroni correction). In these conditions, ownership might have positively affected the sense of agency. We could have discussed the effects of agency or voluntary motion on ownership further, had we investigated the passive RHI conditions under a spatially incongruent environment. Unfortunately, we did not conduct tests on passive conditions.
The mirror-induced enhancement of the illusion could be explained by three reasons.
First, there is the perceptual tolerance to incorrect mirror reflection. People largely pay little attention to the optical correctness of mirrored images [
16,
18,
19]. They also update their internal body models upon seeing their body parts in a mirror [
20,
21]. The mirror reflection was not manipulated and optically correct in the present study; however, because of these intrinsic properties of mirrored images, the seen hand was more easily regarded as the participant’s veridical hand despite the angle incongruency between the fake and veridical hands. In other words, the participants did not see the tilted fake hand in the mirror as unnatural. We think this explanation agrees with the implicit hypothesis in [
13].
The second reason is that visual cues are weakened by a mirror, which accelerates multisensory integration. The body-ownership illusion is considered to be caused by the integration of visual, tactile, and proprioceptive cues in active RHI, and a greater degree of integration would produce a more intense illusion. Multisensory integration is enhanced when the stimuli to individual sensory channels are weak [
22,
23,
50]. This is known as inverse effectiveness [
24] and it is one of the three aspects of RHI: spatial rule, temporal rule, and inverse effectiveness. In our experiment, using a mirror might have weakened the light intensity of the visual stimuli, which could be related to the principle of inverse effectiveness, and multisensory integration might have been enhanced relative to the settings without a mirror.
Third, a mirror reduces the reliability and precision of visual cues and masks the inconsistency of perceived signals. For example, depth perception and judgment of mirrored images are inaccurate [
51,
52]. Provided that the perceived hand is determined by a multisensory integration process of sensory cues [
53,
54,
55,
56], less reliable or imprecise images in a mirror may alter the weightings of unisensory signals for multisensory integration [
15] to maximize the resultant reliability after integration. For instance, in a visual–haptic integration task, a decrease in the reliability of visual cues led to a relative increase in the contribution of haptic cues [
25,
26,
27]. Therefore, illusory body ownership would be influenced by the change in weightings of multisensory cues. In our experimental settings, visual, tactile, and proprioceptive cues were involved, and using a mirror degraded the reliability in terms of the visually provided hand posture. The weightings of sensory cues would be different between conditions with and without a mirror. Such an effect of less reliable images in a mirror was noted in [
14], in which even laterality was masked using mirror images of a fake hand. In other words, in classical RHI settings where the fake hand is gazed at directly, because of the greater contribution of visual cues, visible incongruency may cause the visual sense to fail to capture other cues and prevent the occurrence of the illusion.
Some limitations of the present study lie in the experimental design. We note that the general disadvantages of the within-subjects design hold true for our study as well—i.e., problems associated with learning or after-effects. In our paradigm, a trial in which the illusion was elicited might have affected the subsequent trials [
44]. Our hypothesis is that using the mirror influenced the incongruency between the visual and proprioceptive cues. However, in our experiments, tactile cues were also involved, and it is unclear that how visuotactile congruency/incongruency affected the results. In the future, the experiment will be repeated with either active hand motion or tactile stimuli, whereas tapping or rubbing in the present study caused both motion and tactile stimuli simultaneously. Another limitation may be the weak control of active hand motions. We cannot deny that the results could have been influenced by the differences in the individual motions—i.e., tapping or rubbing and the frequency of their motions.