I am the metre: The representation of one’s body size affects the perception of tactile distances on the body

Abstract

Humans must ground the perception of one’s body in a mental representation to move in space and interact with objects. This representation can be temporarily altered artificially. In the full-body illusion (FBI), participants see a virtual (or filmed) body receiving a tactile stimulation. When participants receive touches on their body similarly to the seen one (i.e., homologous location and synchronous timing), they embody the seen alien body. While the subjective embodiment of alien bodies of different sizes has been already manipulated with the FBI, it remains unexplored whether the body-metric perception is impacted too. We first developed a new setup for the FBI using 360° videos to favour the embodiment. The FBI was induced for bodies of three sizes adopting anatomical and non-anatomical viewpoints, and we measured the subjective embodiment. The results suggest that humans can embody normal size or bigger bodies seen from anatomical viewpoints, but not smaller ones. We then investigated if the FBI modulates the body-metric representation. We found that the resized bodies’ vision affects the perception of one’s body-metric representation, but this was independent of the embodiment, suggesting that the FBI alters the body representation at different levels with a specific impact.

Keywords

Body illusion body perception body metric tactile distance task

This study put forward our understanding of the plastic features of one’s body perception and representation. This study’s novelty stood in using a new method to induce the embodiment illusion with photorealistic resolution (360° videos), which is more immersive than the computer rendering. Moreover, the subjective illusory embodiment was paired with the legs’ perceived changes, exploring the embodiment impact at a new perceptual level, namely, the body metric. Results show that the body representation is malleable at different levels (i.e., subjective experience, body metric perception) with specific mechanisms.

Introduction

Humans must ground the perception of their bodies (i.e., their size and shape) in a mental representation to move, and interact with objects and space (De Vignemont et al., 2007). There is an agreement that the body representation is not a unitary system, but different theoretical models accounted for its components. Longo and Haggard (2010) proposed a model where body representation is split into the somatorepresentation, the cognitive reference model of the body, and the somatoperception, which would be the body’s perceptive representation as to the source of first-person experiences. The latter is particularly relevant for the this article. The somatoperception is supposed to receive information from three sources: (a) an online postural schema, updated with movements; (b) a body model of size and shape; and (c) a superficial schema—a map of the body surface based on somatosensory processes (Longo et al., 2010). These representations are plastic, and they can change during the lifespan, adapting to the different experiences (Flor et al., 2006; Sposito et al., 2012). Crucially, they can also be temporarily altered through experimental procedures collectively named “body illusions” (Botvinick & Cohen, 1998; Ehrsson, 2007; Gandevia & Phegan, 1999; Giurgola et al., 2019; Lenggenhager et al., 2007; Ramachandran & Altschuler, 2009; Slater et al., 2009; Tosi et al., 2018). Body illusions strongly support the idea that body representation is dynamic and that it can be modified through the proper balancing of multisensory information (Tsakiris, 2010).

Body illusions are entangled to the concept of embodiment (Carruthers, 2008; De Vignemont, 2014; Garbarini et al., 2015; Romano et al., 2013), which is thought to be the core mechanism accounting for the efficacy of these experimental procedures. The embodiment can be defined as follows: “(an object) E is embodied if and only if some properties of E are processed in the same way as the properties of one’s own body” (De Vignemont, 2011). The embodiment related to bodily illusions encompasses three facets: the sense of ownership (i.e., the perception that an external object is part of one’s body), the sense of agency (i.e., the feeling of being able to move and control the object as a part of the body), and the sense of location (i.e., the sensation that the object and the body are situated in the same place) (Longo et al., 2008). Besides the contribution at the theoretical level, body illusions can help understand the representational distortions in clinical situations (e.g., phantom limb and hemiplegia) with a potential impact in rehabilitation (Dohle et al., 2009; Flor et al., 2006; Hallett, 2001; Tosi et al., 2018).

A frequently used illusion is named full-body illusion (FBI). The success of the FBI is due to the spread of virtual reality settings. In the FBI, participants see a virtual (or filmed) body that receives tactile stimulation. Congruently with the seen touch, the participant receives a touch on his or her body that mimics the virtual one. The congruency of multisensory stimulation induces a sense of embodiment towards the virtual body (Ehrsson, 2007; Lenggenhager et al., 2007). The FBI has been replicated using congruent visuomotor stimulation (Kilteni et al., 2012), and using a virtual body seen from a first-person perspective (Ehrsson, 2007; Romano et al., 2016; van der Hoort et al., 2011) or a third-person perspective (Banakou et al., 2013; Lenggenhager et al., 2007; Petkova & Ehrsson, 2008; Romano et al., 2014). Notably, Van der Hoort and collaborators adopted the FBI procedure to induce embodiment with bodies of different size (van der Hoort et al., 2011). Specifically, in their study, they used a life-sized body (180 cm height), a doll’s body (80 cm or 30 cm height), or a gigantic body (400 cm height). Pre-recorded videos of the fake bodies stimulated with a stick were presented to the participants through a set of head-mounted displays (HMDs). The video was coordinated with a synchronised tactile stimulation on participants’ legs producing a congruent visuotactile stimulation. The embodiment was measured by both a questionnaire about the subjective experience and by the increase of skin conductance response (SCR) to an unexpected threat towards the fake body. The authors brought evidence that the participants embodied fake bodies of different sizes. Romano et al. (2016) sustained the embodiment for avatars of different sizes, adopting a slightly different procedure. The authors measured physiological responses (SCR) to nociceptive stimulations while manipulating the visual size of a virtual body finding different levels of SCR as a consequence of the embodiment of bodies of different size.

These studies suggest that the FBI can generate a subjective experience of embodiment towards different sized bodies, with a coherent physiological response to threat and actual pain. However, it remains unclear the impact of embodiment on one’s body-metric representation, that is, the cognitive representation of the metric properties of the body. In other words, is the estimation of the actual size of one’s own body part impacted by the FBI?

Different experimental procedures can be used to measure body size and body-metric representation: to estimate body size by visual comparing a body segment with a line (Longo & Haggard, 2012a); to localise tactile stimuli or anatomic landmarks based on position sense (Longo & Haggard, 2010, 2012a); or to estimate the distances between tactile stimuli (Longo et al., 2015; Longo & Morcom, 2016; Sadibolova et al., 2018). The latter task (i.e., Body distance task, BDT) relies on the ability to both localise tactile stimuli on the body and estimate the tactile percept size. On the one hand, the localisation of tactile stimuli is mediated by a higher-order body representation based on somatosensory processes, known as the superficial schema (Head & Holmes, 1911; Longo et al., 2010; Mancini et al., 2011). Longo and coworkers (2010) defined the superficial schema as a somatosensory map of the body skin. On the other hand, the estimation of the tactile percept size refers to a stored representation of the metric properties of the body, called the body model (Longo, 2015; Longo & Haggard, 2011). The literature suggests that this stored representation of our body size and length is distorted by default (Longo & Golubova, 2017; Longo & Haggard, 2010, 2011; Sadibolova et al., 2018; Stone et al., 2018; Tosi & Romano, 2020). Previous studies have found a compression or an underestimation of the proximo-distal axis of the body as compared with the medio-lateral one for both the hand (Longo et al., 2015; Longo & Morcom, 2016) and the leg (Green, 1982; Stone et al., 2018; Tosi & Romano, 2020). These works suggested that humans represent their limbs shorter than they are. The present work focuses on understanding whether such a distorted map can be modulated using bodily illusions. Else, the map could be so deeply rooted in mental representation that it cannot be altered by the temporary, illusory perturbation of the body representation. It has been noticed after amputation (and motor deficits), a sensation of telescoping occurring when the distal part of the phantom is gradually felt to approach the residual limb (Flor et al., 2006). Such a mechanism can be related to a modified perception of the body metric. We assessed the possibility to change the body metric representation through a body illusion temporarily.

We used a visuotactile FBI procedure, administered with a portable technological setup, to assess whether the embodiment sensation modulates the perception of body-metric representation. In two experiments, we assessed

(a) the feasibility and the replicability of the FBI for bodies of different sizes with a 360° camera setup, controlling for the role of visual perspective in the illusory embodiment;

(b) the impact of an experimentally manipulated body representation in the body metric estimation.

The working hypothesis is that the FBI affects the subjective experience of embodiment. Still, this subjective sensation could be accompanied by a change in the body-metric perception that goes hand-in-hand with the embodied avatar’s size. This finding would set a new step in understanding how embodiment shapes how we represent ourselves in the world and how our representations shape the way we perceive sensory events.

Experiment 1: validity and replicability of a body illusion with bodies of different sizes with a 360° camera setup

In Experiment 1, we tested whether it is possible to induce the embodiment towards mannequins of different sizes through a novel version of the FBI using a 360° camera setup. In addition, we explored the replicability of the illusion within the same sample in separate sessions. This was useful to properly design Experiment 2, understanding if we can reliably design a study that requires multiple sessions. In addition, to the best of our knowledge, it is rarely investigated whether embodiment effects can be induced multiple times within the same participant in multiple sessions. Although our experiment cannot fully clarify this latter aspect, it can give a relevant clue to this point.

Materials and methods

Participants

A total of 20 healthy volunteers took part in Experiment 1 (15 females; mean age = 22.7 ± 3.0, range = 18–27; mean education = 16.1 ± 2.0, range = 13–19). We conducted a power analysis with G*power 3.1.9.2, to calculate the a-priori sample size needed to run a within factors repeated measure ANOVA. We based the effect size on previous findings of FBI (Maselli & Slater, 2014; Romano et al., 2016). To obtain a power of 0.8, with alpha set at .05, and effect size at 0.25, a minimum sample size of 19 participants was required. All participants had normal or corrected to normal vision and were naive to the purpose of the experiment. All participants gave their written informed consent before participating in the experiment. The study was approved by the local Ethics Committee “Commissione per la Valutazione della Ricerca, Dipartimento di Psicologia” of the University of Milano-Bicocca and was conducted by the ethical standards of the Declaration of Helsinki (World Medical Association, 2001).

The experimenter explained to the participants the study general aim and the procedure before collecting the informed consent. At the end of the experimental sessions, the experimenter also described the specific scope during a debriefing moment.

No part of the study procedures or analysis was pre-registered before the research being conducted. Data and analysis code are available on the Open Science Framework platform at the following link: osf.io/vrcyg/.

Procedure

The experiment consisted of two identical sessions separated by a week that served as a washout period. Participants underwent an FBI procedure in each session, induced through a set of HMDs (Samsung Gear VR 2016, Samsung Electronics, field of view = 101°). In the HMD, participants saw pre-recorded videos of three artificial bodies recorded from a first-person perspective (videos are available on the Open Science Framework platform at the following link: osf.io/vrcyg/). We measured the embodiment with two approaches. We registered the skin conductance to collect a physiological index of embodiment (van der Hoort et al., 2011). We administered a questionnaire to obtain a measure of the subjective conscious experience of the illusion. The questionnaire was created ad hoc with a collection of statements taken from previous studies questionnaires (Longo et al., 2008; Petkova & Ehrsson, 2008; van der Hoort et al., 2011) and modified for the current specific setup (see Table 1).

Table 1.

Illusory components of the subjective experience induced by the experimental procedure and questionnaire items designed for their measurement.

Subcomponents		ID	Question
Embodiment	Ownership	Q1	Did it seem like you were looking directly at your own legs?
	Ownership	Q2	Did it seem like the legs in the video belonged to you?
	Agency	Q3	Did it seem like you could have moved the legs in the video if you had wanted?
	Agency	Q4	Did it seem like you were not in control of the legs in the video?
	Location	Q5	Did it seem like the legs in the video were in the location where your legs were?
	Location	Q6	Did it seem like the touch you felt was caused by the stick touching the legs in the video?
Loss of own legs		Q7	Did it seem like your legs had disappeared?
		Q8	Did it seem like you could have moved your legs if you had wanted?
		Q9	Did it seem like your legs were out of your control?
		Q10	Did it seem like you couldn’t really tell where your legs were?
Deafference		Q11	Did it seem like the experience of your legs was less vivid than normal?
Control		Q12	Did it seem like your own legs became “fake”?
Control		Q13	Did it seem like you had four legs?
Size		Q14	Did it seem like the stick was smaller, bigger or the same as the real one?
Size		Q15	Did it seem like the legs in the video were smaller, bigger or the same as your own?
Affect		Q16	Did you find that experience enjoyable?

FBI procedure

Participants sat in an armchair, keeping their arms relaxed, lying behind the back of the chair. During the procedure, participants wore a set of HMDs in which they saw pre-recorded videos, encoding: MPEG-H Part2/HEVC (H.265), resolution: 2,560 × 1,280, 30 fps, of three artificial bodies receiving a tactile stimulation with a stick (induction phase), and later stung by a syringe with an evident needle (to measure the evoked threat). Videos were recorded with a 360° camera, Samsung Gear 360 (2016)—camera resolution:15.0 ×2MP; features: CMOS, f/2.0; video recording resolution: Near 4k1; processor speed, type: Dual-Core, so that the participant could explore the environment during the video presentation visually. Indeed, the 360° videos were recorded in the same room where the experiment was performed and from the same position of the participant’s viewpoint, providing an immersive, realistic and ecological context. During the experiment, participants were invited to look down at their legs and lower abdomen to promote the focus on the fake body.

The mannequins presented with the same size for the upper body (i.e., they wore a t-shirt of a Medium size), and three different sizes for the artificial pairs of legs: a life-sized pair (108 cm long)—a big pair, approximately two times longer (203 cm)—and a small pair, half shorter (49 cm). The mannequins wore the same clothes (a green t-shirt and jeans) to match the appearance across bodies. In each video, participants saw one of the three artificial bodies from a first-person perspective, touched by a wooden stick for 120 s (see Figure 1). Touches were delivered on the upper left leg for 2 min at a frequency of 1 Hz (the pace was given by the metronome). During this period, the experimenter touched the participant’s left leg simultaneously in the corresponding location, generating a synchronous visuotactile stimulation. In order to synchronise the touches as well as possible, the experimenter listened to the audio file of the video and followed the same pace given by the metronome. At the end of this stimulation, a syringe appeared in the video, stinging the artificial leg. The threat was supposed to generate an arousal response which was measured through the skin conductance. The video sequence that includes the syringe threat lasted 15 s and started precisely 125 s after the beginning of the video. In total, the video length was 140 s. During the video presentation, participants heard white noise to avoid any sound interference from the outside of the room.

Figure 1.

Body illusion setting for the anatomical condition (panel A: small legs; panel B: standard legs; panel C: big legs).

Participants underwent two conditions of legs orientation. When participants looked down in the 0° rotation, they saw the mannequin legs holding the same posture as their real legs. Thus, the 0° rotation provided the videos of a body seen from an anatomical viewpoint. This one was our experimental condition, the one for which we expected to induce embodiment. In the second condition, we provided the same videos from a 45° clock-wise rotated angle. When the volunteers looked down with this arrangement, they saw two legs that were not aligned with their own body, resulting in a non-anatomically compatible viewpoint. We used this non-anatomical orientation as a control condition (Guterstam et al., 2011; Pavani et al., 2000; Romano et al., 2016).

By crossing the size and viewpoint of the variable, we obtained six different conditions: standard-anatomical, standard-non-anatomical, small-anatomical, small-non-anatomical, big-anatomical, and big-non-anatomical. In each session, participants experienced all six conditions in a randomised order. The same sequence was administered in both sessions so that if in the first session, a participant did the big-non-anatomical condition as the first one, this would be presented at first also in the second session. The entire Experiment 1 results in a 2 (Session) × 3 (Size) × 2 (Orientation) full factorial, within-subject design.

Skin conductance

The skin conductance level (SCL) was recorded during all the conditions to capture the level and the variability of sympathetic activity. We recorded participants’ skin conductance with a Biopac System MP150 (Goleta, USA), adopting the module dedicated to skin conductance recordings named GSR100c. The transducer (TSD200) electrodes were attached to the third phalanx of the participant’s index and middle fingers of the left hand. Data were digitalised at a sample rate of 100 Hz; the gain parameter was set at 5 µmho/V. The acquired biosignal was processed offline with the Biopac software Acqknowledge for Windows (Version 4.2).

The signal was pre-processed in two steps to obtain phasic changes of the skin conductance (i.e., the SCR): (a) offline smoothing of the signal (mean with Gaussian distribution of 25 samples); (b) high pass filtering at .05 Hz.

We then extracted two indices for each condition. First, we calculated the SCR peak-to-peak (P-P) evoked by the syringe illusory prick. SCR P-P reflects sympathetic activity in response to a threatening stimulus being applied to the artificial leg, and it has been often used as a measure of embodiment (Petkova & Ehrsson, 2008; Romano et al., 2016; van der Hoort et al., 2011). This measure was computed as the difference between the maximum and the minimum value detected in the time window of 10 s, starting with the appearance of the syringe. We then applied a log transformation to the data to improve the fit with normal distribution and reduce the impact of extreme values on the inferential statistics. After the log transformation, participants exceeding three standard deviations have been treated as outliers and then excluded from the analysis. A total of three participants were excluded for this reason from the SCR P-P analysis. Second, we calculated the non-specific fluctuation count (NSFC) during the first 120 s of each video (i.e., the induction phase of the illusion). The NFSC is defined as the number of spontaneous response peaks detected in an extended period. Spontaneous response is recorded if the increase of spontaneous phasic activity exceeds the threshold of .05 micro siemens. Acqknowledge software has an automated algorithm to identify that kind of responses offline. This measure represents the spontaneous fluctuation of the skin conductance signal due to circumstances that influence one’s general arousal (Braithwaite et al., 2013). A recent study by D’Alonzo et al. (2020) showed that non-specific SCRs correlate with the typical embodiment measures in a rubber hand illusion experiment.

Embodiment questionnaire

After each condition, participants completed a questionnaire about the subjective sensations felt during the video presentations. The questionnaire was adapted from previous studies about body illusions (Petkova & Ehrsson, 2008; van der Hoort et al., 2011) and pain perception in bodily illusions (Romano et al., 2014, 2016). Participants rated their agreement on 16 questions on a 7-point Likert-type scale where −3 correspond to the less agreement and +3 the most agreement with the statement. Six items were designed to capture the three components of the general experience of embodiment (Longo et al., 2008). The sense of ownership (i.e., the perception that an external object is part of one’s body—statements Q1, Q2). The sense of agency (i.e., the feeling of being able to move and control the object as a part of the body—statements Q3, Q4). The sense of location (i.e., the sensation that the object and the body are situated in the same place—statements Q5, Q6). Four statements were designed to capture the sense of “loss of their own legs” (Q7, Q8, Q9, Q10). Moreover, one item (Q11) was intended to catch the illusory feeling of deafference, while two control statements (Q12, Q13) were designed to control for task compliance and suggestibility. Two questions about the perceived size of the virtual body and the stick were added with the specific purpose of investigating the subjective modification of the perceived visual size of environmental items (Q14, Q15) and a statement (Q16) was added to capture pleasantness of the whole procedure (see Table 1 for the full list of the items).

Each participant’s responses have been ipsatised (i.e., within-subject normalisation) by centring the responses on the average rate of all the questions in all the conditions and dividing the centred value by the standard deviation of the entire set of responses. Following this procedure, questionnaire data are free from the response set bias (i.e., each participant’s response style) so that each item becomes coded in terms of standard deviations from each participant’s average response (Hofstede, 1984).

Analysis

We conducted all the analysis with the software Jamovi 1.6.6.0 (Jamovi project, 2018). We ran a series of independent repeated measures analysis of variance (rmANOVA) with a within-subject design that encompassed a 2 (Session) × 3 (Size) × 2 (Orientation) full-factorial model. Significant effects have been interpreted inspecting 95% confidence intervals.

To evaluate the embodiment of the fake legs from a physiological point of view, we analysed both the P-P and the NSFC measures with two independent rmANOVAs.

To examine the subjective experience of embodiment, we applied the same rmANOVA design to the questionnaire data. In particular, because the first 6 items are all part of the main factor “embodiment” (Longo et al., 2008; Romano et al., 2021), we clustered all those items averaging their values. The analyses investigating the different subcomponents of the embodiment (i.e., Ownership, Agency, Location) independently are presented in the Supplementary Material.

Results

SCR

Peak-to-peak

We found a significant main effect of session, F(1,14) = 10.64, p ⩽ .05, η² = .43, with lower values in the second session (CI: [−0.01, 0.16]) as compared with the first one (CI: [0.23, 0.39]). No further significant effects emerged (all other p-values > .11).

NSFC

We did not find any significant effect (all p-values > .10). When considering only the first session, a trend emerged for the factor size (Small-anatomical (CI: [2.13, 5.43]) and Small-non-anatomical (CI: [2.73, 6.77]) < Standard-anatomical (CI: [2.80, 6.09]); and Standard-non-anatomical (CI: [2.16, 5.01]) < Big-anatomical (CI: [3.18, 6.48]) and Big-non-anatomical (CI: [3.13, 6.43]). However, the result was not significant, Size: F(2, 34) = 2.74, p = .08, so that we do not have enough evidence to reject the null hypothesis.

Embodiment questionnaire

We found a significant main effect of Size, F(2, 36) = 20.00, p ⩽ .001, η² = .18, as well as Orientation, F(1, 18) = 19.10, p ⩽ .001, η² = .16 (Figure 2). We also found the significant interaction: Size × Orientation, F(2, 36) = 7.34, p < .01, η² = .02. These results revealed greater embodiment values in the anatomical condition with the bigger (CI: [0.38, 0.83]) and the standard legs ([0.52, 0.97]). On the contrary, in the non-anatomical condition, big (CI: [−0.25, 0.20]); standard (CI: [−0.04, 0.41]), and with the smaller legs, anatomical (CI: [−0.25, −0.21]); non-anatomical (CI: [−0.50, −0.04]), subjects always showed weak-to-none embodiment sensations.

Figure 2.

Size × Orientation interaction in the within-subjects 2 (Session) × 3 (Size) × 2 (Orientation) repeated measure ANOVA on the averaged ipsatised answers to embodiment statements. The panel shows all the sizes (from the top: big, standard, and small size). Grey and white column display, respectively non-anatomical and anatomical conditions.

No further significant effects emerged (all other p-values > .21). The results obtained from each specific subcomponents of the questionnaire are reported in the Supplementary Material.

Interim discussion

We found a significant difference between the first and the second session in the SCR P-P, with decreasing values in the second one. Notably, the P-P is susceptible to habituation. Participants who may have already familiarised with the task and the procedure tend to respond less prominently to the appearance of the syringe in the second session. We did not find a significant change of skin conductance because of Size or Orientation, suggesting that we have no evidence to sustain a difference induced by the embodiment or body size at the physiological level of response, neither for the SCR P-P nor for the NSFC. We did not find any effect of size or orientation on the SCR P-P measure. Such results conflict with the literature (Petkova & Ehrsson, 2008; Romano et al., 2016; van der Hoort et al., 2011). A possible explanation could be found in the features of the threatening stimulus. Previous studies administrated unexpected and fast emerging stimuli (Guterstam et al., 2011; Romano et al., 2014, 2016; van der Hoort et al., 2011). In our experiment, the threat was delivered through a stimulus slowly approaching the body along with a 15-s interval. It is possible that such a slowly approaching stimulus was not considered as threatening as those used in previous studies. The syringe threat procedure, as adopted, is ineffective to produce threat and is thus a limitation of the present study. Nonetheless, the syringe represents a new stimulus standing out after 120 s of the same visuo-tactile stimulation. Such unfamiliar stimulus may become more and more usual along with the procedure, thus participants may get used to it in the second session.

Regarding the questionnaire, the main effect of orientation confirmed our expectations. We observed a stronger embodiment in the anatomical condition than the non-anatomical. Moreover, we found a significant effect of Size, obtaining comparable embodiment scores with the big and the standard legs, which were both more embodied than the small ones. In the short legs condition, participants always exhibited weak embodiment ratings, both from an anatomical and a non-anatomical viewpoint. This result is in contrast with the study of van der Hoort and collaborators (2011), who used a similar embodiment procedure. This difference can be explained with substantial dissimilarities between the questionnaire’s statements and the statistical analyses used. In particular, van der Hoort’s questionnaire consisted of three illusion statements designed to capture the feeling of ownership and four control statements. Their results showed that the participants gave significantly higher scores to the illusion statements compared with the control statements, but only during the synchronous condition. Noteworthy, our questionnaire was designed to capture several aspects of the body illusion. Moreover, we did not compare the questionnaire’s statements (illusion vs control), but we preferred to examine the differences between legs’ sizes and orientation by including the legs’ size as a factor in the analysis.

Indeed, we also found a significant interaction between size and orientation, suggesting a different balance between the embodiment scores in the anatomical and non-anatomical conditions through different body sizes. When the standard size legs were presented in an anatomical perspective, participants were able to process them as part of their body since there was no difference between the visual input and their body perception. Remarkably, we found the same effect of the subjective experience of the embodiment of the artificial 203 cm length pair of legs. On the contrary, when the body was presented turned 45° clock-wise rotated, the incongruence between participants’ position and the non-anatomical rotation of the legs prevented the embodiment. This occurred even in the presence of synchronous visuotactile stimulation.

Our results suggested that humans are more prone to incorporate normal size or bigger bodies than smaller ones. The lack of embodiment for the small size body has been previously reported in the literature, together with the evidence that illusions of decreased body size were weaker than illusions of increased body size (Marino et al., 2010; Pavani & Zampini, 2007; Romano et al., 2016). A speculative explanation for this effect is that the body always increases the size along with its ontological development. Our cognitive functions may be more willing to accept an enlargement of body size rather than a reduction, accounting for a top-down influence on the embodiment (Pavani & Zampini, 2007).

In contrast to the SCR results, the questionnaire did not show any habituation effect: subjects reported comparable embodiment sensations in the first and the second session.

Experiment 2: body illusion impacts on body-metric representation

The current study scope was to evaluate if the FBI can impact our perception of body-metric representation (Longo et al., 2010). The key question is if the body-metric representation can be plastically modulated by embodying mannequins of different sizes. In other words, do we perceive our body and tactile events on it according to the way we represent it? To investigate this question, in Experiment 2, we replicated the FBI with different body sizes accompanying the questionnaire to a modified version of the BDT (see Tosi & Romano, 2020).

Materials and methods

Participants

A total of 24 healthy volunteers took part in Experiment 2 (13 females; mean age = 25 ± 5.77, range = 19–48; mean education = 16.13 ± 1.85, range = 13–21). We conducted a power analysis with G*power 3.1.9.2, to calculate the a-priori sample size needed to run a within factors repeated measure ANOVA. We based the effect size on previous findings of FBI (Maselli & Slater, 2014; Romano et al., 2016). To obtain a power of 0.8, with alpha set at .05, and effect size at 0.25, a minimum sample size of 19 participants was required. All participants had normal or corrected to normal vision; they were naive to the purpose of the experiment and did not participate in Experiment 1. All the subjects gave their written informed consent before participating in the experiment.

The study general aim and the experimental procedures were explained to participants before collecting the informed consent. Participants were informed that the experiment aimed to study body perception and undergo two experimental sessions.

Data and analysis code are available on the Open Science Framework platform at the following link: osf.io/vrcyg/.

Procedure

The experiment consisted of two sessions, with 1 week of washout in between. In each condition, we administered a BDT, modified from previous studies (Longo et al., 2015; Longo & Golubova, 2017; Longo & Haggard, 2012b) to investigate a possible distortion of the metric representation of the body. We validated the modified BDT in a previous study to obtain a reliable and efficient procedure to assess body size perception (Tosi & Romano, 2020). In the validation study, we measured the reliability of the effect and the possibility of using verbal estimation of the two-points distances (Tosi & Romano, 2020). Moreover, we used the same questionnaire used in Experiment 1 to evaluate the subjective experience of embodiment felt towards the artificial bodies.

Body illusion

Each participant underwent the same Body Illusion conditions of Experiment 1. The setup and the videos were the same as Experiment 1, except that the syringe threat section was cut from the videos and no longer used. In each session, participants experienced the two orientations (anatomical, non-anatomical) in two different sizes: standard size and either small or big according to the experimental session. In that way, all participants underwent a Small-size session and a Big-size session. The standard legs condition was considered as a baseline condition and repeated in both sessions as the first one. The order of the videos was counterbalanced across participant (24 possible combinations), and Small-size and Big-size sessions were alternated following an ABBA order across participants. This procedure resulted in 8 visuo-tactile stimulations for each participant: standard1-anatomical; standard1-non-anatomical; small-anatomical; small-non-anatomical; standard2-anatomical; standard2-non-anatomical; big-anatomical; big-non-anatomical. The different measures of the standard size stimulations were averaged between the different sessions, defining the final 3 (Size: standard, big, small) × 2 (Orientation: anatomical, non-anatomical) factorial design, replicating Experiment 1 design.

Embodiment questionnaire

We used the same questionnaire adopted for Experiment 1. Data were processed following the same steps of Experiment 1; namely, the scores were ipsatised and then aggregated to obtain an illusion score averaging the items: Q1, Q2, Q3, Q4 (reversed), Q5, and Q6. We expected to replicate our previous results, finding significant effects of both Size and Orientation.

BDT procedure

The BDT proved to be effective in measuring the distortion of the metric representation of the body (Longo et al., 2015; Longo & Golubova, 2017; Longo & Morcom, 2016; Tosi & Romano, 2020). In the BDT, participants sat blindfolded on the armchair, with their arm behind the back. A 3 × 3 grid of points was applied on the upper left leg and fixed to the participant’s leg (see Figure 3). Adjacent points on the grid were separated by 5 cm resulting in a 10 cm × 10 cm grid size. Rows and columns points ran along the medio-lateral and proximo-distal leg axes. Before starting, the length of the leg, from the hip (anterior part of the iliac crest) to the knee (inferior part of the kneecap) was measured to fix the grid in the centre of the leg.

Figure 3.

3 × 3 grid of points (10 × 10 cm²) used to administer the stimuli for the BDT.

On each trial, the experimenter touched with a knitting needle two points of the grid in sequence, with an inter-stimulus interval of approximately 1 second. After each trial, participants were asked to estimate the perceived distance between the stimuli verbally. There were 36 pairs combinations of the nine stimuli, and each pair could be presented in two orders. For example, given the pair AB, the experimenter could touch first the point A and then the point B (AB order) or the point B and then the point A (BA order). The procedure resulted in 72 different trials. We administered the two different orders of presentation in two subsequent blocks (Block 1: AB order, Block 2: BA order) so that we could control for the potential effect of temporal distance from the embodiment procedure. Indeed, the administration of the entire BDT takes about 10 min, and it is not known if the embodiment effect can last for that long after the induction phase. By doing so, we can use the two-level factor Block (Block 1 vs Block 2) as an independent variable in the analysis controlling for a temporal fading effect of the FBI.

BDT: global shape dissimilarity

Multidimensional scaling (MDS) is a method for extracting the spatial structure underlying a set of items given a matrix of pairwise distances between objects (Cox, 2001; Everitt, 1997; Shepard, 1980). To obtain the spatial grid structure, we took the perceived pairwise distances between pairs of points (for each order of presentation: AB vs BA) and created a symmetric matrix with zeros on the diagonal. For each participant, we then used MDS to extract the coordinates of each point of the grid in a nine-dimensional space. Based on the eigenvalues, we extracted the first 2 dimensions of the solution, thus obtaining the coordinates in a bi-dimensional space (Table 2 reports the average variance explained by the first 2 dimensions in each condition). We used the coordinates obtained to reconstruct a perceptual configuration of the grid for each subject in each condition.

Table 2.

Averaged variance explained by the first two dimensions in each condition.

			First dimension (%)	Second dimension (%)	Total (%)
Small	Anatomical	Block 1	30.4	23.0	53.4
	Anatomical	Block 2	30.0	22.0	52.0
	Non-anatomical	Block 1	31.0	22.4	53.4
	Non-anatomical	Block 2	29.8	22.2	52.0
Standard	Anatomical	Block 1	23.1	19.1	42.2
	Anatomical	Block 2	27.2	21.0	48.2
	Non-anatomical	Block 1	23.9	19.5	43.4
	Non-anatomical	Block 2	29.2	21.5	50.7
Big	Anatomical	Block 1	30.7	22.5	53.2
	Anatomical	Block 2	30.0	23.0	53.0
	Non-anatomical	Block 1	29.4	22.4	51.8
	Non-anatomical	Block 2	29.0	22.3	51.3
All conditions averaged			28.6	21.7	50.4

For each condition, we averaged the variance explained by the first 2 dimensions considering the whole sample of participants.

We aimed to investigate the shape of the perceived grid. For this purpose, we generated multiple stretches of the real grid to be compared with the subjects tactile perception. We multiplied the y-coordinates of the real grid by a stretch parameter comprised between 0.33 and 3 with a resolution of 0.0005 units in logarithmic space (see Longo & Golubova, 2017; Tosi & Romano, 2020). We obtained 4415 grids with different stretches: a stretch parameter lower than 1 indicated a stretch in the medio-lateral axis, a stretch parameter equal to 1 indicated a square grid, a stretch parameter bigger than 1 indicated a stretch in the proximo-distal axis (Longo & Golubova, 2017).

We then superimposed each of the 4,415 stretched grids and the participants’ perceptive configurations using the Procrustes Alignment (Goodall, 1991; Rohlf & Slice, 1990). By aligning the grids and removing all non-shape differences, we obtained a dissimilarity index between the two configurations, that is, the Procrustes Distance (Bookstein, 1991). This measure ranges between 0 and 1, depending on the amount of shared spatial structure between two configurations.

For each block in each condition, we computed the stretch that minimised the Procrustes Distance among the 4,415 indices of dissimilarity to identify the shape that most likely corresponded to the perceptual grid. Based on the stretch that minimised the Procrustes Distance, we can determine if the perceived grid was square (stretch parameter equal to 1) or rectangular and if the stretch runs along the medio-lateral or the proximo-distal axis.

BDT: misestimation

The Procrustes Distance offers an index of global shape dissimilarity without discriminating the type of distortion (Longo & Golubova, 2017). For this purpose, we compared the participants’ responses at each stimulus pair to the actual distance between the points, and we calculated the error percentage using the following formula: % misestimation = (perceived distance − actual distance)/actual distance × 100. The misestimation represents a complementary index to the Procrustes Distance because it focuses on the length estimation instead of the shape of the grid (Tosi & Romano, 2020). Former studies (Longo et al., 2015; Longo & Morcom, 2016; Stone et al., 2018; Tosi & Romano, 2020) have found different rates of misestimation for distances running along the proximo-distal or medio-lateral axis. In line with the literature, we considered the direction the tactile stimuli run along (medio-lateral axis/proximo-distal axis) and their distance (near-5 cm/far-10 cm). We used this additional index to examine subjects’ misestimations of the leg’s sizes (small/standard/big) during the BDT.

A few more detail about the BDT and the validity of the two calculated indices (i.e., Global shape dissimilarity and Misestimation) can be found on a recently published paper where we tackled methodological issues and potential confounds (Tosi & Romano 2020).

Analysis

Embodiment questionnaire

We ran a within-subjects 3 (Size) × 2 (Orientation) repeated measure ANOVA. Significant effects have been interpreted by inspecting 95% confidence intervals. Questionnaire analysis has been conducted with Jamovi 1.6.6.0 (Jamovi project, 2018).

BDT: global shape dissimilarity

We run a series of independent one-sample t-test on the stretch that minimised the Procrustes Distance of each condition. We compared the stretch to 1, a value indicating a square grid, to assess if there was any distortion in the perceived metric of the legs. Then, we conducted a within-subjects 3 (Size) × 2 (Orientation) × 2 (Block) repeated measure ANOVA using as dependent variable the stretch that minimised the Procrustes Distances, to evaluate if the FBI and/or the different body sizes modulates the eventual distortion of the BDT. We run the analysis of the global shape dissimilarity with Jamovi 1.6.6.0 (Jamovi project, 2018).

BDT: misestimation

We run a linear mixed model (LMM) two-way ANOVA with the misestimation as the dependent variable. Fixed effect factors were Direction (medio-lateral axis/proximo-distal axis) and Distance (near/far) as within-subject factors. We set participants as a random effect variable. Significant effects have been interpreted by inspecting 95% confidence intervals. We conducted the analysis with Jamovi 1.6.6.0 (Jamovi project, 2018).

Results

Embodiment questionnaire

The 3 (Size) × 2 (Orientation) repeated measure ANOVA resulted in significant main effects of Size, F(2, 46) = 21.89, p ⩽ .001, η² = .48, and a significant main effect of Orientation, F(1, 23) = 9.78, p ⩽ .05, η² = .30, partially replicating the results of the Experiment 1. Once again, the standard legs (CI: [0.16, 0.46]) and the big ones (CI: [−0.02, 0.42]) presented in an anatomical orientation induced a stronger embodiment than the small legs, anatomical (CI: [−0.42, −0.03]); non-anatomical (CI: [−0.66, −0.26]) (Figure 4). The results obtained from all the subcomponents of the questionnaire are reported in the Supplementary Material.

Figure 4.

Results of the within-subjects 3 (Size) × 2 (Orientation) repeated measure ANOVA on the averaged ipsatised answers to embodiment statements. Grey and white columns display Non-anatomical and Anatomical conditions, respectively. Error bars display confidence interval limits.

BDT: global shape dissimilarity

The results of the independent one-sample t-tests are reported in the Supplementary Material, Table 3). The 3 (Size) × 2 (Orientation) × 2 (Block) repeated measure ANOVA resulted in a significant main effect of Size, F(2, 40) = 34.35, p ⩽ .001, η² = .22. The stretches that minimised the Procrustes Distances were larger with the Standard legs (CI: [1.43, 1.69]) than with the Small (CI: [0.78, 1.05]) and Big ones (CI: [0.8, 0.07]), suggesting a greater distortion during the conditions in which the fake legs held the same size as the participants’ ones. We also found a significant main effect of Block, F(1, 20) = 15.79, p ⩽ .001, η² = .06, with stretches closer to 1 in the second block (CI: [0.86, 1.09]), than in the first one (CI: [1.18, 1.41]). This result implies that the perceptive grids were more distorted in the first part of the BDT. The interaction between Size and Block was significant as well, F(2, 40) = 4.76, p ⩽ .05, η² = .03. The stretches that minimised the perceptive grid were higher after seeing the Standard legs, Block 1 (CI: [1.7, 2.07]); Block 2 (CI: [1.04, 1.42]), as compared with the Small ones, Block 1 (CI: [0.79, 1.17]); Block 2 (CI: [0.66, 1.04]) with no differences between the first and the second block. On the contrary, we found a difference between the BDT administered right after the illusion with the Big legs (CI: [0.83, 1.2]), resulted in a square grid, and the BDT administered in the second block (CI: [0.67, 1.04]), which showed a stretch in the medio-lateral axis. The interaction between Orientation and Block was close to significance, F(1, 20) = 4.04, p = .058, but it did not reach sufficient evidence to reject the null hypothesis, Non-anatomical: Block 1 (CI: [1.10, 1.38]), Block 2 (CI: [0.89, 1.17]), anatomical: Block 1 (CI: [1.21, 1.49]); Block 2 (CI: [0.79, 1.07]).

Crucially, we found a significant interaction between Size, Orientation, and Block, F(2, 40) = 6.05, p ⩽ .05, η² = .02 (Figure 5). We found that participants perceived a roughly square grid right after seeing the Big and the Small legs in both orientations, while in the second block emerged a stretch in the medio-lateral axis. On the contrary, after seeing the Standard legs, participants perceived a stretch in the proximo-lateral axis. The amount of stretch was the same between anatomical and non-anatomical orientation during the first block. In contrast, in the second block, we found a lower stretch in the anatomical condition than the non-anatomical one.

Figure 5.

Results of the 3 (Size) × 2 (Orientation) × 2 (Block) repeated measure ANOVA: three-way interaction between size, orientation and block. Grey and white columns display the stretch that minimises the Procrustes Distances in the first block and the second one, respectively; error bars display confidence interval limits. A stretch of 1 indicates a square grid; values greater than 1 indicate a higher underestimation in the proximo-distal axis, while values less than 1 indicate a higher underestimation in the medio-lateral axis.

BDT: misestimation

In the first block, we found significant main effects of Distance in all conditions (see Supplementary Materials, Table 4), showing higher underestimation for long distances (averaged CI: [−50.03, −26.27]) than for short ones (mean CI: [−38.15, −14.75]). Moreover, the Standard-anatomical condition showed a significant main effect of Direction, F(1, 405) = 6.00, p ⩽ .05: the perceived distances between the points running along the proximo-distal axis were more underestimated (CI: [−45.5, −25.1]) than those along the medio-lateral axis (CI: [−41.3, −21]). Therefore, we found a contraction of the proximo-distal axis only for the standard legs, but not for the bigger and the smaller ones if presented in an anatomical orientation.

Looking at Block 2, we found significant main effects of both Direction and Distance in all conditions (see Supplementary Materials, Table 4). The results went in the same direction of the first block, with higher underestimations for long distances (averaged CI: [−55.52, −32.75]) than for short ones (averaged CI: [−44.55, −22.23]) and along the proximo-distal axis (averaged CI: [−54.65, −31.08]) as compared with the medio-lateral one (averaged CI: [−46.42, −23.83). Moreover, we found a significant interaction between Direction and Distance in the Standard-anatomical condition, F(1, 405) = 7.17, p ⩽ .05.

Discussion

Experiment 2 showed that the FBI induces a robust perceptual illusion of embodying the alien body segments of equal or bigger size than one’s own. Our participants experienced higher subjective embodiment with both the standard and the big size legs when they were presented from an anatomical point of view than any other condition, a result in full agreement to Experiment 1. In Experiment 2, we also explored if the embodiment of different sized bodies affects the perception of body-metric representation.

We processed tactile distance estimations with the MDS to generate a perceptive configuration of the grid to be compared with the actual one. By doing so, we had a measure of body distortion that was independent of a generalised effect of larger/smaller precepts (e.g., I see bigger legs, then I scale everything bigger). The stretches that minimise the Procrustes Distances suggested that subjects’ perceptual maps significantly differed from the real grid in the first block and the standard size legs.

It was shown that the body model underlying tactile and proprioceptive information about the body is distorted net of any other effect or manipulation (Longo et al., 2015; Longo & Morcom, 2016). The basic distortion was found for both the hand (Longo et al., 2015; Longo & Morcom, 2016) and the leg (Stone et al., 2018; Tosi & Romano, 2020). In line with this evidence, when we did not introduce any size manipulation (i.e., in the Standard legs conditions), we recorded a distorted map of the leg.

Crucially, we found a three-way interaction between Size, Orientation and Block, which suggests that our manipulations nudged the leg metric maps to different distortions. Specifically, we found that in the first block of the BDT, the stretch that minimises the Procrustes Distances was closer to 1 after seeing the small and big legs. This effect suggests a perceptive grid similar to the real one and a less distorted map during the conditions in which the fake legs held a different size. Notably, a reduction of the distortion is a deviation from the norm. Thus these conditions are those that most effectively impacted the body-metric representation. In other words, a body illusion affecting the visual input from the body (i.e., the different size of the legs) has the potential to break the natural distortion that humans have for the body metric representation.

The vision of one’s body is known to alter the perception of tactile (Kennett et al., 2001; Longo, Cardozo, & Haggard, 2008; Press et al., 2004) and painful (Longo et al., 2009; Romano & Maravita, 2014) stimuli. Here we propose that it can also influence the perception of body size. Previous studies have found in the absence of any manipulation a compression or an underestimation of the proximo-distal (longitudinal) axis (Green, 1982; Longo et al., 2015; Longo & Morcom, 2016; Stone et al., 2018; Tosi & Romano, 2020) as compared with the medio-lateral (transversal) one. These works suggested that humans represent their limbs shorter than they are, perceiving a square grid on our skin as a wider and shorter configuration. When we manipulated the length of the fake legs presented during the body illusion, our participants recalibrated their perception. The distorted perception of the two axes was rebalanced by presenting a pair of abnormally long (or abnormally short) legs. On the contrary, when the legs held the same size of the real ones, participants showed the same distortion recorded in the literature (Green, 1982; Stone et al., 2018; Tosi & Romano, 2020).

The absence of a statistical difference between the anatomical and the non-anatomical conditions suggests that we cannot interpret the effect onto body-metric as depending on the subjective feeling of embodiment. As a matter of fact, we observed a modulation of body metric maps after the body illusion with the Small legs, a condition that did not show subjective feelings of embodiment. This is not the first dissociation between different types of measures in the literature of embodiment. Indeed, the overlap between embodiment and implicit measures, such as the proprioceptive location of the body, is not always detected (Maselli & Slater, 2014; Romano et al., 2015; Tosi & Romano, 2020). It is possible to induce perceptual changes in body representation (the body metric in our case) without any conscious illusory experience of embodiment.

These findings suggest that the vision of one’s body can impact the metric representation of it, and crucially to our purposes, this can be modulated by the seen size of the to-be-embodied body. The misestimation results further support this conclusion. Indeed, participants underestimated the distances running along the proximo-distal axis as compared with the medio-lateral axis only during the standard anatomical legs in the first block. After seeing the fake legs holding an unnatural size, our participants perceived an undistorted (which again is unusual) squared grid.

The BDT is believed to rely on both the superficial schema (Head & Holmes, 1911; Longo et al., 2010) and the body model (Longo, 2015; Longo & Haggard, 2011). Previous works have found that altering the perceived body size produce corresponding changes in tactile size perception (Taylor-Clarke et al., 2004; Trojan et al., 2006, 2009). Similarly, the FBI affects the perception of body size.

The key finding of the study is thus the experimental evidence that bodily illusions can affect the perception of body-metric. The results of the BDT showed the characteristic underestimation along the proximo-distal axis when the standard fake legs are presented from an anatomical viewpoint. This is something that should be expected from the vision of legs that have the same size as the participant. Crucially, we did not find such a distortion with the longer and shorter legs. After seeing the resized bodies, our participants recalibrated their body size perception. Importantly, this effect was independent of the subjective feeling of embodiment, and in fact, could also be induced by the vision of a small body. These results are partially in line with a recent paper by Serino and collaborators (2020) revealing that the embodiment over a smaller body induced a significant reduction in participants’ body image.

Previous studies showed that the visual enlargement of body size could affect pain processing (Mancini et al., 2011; Romano et al., 2016; Romano & Maravita, 2014), kinematic parameter during grasping (Marino et al., 2010), and judgement of an object size contacting to the body (Taylor-Clarke et al., 2004). More recently, with a similar FBI procedure, we proved that body affordances during motor imagery in virtual reality can be affected too (Tosi et al., 2020). The present study adds another piece to the puzzle, clarifying that transient modification of a seen body, during illusory procedures, can also produce an ongoing modulation of the representation of our body-metric.

Supplemental Material

sj-docx-1-qjp-10.1177_17470218211044488 – Supplemental material for I am the metre: The representation of one’s body size affects the perception of tactile distances on the body

Supplemental material, sj-docx-1-qjp-10.1177_17470218211044488 for I am the metre: The representation of one’s body size affects the perception of tactile distances on the body by Giorgia Tosi, Angelo Maravita and Daniele Romano in Quarterly Journal of Experimental Psychology

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Giorgia Tosi

Data accessibility statement

The data and materials from the present experiment are publicly available at the Open Science Framework website:

Supplementary material

The Supplementary Material is available at:

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