Participants

One hundred twenty-five naïve, healthy volunteers participated in the study. None of the subjects took part in more than one experiment. Based on the results of previous studies using similar measurements16,20,21, we estimated that the sample size necessary to achieve statistical significance would be approximately 20 participants. We recruited 25 participants in each experiment because, based on previous experience, we estimated that about 20% of the scheduled subjects would not show up. In Experiment 4, we recruited six additional participants because of a technical issue with the heart rate data acquisition (see below). The data collection was stopped when the originally scheduled participants had been tested (no-shows were excluded from any further analysis). All of the participants were instructed to wear a pair of trousers and a t-shirt. The study was approved by the Ethical Review board at Karolinska Institutet, the methods were carried out in accordance with the approved guidelines and all participants gave their written informed consent.

Experimental setup and illusion induction procedure

The participants were asked to stand in an upright position with their head tilted and to look down at their body. They were then fitted with a set of HMDs (VR1280, Virtual Research Systems Inc., California, USA; 1280 x 1024 display resolution per eye, 60° field of view). The HMDs presented the participants with video streams of two high-performance, industrial, digital cameras (Stingray F-046, Allied Vision Technologies, Stadtroda, Germany; 780 x 580 resolution, 60 fps) that were attached side by side with a custom-made tripod mounted on the wall at the level of the participant’s head and that faced the floor (Figure 1, left panel). The video streams from the left and right cameras were transmitted to the left and right displays of the HMDs, respectively, through a computer that ran in-house, 3-D, video-displaying software. The total video delay was non-noticeable (89 ms). Thus, through the HMDs, the participants viewed the empty space below the cameras in 3-D and in approximately real-time. To induce the illusion, the experimenter stroked the participant’s body with a large paintbrush while simultaneously moving another paintbrush in the corresponding location in the empty space below the cameras, as if he were touching an “invisible body” that was in this location (illustrated by the grey body in Figure 1, left panel). The experimenter applied touches to five different body parts: the abdomen, the left and right lower arms and the left and right lower legs and feet. The duration of each individual brushstroke was 1 s and the time between the offset of one touch and the onset of the next touch was 1.5 s (to ensure that there were no overlaps between the seen and felt touches in the asynchronous condition). The brushstrokes were delivered in the following pre-determined sequence (A = abdomen, RLA = right lower arm, LLA = left lower arm, RLF = right lower leg and foot, LLF = left lower leg and foot):

To identify the portions of empty space representing the abdomen, arms, legs and feet of the invisible body, we used the mannequin as a template. Visual landmarks were placed outside the field of view of the cameras (i.e., hidden from the participants’ view) that indicated the starting and stopping points of the brushstrokes for each individual body part. We then removed the mannequin and the experimenter (ZA) practiced extensively on delivering the brushstrokes in the empty space, until he could deliver spatially well-defined strokes with a high level of synchrony and stability. In the questionnaire experiments (Experiments 1a, 2a and 3), the duration of one experimental block was 2 min and each condition was repeated only once16,20,21. In the SCR experiments (Experiments 1b and 2b), the duration of one block was 1 min and each condition was repeated three times in a pseudo-randomized order16,20,21. In Experiment 4, which included measures of both heart rate and questionnaire responses, the block duration was 1 min and each condition was repeated twice. The heart rate was recorded in the first set of repetitions and the questionnaire responses were given in the second set of repetitions. The presentation order of the experimental conditions was balanced across participants in all of the experiments.

Experiments 1a and 1b: Temporal and spatial congruence

The aim of Experiments 1a and 1b was to provide subjective and objective evidence for the invisible body illusion. In our previous study, we demonstrated that the illusion of having a single invisible limb is dependent on spatio-temporal congruence of visual and tactile signals16. We therefore hypothesized that the brushstrokes applied to the empty space and the touches delivered to the participant’s body would need to be synchronous and spatially aligned for the invisible body illusion to be elicited. In two independent experiments, we combined questionnaire (Experiment 1a) with SCR measurements (Experiment 1b) and included the following three experimental conditions: synchronous (illusion condition), asynchronous (control) and spatially incongruent (control) visual and tactile stimulation. In the asynchronous condition, the brushstrokes observed in the empty space were delayed by 1.25 s with respect to the tactile stimulation, while keeping all the other factors constant. The incongruent condition consisted of synchronous visuo-tactile stimulation; however, the touches on the participant’s real body were applied in the opposite direction and on another body part. The questionnaire data were first analyzed using a 3 × 2 ANOVA with the factors condition (synchronous, asynchronous, incongruent) and statement type (illusion, control) to examine the main effect of condition. To investigate the contrasts synchronous vs. asynchronous and synchronous vs. incongruent, we ran two separate 2 × 2 ANOVAs for the factors condition (synchronous, asynchronous or synchronous, incongruent) and statement type (illusion, control). The SCR data were analyzed with a one-way ANOVA and two paired t-tests for the planned comparisons of synchronous vs. asynchronous and synchronous vs. incongruent. We included 15 participants in Experiment 1a (six females, mean age 27 ± 7 years) and 22 participants in Experiment 1b (19 females, mean age 22 ± 4 years).

Experiments 2a and 2b: Invisible versus solid body

The aim of Experiments 2a and 2b was to compare the invisible body illusion with the previously published “body-swap” mannequin illusion, in which the participants experience ownership of a mannequin’s body17,19. The results of Experiments 1a and 1b demonstrated that the invisible body illusion appears to obey the same spatio-temporal multisensory rules as the mannequin illusion (Figure 2). We therefore predicted that the subjective strength and the threat-evoked SCR of these two full-body illusions would be correlated and of similar magnitude. In two independent experiments, we compared the illusion strengths in terms of questionnaire ratings (Experiment 2a) and threat-evoked SCR (Experiment 2b) using a 2 × 2 factorial design, with the main factors being visuo-tactile temporal congruence (synchronous, asynchronous) and body type (invisible, mannequin). In the two mannequin conditions, the body of a male mannequin was placed below the cameras, keeping all of the other factors equivalent to the invisible body conditions (Figure 1, right panel). The questionnaire data were analyzed using a 2 × 2 × 2 ANOVA, with the factors being body type (invisible, mannequin), visuo-tactile temporal congruence (synchronous, asynchronous) and statement type (illusion, control). The SCR data were entered into a 2 × 2 ANOVA, with the factors being body type (invisible, mannequin) and visuo-tactile temporal congruence (synchronous, asynchronous) (Figure 3). Eighteen subjects participated in Experiment 2a (13 females, mean age 25 ± 10 years) and 21 participated in Experiment 2b (7 females, mean age 27 ± 10 years).

Figure 2 Results of Experiments 1a and 1b. (a) The results of Experiment 1a showed that the participants rated the statements reflecting the illusory experience (S1-S3) significantly higher in the synchronous illusion condition than in the asynchronous and spatially incongruent control conditions. No such differences were observed for the control statements (S4-S6). (b) The results of Experiment 1b showed that the skin conductance response (SCR) evoked by a knife entering the field of view and threatening the invisible body was significantly stronger in the synchronous condition than in the asynchronous and spatially incongruent conditions. Together, these results suggest that the invisible body illusion is dependent on temporally and spatially congruent visuo-tactile stimulation. *P < 0.05. Full size image

Figure 3 Results of Experiments 2a and 2b. (a) The results of Experiment 2a show that strength of the invisible body illusion was equal to that of the mannequin illusion (the three-way interaction of body type × visuo-tactile temporal congruence × statement type was non-significant). (b) The SCR results of Experiment 2b show a significant main effect of visuo-tactile temporal congruence and a non-significant interaction of body type × visuo-tactile temporal congruence, corroborating the conclusion that the invisible body and mannequin illusions are of similar strengths. (c) There was a positive correlation between the magnitudes of the invisible body and mannequin illusions in terms of subjective ratings (left graph) and threat-evoked SCRs (right graph). These results imply that the elicitation of the illusions is dependent on analogous neural processes. *P < 0.05. Full size image

Experiment 3: Effects on the perceived body image

The aim of Experiment 3 was to quantify potential effects on the conscious body image, which was not fully captured by the questionnaire and SCR measurements. To address this question, we developed a body image test consisting of a perceptual matching task that assessed the participants' perceived degree of body transparency (see Body image task below and Figure 4, upper panel). We used a 2 × 2 factorial design, with the main factors being body type (invisible, mannequin) and visuo-tactile temporal congruence (synchronous, asynchronous). The inclusion of the mannequin conditions allowed us to control for unspecific effects related to experiencing a full-body illusion. We hypothesized that we would find a significantly higher rating in the synchronous condition than in the asynchronous condition for the invisible body, but we did not expect to find such a difference in the mannequin conditions (i.e., a significant body type × visuo-tactile temporal congruence interaction). Additionally, the participants performed a separate perceptual matching task to control for their suggestibility and task compliance. In this control task, the spatial orientation of the body relative to the environment was systematically altered (Figure 4, upper panel). We did not expect significant differences between any of the conditions in this control task. We included 20 participants in Experiment 3 (10 females, mean age 28 ± 7 years).

Figure 4 Results of Experiment 3. The upper panel shows that the participants reported that the invisible body illusion induced a significant shift in the perceived body image toward greater transparency, in contrast to the mannequin illusion’s effect (there was a significant body type × visuo-tactile temporal congruence interaction). The lower panel displays the results of the control task, in which the participants estimated the perceived orientation of their body, which revealed no significant differences between conditions. *P < 0.05. Full size image

Experiment 4: Effects on social anxiety

Studies on social anxiety within virtual environments have shown that exposing an individual to virtual social situations elicits anxiety responses that mimic the responses to analogue real-world social situations22,23,24. Standing in front of an audience is generally acknowledged as a stressful event and is associated with increased heart rate and levels of social anxiety25,26. The aim of the final experiment was to test the hypothesis that the feeling of invisibility would reduce the perceived anxiety related to experiencing a stressful social situation. We based this prediction on the assumption that if the body is represented as an invisible entity, it will be represented as being invisible to outside observers as well, which, in turn, should reduce the brain’s social anxiety response to being the center of other people’s attention. To test this hypothesis, we recorded the participants’ heart rate and their subjectively rated stress level in response to standing in front of an audience after exposure to 1 min of visuo-tactile stimulation. Again, we employed a 2 × 2 factorial design, with the main factors being body type (invisible, mannequin) and visuo-tactile temporal congruence (synchronous, asynchronous). First, we repeated the conditions once in a randomized order while recording the heart rate. We then repeated each condition again and asked the participants to verbally report their level of stress when looking up and seeing the crowd, using a visual analogue scale (presented on the HMDs immediately after each repetition) ranging from 0 (“I felt fully relaxed.”) to 100 (“I felt extremely stressed.”). We hypothesized that the perceived level of stress would only be elevated in the condition in which the participants experienced being physically present in front of the crowd (i.e., during the mannequin synchronous condition). We thus predicted a significant visuo-tactile temporal congruence × body type interaction to be driven by a significant difference between invisible body synchronous vs. mannequin synchronous, whereas we expected the contrast invisible body asynchronous vs. mannequin asynchronous to be non-significant. We included 29 participants (10 females, mean age 28 ± 7 years) in the experiment. Six subjects had to be excluded from the heart rate data analysis owing to severe artifacts in the ECG recording.

Questionnaires

We used questionnaires to quantify the subjective experience associated with the illusion in Experiments 1a and 2a8. Immediately following each experimental condition, the participants were asked to remove the HMDs and rate six different statements (Table 1) concerning their experience using a seven-point Likert scale, ranging from -3 (“I completely disagree”) to +3 (“I completely agree”), with 0 corresponding to “I neither agree nor disagree.” Three of the statements (S1-S3) examined the perception of the illusion and the other three statements (S4-S6) were designed to control for suggestibility and task compliance. The statistical analyses of the questionnaire data were performed on the average rating of the illusion statements (S1-S3) and control statements (S4-S6).

Table 1 Questionnaire statements. Full size table

Skin conductance responses and knife threat

We threatened the invisible body with a knife and measured the evoked skin conductance responses (SCRs) in Experiments 1b and 2b to provide an objective physiological measure of the illusion. Previous studies have shown that the threat-evoked SCR is a reliable proxy for changes in bodily self-attribution16,17,20,21,27 and that a stronger feeling of ownership of a seen limb is directly related to increased threat-evoked neuronal responses in the areas that reflect pain anticipation14,28. Here, a hand that was holding a knife entered the participant’s field of view from above and performed a slow, continuous motion toward the abdomen of the invisible body (or the mannequin’s body in Experiment 2b), as illustrated in (Figure 1). The knife stopped just before “hitting” the abdomen of the invisible/mannequin body, changed direction (-180°) and then disappeared out of the field of view in the same motion that it entered. The duration of the entire event was approximately 2 s. The threat-evoked SCR was identified as the peak of conductance that occurred within 5 s of the onset of the threat stimulus (from the first moment that the knife entered the participant’s visual field) and was flagged in the SCR recording file. The investigator who performed the analysis was blind to the condition (i.e., illusion or control). We used a Biopac System MP150 (Goleta, California, USA) to record the SCR and all of the data acquisition parameters and recording procedures were identical to those used in our previous studies16,17,20,21,29.

Body image task

For Experiment 3, we developed a test to examine the potential effects of the illusion on perceived body image. The test consisted of a perceptual matching task in which the participants were presented with a range of schematic drawings of seven bodies with different degrees of invisibility (Figure 4, upper panel). The series of bodies constituted a seven-point scale, ranging from a solid body (1), to increasingly transparent bodies (2–4), to a hollow body that merely had contours (5–6), to a completely invisible body (7). The participants’ instructions were the following: “How did you experience your body? Below you will find seven schematic drawings of your body during the experiment. Please select the body below which best corresponds to your experience.” To control for suggestibility and task compliance, the participants performed a separate perceptual matching task in which the spatial orientation of the body relative to the environment, rather than the transparency of the body, was systematically altered. In this task, the seven-point scale represented different angular rotations of a solid body (Figure 4, lower panel).

Heart rate measure and social stress stimuli

In Experiment 4, we recorded the participants’ heart rate while they were exposed to a stressful social situation to examine the potential effects of the illusion on social anxiety. The “social stress event” consisted of standing in front of a crowd of unknown people for 13 s, which immediately followed 1 min of visuo-tactile stimulation (see Figure 5a). The crowd comprised of 11 lab group members who were instructed to put on a skeptical and serious-looking face and look directly toward the position of the cameras (i.e., the position from which the participants viewed the room). The specific time interval of 13 s was the result of approximations and informal pilot experiments: the primary aim was to maintain the duration of the social stress event sufficiently long for the participants to comprehend the situation and have time to inspect the faces of the crowd of strangers, but at the same time not too long, because one would expect that the illusory feeling of invisibility and its potential stress-reducing effect decreases over time when not maintained through visuo-tactile stimulation. For practical reasons, we used pre-recorded 3-D videos of the visual stimuli (using two identical cameras mounted in parallel 8 cm apart; CamOne Infinity HD, resolution 1920 × 1080, Touratech AG, Germany) instead of the real-time setup used in Experiments 1–3. The experimenter wore headphones and listened to a pre-recorded audio track to synchronize the tactile stimulation with the videos. The participants were blind-folded and wore ear plugs when they entered the experiment room. This precaution was taken in order to minimize the risk of the participants discovering that experiment room was in fact empty of a crowd of people and that the visual stimuli were pre-recorded. In order to synchronize the lifting of the participants’ gaze with the prerecorded videos, the participants were instructed that at certain occasions during the experiment the experimenter would gently hold and lift the HMDs and that they should follow this motion with their head. Thus, by lightly holding and lifting the HMDs in a single controlled movement, the experimenter slowly lifted the participant’s gaze in synchrony with the pre-recorded visual input. Unfortunately, we did not formally quantify the feeling of presence in the virtual environment, as one anonymous reviewer pointed out. Anecdotally, however, most participants were surprised to find the experiment room empty of people when removing the HMDs after concluding the experiment and the results of the experiment speak in favor of a high degree of presence felt (see Results). Nevertheless, we recommend that future studies include a post-experiment questionnaire regarding the participants’ feeling of presence in the spatial environment presented in the HMDs.

Figure 5 Setup and results of Experiment 4. (a) The experiment timings and five representative frames from the visual stimuli are shown. Following 60 s of synchronous or asynchronous visuo-tactile stimulation that featured the invisible body or the mannequin’s body, the participants slowly lifted their gaze to discover that they were standing in front of an audience. The audience consisted of 11 scientists who were instructed to look directly at the participant (i.e., the cameras providing visual input to the HMDs) with a stern, serious face. The participants’ heart rate and subjective level of stress were measured. (b) The invisible body illusion was associated with significantly lower stress ratings than were found in the mannequin illusion. No significant difference was observed in the asynchronous control conditions. (c) In accordance with the subjective data, the heart rate response was significantly lower in the invisible body condition than in the mannequin condition - but only in the synchronous illusion condition. These results suggest that the illusion of owning an invisible body reduces the social anxiety associated with the experience of standing in front of an audience. Full size image

We used the Biopac System MP150 (Goleta, USA) and three electrodes (attached the left arm, right arm and left foot of the participant) to register a single-lead electrocardiogram (ECG). The R-wave detector function, which removes any components of the waveform that might be mistaken for peaks, was activated to optimize the ECG data for heart-rate calculation. The heart rate was calculated for every heartbeat based on the R-R interval with respect to the preceding beat. As a measure of general autonomic arousal in response to the social stress event, we calculated the mean heart rate for the 13 s that corresponded to the social stress event. To control for potential condition-specific effects on the heart rate that are unrelated to standing in front of the crowd, we analyzed the mean heart rate for the 13 s preceding the social stress event onset.

Statistical analysis

The Kolmogorov–Smirnoff test was used to check the normality of the data. For normally distributed data sets, we used t-tests and repeated-measures ANOVAs. For data sets that were not normally distributed, we used the non-parametric Wilcoxon signed-rank test. Although the data were not normally distributed, we investigated the interaction effects between two main factors in a 2 × 2 factorial design and calculated the “nonparametric interaction” (referred to as “interaction”) by calculating the numeric difference between the two levels of each factor and then statistically comparing these differences using a two-tailed Wilcoxon signed-rank test. We used two-tailed tests for all the analyses except the ones for which we had strong a priori hypotheses (the planned pair-wise comparisons in Experiment 1b and correlation analyses in Experiments 2a and 2b), in which one-tailed tests were used. The alpha was always set at 5%.