# Does virtual threat harm VR experience?: Impact of threat occurrence and repeatability on virtual embodiment and threat response

Rebecca Fribourg, Evan Blanpied, Ludovic Hoyet, Anatole Lécuyer, Ferran Argelaguet

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Rebecca Fribourg, Evan Blanpied, Ludovic Hoyet, Anatole Lécuyer, Ferran Argelaguet. Does virtual threat harm VR experience?: Impact of threat occurrence and repeatability on virtual embodiment and threat response. Computers and Graphics, 2021, 100, pp.125-136. 10.1016/j.cag.2021.07.017. hal-03468505

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<figure><img src="image_1.png" /></figure>

# Does Virtual Threat Harm VR Experience?: Impact of Threat Occurrence and Repeatability on Virtual Embodiment and Threat Response

Rebecca Fribourg<sup>a</sup>, Evan Blanpied<sup>b,1</sup>, Ludovic Hoyet<sup>a,1</sup>, Anatole Lécuyer<sup>a,1</sup>, Ferran Argelaguet<sup>a,1</sup>

## ARTICLE INFO

**Article history:**

Accepted 30 July 2021

DOI: https://doi.org/10.1016/j.cag.2021.07.017

© 2021. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/

**Keywords:** Virtual Reality, Avatar Embodiment, Threat Response

## ABSTRACT

This paper is an extended version of a previous paper [1] published at ICAT-EGVE 2020. We explored the potential impact of threat occurrence and repeatability on users' Sense of Embodiment (SoE) and threat response. To that aim, we conducted an experiment in which participants were embodied in a virtual avatar, and performed a task in which a threat towards the virtual body was introduced a first time, then repeated several times through the experiment (in total 5 times). The SoE of participants as well as their subjective response to the threat were assessed through subjective questionnaires before the introduction of the threat, after a first introduction of the threat and at the end of the experiment. A control group did the same experiment with no threat introduced during the task. The main findings of our experiment are that the introduction of a threat does not alter users' SoE but might change their behaviour while performing a task after the threat occurrence. In addition, threat repetitions did not show any effect on users' subjective SoE, or subjective and objective responses to threat. Taken together, our results suggest that embodiment studies should expect potential change in participants behaviour while doing a task after a threat was introduced, but that threat introduction and repetition do not seem to impact the subjective measure of the SoE (subjective ratings) nor the objective measure of the SoE (physical reaction to threat).

© 2021 Elsevier B.V. All rights reserved.

## 1. Introduction

It has now become widely considered that the success of VR experiences involving avatars relies greatly on users' embodiment towards them [2, 3]. Achieving a satisfying user embodiment then has become a striking constraint in the development of such applications and has reinvigorated the need to understand the process underlying the perception of avatars. For this reason, the study of virtual embodiment has received much attention as well as the different possible methods to measure it. Among them, the study of the Sense of Embodiment (SoE) is widely used to assess how users perceive their avatars and whether they accept or reject their virtual body. The SoE is usually determined by the use of subjective questionnaires such as the one suggested by González-Franco and Peck [4]. However, the use of objective

measures of the SoE is being increasingly frequent in embodiment studies. For instance, Kilteni et al. showed that people with higher SoE experienced high behavioural changes [5]. Yet, the more common objective measure of the SoE remains to this day the response to a virtual threat towards the avatar.

Indeed, some research successfully showed that the SoE was correlated with the response to a virtual threat towards the virtual body [6, 7], and that this response could be objectively measured by galvanic skin conductance [6], change in user body motion [8] or brain imagery [9, 10]. Such findings are thus particularly seducing as they suggest that response to a virtual threat can be used as an objective measure of the SoE. Nevertheless, while the introduction of a virtual threat in virtual embodiment studies is widely used, no research has specifically evaluated

<footnote><sup>a</sup>Univ. Rennes, Inria, INSA, CNRS, IRISA, Rennes, France</footnote>

<footnote><sup>b</sup>Davidson College, Davidson, North Carolina, USA</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1. Overview of the virtual environment representing a factory (left), an avatar representing a user placing an ingot on the plate arrived on the conveyor lay (center) and the crusher threatening the user by suddenly going down while the user's hand is under it.</figcaption></figure>

the impact of the virtual threat on the SoE. In other words, is the SoE modulated by the actual occurrence of the threat? For example, the stress induced by threats can be detrimental to cognitive functions such as spatial working or memory [11]. More precisely, a study from Christensen et al. [12] showed that fear induction was detrimental to the sense of agency of users towards their actions. While these studies were not conducted in VR, we may wonder if a virtual threat would impact similarly user cognitive functions and possibly their SoE. In addition, both immersion and affective content had been shown to impact the sense of presence in virtual environments [13, 14], a cognitive feeling also widely studied to assess users' perception of virtual environments.

Moreover, a virtual threat has no nociceptive feedback corresponding to the event, unlike a real threat, although visual, acoustic and haptic feedback can be provided. While in most studies a threat is only introduced at the end of the experiment [15, 16], in other studies the threat can be repeated multiple times [7]. Hence, the repetition of a threat in virtual reality may lead to decreased relevance of the illusion and thus less response from the participants. The main scope of this paper is therefore to explore the impact of threat occurrence and repeatability on the SoE and threat response.

To that aim, we conducted an experiment (n = 60) in which participants were embodied in a virtual avatar, and performed a task in which a threat towards the virtual body was introduced a first time, then repeated several times through the experiment (see Figure 4). The SoE of participants as well as their subjective response to threat were assessed through subjective questionnaires before the introduction of the threat, after a first introduction of the threat and after all the repetitions of threat occurrence. In addition, threat response was also assessed through objective measures by observing participants' physical response to the threat stimuli, as well as potential behaviour adaptations while performing the task after a threat was introduced. A control group did the same experiment with no threat introduced during the task. We were also interested in the potential impact of presence and individual differences between participants on the results, which is why participants also answered demographic, presence and personality questionnaires. More precisely, we decided to include a personality questionnaire for assessing the neuroticism trait, as it relates to anxiety which could have impacted our study results due to the stressful nature of the threat introduction.

The main findings of our experiment are that the subjective

measures of SoE are not altered by the introduction of a threat, nor its repetition. On the one hand, this result is interesting as it suggests that the response to a threat can be safely used in embodiment studies as it does not change the subjective measure of embodiment and would then not bias the experimental results. On the other hand, it also shows that a threat can be introduced several times without impacting the subjective measures of the SoE of users. In addition, the repetition of the threat did not show a significant effect on user physical response to threat, meaning that this objective measure of the SoE does not seem to be impacted by threat repetition. However, another result shows that users may have a different behaviour while performing a task after a threat was introduced, even in safe conditions. Finally, we found that individual differences between users, such as users' neuroticism trait, gender or game experience, could influence their threat responses and also their SoE. Taken together, our results suggest that embodiment studies should expect potential change in participants behaviour while doing a task after a threat was introduced, but that this threat introduction and repetition do not impact the subjective measure of the SoE (user responses to questionnaire) nor the objective measure of the SoE (physical reaction to threat towards the virtual body).

## 2. Background

The Sense of Embodiment (SoE) is widely used to assess how users perceive their avatars and whether they accept or reject this virtual body. As defined by Kilteni et al. [17], it can be divided into three subcomponents: the sense of ownership (one's self-attribution of a body), the sense of agency (feeling of control over actions and their consequences) and the sense of self-location (one's spatial experience of being inside a body). While the SoE is commonly assessed by the use of subjective measures such as questionnaires [4], recent research tend to explore alternatives to assess this perception of the virtual body. Among them, the introduction of a threat has become a popular means to assess if users are well embodied in their avatar. This practice relies on the assertion that if users react to a virtual threat towards their virtual body, they must have a strong sense of embodiment towards it. Indeed, several studies showed that the sense of ownership towards a body was connected with increased affective response to threat towards the body [6, 7].

### 2.1. *Threat Response and Body Ownership*

The first studies exploring the relation between body ownership and response to threat were based on the rubber hand
illusion. Armel et al. [18] were among the first ones to show
that response to a threat towards a rubber hand was linked to the
assimilation of the rubber hand as into one's own body image.
The threat response was in that case assessed by skin conduc-
tance response (SCR), e.g., if the rubber hand was "injured",
participants displayed a higher skin conductance. Rapidly, the
use of a threat has been extended to rubber hand illusions target-
ing deeper exploration of the body sense of ownership. Indeed,
SCR measures after a threat introduction have been used to show
that amputees of an upper-limb could feel ownership towards
a rubber hand prothesis [19] but also that it was possible to
feel ownership towards two rubber hands in supplementary to
the physical hand[20] or to a third rubber arm supplementary
to the two physical arms [21]. Studies exploring the concept
of body-swapping also used SCR as an objective measure of
ownership towards another body (either a manikin or someone
else's body) in a context of out-of-body experiences, using an
HMD coupled to a video camera oriented down at the manikin
or someone else's body [15, 16]. Quickly, research exploiting
the use of a threat to measure the sense of ownership have been
brought to virtual reality. Slater et al. [22] were the first ones to
transpose the rubber-hand illusion in Immersive Virtual Reality
(IVR) and by the same time the first ones to use SCR as a mea-
sure of ownership when a threat is introduced. Ma et al. [23]
however questioned their findings in that they did not consider it
succeeded in proving that SCR to threat was linked with owner-
ship, because one can have a similar affective resonance when
someone else is hurt. Ma et al. [23] also showed in their own
study that if response to a non dangerous impact was linked to
the sense of ownership, response to a threat appeared to be inde-
pendent of the sense of ownership, which is in contradiction with
other researches using SCR to threat as an objective measure of
ownership [24, 25].

## 2.2. Measures of Threat Response

While SCR had been mainly used in early studies [24, 25], other researches tried to use different methods to measure threat response. For instance, Slater et al. [26] reproduced an out-of-body experience in IVR exploiting response to a virtual threat as an objective measure of ownership but using this time another measure: the heart-rate deceleration in response to threat. Indeed, while SCR was the most common way to objectively measure threat response, some limitations were raised by Zhang et al. [7] saying that it cannot be differentiated if SCR measured general level of arousal or specific fear emotion. For this reason other measures of threat response have been explored. Erhsson et al. [9] explored another measure than SCR to assess the link between the sense of ownership towards a rubber hand and response to a threat towards it. They successfully showed that threatening a rubber hand that feels “owned” can induce brain-activity patterns that are associated with anxiety. They also highlighted that this brain activity was similar to when the participant’s real hand was threatened. This heightened the evidence that brain activity after a threat towards a rubber hand can provide an objective measure that the rubber hand is fully incorporated into the body. While this previous study was not conducted in VR, González-Franco et al. [10] showed in another

study that when a threat is introduced towards a virtual body
in IVR, brain activity (motor cortex activation) is correlated to
the sense of ownership. Physical avoidance of a threat towards
the virtual body was also used in IVR as a measure of owner-
ship [27], where participants had to avoid a virtual fan going
down in their direction. Kilteni et al. [28] also used physical
avoidance of threat, such as body defensive mechanisms, as an
evidence of ownership towards virtual body parts. Other re-
searches used subjective questions [29] to assess the perception
of threat and its link with the sense of ownership, such as an
anxiety inventory (SA-I) [7].

## 2.3. Types of Threat in Embodiment Studies

While the methods to assess response to threats towards a
virtual body thus appear very numerous, the types of threat that
were implemented so far also tend to vary. In rubber hand studies,
the threat was often induced by an experimenter (e.g. bending a
finger into a painful position [18] or introducing a sharp needle
into the rubber hand [19]). Yet, threats in virtual embodiment
studies are rarely introduced by a third party, although some stud-
ies did use virtual characters in order to introduce a threat (e.g.
a virtual character slapping the face of one's virtual body [26]
or stabbing one's virtual hand [30]). It is thus more common in
virtual reality to have the threat induced "by itself", like a virtual
knife flying in the air and stabbing the virtual body [10, 24, 7].
In addition, the threats may also differ by the way they are in-
troduced. A threat may be introduced with a goal of "surprise",
in order to observe the direct physical response of participants
to a sudden threat towards their virtual body [24], and may be
characterized as "active threat". Otherwise, threats can also
be "passive", being present in the virtual environment from the
beginning, with participants needing to avoid them in order to
perform the task [31]. We may also characterize participants
as "active" or "passive" considering that in experiments partici-
pants are asked to stand motionless while a threat is introduced
towards their virtual body [10], while in other experiments par-
ticipants perform a task and are therefore in movement when a
threat is introduced towards them [7]. Moreover, virtual threats
in embodiment studies also vary by their frequency and time of
occurrence. Most of the time, the threats are introduced at the
end of the experiment [16, 21] but they sometimes occur repeat-
edly [10, 23]. Virtual threats may also vary in terms of duration.
For instance, Steptoe et al. [29] used a threat that consisted of
fire burning virtual body parts, and lasted 30 seconds. This can
be considered as a rather "slow" threat compared to more direct
ones such as stabbing virtual threats [10]. Finally, we may con-
sider the differences of feedback used in embodiment studies
to accompany the threat, which may be strictly visual [30] or
associated with tactile stimulation [23] or sound [7].

## 2.4. Impact of Virtual Threat on the SoE

The introduction of threat in embodiment studies has thus
already been widely used as an objective measure of the SoE. Yet,
no research has been conducted to evaluate the actual effect of
introducing a virtual threat on the subjective measures of the SoE.
Indeed, while the response to a virtual threat is used as a measure
of the SoE, to our knowledge, it has never been considered as
a possible influencing effect. In other words, the response to a virtual threat is associated to a strong SoE towards an avatar, but it was never verified whether its introduction could actually impact an initial SoE. However, some studies showed that stress induced by threats can be detrimental to cognitive functions such as spatial working or memory [11]. More precisely, a study from Christensen et al. [12] showed that fear induction was detrimental to the sense of agency of users towards their actions. While these studies do not depict the context of VR, we may wonder if a virtual threat would impact similarly user cognitive functions and possibly their SoE. Furthermore, both immersion and affective content had been shown to impact the sense of presence in virtual environments [13, 14], a cognitive feeling also widely studied to assess users' perception of virtual environments.

Additionally, in most studies a threat is only introduced at the end of the experiment [15, 16], although sometimes it is repeated and occurs randomly[7]. Nevertheless, to our knowledge the impact of threat repeatability on its efficiency has never been assessed. Yet, when a virtual threat is induced to users in virtual reality, they may see their virtual body visually impacted by the threat (collision or even virtual blood), but have no nociceptive feedback corresponding to the event. Hence, it is possible that the repetition of a threat in virtual reality may lead to a decreased relevance of the illusion and thus a diminished response from participants.

# 3. Experiment

The main scope of this paper is therefore to explore the impact of threat occurrence and repeatability on the SoE and on threat response. The first goal was to study the potential impact of a first threat occurrence on the SoE. The second goal was to observe if the repetition of a threat would impact the way it is perceived by participants, and potentially testify of a modification on users' SoE. Therefore, in this experiment participants experienced multiple threats occurrences and their SoE was assessed through subjective questionnaires before the first threat occurrence, right after the first occurrence, and finally after all the occurrences at the end of the experiment. A control group did the same experiment with no threat introduced during the task. We also assessed participants sense of presence through subjective questionnaires at the end of the experiment in both groups. Presence has an interesting bidirectional relation with anxiety and emotional state as previous work showed that fear in VR could be influenced by the sense of presence [32], and that both immersion and emotional content can also impact the sense of presence [13, 14]. We thus expected that the way participants would react to the virtual threat would be influenced by how much they believed to be in the virtual factory. We also expected that ratings of presence would be higher in the group that experienced the threat compared to the control group. Motivated by previous works which explored the influence of personality traits on subjective feelings in VR [30, 33], we decided to include as well questions related to one of the Big Five Inventory traits, the neuroticism trait, as it relates to anxiety which could have impacted our study results due to the stressful nature of the threat introduction.

## 3.1. Participants

Sixty participants volunteered to take part in the experiment (30 males and 30 females; mean/S.D. age: 34.1±10.6). They were recruited from the university campus, were naive with respect to the purpose of the experiment and had normal or corrected-to-normal vision. The studies conformed to the declaration of Helsinki. Among the participants, 19 subjects had never tried VR, 33 had limited experience with VR and 8 had knowledgeable experience with VR. The experiment was approved by an ethical committee and every participant signed an informed-consent form before the experiment.

## 3.2. Apparatus

The experiment was developed using Unity software (version 2018.2.19f1). Participants were immersed in VR using a HTC Vive PRO Head-Mounted-Display (HMD) and equipped with two Vive controllers (one in each hand) and two Vive trackers (one attached to each foot). There were embodied in a gender-matched avatar that was animated using inverse kinematics (Unity FinalIK plugin) using the positions of the HMD, the controllers and the trackers.

## 3.3. Task & Threat

In order to increase the coherence of a potential threat occurrence, we chose to put participants in a virtual environment that represented a factory where potential incidents might happen, e.g., a malfunction of a dangerous machine (see Figure 1, left). More precisely, participants had to perform a task that consisted in grabbing a metallic ingot, putting it on a plate coming on a conveyor lay, then pressing a button so that a crusher smashed the ingot to transform it into a metallic pinion (see Figure 5). Before the ingot was placed on the plate, the button remained red, and only if the ingot was correctly placed within rectangular boundaries drawn on the plate, the button would turn green and become pressable. Therefore, participants had to be precise in their gesture.

All the task interactions were performed by participants using their dominant hand. Depending of whether participants were left or right-handed, the environment was mirrored symmetrically, e.g., the box containing the ingots as well as the button were placed on the opposite side. Using the original 3D model of the HTC Vive controller, we attached a 3D magnet on top, which participants used to grab the virtual ingot by pressing the controller trigger. To release the ingot, participants simply released the controller trigger.

As for the threat, we decided to have an active short threat with tactile feedback that would lead to a surprise effect with participants being active in a task. It consisted in a malfunction of the crusher, which would suddenly activate while participants were positioning the ingot on the plate (i.e., the participants' hand was still under it). It was accompanied by a threatening sound of a "machine crash", working not only on the visual but also on the auditive dimension. The crusher would go down to the plate, to increase the chances to collide with the virtual arm, by the speed of 2 m/s. The threat was thus designed in a way that would make it plausible for the participants, in order to ensure its efficiency in virtually threatening them. Moreover, a
<table><caption>**Table 1. Questionnaire used in the experiment. Questions in italics are control questions.**</caption><thead><tr><th>Variable</th><th>Question</th></tr></thead><tbody><tr><td>Ownership</td><td>O<sub>1</sub>) I felt as if the virtual body I saw when I looked down was my body.<br>O<sub>2</sub>) It felt as if the virtual body I saw was someone else.<br>O<sub>3</sub>) It seemed as if I might have more than one body.</td></tr><tr><td>Agency</td><td>A<sub>1</sub>) It felt like I could control the virtual body as if it was my own body.<br>A<sub>2</sub>) The movements of the virtual body were caused by my movements.<br>A<sub>3</sub>) I felt as if the movements of the virtual body were influencing my own movements.<br>A<sub>4</sub>) I felt as if the virtual body was moving by itself.</td></tr><tr><td>Self-Location</td><td>SL<sub>1</sub>) I felt as if my body was located where I saw the virtual body.<br>SL<sub>2</sub>) I felt out of my body.<br>SL<sub>3</sub>) I felt as if my (real) body were drifting towards the virtual body or as if the virtual body were drifting towards my (real) body.</td></tr><tr><td>Appearance</td><td>AP<sub>1</sub>) It felt as if my (real) body were turning into an “avatar” body.<br>AP<sub>2</sub>) At some point it felt as if my real body was starting to take on the posture or shape of the virtual body that I saw.<br>AP<sub>3</sub>) At some point it felt that the virtual body resembled my own (real) body, in terms of shape, skin tone or other visual features.<br>AP<sub>4</sub>) I felt like I was wearing different clothes from when I came to the laboratory.</td></tr><tr><td>Threat</td><td>T<sub>1</sub>) I felt that my own body could be affected by the crusher.<br>T<sub>2</sub>) I felt a fear sensation in my body when the crusher malfunctioned, if it did.<br>T<sub>3</sub>) When the crusher malfunctioned, if it did, I felt the instinct to move my hand.<br>T<sub>4</sub>) I had the feeling that I might be harmed by the crusher.</td></tr></tbody></table>

vibration was given through the HTC Vive controller each time the crusher smashed the ingot or malfunctioned.

### 3.4. *Experimental Protocol*

Upon their arrival, participants read and signed the experiment consent form and filled in a demographic questionnaire (collecting age, gender and experience in video games and VR). They were then briefed about the experiment and equipped with the HMD, controllers and trackers. Afterwards, avatars were re-scaled so that the dimensions matched the participant's eye-height, as well as arm span, which were computed from the position of the HMD and controllers while the participant held a N-pose. Finally, participants were immersed in the virtual environment. They all started the experiment facing a virtual mirror in the virtual factory (see Figure 2), giving them the opportunity to see their full virtual body animated by their own motions. When they were ready to start, the mirror disappeared

<figure><img src="image_3.png" /><figcaption>**Fig. 2. Phase of familiarization with the virtual avatar facing a mirror, captured from a third person point of view (left) and a first person point of view (right).**</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3. Television displaying the subjective questions asked to participants between blocks.**</figcaption></figure>

by mechanically sliding towards the ceiling, and the experiment began. From this point, the experimental flow was divided into three blocks that involved 12 trials each. One trial consisted in performing the task once. The overall organization of the experimental flow is summarized in Figure 4.

A threat was introduced at the end of the second block (in the 24th trial). The same threat was then introduced again in the third block during trials 26, 30, 33 and 34. A control group of participants was considered for the experiment, for which no threat was ever introduced, meaning that all trials were similar. At the end of each block, participants answered an embodiment questionnaire (an adapted version of González-Franco and Peck's embodiment questionnaire [4]) while being immersed in the virtual environment. A virtual television appeared in the factory with questions written on it, and participants answered the questions with the trackpad and trigger of their right controller (see figure 3). Finally, after the last block, participants were unequipped and answered a final questionnaire which included a presence questionnaire [34] as well as questions extracted from the Big Five Inventory questionnaire [35] regarding neuroticism personality trait. Each trial lasted approximately 5 seconds and participants performed in total 36 trials each. The whole experiment, including welcoming of participants, reading and signing the consent form, and answering questionnaires lasted approximately thirty-five minutes.

### 3.5. *Experimental Design*

A mixed-design was adopted for the experiment, considering two independent variables: Group and Block. Group was a between-subject factor with two levels (*threat* and *control*), corresponding respectively to half of the participants (n=30: 15 women and 15 men) that encountered a threat during the experiment and the other half of the participants (n=30: 15 women and 15 men) which performed the whole experiment without experiencing a threat. Block was a within-subject factor with
<figure><img src="image_5.png" /><figcaption>Fig. 4. Summary of the experimental design. In the *control* group, threat trials were replaced by safe trials without any threat</figcaption></figure>

three levels corresponding to the blocks of the experiment flow: *first*, *second* and *third*.

Regarding dependent variables, both objective and subjective data were collected during the experiment to assess participants' SoE as well as threat responses.

### 3.5.1. *Subjective Data*

Each participant answered a subjective embodiment questionnaire at the end of each block, inspired from the questionnaire proposed by González-Franco and Peck [4]. The questions were divided into five categories (Ownership, Agency, Self-Location, Appearance and Threat). However, since one group did not encounter any danger, only Ownership, Agency and Self-Location and Appearance were used to compute SoE scores. For the same reason, questions related to the threat event were only analysed for the group with danger. However, other questions were related to the threat but not related to the occurrence of the threat event, and were therefore analysed in two groups. All the questions were answered on a 7-point Likert scale, from -3 (strongly disagree) to 3 (strongly agree), and can be found in Table 1.

In addition, participants answered a final questionnaire at the end of the experiment, which contained an additional presence questionnaire (7-point Likert scale) [34].

### 3.5.2. *Objective Data*

In order to assess participants' physical response to the threat, as well as potential changes in their behaviour while performing the task after the threat was introduced and repeated, the motion (position and orientation per frame) of the participants' dominant hand was recorded. In addition, the time during which the dominant hand was under the crusher was also recorded for each trial. To gain some insight regarding the objective reaction to threat during the experiment, speed profiles were computed for each participant and each trial. More precisely, we were interested in the direct physical reaction from participants to the threat stimuli, but also the potential impact on user behaviour while performing the task in the safe trial (a safe trial is a trial with no threat occurrence, independently of the group of participants).

### 3.5.3. *Hypotheses*

In this experiment, we were interested in evaluating the impact of threat occurrence and its repeatability on the SoE and threat response in VR. Indeed, while the first occurrence of a threat in a virtual environment might elicit a strong fear reaction accentuated by a potential surprise effect, we hypothesized that its repetition would tend to decrease its effect due to the absence of nociceptive feedback and thus credibility.

Regarding the influence of the first threat occurrence, we first hypothesised that it would impact positively the subjective measure of the SoE. Indeed, previous research showed that experiencing emotional response such as fear in a virtual environment leads to a stronger sense of presence [14], another subjective perception of the virtual experience. While, the sense of presence differs from the SoE, previous works showed that they are not likely to be totally independent [4]. However, other studies also showed that the stress induced by threats can be detrimental to cognitive functions such as spatial working or memory [11]. More precisely, a study from Christensen et al. [12] showed that fear induction was detrimental to the sense of agency of users towards their actions. While these studies were not conducted in VR, we may wonder if a virtual threat would impact similarly user cognitive functions and possibly their SoE. Fear was thus both shown to be detrimental to the sense of agency of users towards their actions as well as leading to a stronger sense of presence. Nevertheless, as agency is related to embodiment, we considered that the second hypothesis was more likely to be accepted. For this reason, we argue that the SoE could be negatively impacted by the first occurrence of the threat, i.e., that participants would experience a lower SoE after experiencing a threat. We also hypothesised that this first threat occurrence would have an impact on participants' behaviour while performing the task afterwards, because of the anxiety being raised by the threat. More precisely, we believed those changes would be visible either by a accelerated speed while doing the task or a decreased time of their dominant hand spent under the crusher. However, when considering the repeatability of the threat introduction, the expected impact on the SoE and Threat response to be different. Indeed, when experiencing a virtual threat in VE, participants encounter visual feedback as well as sometimes auditory or tactile feedback. However, no nociceptive feedback is associated with the virtual threat, which might at some point break the illusion. Hence, because we expected the repetition of the threat to decrease its efficiency in making participants react, we supposed their physical reaction to it would decrease along the repetitions and that their subjective response to the threat (answers to subjective questions about how the threat was perceived) would also be diminished. In addition, we expected the loss of plausibility of the virtual threat to impact negatively the SoE, e.g., that if participants lost conviction of the virtual environment they might also lose conviction of their virtual body. Finally, we expected that these effects would not be present in the control group and therefore not related to the exposure time.

In summary, considering our experimental design, our main hypotheses are as follows.

**H1:** In the *threat* group, the SoE scores will be lower after the first threat (i.e., lower after the *second* block than after the *first* block.)

**H2:** In the *threat* group, the SoE scores will be lower after several repetitions of the threat (i.e., lower after the *third* block than after the *second* block as well as after the *first* block.)

**H3 (control):** In the *control* group, the SoE scores will remain similar between all blocks.

**H4:** In the *threat* group, the scores of subjective threat responses (Threat category of subjective embodiment question-
<figure><img src="image_6.png" /><figcaption>Fig. 5. Different steps of the task: grab the ingot (first), place it on the plate (second), press the button (third), the ingot was smashed into a pinion (fourth).</figcaption></figure>

naire) will be lower in the *third* block than in the *second* one.

**H5:** In the *threat* group, the physical response to the threat will decrease along the repetitions of the *third* block.

**H6:** The sense of presence will be higher in the *threat* group compared to the *control* group, and participants in *threat* group with higher sense of presence will have higher subjective threat responses.

$(p<.05)$ and that $O_3$ scores were higher in block 2 that in block 1 ($p<.05$) and higher in block 3 that in block 1 ($p<.001$). These results suggest that the repetition of the experimental blocks had a negative impact on some questions related to subjective Ownership, independently of whether a threat was introduced or not during the experiment, which does not allow the validation of **H3**.

# 4. Results

Mixed two-way ANOVA analyses were performed when comparing scores of SoE between the blocks (within-subjects) and the two groups (between-subjects). The normality assumption was tested using Shapiro-Wilk test and when not verified, an Aligned Rank Transformation (ART) was applied on the data. Tukey's post-hoc tests ($\alpha = .05$) were conducted to check significance for pairwise comparisons. When comparing scores of threat subjective questions, Friedman test was performed between blocks as normality assumption was not verified. As for correlation analyses, Pearson's r ($r$) was used for parametric data and Spearman's r ($r_s$) was used for non-parametric data.

## 4.1. Subjective measure of the SoE

The embodiment scores were computed by averaging the scores of Ownership, Agency, Appearance and Self-Location according to Gonzalez-Franco and Peck [4] computation method: Ownership = $(O_1 - O_2 - O_3)/3$, Agency = $(A_1 + A_2 + A_3 - A_4)/4$, Self-Location = $(SL_1 - SL_2 + SL_3)/3$, Appearance = $(AP_1 + AP_2 + AP_3 + AP_4)/4$ and Embodiment = $(Ownership * 2 + Agency * 2 + Self-Location * 2 + Appearance)/7$. As previously said in Section 3, Threat scores were not included in the SoE computation since one group did not encounter any threat. A mixed-two way ANOVA (group, block) analysis was performed on embodiment scores as well as on each sub-component. We did not find significant differences between the embodiment scores depending on the block or the group, or their interaction, which thus does not support **H1** nor **H2**. The general mean score are $1.46\pm0.61$ (S.D.) for embodiment, $1.12\pm1.16$ (S.D.) for Ownership, $2.30\pm0.58$ (S.D.) for agency, $1.87\pm0.91$ (S.D.) for self-location and $-0.37\pm1.19$ (S.D.) for appearance.

Although not significant, the here-above analysis highlighted a tendency for Ownership scores to decrease from block 1 to block 3. We thus decided to perform a mixed-two way ANOVA (group, block) analysis on each question of Ownership independently, which highlighted a significant order effect for $O_1$ and $O_3$ from Ownership questions ($[F_{2,116}=4.26, p < .05]$ for $O_1$ and $[F_{2,116}=8.55, p < .001]$ for $O_3$). Post-hoc tests showed that $O_1$ scores were significantly lower in block 2 than in block 1

## 4.2. Subjective Response to Threat

Subjective responses to the threat were analysed in two groups: Event Related questions (**ER**) refers to the two questions directly related to the occurrence of a threat ($T_2$ and $T_3$), and Non Event Related questions (**NER**) refers to the questions related to general fear towards the crusher ($T_1$ and $T_4$). Friedman tests were performed to analyse responses to **ER** questions only in the *threat* group, as no threat was introduced in the *safe* group. Significant differences depending on block were found for each question ($T_2$: $\chi^2=34.7, p < .0001$, $T_3$: $\chi^2=42.0, p < .0001$). Wilcoxon tests were thus conducted and showed that threat scores were significantly lower in the *first* compared to the *second* block for all four questions ($p<.0001$) and in *first* compared to *third* block ($p<.0001$). However, no significant difference was found between blocks *second* and *third* (see Figure 6). Subjective **ER** threat response thus increases after a first threat occurrence, but does not further increase nor diminishes after several repetitions.

**NER** questions were analysed for both groups, and a mixed-two way ANOVA analysis was also performed on both questions independently. For both questions, significant effects of group ($[F_{2,58}=19.37, p < .001]$ for $T_1$ and $[F_{2,58}=14.03, p < .001]$ for $T_4$), block($[F_{2,116}=5.41, p < .01]$ for $T_1$ and $[F_{2,116}=20.22, p < .001]$ for $T_4$) and interaction between the two ($[F_{2,116}=11.49, p < .001]$ for $T_1$ and $[F_{2,116}=8.56, p < .001]$ for $T_4$) were found. For $T_1$ and $T_4$, post-hoc tests showed that ratings in the *second* and *third* blocks were higher than in the *first* block ($p<.0001$) for group *threat*, but not for *safe* group (see Figure 7). Similarly to **ER** response, these results suggest that subjective **NER** threat response increases after a first threat occurrence, but does not further increase nor diminishes after several repetitions. Hence, these results do not support **H4**.

## 4.3. Objective Response to Threat

In this analysis, we were interested in comparing objective data depending on trials to search for potential evolution in user behaviour due to threat introduction and repetitions. We thus considered Trial as another independent variable.
<figure><img src="image_7.png" /><figcaption>Fig. 6. Mean scores of ER threat subjective questions for the threat group.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7. Mean scores of NER threat subjective questions.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Averaged speed profiles of participants during threat trials (up) and SPM analysis highlighting significant differences between trials (down). The sign of the y-axis represents the direction of the motion, positive speeds represent the user moving his hand forward to place the ingot and negative speeds represent the user moving his hand away from the crusher.</figcaption></figure>

### 4.3.1. Time of the dominant hand being under the crusher

We were interested in the time the participant's dominant hand spent under the crusher during each trial (see Figure 9), as an information of how “scared” they might be of their hand being potentially crushed while doing the task. More precisely, we were interested in all the safe trials (in which no threat was introduced) ranging from the last safe trial before a first threat was introduced to the last safe trial of the experiment (23, 25, 27, 28, 29, 31, 32, 35 and 36). Five outlier samples were removed for this analysis due to abnormal time values in a few trials, corresponding to a time of either 0s (rare cases in which participants threw the ingot and had it placed perfectly on the plate) or an

excessively abnormal time under the crusher (in some situations where participants were scared of the crusher and had to make several attempts to place correctly the ingot under the plate).

A one-way ANOVA was performed on the data from the *threat* group to investigate differences of time among the selected safe trials, and highlighted a significant effect ($[F_{8,232}=4.05, p < .0001]$). Post-hoc tests only showed significant effects between trial 23 and all other trials, except 25: the time that the dominant hand spent under the crusher was not significantly lower in the trial following the first threat (25), compared to the trial preceding it (23), but the time in the other safe trials were all significantly lower than trial 23 ($p < .05$). A one-way ANOVA was also performed in *control* group and did not show any significant differences between the investigated trials ($[F_{8,232}=0.53, p = .83]$). This result suggests that after the threat was introduced twice, participants left their hand a shorter amount of time under the crusher, and thus performed the task faster. The fact that this change of behaviour is not visible in the control group also suggests that this change is due to participants' reaction to the threat.

### 4.3.2. Speed Profiles

For each trial, the speed profiles of the dominant hand while performing the task were computed for each participant, then averaged across participants. More precisely, for the trials in which a threat occurred, the speed profiles were computed from the time the threat occurred and for the trials with no threat, the speed data were aligned between participants on the time the virtual ingot was released from the dominant hand. Data were then cropped in order to ensure having the same length of data for each participant (for trials with a threat, we kept the 80 frames following the frame of the threat introduction, and for safe trials we cropped from 50 frames before the ingot was released to 50 frames afterwards). In addition, to include information about the direction of the hand movement in the analysis, we considered speed values of movements away from the participant (along the X axis, i.e., towards the machine) to be positive, while speed values of movements towards the participant (along the -X axis, i.e., away from the machine) to be negative. This representation enabled us to observe simultaneously the magnitude of the movement of participant's dominant hand, as well as its direction (see Figure 8). To analyse the speed profiles, we resampled them at a frequency of 60 Hz, then we filtered the data with a butterworth low-pass filter with a cutoff frequency of 30 Hz to reduce the noise. We evaluated the effect of Trial on the speed profiles using Statistical Parametric Mapping (SPM) methods [36]. This process allows comparing time series data taking into account their variability at each time frame.

**Motion data in threat trials.** We used SPM analysis to compare speed profiles of threat trials (Trial = 24, 26, 30, 33 and 34) in the *threat* group, which showed a significant effect of Trial ($p < .001$). Post-hoc tests revealed a significant difference between trial 24 (first threat introduction) and all other trials (26, 30, 33 and 34) ($p < .05$). Qualitatively, we can notice that the maximum speed remains comparable among trial 24 and the others, while displaying a temporal shift: on trial 24, participants
<figure><img src="image_10.png" /><figcaption>Fig. 9. Mean time of dominant hand spent under the crusher per trial for each group. Trials with an asterisk correspond to the trials in which a threat was introduced, and are therefore masked for that the time spent under the crusher in those trials is not comparable with other trials.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10. Averaged speed profiles of participants during selected safe trials in the control group (left) and the threat group (right). The sign of the y-axis represents the direction of the motion, positive speeds represent the user moving his hand forward to place the ingot and negative speeds represents the user moving his hand away from the crusher.</figcaption></figure>

reacted significantly later to the threat than in the other trials
(see Figure 8).

Motion data in safe trials: We used SPM analysis to compare speed profiles of specific safe trials in both *threat* and *control* groups. More precisely, since we were interested on the impact of threat repetitions on behaviour in safe trials, we compared the first trial (1), the trial before the first threat (23), the trial after the first threat (25) and the last trial (36). SPM analysis showed a significant effect of Trial on speed profiles (*p*<.001) in both groups, on two distinct phases: the forward motion of the dominant hand, and the backward motion. In the *threat* group, post-hoc tests showed significant differences between the last trial of the experiment and trials 23 and 25, yet only in the forward motion (*p*<.05) (see Figure 10). This result shows that in average the approaching speed was higher for the *threat* group for the last trial. Overall, those results do not support **H5**, as the repetition of threats did not impact physical threat response.

4.4. *Presence, Personality and Threat*

From the final questionnaire we computed the mean scores for
presence regarding the global experiment for each group (*threat*,
*control*). While presence questionnaires were only collected at
the end of the experiment and thus could not show the impact of
threat introduction and repetitions on presence, it enabled us to
compare presence score between our two groups of participants.
Presence scores were relatively high and were similar for both

groups (5.24±1.09 (S.D.) for *threat* and 5.08±1.11 (S.D.) for *control*). They were also not statistically different after performing a Mann-Whitney test ( *p*=.21), showing that the sense of presence was not significantly different between the group that encountered several threat introductions and the control group. These results therefore do not support **H6**.

Neuroticism scores were also similar between both groups (2.42±0.65 (S.D.) for *threat* and 2.44±0.65 (S.D.) for *control*). Presence and neuroticism scores were also used to compute correlations with the subjective scores of each threat question in the *threat* group. Correlations between neuroticism and subjective threat response were computed for each block. In the *first* block (before a threat was introduced), T₁ and T₄ scores were both correlated with participants’ scores of neuroticism (respectively rₛ=0.46, *p*<.01 and rₛ=0.51, *p*<.01). In the *second* block, only T₄ scores were correlated with neuroticism scores (rₛ=0.65, *p*<.0001). In the *third* block, only T₁ scores were correlated with neuroticism scores (rₛ=0.41, *p*<.05). Correlations between threat response and presence scores were also computed. Only correlations in *first* block were found: T₄ scores were correlated with presence scores (r=0.36, *p*<.05).

Considering previous work [30] that explored the potential
relation between the SoE and personality traits, we as well per-
formed correlations between our SoE dimensions scores and
neuroticism scores. Correlations were performed for both *threat*
and *control* groups separately. A strong positive correlation was
found between appearance score and neuroticism scores in the
threat group ($r=0.58, p<.001$) which was also present in the control group but in a lower strength ($r=0.23, p<.05$). Additionally, the sense of self-location was found to be negatively correlated with neuroticism scores in the threat group ($r_s=-0.36, p<.001$).

## 4.5. Demographic Data

From the demographic questionnaire was extracted participants' experience in video games. Descriptive analyses were done to observe the responses of NER questions (about threat but not related to the threat event) and the SoE depending on participants game experience (from “Never Play” to “Expert Player”) (see Figure 11). Game experience was then transformed into a numerical score in order to perform correlations. $T_1$ scores were found to be slightly negatively correlated with game experience ($r_s=-0.17, p<.05$), while Ownership was found to be positively correlated with game experience ($r_s=0.24, p<.001$).

Because previous works highlighted the potential impact of gender on the SoE [30], a two-way anova analysis was performed to investigate its effect on the SoE scores with the between subjects factors Gender and Group. An interaction effect was found between Group and Gender for agency scores ($[F_{1,56}=6.78, p < .05]$) while post-hoc tests showed that agency scores in the threat group were higher for male than for female participants ($p<.05$, see Figure 12).

# 5. Discussion

The main objective of this paper was to investigate the potential impact of threat occurrence and repetition on users' SoE and threat response. More precisely, we hypothesised that a single threat occurrence would decrease participants' SoE towards their avatar, but that threat repetition would tend to decrease it. In addition, we expected threat repetition to cause a decrease in participants response to threat. In this section, we discuss our results regarding the impact on threat responses and the impact of the threat on the SoE. Finally, we discuss the potential impact of threat on participants' behaviour as well as the link between presence and threat.

## 5.1. Threat Responses

Subjective and objective data of threat response were collected for two main reasons. First, we wanted to verify that participants reacted to the threat we had designed, which was validated by both the subjective and behavioral responses. Participants from the threat group significantly reacted to the threat introduction by a fast withdrawal of their hand, visible in the results by a significant speed peak of their dominant hand when the threat occurred (Figure 8). They also rated a strong subjective feeling of fear towards the crusher when it malfunctioned (Figures 6 and 7). Second, we were interested in the impact of threat repetition on the way it was perceived by participants. We indeed had the hypothesis that the repetition of the threat would impact its credibility due to the absence of nociceptive feedback, and that in consequence participants would lose faith in it and stop reacting. However, this was not observed in our results. The subjective ratings regarding the fear induced by the threat were in the third block as high as the ratings in the second block, which

did not support **H4**. Regarding the objective data, speed profiles only highlighted a difference between participants speed profiles in the first threat introduction and all the other threat occurrences. More precisely, the average speed peak remained similar for all threat trials (around 2m/s), but the peak was shifted: the first time the threat occurred, participants took more time to react to the threat than in the other threat trials. While we would have expected the speed peak to decrease along the repetitions of threat, we can notice in Figure 8 that although not significant, the speed peak tends to diminish in the last threat trials (30, 33 and 34). Although some adaptation is observed along the experiment, the current results do not support that the repetition of a threat alters physical threat response (**H5**). Nevertheless, we may wonder whether the number of threat repetitions was sufficient, which is why we address this matter in Section 5.5.

As we can see in Figure 10, our results also highlighted changes in user behaviours in the safe trials that occurred after the threat occurrences. Before the ingot was released ($t \approx 0.6s$), we can observe that the approaching speed increased in both groups. Yet, we can observe that this effect is higher, and significant, in the last trial of the threat group. By increasing the approaching speed, participants seem to have tried to avoid “more” the threat after several threat occurrences. Yet, interestingly the subjective data does not support an increased fear towards the crusher by the end of the experiment. This result is also coherent with results regarding the time that the dominant hand stayed under the crusher. In addition, these time results suggest that an effect on participants behaviour was mainly visible after the threat was introduced twice, not after the first time. We may wonder if this is due to participants believing that the threat will not be introduced again, and therefore not changing their behaviour, while a second occurrence may confirm that they will be several of them. It would thus be interesting to replicate this experience with only one threat and same duration of experience to verify if indeed a certain repetition of threat is needed to trigger change in behaviour.

## 5.2. Threat Occurrences and SoE

The results regarding the subjective measure of the SoE did not show any impact of the threat first occurrence nor of its repetition, which thus does not fulfill our hypotheses (**H1** and **H2**). According to the work of Christensen et al. [12], we expected the fear induced by the crusher malfunctioning to negatively impact the sense of agency of participants. Christensen et al. have indeed shown that fear expectation alters users' sense of agency. Their study was inspired from the work of LeDoux [37], which states that fear is associated with automated behavioural patterns. Indeed, fear commonly induces automatic withdrawal responses or action inhibition (e.g., fleeing or freezing) [12]. We indeed observed such patterns in the participants' response to the crusher malfunction, as visible for instance in Figure 8, which highlights a speed peak when participants moved their hand backward from the machine after a threat was introduced. However, while Christensen et al. found an impact on users' sense of agency, no impact was found in our study on users' sense of agency towards their avatar, nor over their SoE. Nevertheless, we must emphasize two main differences between the
<figure><img src="image_12.png" /><figcaption>Fig. 11. NER questions and Ownership scores depending on participants' game experience.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. Agency scores depending on Gender and Group for all blocks combined.</figcaption></figure>

study of Christensen et al. and our study. First, their study was not conducted in virtual reality, and the sense of agency thus did not refer to the control of a virtual avatar. Moreover, in their study they specifically informed participants that in some blocks of trials, no threat would ever be introduced, and that in other blocks one or several threats might occur. Participants were thus perfectly aware of when they were to expect a threat or feel safe. In our implementation, this was not transparent for participants. In the consent form participants signed, they were briefed that a “malfunction of the crusher” could occur, with no more precision. We must consider that, entering the experiment, participants might have been in a “threat expectation” state. It would thus be interesting to replicate this study being transparent with participants on when a threat could occur or not, e.g., to measure whether we are able to replicate Christensen et al.’s results.

Moreover, our threat was designed as in most embodiment
studies [10, 24], in a way that it would visually affect the in-
tegrity of the virtual body by colliding with it. After verification
in the analysis, we found that over 150 trials with a threat, the
crusher collided 128 times with the dominant hand of partic-
ipants (mean/S.D. time of collision in seconds: 0.21±0.10).
Other times, participants might have withdrawn their hand too
fast, but in all cases participants experienced a vibration on the

controller when the threat happened. This vibration was impor-
tant as it is a common fact that mismatches between what you
see (e.g. an object touching your avatar) and what you feel (e.g.
tactile feedback) decrease the SoE towards the avatar [17]. How-
ever, we must acknowledge that the coherence between visual in-
put and tactile feedback differs within experiences. For instance,
the coherence between visual and tactile is not the same whether
the participants’ hand is virtually brushed while being brushed
simultaneously in the physical world [38], or if the participants’
hand is virtually harmed by a knife while receiving a vibration
in the physical world [23]. The notion of coherence in virtual
environments has been shown to be of great importance to have
participants react realistically to the virtual environment [39]. In
our experiment, no nociceptive feedback was associated with the
virtual threat. For this reason, we expected this lack of coherency
to negatively impact threat response along the threat repetitions
(H5). However, even though participants noticed and reacted to
the threat, the quickness of the threat in our experiment might
have prevented participants from observing the actual collision,
which could be a possible reason why we did not observe a
decrease of the physical response to the threat in the last block.
It would thus be very interesting in future work to investigate
the potential impact of mismatch between tactile feedback and
virtual threat on the SoE.

Our results also highlighted a sequential effect of the repetition
of the blocks on two questions regarding ownership, which was
also present in the control group and therefore did not allow
the full validation of **H3**. However, it remains unclear weather
the scores were impacted by the duration of the experiment
(i.e., the duration would then impact negatively the illusion of
ownership), or by the repetition of questionnaires regarding the
sense of embodiment (i.e., answering the questions may lead to
an increased attention given to the virtual body, which may put
in evidence artefacts that would affect the illusion).

5.3. *Presence, Personality and Threat*

Presence has an interesting bidirectional relation with anxiety
and emotional state as the sense of presence was shown to be
increased by affective content [14, 13], but also to influence
the perception of fear in VR [32]. Nevertheless, our results did
not highlight such impacts, while we must acknowledge that
we only compared between threat and control groups presence scores that were gathered at the end of the experiment. Although this was not the main focus of this paper, we believe it would be interesting to deeper investigate in future studies the relation between affective content and the sense of presence in VR.

Our results highlighted several correlations between neuroticism scores and threat subjective response, showing that overall, participants with a higher tendency to experience anxiety seem to be more likely to fear a virtual threat, which is coherent with previous work [30]. We also found that presence could be correlated with neuroticism scores, which goes in the sense of previous literature on the subject [40].

Interestingly, some correlations were found between two dimensions of the SoE and neuroticism scores. The appearance scores were positively correlated with neuroticism scores, meaning that participants that tend to experience anxiety as well as anger and depression were more likely to feel closer in appearance to their avatar, contributing to higher SoE. However, neuroticism scores were also found negatively correlated with the sense of self-location, showing that the same type of persons that would possibly experience higher appearance resemblance with their avatar, are also likely to experience lower sense of self-location towards their avatar, which would tend to lower their SoE. It is to our knowledge the first time that neuroticism is correlated with SoE dimensions. While it is at this stage difficult to understand why such personality trait could influence users' SoE, we believe it is important to go towards a better understanding of individual differences influencing the SoE, in order to potentially be able in the future to design avatars specifically adapted to certain types of users. We thus think this should be investigated further in future works with higher sample of participants, since the number of participants remained limited in this study to support strong evidence from the results.

## 5.4. Game Experience and Gender

The potential influence of users' game experience of the way they perceive several entities of virtual environments is becoming more and more studied in latest works. The sense of presence in immersive VR was found to be positively correlated with users' game experience [41], while the sense of agency towards a virtual avatar was found to be positively correlated with game experience. In this study, we found that participants with high game experience seemed less likely to experience fear towards the virtual threat, although it is a very small effect. While some video games can be very violent and intense in terms of fear induction, we may wonder how a long exposure to these kind of games could impact the perception of virtual threats, which would be interesting to explore in future work. Furthermore, while no correlation was found between agency and game experience in this paper, such user characteristics appeared positively correlated to the sense of ownership, reinvigorating the need to explore how game experience may influence users' SoE in VR. Finally, our results also highlighted a small effect of gender on the sense of agency, which corroborate previous works' findings on the influence of gender on the SoE [41].

## 5.5. Limitations

When designing our experiment, a number of choices were made regarding the implementation of the threat. As presented in Section 2, there exist many different kinds of threats in the literature in embodiment studies. We decided to make coherence the main aspect of our threat, placing it in a realistic context where an accident is likely to happen. In addition, our threat was associated with auditory and tactile feedback and was conceived to collide with the virtual body. All those choices made in the experiment can potentially bias the results, and therefore, it would be interesting to validate that our results generalize to other threats, or at least to similar types of threats. For instance, while we expected participants to be conscious of the collision of the threat with their virtual body, we believe that replicating this experiment with a threat that makes the collision more obvious would be interesting.

Furthermore, the length of the experiment could have also played a role in the results. Indeed, although not significant, we observed adaptation patterns that appeared in the motion profiles. In the experiment, we decided to keep a low number of threat repetitions as most experiments keep a low number of threat repetitions and to reduce fatigue. Nevertheless, changes in the physical reactions of the participants might become more obvious with a longer exposure, and it remains unclear if these changes would remain between VR sessions.

# 6. Conclusion

In this paper, we explored the potential impact of threat occurrence and repeatability on users' Sense of Embodiment (SoE) and threat response. The main results show that the introduction of a threat does not alter users' SoE but might change their behaviour while performing a task after the threat occurrence. In addition, threat repetitions did not show any effect on users' subjective SoE, or subjective and objective responses to threat. Furthermore, it seems that the changes on physical responses to threat are modulated by threat repetitions, although the results are not always significant. We may therefore consider that there is an adaptation associated with threat repetition, and that it remains unclear if this adaptation would continue if the experiment would last longer. Finally, we found that individual differences between users, such as users' neuroticism trait, gender or game experience, could influence their threat responses and also their SoE. Taken together, our results suggest that embodiment studies should expect potential changes in participants behaviour while doing a task after a threat was introduced, but that threat introduction and repetition do not seem to impact the subjective measure of the SoE (user responses to questionnaires) nor the objective measure of the SoE (behavioural responses to threat towards the virtual body).

## Acknowledgments

We wish to thank participants of our experiment. This work was sponsored by the Region Bretagne and the Inria Avatar Challenge.
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