Nawel Khenak
email: nawel.khenak@limsi.fr

Jeanne Vézien

David Théry

Patrick Bourdot
email: patrick.bourdot@limsi.fr

Spatial Presence in Real and Remote Immersive Environments and the Effect of Multisensory Stimulation 1

. Besides, objective metrics based on user performance and behavioral indicators were logged. Results indicate that participants experienced a higher sense of spatial presence in the remote environment (hyper-presence), and a higher ecological validity. In contrast, objective metrics prove higher in the real environment, which highlights the absence of correlation between spatial presence and the objective metrics used in the experiment. Moreover, results show the benefit of adding audio rendering in both environments to increase the sense of spatial presence, the performance of participants, and their engagement during the task.

Introduction

Spatial presence is commonly defined as the sense of "being there" [START_REF] Biocca | The cyborg's dilemma: Progressive embodiment in virtual environments[END_REF]. The term "there" refers either to a virtual place or a real remote place [START_REF] Wirth | A process model of the formation of spatial presence experiences[END_REF].

Therefore, spatial presence encompasses the ability of users to experience a sense of presence in any environment in which they are transported. Namely, (i) if the environment is real and non-mediated, the user experiences a natural presence [START_REF] Biocca | Criteria and scope conditions for a theory and measure of social presence[END_REF]; (ii) if it is real and mediated, the user experiences a remote presence or "telepresence" [START_REF] Steuer | Defining virtual reality: Dimensions determining telepresence[END_REF]; and (iii) if the environment is virtual, i.e., computer generated, the user experiences a virtual presence [START_REF] Sheridan | Musings on telepresence and virtual presence[END_REF]. This classification was proposed by [START_REF] Zhao | Reconceptualizing presence: differentiating between mode of presence and sense of presence[END_REF], who emphasized the importance of comparing the sense of presence between the different types of environments in order to better understand this phenomenon.

Thus, previous studies have focused on the evaluation of the sense of virtual presence. [START_REF] Usoh | Using presence questionnaires in reality[END_REF] carried out an experiment in which 10 participants had to find a box. The participants were located either in a real office or in a simulated virtual environment (VE) of the same office rendered over the Virtual Research VR4 head-mounted display (HMD). The authors assessed spatial presence by using the ITC-SOPI questionnaire [START_REF] Lessiter | A cross-media presence questionnaire: The ITC-Sense of Presence Inventory[END_REF] and found significantly higher presence values for the real environment.

Similarly, [START_REF] Mania | Connections between lighting impressions and presence in real and virtual environments: an experimental study[END_REF] used Slater's presence questionnaire [START_REF] Slater | The influence of body movement on subjective presence in virtual environments[END_REF] to assess the level of presence in a memory task experiment. They compared a real seminar room and a simulated virtual environment of the same room rendered over an HMD. The results showed a higher presence score in the real environment.

More recently, [START_REF] Busch | Being there for real: presence in real and virtual environments and its relation to usability[END_REF] assessed spatial presence using the ITC-SOPI questionnaire. They evaluated the use of a mobile navigation application in a real laboratory environment and a virtual simulation of the same laboratory rendered in a five-sided Cave Automatic Virtual Environment or CAVE [START_REF] Cruz-Neira | The CAVE: audio visual experience automatic virtual environment[END_REF]. They experimented with 65 participants and found no statistical differences in the sense of spatial presence between the environments. Following the same procedure, [START_REF] Brade | Being there again-presence in real and virtual environments and its relation to usability and user experience using a mobile navigation task[END_REF] compared a virtual environment rendered in a CAVE and a real environment (a city centre). The results showed no significant differences in the sense of spatial presence between these environments.

On the other hand, regarding remote environments, the evaluation of spatial presence suffers from a lack of interest. This lack could be explained in part by the blurred boundaries between virtual and remote environments that led to confusion between them. Indeed, the rendering of both environments relies on similar user-interface technology. Also, it shares the common feature that the relevant parts of the user's experience at some stage in the process will be transmitted via a digital representation. However, the difference is that physical representation exists in the case of remote environments. Therefore, they have the possibility to induce real-life consequences in contrast to pure virtual environments. Indeed, virtual environments have a few real-life consequences (e.g., objects that can break) as the results of the actions remain restricted to the computer-generated space [START_REF] Lee | Presence, explicated[END_REF].

Researches in other areas of virtual reality (VR) have shown that the users' actions during an experiment can be influenced by the perceptions associated with them [START_REF] Slater | A virtual reprise of the Stanley Milgram obedience experiments[END_REF]. For example, a "perceived" agency can matter more during interactions with virtual humans than a "real" agency [START_REF] Nowak | The effect of the agency and anthropomorphism on users' sense of telepresence, copresence, and social presence in virtual environments[END_REF]. Therefore, this awareness of a real place is essential. Users might be aware that all their actions have a tangible impact on the real world, which in turn may influence their experience of spatial presence. Consequently, an environment is considered remote as soon as it satisfies two criteria: Criterion 1: It allows the remote perception of a real place, either through:

(a) The mediation of sensory stimulation of the space depending on its nature and the activity, and when the technologies afford it (e.g., manipulating objects with haptic force feedback gloves (Fernando, Furukawa, Minamiza, & Tachi, 2013)).

(b) The virtual representation of the space that may be highly realistic (3D reconstruction of the scene and/or physical modeling of real properties), or just symbolic depending on the focus (learning, task efficiency, entertainment [START_REF] Mestre | Does virtual reality enhance exercise performance, enjoyment, and dissociation? an exploratory study on a stationary bike apparatus[END_REF], etc.) (c) Combination of (1.a) and (1.b). This case is more common in augmented reality environments when the goal is to provide remote assistance (e.g. surgical operations in [START_REF] Shenai | Virtual interactive presence and augmented reality (VIPAR) for remote surgical assistance[END_REF], or when there is no real-time data flowing (e.g., space operations in NASA Robonaut: [START_REF] Ambrose | Robonaut: NASA's space humanoid[END_REF], or when the nature of the space makes it difficult to perceive (e.g., submarine and subterranean operations in Remote Operated Vehicles: [START_REF] Lichiardopol | A survey on teleoperation[END_REF].

Criterion 2: Users' actions must have real consequences on the real place, with varying immediacy according to the constraints of teleoperation systems. In addition, users have to be aware of this real impact.

If the first criterion is usually met, the second is often overlooked, creating confusion between remote and virtual environments. For instance, in the [START_REF] Mccall | Measuring presence in virtual environments[END_REF] study on presence, participants were either physically located in a real garden or in a computer-generated environment representing the same garden (viewed inside an HMD). This environment was generated through the rendering of photographic images of the real garden (criterion 1.b met). However, the participants had no impact on the actual garden because the captured images were processed before the experiment with no real-time rendering (criterion 2 not met). Consequently, the photo-realistic environment was virtual, not remote.

Another reason that may explain the lack of studies on spatial presence in remote environments is the low level of sensory fidelity that systems could provide at that time. The sensory fidelity, also referred to as immersion (Slater, Likanis, Usoh, & Kooper, 1996), is determined by the quality, the richness, and the consistency of each sensory channel used by the system (Schubert, Friedmann, & Regenbrecht, 1999a). Its relationship with spatial presence has been a topic of considerable theoretical discussion and empirical investigations for over 25 years. The reader is referred to Cummings & Bailenson's paper (2016) for a detailed review of the effect of immersive system technology on presence. Thus, it is now generally accepted that immersion plays a significant role in the emergence of spatial presence through the construction of a mental representation of the environment in which users are located [START_REF] Bowman | Virtual reality: how much immersion is enough?[END_REF][START_REF] Wirth | A process model of the formation of spatial presence experiences[END_REF]. In other words, whether in virtual or remote environments, systems of higher immersive quality elicit a greater sense of presence. Yet, the impact of immersion is more valuable when assessing spatial presence in remote environments. This is because of the importance of providing high quality natural sensory channels to allow users to experience an immersive situation similar to that in a real (non-remote) environment [START_REF] Sheridan | Musings on telepresence and virtual presence[END_REF]. However, tools were missing that enable researchers to develop remote environments facilitating a high sensory fidelity experience. With recent technological advances, including visual quality of HMDs, sound spatialization, and overall system latency reduction, researchers have been able to develop modern remote environments with high sensory fidelity. Such environments could provide a high perception of the real world. Therefore, assessing spatial presence in real and remote highly immersive environments could be very interesting. By comparing them, one can isolate the effect of modern VR technology on the sense of presence and truly assess the impact of immersion.

Hence, the main goal of the present study is to assess the user's feeling of spatial presence in a remote immersive environment (presented with an HMD, following the criteria (1.a) and ( 2)) and compare it with the sense of spatial presence in a real environment. Also, the study aimed to evaluate the effect of immersion by comparing the environments in almost similar immersive (visual and auditory) conditions.

The remainder of this paper is divided into three main sections. The first section provides an overview of related work highlighting the research focus of the paper. The second section describes the design of the user experiment, followed by the evaluation protocol and the results. The third and last section concludes with an interpretation of these results and proposes new perspectives.

Related Work

Spatial presence in remote environments: Telepresence

Spatial presence in remote environments or remote presence is about the user's perception of a real environment and real consequences when it is mediated and/or virtually represented by means of technologies. The term takes its origin from "telepresence" [START_REF] Steuer | Defining virtual reality: Dimensions determining telepresence[END_REF] which was first employed in 1980 in the context of teleoperation systems, designating remote task manipulation as the "illusion of being transported via telecommunication systems to a real, physical location, which could then be experienced synchronously from afar" [START_REF] Minsky | Telepresence[END_REF]. Later, Akin, Minsky, Thiel, and Kurtzman (1983, vol n°1, pp. 3) used telepresence to describe the specific situation when "the manipulators have the dexterity to allow the operator to perform normal human functions" and argued that the feeling of presence is reached when "the operator receives sufficient quantity and quality of sensory feedback". Since then, telepresence has been of particular importance to robotics researchers.

Many studies highlighted its value in the transmission of information, especially concerning the spatial perception of remote sites [START_REF] Sheridan | Musings on telepresence and virtual presence[END_REF][START_REF] Schloerb | A quantitative measure of telepresence[END_REF]. Also, they emphasized the importance of providing teleoperators with spatial cues to enhance their sense of telepresence. It is within this context that [START_REF] Muhlbach | Telepresence in videocommunications: A study on stereoscopy and individual eye contact[END_REF] first coined the term "spatial presence" in remote environments. They assumed that telepresence was affected by two variables: (1) communicative presence, defined as the degree to which reciprocal signals (verbal and nonverbal) are transmitted, and (2) spatial presence, defined as the degree to which spatial cues about the remote site are transmitted. They conducted an experiment to investigate how spatial (auditory and visual) cues affect the impression of spatial presence and telepresence in a video conference system. They collected questionnaire data and concluded that spatial cues enhance spatial presence as well as telepresence.

Similarly, in the context of virtual environments, [START_REF] Biocca | The cyborg's dilemma: Progressive embodiment in virtual environments[END_REF] introduced the term "physical presence" and defined it as the sense of being physically transported in virtual or mediated environments. This definition was quickly adopted in the VR community as referring to spatial presence in virtual environments (Schubert et al., 1999a).

Consequently, spatial presence is now referring to the sense of "being there" in both virtual and remote environments [START_REF] Regenbrecht | Real and illusory interactions enhance presence in virtual environments[END_REF][START_REF] Schuemie | Research on presence in virtual reality: A survey[END_REF][START_REF] Wirth | A process model of the formation of spatial presence experiences[END_REF]. Furthermore, telepresence, which was initially the origin of the concept, is now used to define spatial presence in the specific context of remote environments as remote spatial presence.

Assessing the degree of spatial presence

Research in remote environments has mainly focused on optimizing the technological aspects of teleoperation systems to improve user performance [START_REF] Yamaashi | Beating the limitations of camera-monitor mediated telepresence with extra eyes[END_REF]. These systems were designed in the robotic field (e.g., space operations, [START_REF] Ambrose | Robonaut: NASA's space humanoid[END_REF], the medical field (e.g., surgical applications, [START_REF] Taylor | Medical robotics and computer-integrated surgery[END_REF], and the industrial domain (e.g., assembly operations, [START_REF] Radi | Telepresence in industrial applications: implementation issues for assembly tasks[END_REF] to name a few. The evaluation of spatial presence as a subjective feeling was instead the focus of researchers in virtual environments. Indeed, a large part of these studies was concerned about identifying the factors influencing presence, its usefulness in VR applications, and especially, the tools for assessing this feeling [START_REF] Mestre | On the usefulness of the concept of presence in virtual reality applications[END_REF]. Therefore, the measures proposed in the literature are virtual context-related.

Moreover, the way presence is assessed depends on the framework used to define it [START_REF] Schuemie | Research on presence in virtual reality: A survey[END_REF]. As the notion of spatial presence has been redefined multiple times, the methods of evaluation have also changed. Consequently, several methods to measure presence in the context of virtual environments exist. These methods can be divided into subjective (mostly using questionnaires) and objective measures, with post-experiment questionnaires being the most common approach for assessment.

The following subsections summarize the most widely used methods and measurements (for more details, see Lombard, Biocca, Freeman, IJsselsteijn, & Schaevitz (2015, pp. 139-185). The idea of this paper is to use the same measurements to assess spatial presence in remote environments.

Subjective questionnaires. Many presence questionnaires have been proposed since the early days of presence research. [START_REF] Slater | The influence of dynamic shadows on presence in immersive virtual environments[END_REF] proposed a onedimensional questionnaire in which the presence score is taken as the number of answers that have a high score. [START_REF] Kim | Telepresence via television: Two dimensions of telepresence may have different connections to memory and persuasion[END_REF] designed a questionnaire based on their metaphor of transportation comprising two dimensions: (i) arrival, being present in the mediated environment, and (ii) departure, not being present in the unmediated environment.

Shortly after, [START_REF] Witmer | Measuring presence in virtual environments: A presence questionnaire[END_REF] designed the Presence Questionnaire (PQ), based on three factors: (i) involvement, (ii) behavioral fidelity of interactions and control of locomotion and (iii) the user's ability to concentrate on the tasks. [START_REF] Schubert | Decomposing the sense of presence: Factor analytic insights[END_REF] constructed the Igroup Presence Questionnaire (IPQ) by combining both the Slater et al.'s (1995) and Kim & Biocca's questionnaire. It is based on eight factors. Three of them are directly concerned with presence: (i) the sense of spatial presence, (ii) the involvement into the environment, and (iii) the sense of reality attributed to the virtual space.

The others are considered as immersion and interaction variables that may influence presence. [START_REF] Usoh | Using presence questionnaires in reality[END_REF] developed the SUS (Slater-Usoh-Steed) questionnaire based on several questions all variations on one of three themes: (i) the sense of being in the virtual environment, (ii) the extent to which the environment becomes the dominant reality, and (iii) the extent to which the environment is remembered as a "place".

However, one of the most validated questionnaires is the Independent Television Commission Sense of Presence Inventory (ITC-SOPI) by Lessister et al. (2001). It has been widely used within the literature and was shown to produce reliable results [START_REF] Busch | Being there for real: presence in real and virtual environments and its relation to usability[END_REF][START_REF] Gorini | The role of immersion and narrative in mediated presence: the virtual hospital experience[END_REF][START_REF] Tang | Comparing differences in presence during social interaction in augmented reality versus virtual reality environments: An exploratory study[END_REF][START_REF] Usoh | Using presence questionnaires in reality[END_REF]. It is based on four factors: (i) the sense of spatial presence, (ii) the user's engagement, (iii) the ecological validity of the environment, and (iv) the negative effects (such as cybersickness). This questionnaire has the advantage to be quite easy to administer and score (Lombard et al., 2015).

Despite having proved their usefulness for measuring the sense of presence, questionnaires are subjective measures highly dependent on users' experiences. In addition, they do not provide a continuous measurement of presence during the experiment because users complete questionnaires at the end of their experience, so as not to cause breaks that reduce the sense of presence. Therefore, objective non-invasive metrics have been designed to assess the sense of presence continuously during the experiment. These metrics are generally combined with questionnaires.

Physiological measures.

In parallel to subjective questionnaires, an approach based on physiological indicators sought to establish reliable and validated measures for spatial presence. Thus, studies on the reliability of physiological indicators to measure presence such as changes in Heart Rate (HR) [START_REF] Meehan | Physiological measures of presence in stressful virtual environments[END_REF] and skin temperature and conductance-based on Electrodermal Activity (EDA) [START_REF] Wiederhold | An investigation into physiological responses in virtual environments: an objective measurement of presence[END_REF][START_REF] Wiederhold | Fear of flying: a case report using virtual reality therapy with physiological monitoring[END_REF] provided promising results. For example, Meehan and colleagues [START_REF] Meehan | Review of four studies on the use of physiological reaction as a measure of presence in stressful virtual environments[END_REF] showed that in a stressful virtual environment depicting a pit room, changes in HR correlated positively with self-reported presence.

However, these measurements require a baseline comparison for each participant, which means a considerable effort in study design. Besides, it has been shown that additional equipment to measure physiological responses, such as Electroencephalography (EEG) to measure brain responses [START_REF] Baumgartner | Neural correlate of spatial presence in an arousing and noninteractive virtual reality: an EEG and psychophysiology study[END_REF], can be a cause of breaks in presence [START_REF] Brogni | More breaks less presence[END_REF]. Moreover, this equipment is more efficient when participants do not move [START_REF] Nalivaiko | Cybersickness provoked by head-mounted display affects cutaneous vascular tone, heart rate and reaction time[END_REF], which reduces the scope of possible experiments.

Thus, physiological indicators exist that could become potentially reliable measures for presence (for more details, see Lombard et al., 2015, pp. 150-185). Nevertheless, the correlation between them and the sense of presence is not yet firmly established [START_REF] Freeman | When presence and emotion are related, and when they are not[END_REF]. A common approach to achieve this goal is to compare results from this kind of measures with outcomes from presence questionnaires [START_REF] Bracken | Revisiting the use of secondary task reaction time measures in telepresence research: exploring the role of immersion and attention[END_REF][START_REF] Freeman | Using behavioral realism to estimate presence: A study of the utility of postural responses to motion stimuli[END_REF].

Behavioral measures.

Researchers also investigated the relationship of behavioral indicators with presence [START_REF] Blascovich | Immersive virtual environment technology as a methodological tool for social psychology[END_REF][START_REF] Slater | A note on presence terminology[END_REF]. In a survey, [START_REF] Schuemie | Research on presence in virtual reality: A survey[END_REF] showed that virtual experiences could evoke the same reactions as real experiences.

These reactions can be represented by postural adjustments and body movements. For example, [START_REF] Freeman | Using behavioral realism to estimate presence: A study of the utility of postural responses to motion stimuli[END_REF] measured postural responses (lateral movement) to a video sequence filmed from the hood of a car traversing a rally track. Results showed no significant correlation between the postural response and presence. However, the degree of presence was determined by a single question. In addition, presence may have been biased by the novelty of the technology used (stereoscopic display consisting of two-color monitors equipped with polarized filters). They concluded that such postural responses could be useful in corroborating subjective ratings of presence, but not in replacing them.

Similarly, [START_REF] Lepecq | Afforded actions as a behavioral assessment of physical presence in virtual environments[END_REF] measured body movement during an experiment in which participants had to walk through either a virtual or a real aperture. Results showed that participants swiveled their bodies similarly in both real and virtual situations. Moreover, the body rotation was a function of aperture and shoulder width. [START_REF] Bailenson | Interpersonal distance in immersive virtual environments[END_REF] conducted two experiments. In the first experiment, participants were standing while a walking virtual human approached them and invaded their personal and body space. In the second experiment, the virtual human was stationary, while the participants had to approach him. The metric used was participants' movements and posture changes as the virtual human invaded their space (how far participants moved away from the virtual human in the first experiment), or as they were approaching it (in the second experiment). More precisely, researchers examined the degree of interpersonal distance that participants maintained between themselves and virtual humans in both situations. Results from both experiments showed that participants exhibited patterns of interpersonal distance behavior relative to virtual humans similar to those from researches with actual humans. However, these measurements are more related to the user's sense of social presence (the sense of "being with", [START_REF] Biocca | The cyborg's dilemma: Progressive embodiment in virtual environments[END_REF], which is beyond the scope of this paper. For more information about interpersonal space in virtual environments, see [START_REF] Lachini | Peripersonal and interpersonal space in virtual and real environments: Effects of gender and age[END_REF].

The reactions evoked by the environment can also be reflex responses to virtual danger [START_REF] Slater | Place illusion and plausibility can lead to realistic behaviour in immersive virtual environments[END_REF]. [START_REF] Maïano | Affective, anxiety and behavioral effects of an aversive stimulation during a simulated navigation task within a virtual environment: A pilot study[END_REF] found a correlation between aversive stimuli (fire, smokescreen, and warning alarm) appearing in a virtual environment, the degree of selfreported anxiety of participants navigating in this environment, and the way they moved away from "danger".

Other measures that have been studied as potential behavioral indicators are the attention-based measures. In their studies, [START_REF] Bracken | Telepresence and attention: Secondary task reaction time and media form[END_REF], 2014) investigated the relationship between participants' sense of presence measured by a questionnaire and their reaction time to some audio distraction cues while watching a movie. A significant correlation was found between this reaction time and two subcomponents of presence: immersion and attention.

Therefore, observational methods to measure presence are promising. Yet, more investigation is still needed to establish reliable measures based on people's reaction and behavior.

Performance measures. Close to behavioral measures (and, sometimes even considered as a category of the latter), performance measures may provide an objective indicator of spatial presence in immersive environments [START_REF] Cummings | How immersive is enough? A meta-analysis of the effect of immersive technology on user presence[END_REF][START_REF] Laha | Effects of immersion on visual analysis of volume data[END_REF]. However, care must be taken to compare environments with the same degree of similarity.

Thus, the degree of similarity, also referred to as "the fidelity of the environment" [START_REF] Dalley | The use of high-fidelity human patient simulation and the introduction of new anesthesia delivery systems[END_REF]) can strongly affect performance. For example, [START_REF] Mcmahan | Separating the effects of level of immersion and 3D interaction techniques[END_REF] found that object manipulation can be successfully performed with lower fidelity VR technologies, such as less costly displays, with no loss of efficiency. In particular, it is essential to use similar interactive techniques across environments as the way people interact in the environment can influence their performance [START_REF] Carvalho | Different interaction paradigms for different user groups: an evaluation regarding content selection[END_REF][START_REF] Mcmahan | Separating the effects of level of immersion and 3D interaction techniques[END_REF]. Indeed, some interaction paradigms make better use of human's innate skills rather than imposing new learning processes (such natural gesture-based interaction), and others rely on acquired skills and previous users' experiences to improve performances (such as joystick interaction in games), without affecting the sense of presence.

In addition, different types of tasks can be used to evaluate performance. The most common types are the memory/learning tasks and the manipulation tasks (for a taxonomy, see [START_REF] Poupyrev | A framework and testbed for studying manipulation techniques for immersive VR[END_REF]. Depending on the task, some interaction paradigms may be more efficient, which may increase task performance.

Furthermore, the relationship between presence and performance may also be highly dependent on the measures of performance used [START_REF] Picciano | Beyond student perceptions: Issues of interaction, presence, and performance in an online course[END_REF]. Many studies investigated this relationship with different measures of performance. For example, [START_REF] Stanney | Human performance in immersive virtual environments: Effects of exposure duration, user control, and scene complexity[END_REF] conducted a study in which participants located in a virtual environment had to complete the maximum amount of basic tasks. Results showed that best performances -measured by the number of tasks completed and the time needed to complete each task -resulted in a higher sense of presence. [START_REF] Youngblut | The relationship between presence and performance in virtual environments: Results of a VERTS study[END_REF] found a significant relationship between presence measured by the SUS questionnaire and user's performance in a learning task.

In contrast, Mania and Chalmers' study (2004) showed that the sense of presence was not correlated with task performance measured by the ability to acquire knowledge during a lecture. More recently, [START_REF] Stevens | The relationship between presence and performance in virtual simulation training[END_REF] studied the relationship between presence and performance in virtual simulation-based training. Participants had to destroy as many virtual enemy forces as possible. Results showed a moderate relationship between the degree of presence measured with the PQ questionnaire and participants' performance, regardless of their level of expertise and the visual display used (HMD or flat-screen display). [START_REF] Cooper | The effects of substitute multisensory feedback on task performance and the sense of presence in a virtual reality environment[END_REF] conducted a study to evaluate the effect of different sensory cues on performance.

Participants (n = 17) performed a wheel change in an immersive virtual environment (rendered on a planar stereoscopic display screen 6m in length and 2.1m in height). Auditory, haptic and visual cues were signaling critical events during the task. The time taken to complete the task was used as an objective performance measure. Results indicated a moderate relationship between the performance measure and presence.

Thus, studies on presence and performance provided promising results. With technological advances in high fidelity user-interfaces in term of interaction, the difference between real and remote environments is narrowing, leading researchers to further explore the relationship between presence in remote sites and task performance.

Multisensory stimulation effect on spatial presence

Another particularly relevant aspect of presence research for immersive environments is the evaluation of potential positive effects of multisensory stimulation. For instance, Mania and Chalmers (2001) compared the sense of presence -using Slater's questionnaire -between a real environment, an immersive (visual and audio) virtual environment, and an audio-only virtual environment. Participants (n =18) reported higher levels of presence for the real environment as compared to the immersive virtual environment, and a higher level of presence for the immersive (visual and audio) virtual environment as compared to the audioonly virtual environment. [START_REF] Larsson | When what you hear is what you see: Presence and auditory-visual integration in virtual environments[END_REF] hypothesized that consistency across sensory renderings in terms of matching the visual space to the auditory space is crucial for the sense of presence. They carried out an experiment in which 30 participants were exposed to four different audio-visual conditions with varying degrees of auditory-visual consistency (one purely visual and three auditory-visual). In a presence questionnaire, participants rated the auditory-visual conditions as inducing significantly higher presence than the condition with only visual information. However, no significant differences in presence ratings between the three auditory-visual conditions were found. [START_REF] Cooper | The effects of substitute multisensory feedback on task performance and the sense of presence in a virtual reality environment[END_REF] also examined the effect of the different sensory cues (audio, visual and haptic cues) on subjective ratings of presence.

Results indicated significant main effects of audio and tactile cues on presence.

Thus, the benefits of multisensory cues have been demonstrated in virtual environments.

The next step should be to investigate to what extent the results hold for real and remote environments, and whether the multisensory stimulation effect is different between such environments to improve presence.

Objective of the study

As previously mentioned, this paper aims at evaluating the sense of spatial presence in remote environments. A user experiment is presented that compares the sense of spatial presence in a real place with the presence experienced in a remote mediation of the same place. Real-time sensory restitution in the remote location is achieved using an HMD coupled with a 360° camera and a 3D audio system. Besides, the paper investigates the effect of sensory channels on spatial presence by comparing a "visual-only" and a "visual and audio" immersive condition in such real and remote environments. The task performed during the experiment is a 3D pointing task. Spatial presence and three subcomponents (ecological validity, engagement, and negative effects) are evaluated using the ITC-SOPI questionnaire [START_REF] Lessiter | A cross-media presence questionnaire: The ITC-Sense of Presence Inventory[END_REF]. A performance measure and behavioral observations on users' activity are also logged to study their relationship with the feeling of spatial presence and whether they can be conceivable tools for objective assessment of this sense.

The main hypotheses are that the environments and sensory renderings will affect the sense of spatial presence and its components, as well as the task performance and user's behavior, with a positive effect of the real environment and the "visual and audio" condition.

The experiment is described in more details below.

Experiment

Experimental Design

The real and the remote environment. Two rooms with a very similar layout (dimension = 4,25m x 3,85m) were used to represent the real and the remote environment in the experiment:

(1) An "operating room" (see Figure 1), representing a rectangular office where 12 tablets were attached to the four walls at fixed positions throughout the experiment. These positions were determined using a uniform sampling procedure.

(2) A "teleoperating room" where a teleoperation system was located that allowed participants to be remotely transported in the operating room. The pointing task. Pointing tasks have been the focus of many studies that showed their effectiveness to evaluate users' behavior and capture performance (see [START_REF] Raynal | Towards unification for pointing task evaluation in 3D desktop virtual environment[END_REF], for a detailed review). Consequently, a 3D pointing task was chosen for the experiment. More specifically, the task was to point as fast as possible a sequence of images that were displayed sequentially (i.e., one image at a time) on the tablets. The order of the images displayed was determined randomly; nevertheless, it remained the same for all participants. In addition, the images randomly depicted different types of worldwide well-known animals: cat, dog, duck, and pig, and sounds corresponding to the animal displayed were played from the tablets. Thus, the participants were seated in the middle of one of the two rooms on a swivel chair. This chair allowed them to see the different walls of the room (by swiveling), and therefore see the tablets. Then, the participants had to perform the pointing task (see Figure 2):

(1) Either directly if they were located in the operating room, in which case they were experiencing a sense of natural presence.

(2) Or remotely through the teleoperation system if they were in the tele-operating room, in which case they were experiencing a sense of telepresence.

Method

Participants. In total, 28 participants took part in the evaluation (19 men, nine women, M age = 27.4 years, SD = 4.4 years, age ranging 22-39 years). All participants had a normal or corrected view and non-impaired hearing. In addition, they all came from academic or scientific fields: 18 participants were students while the remaining 12 came from different scientific fields of the university. Regarding the level of experience in VR, only four participants had never used an HMD before and reported no experience in VR systems, 17 reported that they were beginners, four reported intermediate expertise, and five considered themselves experts. Materials. The general setting is described in Figure 3.

The visual set-up. In the operating room, the participants experienced a natural visual and auditory stimulation. For the tele-operating system, remote visual capturing was obtained with a Ricoh Theta-V 1 360° panoramic camera placed at the center of the operating room.

Images were streamed at a resolution of 3840 x 1920 at 30 Frames Per Second (FPS) to Unity's real-time, the real-time rendering engine 2 v2018.1.0 (consisting basically in a large spherical texture), then visualized in an HTC Vive Virtual Reality headset 3 , provided a 90Hz refresh rate. Remote action was made possible by tracking participants' hand with a Leap Motion sensor attached to the headset. Then, a corresponding hand avatar was displayed to visually simulate the real hand as it was acting in the scene of the operating room (see Figure 3[d]). Informal interviews were collected from pilot tests on five participants. These interviews showed that all participants considered the avatar hand as their personal hand. Because the field of view of the VR headset was limited to about 110°, similar viewing conditions were created in the operating room by making the user wear a headset mockup.

The mask of the headset mockup was configured using a calibration procedure summarized in Figure 4. With the head still and one eye closed, a trained operator looked at a calibration target consisting of vertical lines drawn on a vertical board, positioned at a fixed distance, thus providing tangible support for measuring the field of view. Looking straight to the zeroline, different sizes of eye apertures were trialed, until the perceived field of view matched the 55° line on each side. The measure is subjective, but actually closely matches the one achieved in an identical virtual setup with the Vive HMD. The procedure was repeated for each eye, to create the finale mockup of Figure 5. This calibration procedure ensured that the potential FOV effects were controlled. In particular, the number of tablets that were visible at any given point of the evaluation was the same regardless of the room in which the participants were located. Again, the informal interviews showed that the mockup suited well the participants.

Figure [5] here.

The tracking system. In order to record the motion of each participant in the operating room, an infrared tracking system was set up, consisting of a six-camera ARTTrack5 4 tracking network. Participants wore infrared markers on both their pointing hand and the headset (Figure 3[a,c]). The pointing gestures were recorded at 60Hz with millimetric accuracy. In the teleoperating setting, head tracking was obtained with the Vive lighthouse system associated with the HTC-Vive headset, while hand tracking was provided by the Leap Motion sensor attached to the headset that recorded the hand motion in the headset coordinate frame, and then translated into the world coordinates.

To determine where the participants were pointing, a vector was calculated based on two virtual points: a first point located just in front of the participants' eyes and a second point located on the top of their pointing hand. In order to avoid the misinterpretation of the pointing gesture and to obtain the correct pointing vector [START_REF] Herbort | Spatial (mis-) interpretation of pointing gestures to distal referents[END_REF], participants were instructed to close their hands while targeting the images so they do not use one of their fingers to point. Then, pointing to an image was considered by the system as successful as soon as an intersection between the pointing vector and an invisible virtual sphere representing the location of the corresponding tablet was detected.

The audio setup. In the operating room, the participants experienced a natural 3D sound, as each tablet surrounding them was effectively emitting sound. To reproduce a similar auditory stimulation in the tele-operating room, a 1st-order Ambisonic microphone (Tetramic 5 ) was used to capture the sound in the operating room. This sound was then rendered binaurally over headphones (Closed Sennheiser HD 280) in the tele-operating room.

A patch designed in the Max 6 environment managed the entire audio processing pipeline. In particular, the SPAT library [START_REF] Carpentier | Twenty years of Ircam Spat: looking back, looking forward[END_REF] was used to perform the conversion from the Tetramic's recording format (A-format) to the widely accepted Ambix format, and subsequently the decoding from ambisonic to binaural rendering using the virtual speaker array approach [START_REF] Noisternig | 3D binaural sound reproduction using a virtual ambisonic approach[END_REF]. The SPAT also added a small amount of reverberation to simulate the acoustics of the operating room (1.2 second of reverberation). Besides, the source aperture was fixed to 90°. Finally, the RMS level was set equal in both rooms to 75 dB (A).

Procedure.

Independent variables. A mixed-design study was run with two fixed factors:

ENV: The type of environment with two modalities labeled REAL and REMOTE, representing respectively the situation where a participant is performing the task while being physically located in the operating room (i.e., experience natural presence), and the situation where a participant is operating remotely from the tele-operating room (i.e., experience telepresence). This factor was run following a between-subject design, first to minimize the learning effect on the task between the two conditions, but also to avoid tiring the participants. SEN: The sensory mode with two modalities labeled WITH SOUND and NO SOUND, representing respectively the multisensory condition -visual with audio rendering -, and thevisual-only -silent condition. This factor was run following a within-subject design.

The order of both factors was counterbalanced across participants using a Latin Square Design 7 to control the effects of the assignment of participants and condition order.

User evaluation. The evaluation was approved by the local ethics and research committee of the university (CER 8 ). Each participant performed the task in only one of the two environments (REAL or REMOTE condition) depending on its order. However, they had to go successively through the two sensory modes WITH SOUND and NO SOUND, no matter the environment in which they operated. Therefore, all participants performed the task twice, which consisted of three parts:

• The training: The participants took a training session in order to get familiar with the setup, the instructions of the evaluation, the pointing task, and the constraints (keeping a fixed position and the use of the same closed hand to point all along with the evaluation).

• The experiment: The participants had to find and point on the images displayed on the tablets one after the other (as soon as an image was pointed at, it disappeared, and another one appeared in another location). They had to point to the maximum number of images, in a time limit of 3 minutes.

• The post-assessment (questionnaire): Once the evaluation was over, the users' feeling of presence was assessed by administering the ITC-SOPI questionnaire.

The duration of the evaluation (training, experiment, and post-questionnaire) was approximately 20 minutes. As the participants repeated the task twice (one for each sensory modes), the total duration to complete the experiment was 20 x 2 ~ 40 minutes.

Data collection. In total, 56 trials were registered: 1 [ENV] x 2 [SEN] x 28 participants.

For each trial, the total number of images pointed, and the trajectory path of both head and hand of participants were logged in the room coordinate frame. Finally, aside from the ITC-SOPI questionnaire, personal demographic variables (age, gender, profession/education, and experience with VR systems) were also collected. Therefore, the study comprised seven different dependent variables which can be grouped as follows:

• Four subjective variables:

Responses to the 5-point Likert scale ITC-SOPI (1 = strongly disagree, 5 = strongly agree) yielded scores for each of the four components of the questionnaire:

(1) SP: perceived spatial presence.

(2) ENG: personal engagement.

(3) ECO: ecological validity of the environment.

(4) NEG: negative effects.

• Three objective variables:

(1) SCORE: the participant's performance that reports the total number of images pointed during the evaluation.

(2) HRHI: the head-related hesitation indicator that reports how many times a participant swiveled in the wrong direction while trying to find the location of an image, and then decided to turn in the opposite direction (i.e., participant's turnaround). A smaller value means that the participant had less hesitation (or very little) to locate the displayed images.

(3) STCI: the smoothness of the trajectory curve indicator that reports how much the participant pointing gesture was "fluid".

The SCORE was directly computed from the participants' results during the pointing task.

Concerning HRHI, it was extracted from participants' head trajectory. Precisely, the indicator was processed -for each tablet to be localized -by calculating the number of times the head trajectory exceeded an angle higher than 90° in one direction, and then higher than 90° in the other direction.

Finally, STCI was extracted by processing the data cloud of participants hand trajectory as follows:

i. The 3D data set was projected on each wall of the room to get 2D data.

ii. A principal component analysis (PCA) was computed on the raw 2D data set to get the variance of each principal components (one for the coordinates on the axis X and the other for the coordinates on the axis Y.

iii. Finally, the ratio of these variances was calculated. The latter represents the smoothness of the trajectory curve indicator.

Hypotheses. The hypotheses were as follows:

H1: The environment will significantly affect the reported responses to the spatial presence questionnaire, the performance measure, and the behavioral indicators. More precisely, it is expected that SP, ENG, ECO scales will be higher in the REAL condition, while the NEG scale will be higher in the REMOTE condition (H1a). SCORE will be higher in the REAL condition (H1b). HRHI will be lower in the REAL condition (H1c). And STCI will be higher in the REAL condition (H1d).

H2:

The sensory channels will significantly affect the reported responses to the spatial presence questionnaire, the performance measure, and the behavioral indicators. More precisely, SP, ENG, and ECO scales will be higher in the WITH SOUND condition, while the NEG scale will be higher in the NO SOUND condition (H2a). SCORE will be higher in the WITH SOUND condition (H2b). HRHI will be lower in the WITH SOUND condition (H2c). STCI will be higher in the WITH SOUND condition (H2d).

Results

This section is divided into four parts. The first part describes the interaction effects of the environments [ENV] and sensory modes [SEN] factors. The second and the third part describe respectively the environments and the effect of the sensory modes on each measure.

The last part describes the correlation between subjective responses to the questionnaire and objective performance and behavioral measures.

The result of the statistical parametric and nonparametric tests of each measure is reported. For statistically significant effects (p < .05), the effect size estimate r is given, which is interpreted as follows: a value under +-.30 represents a small effect, a value over +-.30 and under +-.50 represents a medium effect, and a value over +-.50 represents a large effect.

The same goes for the correlation coefficients. All the analyses were performed using RStudio version 3.4.1.

Interaction effect.

In order to analyze the interaction effect of ENV and SEN, a two-way analysis of variance (ANOVA) was computed on each measure. An aligned rank transform was used on nonparametric measures (followed by (*)) before running the analysis. The results showed no significant interaction effect of ENV and SEN on any dependent variables: SP*, F(1,52) = 1; p = .32, ENG*, F(1,52) = .21; p = .64, ECO*, F(1,52) = .6; p = .44, NEG*, F(1,52) = .12; p = .73, SCORE, F(1,52) = 3.7; p = 0.6, HRHI*, F(1,52) = .67; p = .42, STCI*, F(1,52) = 1.44; p = .24. The focus was then given to the effect of each factor regardless of the other factor.

Environments effect.

Subjective questionnaire: SP, ENG, ECO, and NEG scales. The means of the four scales were higher in the REMOTE condition than in the REAL condition, especially the SPand ECO-scale (see Figure 6). In absolute terms, mean levels in the REMOTE condition of the SP-, the ENG-, and the ECO-scale were high (above 3). A Wilcoxon signed-rank independent test was performed for every scale between the REAL and the REMOTE condition. The onetailed significance values are reported. There were no significant differences in the ENGscale, W = 77; p = .35 and the NEG-scale, W = 84; p = .53. However, statistically significant differences were found in the SP-scale, W = 39; p < .01; r = 1.18 and in the ECO-scale, W = 29; p < .01; r = 1.61, with unexpectedly, the REMOTE condition outperforming the REAL condition. These results go against (H1a), which expected that REAL condition would be better than REMOTE one. Objective measures: SCORE, HRHI, STCI indicators. A Shapiro-Wilk normality test was performed on the data set of each measure. Results indicated that only SCORE and HRHI followed a normal distribution (SCORE : W = .96; p = .44, HRHI : W = .98; p = .9, STCI : W = .92; p = .04). Therefore, a Welch two-sample independent t-test was performed on the average SCORE and HRHI, and a Wilcoxon signed-rank independent test was performed on the average STCI. The tests indicated statistically significant results with REAL condition overcoming REMOTE one for each measure : SCORE, t = 3.89; df = 25.87; p < .001; r = 1.47; CI = [-21.61;-6.67], HRHI, t = -2.72; df = 25.99; p < .05; r = 1.03; CI = [-3.45;-0.48], STCI, W = 170; p < .000; r = 1.4), which validates H1{b,c,d} hypotheses. See Figure 7.

Sensory channels effect.

ITC-SOPI questionnaire: SP, ENG, ECO, and NEG scales. Except for the NEG-scale, the means of SP-, ENG-, and ECO-scales were all higher in the WITH SOUND condition (see Figure 8). A Wilcoxon signed-rank dependent test between the NO SOUND and the WITH SOUND condition was performed on each scale. There were significant differences in the SPscale, V = 14.5; p < .000; r = .9 and the ENG-scale, V = 43; p < .000; r = 1.1, which validate the (H2a) hypothesis. However, no significant statistical differences were found in the ECOscale, V = 88; p = .13, and the NEG-scale, V = 139; p = .99.

Discussion

The first part of this paper highlighted the importance of the immersive fidelity between environments in order to evaluate the potential of objective measures to assess spatial presence. In particular, the reproduction of accurate remote auditory component was of major concern to reach renderings comparable to the real condition [START_REF] Postma | Perceptive and objective evaluation of calibrated room acoustic simulation auralizations[END_REF].

Nevertheless, the spatial resolution of the First-order Ambisonic microphone used for the teleoperation system was lower than the quality of the natural hearing in the real environment. This disparity between the environments could bias the results as hypothesized in (Khenak, Vézien, Thery, & Bourdot, 2019). In order to explore this possibility, the interaction effect of both environments and sensory renderings factors was analyzed. It showed no significant effects of sensory renderings regarding the real and the remote environment, meaning that the audio setup (empiric evaluation of reverberation, first-order 3D microphone), despite its shortcomings, provided sufficient accuracy localization.

Consequently, the analysis has centered on evaluating the effect of the two factors independently of each other. This analysis was performed on the four scales of the ITC-SOPI questionnaire (spatial presence, personal engagement, ecological validity, and negative effects), the average score of participants, their degree of head hesitation, and the smoothness of their hand trajectory during the pointing task.

Concerning the effect of the environments, the responses to the questionnaire indicated a significantly higher level of spatial presence and ecological validity in the remote environment compared to the real environment. This sense of a higher presence in the remote situation than in reality can be viewed as "hyper-presence", which was mentioned by [START_REF] Biocca | The cyborg's dilemma: Progressive embodiment in virtual environments[END_REF]. A possible cause could be a "technological effect" resulting from the lack of familiarity of participants with the VR HMD (HTC-Vive), especially in such a telepresence configuration. This effect could have led participants to appreciate more the remote experiment and consequently give better ratings. Indeed, most of them were beginners (66,7%) in the use of VR headsets and some informal interviews showed the great enthusiasm of participants in the experiment. This means that the responses to the questionnaire could be highly dependent on prior users' experiences (whether during the experiment at hand or beforehand) and interests. It encourages then the use of additional measures that are more objective.

This "hyper-presence" may also be related to the fact that the environment was a between-subject factor. A between-subject design was chosen to mitigate the learning effect on the task. Since the sense of spatial presence is a subjective user's feeling, every participant could perceive it and rate it differently based on their personal interpretation of the feeling and their "inner" scale. Nevertheless, the use of ITC-SOPI questionnaire, which has already proved to be a valid questionnaire [START_REF] Lessiter | A cross-media presence questionnaire: The ITC-Sense of Presence Inventory[END_REF], allows reducing this potential bias.

In contrast, the engagement scale showed no significant difference between the environments, which can be explained by the similarity of the task and content in both environments, as the goal of the remote environment was precisely to remotely allow the participants to operate in the real environment. The same applies to the negative effects scale that showed no statistically significant difference between the environments, which demonstrates the potential growth of such a teleoperation system in terms of visual quality, tracking precision and latency reduction.

Thus, the hypothesis (H1.a) that the real environment provides a higher sense of spatial presence, ecological validity, and engagement, and a lower negative effect was not confirmed and even took an opposite direction through the emergence of a sense of "hyper-presence" in the remote environment.

On the other hand, objective results related to participants' performance showed a higher task efficiency in the real environment, which meets the expectation of the hypothesis (H1.b). The most likely explanation is that in the remote environment, the participants felt they had fewer facilities to move because of the equipment of the teleoperation system (headset, headphones, wires, etc.). Therefore, they moved more slowly and had more restricted gestures while performing the pointing task. This assumption tends to be confirmed by the two ad-hoc indicators extracted from the head and hand participants' trajectory:

-a higher level of hesitation of participants when turning their head to find the location of images; -and less smoothness when pointing on images in the remote environment.

Concerning the effect of sensory renderings, the sense of spatial presence and the degree of ecological validity were higher when providing audio rendering, while the negative effects related to the disorientation of participants was lower. In particular, the results of the experiment provided strong evidence on the benefits of audio rendering in increasing the engagement of participants. These results which support the hypothesis (H2.a) are in agreement with previous studies that showed the usefulness of sensory cues in the improvement of presence [START_REF] Lipscomb | Cross-modal integration: Synchronization of auditory and visual components in simple and complex media[END_REF][START_REF] Mania | The effects of levels of immersion on memory and presence in virtual environments: A reality centered approach[END_REF], and user satisfaction [START_REF] Lee | A usability study of multimodal input in an augmented reality environment[END_REF] in virtual environments. Similarly, the participants' performance and the smoothness of their pointing gesture trajectory was significantly higher when the audio sensory channel was provided, reflecting less hesitation. It should be noted that these results were found regardless of the type of environment (as no interaction effect was found).

Furthermore, previous papers raised the question of an existing correlation between the sense of presence and performance (see Section 1). Indeed, a common assumption is that immersive environments increase both presence and performance [START_REF] Cummings | How immersive is enough? A meta-analysis of the effect of immersive technology on user presence[END_REF][START_REF] Tan | Physically large displays improve performance on spatial tasks[END_REF]. This assumption suggests a positive link between presence and performance (Lombard et al., 2015, pp. 139-185). In this study, no correlation was found between the perceived feeling of spatial presence and the actual pointing task performance. This result echoes some evaluations [START_REF] Picciano | Beyond student perceptions: Issues of interaction, presence, and performance in an online course[END_REF][START_REF] Welch | How can we determine if the sense of presence affects task performance?[END_REF] that tended to argue that presence and performance are not related. Yet, it contradicts more recent studies [START_REF] Cooper | The effects of substitute multisensory feedback on task performance and the sense of presence in a virtual reality environment[END_REF] and encourages further evaluations. Besides, no correlation was found between spatial presence and the participants' behavior represented by the two ad-hoc indicators. However, the different body representations in each environment (the participant's hand in the real environment, and the 3D representation of participant's hand in the remote environment) may explain the results as it was proved that body representation could affect task performance [START_REF] Medeiros | Keep my head on my shoulders!: why third-person is bad for navigation in VR[END_REF].

Therefore, more investigation is needed to understand the reliability of performance and behavioral analysis as potential tools to measure spatial presence.

Conclusion

This paper presented a user evaluation assessing the feeling of spatial presence, by comparing a real and a remote immersive environment representing respectively an actual office setting and its remote representation captured by tele-sensing technologies. In an experiment, participants performed a 3D pointing task while being directly located in the office or being remotely transported in the same office through a tele-operation system. The study evaluated the sense of spatial presence and three subcomponents (engagement, ecological validity, and negative effects) using the ITC-SOPI questionnaire [START_REF] Lessiter | A cross-media presence questionnaire: The ITC-Sense of Presence Inventory[END_REF]. In addition, objective metrics were recorded based on the users' performance to achieve the task and their behavior obtained by extracting ad-hoc indicators that represented the hesitation and smoothness of users' pointing gestures.

Results from the questionnaire highlighted an unexpected higher sense of spatial presence, or "hyper-presence", in the remote environment compared to the real environment.

A potential explanation could be the "technological effect" resulting from the lack of familiarity of participants with the telepresence technologies used. Therefore, future work will address this assumption by comparing the sense of spatial presence between different environments with the same telepresence technologies. More precisely, the next studies will evaluate spatial presence between remote and virtual environments. Particular attention will be paid to rely on similar VR user-interface technologies for the rendering of both types of environments in order to mitigate the influence of technological effects. Such studies will also allow investigating the effect of users' awareness of real consequences on the physical place that characterizes remote environments over virtual ones [START_REF] Slater | A virtual reprise of the Stanley Milgram obedience experiments[END_REF]. Therefore, it will be essential to design tasks that have a significant impact on the physical place in order to increase this awareness.

Moreover, the future comparisons between environments will have to be carried out following a within-subject design in order to ensure that there is no influence of the user's interpretation of the sense of spatial presence. Results collected from these comparisons will help to clarify the boundaries between remote and virtual environments.

On the other hand, task performance and behavioral indicators provided results that were clearly in favor of the real environment. So that no correlation between the objective measures and the feeling of spatial presence was found, which calls for a more in-depth investigation on the use of reliable, objective behavioral tools to measure spatial presence. More specifically, the next studies will aim to develop behavioral indicators related to kinematic metrics extracted from users' trajectories (e.g., user's backtrack during a navigation task, [START_REF] Martin | Sensorimotor feedback for interactive realism: Evaluation of a haptic driving paradigm for a forklift simulator[END_REF]. Thus, a possible experiment would be to analyze the trajectory of participants when traveling on a circuit and their reaction when facing obstacles in a remote or virtual environment (i.e., avoidance of obstacles, [START_REF] Lynch | Collision avoidance behavior between walkers: global and local motion cues[END_REF]. This kind of experiment would also be very interesting to study the influence of participants' awareness of the real consequences of their actions on the physical location (in the case of remote situations). Thus, one could wonder whether the participants will break or bypass the obstacles depending on the environment in which they are located. This paper also evaluated the effect of providing spatialized audio rendering consistent with visual rendering in both real and remote environments. Results provided evidence of the importance of audio rendering to enhance the sense of spatial presence and its components.

More specifically, the engagement component proved to be highly affected by audio rendering. Performance and behavioral indicators also confirmed this assumption regardless of the type of environment.

To summarize, this paper is a first step in understanding the sense of spatial presence in remote environments. It compared a real and a remote environment and raised the question over the difference between virtual and remote environments. In addition, it sought to introduce new behavioral and performance tools to measure spatial presence, and it identified new issues about hyper-presence and potential effects of VR technologies (the technological aspect is a significant difference between real and remote environments). Such studies will certainly benefit the presence research community as well as the designers of teleoperation technology.

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Figure 1 .

 1 Figure 1. The operating room representing a real office.

Figure 2 .

 2 Figure 2. 3D overview of the two rooms. (a) A participant in the operating room. (b) A

Figure 4 .

 4 Figure 4. A diagram summarizing the calibration procedure.

Figure 6 .

 6 Figure 6. Effect of environments conditions on mean scores for each scale of the ITC-SOPI

Figure 7 .

 7 Figure 7. Effect of environments conditions on mean scores of the total number of images

Figure 8 .

 8 Figure 8. Effect of sensory conditions on mean scores for each scale of the ITC-SOPI

Acknowledgments

This research was partially supported by RTA Digiteo and Labex DigiCosme (Idex Paris-Saclay ANR-11-IDEX-0003-02), and by EquipEx DIGISCOPE (ANR-10-EQPX-26-01) operated by the French National Research Agency (ANR), as part of the program Investissements d'Avenir.