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This article is a technical report without peer review, and its polished and/or extended version may be published elsewhere.

第 28 回日本バーチャルリアリティ学会大会論文集（2023 年 9 月）

Strengthening Avatar Embodiment in Virtual Reality:
A Pre-Exposure Mental Preparation Approach
Adelaide Genay2) , Erika Kimura1) , Anatole Lecuyer3) , Martin Hachet2) , Yutaro Hirao1) ,Monica Perusquı́a-Hernández1) , Hideaki Uchiyama1) , and Kiyoshi Kiyokawa1)1) 奈良先端科学技術大学院大学 先端科学技術研究科（〒 630-0192 奈良県生駒市高山町 8916-5,{kimura.erika.ki3, yutaro.hirao, m.perusquia, hideaki.uchiyama, kiyo}@is.naist.jp）
2) Inria, Bordeaux, France （{adelaide.genay, martin.hachet}@inria.fr）
3) Inria, Rennes, France （anatole.lecuyer@inria.fr）
概要:

We aim to help users build a stronger sense of embodiment (SoE) toward their virtual

reality (VR) avatars. To do so, we designed a mental preparation protocol that involves guided
imagery and acting exercises to help participants imagine that they are their avatars before seeing
themselves in its body. This article introduces a pilot study that tests our approach with an avatar
representing the Hulk, from the Marvel franchise. We hypothesize that this preparation process will
help users build a stronger SoE and enhance Proteus Eﬀects. As the pilot study is still underway,
the results will be presented and discussed at the conference.
キーワード： Sense of embodiment, Proteus eﬀect, priming eﬀect

1.

Introduction

selves in its body. We decided to test this protocol onThe Proteus Eﬀect is a psychological phenomenon wherean avatar representing the Hulk (Marvel franchise). We

individuals’ behavior and self-perception are altered byhypothesize that this preparation process will help users

the characteristics of their virtual avatars [1]. This eﬀectbuild a stronger SoE and reduce individual diﬀerences.

usually occurs when users experience a sense of embod-

As the pilot study is still underway, the results will beiment (SoE) toward their avatars and may be exploitedpresented and discussed at the conference.

to improve their performance in speciﬁc tasks. However,there are some situations where avatars are being rejected2.

Mental Preparationby their users, even with state-of-the-art Virtual Reality

Embodiment experiences can be divided into three main(VR) technology. This diﬃculty is often explained byphases: pre-embodiment, embodiment, and post-embodiment.

the existence of subjective factors, such as personality

In most experiments on the SoE, the stimulation to in-traits [2]. Depending on the user, they can lead to aduce the BOI is typically applied after entering the em-propensity to body ownership illusions (BOIs) or to abodiment phase, once the user can already experience

reluctance to feel embodiment. Therefore, an embodi-

their avatarization. Then, the post-embodiment phase isment system cannot be guaranteed to generate the sameoften used to assess the resilient eﬀects of the avatar on

experience and eﬀects across users.

the users or collect additional subjective data. The pre-While removing subjective factors is impossible, strate-embodiment phase is rarely exploited: during that time,gies can be elaborated to reduce the variability they cause.

participants are usually told about their tasks and are

We previously explored one of these strategies, using

confronted with their avatars without further ado. Con-graphical transitions [3]. We found that smoothing thesequently, users must instantly adapt to their new bodies

experience of the switch between the real and virtual bod-and learn what they can aﬀord.
To make this change less unsettling, we propose toies by applying visual eﬀects improved the SoE.
This article explores another way to enhance the SoE

combine guided imagery and acting exercises over a 10-in VR by mentally preparing users ahead of the embod-

minute protocol before embodying the avatar. This pro-iment. We designed a protocol that involves guided im-

tocol aims to guide users into imagining they have be-agery exercise and acting to help participants imagine

come their avatars and is meant to be used with avatars

that they have become their avatar before seeing them-that represent speciﬁc characters bearing a personalityⒸ 2023 日本バーチャルリアリティ学会-1B2-01-

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図 1: A) Equipment, B) Hulk avatar, C) VR environment, D) Visuomotor task for SoE induction, E)

User perspective, F) Rock crushing task, and G) Apple object used for the grip strength measures. Topand bottom images show the scene and user perspective before and after the embodiment, respectively.
or stereotypes that can be imitated.

Therefore, it is

6GB graphics card, an Intel Core i5-8400 processor, andnot compatible with abstract avatarization forms or with32 GB of RAM, is used. The HTC lighthouses are placed

avatars that are not diﬀerent from the users. Readers

in a rectangle-shaped room securing the tracking area ofshould note that the experiment we present does notabout nine square meters. Participants wear and oper-

cover every case but acts as an entry point to investi-

ate the VR headset, and two hand controllers. Also, twogate how to exploit pre-embodiment time better.

Vive trackers are attached to their feet to perform a 5-

The protocol we propose can be summarized in thefollowing steps:

point body tracking (see Figure 1A). The experimental
software was built on Unity using free assets.

1. Explanation: the user is shown a picture of the3.2Avatar

avatar and is explained who it is and how it typ-

We selected an avatar model representing a character

ically behaves. The experimenter checks that thethat would not be hard to imitate for non-actors. A char-user understood correctly by asking questions, e.g.,acter that is familiar to the user and displays a simple“How would you describe this character?”. An-

behavior would therefore be a plus. Therefore, we used

swers are then orally validated or corrected to en-

the Hulk from the Marvel franchise (See Figure 1B). Thesure a baseline knowledge of the avatar’s traits.

Hulk is characterized by anger and brutality, and the2. Guided imagery: the user actively imagines being the avatar’s character. To help them, the experimenter provides instructions on what to imagine visually and emotionally. E.g., “Think of something that made you angry or stressed recently” or“Imagine that your body is growing muscly.”

character’s image is easily expressed through body language, making it easy to imitate even for those unfamiliar
with acting. The model we used was rigged for animation and included detailed body textures. We mapped
the action of pressing the hand controllers to animation
clips that put the user’s ﬁngers into a ﬁst. The rest of
the body was entirely animated with body tracking and3. Acting: the user physically imitates their avatars’

inverse kinematics (IK).

character. They also receive instructions to help

The calibration of the avatar is done in two stages: (1)them perform. For example, “Clench your ﬁsts

adjusting the height of the avatar and the user by theand breath deeply.” After going through this step,tracking position of the headset; and (2) matching thevirtual avatarization is immediately applied.

arm and leg lengths between the avatar and the user using the FinalIK plug-in VRIK calibration scripts. Man-3.

System Overview

ual adjustments are quickly made by moving the trans-

We implemented a VR system allowing users to embody the Hulk in the ﬁrst person to test whether the approach described in Section 2. may eﬀectively reinforce
the SoE.
3.1

forms of the head, hands, and feet target points if they
are not perfectly aligned with the user. Lastly, the resulting avatar is scaled back to Hulk’s model size (2.5meters tall).

Apparatus

3.3

Figure 1A illustrates the equipment in the experiment.
An HTC Vive Pro head-mounted display (HMD) con-

The virtual environment is a realistic industrial area(see Figure 1C). Ambient noise is added to increase im-

nected to a PC with an NVIDIA GeForce GTX 1060

Ⓒ 2023 日本バーチャルリアリティ学会

Environment-1B2-01-

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mersion (low hum of construction work). To avoid col-Sense of Embodimentlisions with the real environment, participants can only

The SoE towards the avatar is assessed with the revised

walk within a restricted area delimited by a green circle

16-item questionnaire of Peck and Gonzalez-Franco [4].

on the ﬂoor. The environment included life-size objects,

The items are adapted to our experiment’s context andsuch as soda cans, to provide a visual reference for theapplied during the VR embodiment.

user to understand their size and the content of the en-

Proteus Eﬀects

vironment. A mirror is placed in front of them to allow

Proteus Eﬀects indicate the success of BOIs. To de-them to observe their entire body (see Figure 1D). Twotect them, we measure the participant’s grip strength

conveyor belts carrying stones for the main task of the ex-

before and during the embodiment in VR. Stronger grip

periment are positioned on the right side (see Figures 1E

strengths during the embodiment should indicate the pres-and 1F). When touching or breaking the stones, feedback

ence of a Proteus Eﬀect. A relative grip strength (RGS)

is incorporated through the sound and vibration of theis obtained by measuring the participant’s maximum grip

hand controllers. The intensity of this feedback varies

strength (MSG) and strength when asked to crush an ap-

with the intensity of the applied touch.

ple with just enough force. The RGS will be a percentage
calculated by dividing the apple-crushing strength by the4.

Experiment

MGS.

This experiment will test the eﬀects of the mental

Participants are additionally asked to perform a punch-

preparation protocol detailed in Section 2. on the BOI ofing task in VR. Punching speed, hand agitation, anda Hulk avatar and on resulting Proteus Eﬀects. To do so,punching posture while crushing rocks are measured bywe compare two conditions in a between-subjects design.

videography and compared across conditions.

These conditions only diﬀer in their pre-embodiment phase

Emotional state

as follows:

Mood may be informative regarding the participant’s• Condition “NO PREP” (no preparation) isimmersion in the Hulk character, which is expected to be

the control condition. Participants perform dot-angry and aggressive. Emotional state is measured withconnecting tasks on paper for 12 minutes. They

the Self-Assessment Manikin (SAM) [5]. This scale eval-

are told that their performance is not assessed anduates a person’s aﬀective state’s valence, arousal, andthat they will embody an avatar representing thedominance dimensions. We apply it upon the partic-

Hulk in VR after that (without further details).

ipant’s arrival, during the VR embodiment, and after

After completing the task, they embody the Hulk

leaving VR.

avatar in VR.

Simulator sickness
Simulator sickness is controlled as a potential confound-• Condition “PREP” is the test condition. Instead of the dot-connecting task, participants are
explicitly told that their goal is to imagine being Hulk as strongly as possible. They are briefed
about the preparation process and the avatar they
will embody in VR afterward. Next, they go throughthe three-step mental preparation process describedin Section 2. for about 12 minutes. Finally, theuser embodies the Hulk in VR like in condition
NO PREP.

ing variable by applying the 16-item questionnaire ofKennedy et al. [6].
Post-experiment feedbackOther subjective feedback is obtained through a 7-item
custom questionnaire on a 7-point Likert scale. It contains an optional comment box for participants to write
open-ended feedback and is applied at the end of eachsession after VR exposure.
4.2

Procedure

After a brief outline of the experiment’s process, par-The Hulk embodiment phase in VR that follows eachticipants complete a demographic survey collecting theircondition is identical and serves to compare the SoE in-age, gender, VR experience, weekly video game playing

duced by the avatar with and without mental prepara-time, participation in acting activities, and understand-tion. This phase includes several tasks to stimulate theing of the character image of the Hulk. This information

SoE towards the avatar and allow data collection.

is collected to characterize the tested population and con-4.1trol for potential confounding variables. They are also

Measures

To understand the impact of mental preparation during the VR phase, user experience is measured in severalways.

Ⓒ 2023 日本バーチャルリアリティ学会

asked to ﬁll out the SAM scale.
Then, the participant is equipped with the HMD tomeasure relative grip strength. The participant’s maxi--1B2-01-

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mum strength is ﬁrst measured by asking them to grip a

4.4

Participants

dynamometer as ﬁrmly as possible. After a one-minute

We plan to recruit 40 participants via e-mail on cam-break, participants are shown a virtual apple colocatedpus. They will be equally divided into two groups corre-

with the dynamometer, and their strength is measured

sponding to the two conditions, PREP and NO PREP.

again while asked to crush the apple with just enoughThey will be paid 1000 JPY in cash after the experi-force (see Figure 1G top). Note that they see the virtualment. The experiment was approved by a local ethicalenvironment in life size without any avatar in this phase.

committee (2022-I-24).

Next, the participant removes the HMD proceeds tothe dot-connecting task (NO PREP) or the mental prepa-5.

Conclusion

ration protocol (PREP), depending on the condition. Once

In this article, we introduced a pilot study we are con-done, the participant wears the trackers and HMD again

ducting to test a mental preparation protocol applied tobut are asked to close their eyes while the experimenteran avatar representing the Hulk. This protocol aims atlaunches the software and calibrates their avatars. The

reinforcing BOIs of avatars in VR. It involves a guided

VR embodiment phase of the experiment then begins.

imagery exercise and an acting activity to help partici-Participants are brieﬂy explained how to use their avatar

pants imagine that they are their avatars before seeing

and are asked to face a virtual mirror to perform a visuo-themselves in its body. We hypothesize that this prepa-

motor task, aiming to induce an SoE. This task consists ofration process will help users to build a stronger SoE,

performing gestures they think the Hulk would do while

and thereby beneﬁt from more intense Proteus Eﬀects.

looking at themselves. We expect that participants who

We hope to present interesting experimental results atwent through the PREP condition will reproduce the ges-the conference and argue future directions.

tures from the introductory video they were shown. Af-

参考文献

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Hypotheses

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H1 The SoE will be higher in the PREP condition

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H2a MGS will be higher in the PREP condition com-

[4] T. C. Peck and M. Gonzalez-Franco. Avatar embodiment. a standardized questionnaire. Frontiers in Vir-

pared to the NO PREP condition. We expect it astual Reality, 1:575943, 2021.

a result of higher SoE in PREP condition.
H2b RGS will be lower in the PREP condition com-

[5] M. M. Bradley and P. J. Lang. Measuring emotion:

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that participants would think a smaller output isthe self-assessment manikin and the semantic diﬀer-[6] R. S. Kennedy, N. E. Lane, K. S. Berbaum, andenough to crush the apple.
H3 Rock crushing performance (e.g., the punching speed)
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H4 Arousal and valence will be higher in the PREP
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Ⓒ 2023 日本バーチャルリアリティ学会-1B2-01-

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