Test of independence model between bodyownership and agency in a virtual reality
environment
Asaki Kawaguchi

Shogo Okamoto

Masayuki Hara

Tokyo Metropolitan UniversityHino, JapanTokyo Metropolitan UniversityHino, JapanSaitama University
Saitama, Japan

Abstract—Body ownership refers to the feeling that one’s
body belongs to that person. Agency is the sense of being
able to control one’s own body parts. Three major hypotheticalmodels describe the relationship between body ownership andagency. In this study, we examined whether one of the threemodels—the independent model—was accurate in describing theaforementioned relationship. We prepared for two conditions
using an immersive virtual reality setting. In one condition, bothbody ownership and agency were evoked. In the other condition,
only agency was evoked. We compared the effects of motion
delays between these two conditions. The effect of delay on
agency did not depend on ownership intensity, supporting theindependence of body ownership and agency. However, the higher
the reported body ownership was, the higher the reported agency
could be, negating their independence. Holistically, the results are
not in favor of the independence model.
Index Terms—Body ownership, Agency, Virtual reality

II. M ETHODS
A. Participants
Eleven university students who were unaware of the objective of this study, participated in the experiment.

I. I NTRODUCTION
Body ownership refers to the feeling that the limbs and otherbody parts belong to one’s own body. Agency refers to thefeeling of being in control of a limb or a tool. To date, at least
three theories have been proposed regarding the relationshipbetween a sense of body ownership and agency [1]–[3]. In
the additive model, body ownership is considered part ofagency. The independent model assumes that agency andbody ownership do not interfere with one another. In theinteractive model, agency and body ownership interact witheach other. However, researchers are still debating the validityof these models. In this study, we investigate the validity of
the independent model.
In this study, both body ownership and agency were generated by displaying computer graphics (CG) of a hand linked
to actual hand movements in a virtual reality environment.
Additionally, we prepared a condition in which only agency
was generated by displaying a banana instead of a hand.
Delays between actual and CG hand movements make it more
difﬁcult to experience body ownership and agency [4]. If theindependent model is valid, the effect of delay on agency
should not change depending on the ownership condition.
Thus far, the effects of delays on body ownership and agency
have yet to be investigated under the two conditions describedabove. The results of this study will add positively to theliterature on the understanding of embodiment.

979-8-3503-4017-4/23/$31.00 ©2023 IEEEFig. 1. Computer graphics presented to participants. (a) Authentic-looking
left hand. (b) Banana displayed at the position of the actual left hand.

B. Apparatus
Oculus Quest 2 (Oculus VR, LLC., CA, USA) was used as
the virtual reality goggles. The left and right wrists and ﬁngertips were tracked using the built-in goggle cameras. The virtualenvironment was implemented using Unity 2020.3.35.f1.
C. Stimuli
Two different experimental conditions were used in thisstudy. Under the ﬁrst condition, the CG of an authentic-looking
hand was displayed at the position of the actual hand. In thesecond condition, the CG of a banana was displayed at theposition of the actual hand. A sense of bodily possession
was expected to occur for the authentic-looking hand CG
and not for the banana CG. Under both conditions, the CGhands moved synchronously with the participants’ actual lefthand. Hence, similar to the moving rubber hand paradigm [5]–[7], both the body-ownership and agency can be felt for theauthentic-looking hand.
The sense of body ownership and agency were barely felt,
with a motion delay of 500 ms [4]. Hence, 0 ms (minimumdelay) and 500 ms delay conditions were also set.
Individual participants experimented with four conditions(two types of CG × and two levels of delay), each of which
was replicated three times in a randomized block design.

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TABLE I
Q UESTIONNAIRE TO PARTICIPANTS .
Q1
Q2
Q3
Q4

I felt as if the visible hand (or banana) was my own left hand.
I felt I was controlling the visible left hand (or banana).
I felt as if I had more than one left hand.
I felt that I had lost my left hand.

D. Procedure of experiment
A 1-minute adaptation session was performed to familiarize
the participants with the visible left hand or banana. Participants continuously moved their left hand to trace a circle
parallel to the ground for one minute, during which they gazedat the left hand or banana. During the experiment, participants
were instructed to place their right hand on their lap andmaintain their face and body orientation, while refraining fromtouching something with their left hand.
After the adaptation task, participants completed the fourquestionnaires listed in Table 1 on a nine-point Likert scale(1 = not agree, 5 = neutral, 9 = largely agree). Among thefour questionnaires used, Q1 pertains to body ownership,
Q2 focuses on agency, and Q3 and Q4 serve as control
questions to minimize suggestibility. The participants took a 1minute break before repeating the abovementioned tasks under
different conditions.

Fig. 2. Mean scores for the questionnaire of body-ownership (Q1) and agency(Q2). * and ** mean signiﬁcant differences at p < 0.05 and 0.01, respectively.

E. Data analysis
For each stimulus condition and questionnaire, the meanof three repeated trials were calculated for each participant.
For Q1 and Q2, the mean scores were compared between theconditions with and without delay using a t-test. Furthermore,
a two-way analysis of variance (ANOVA) was applied to theresults of the Q2 questionnaire, with delay and type of CG asfactors.
III. R ESULTS
Fig. 2 presents the results for Q1 and Q2. The delayinﬂuenced the scores of Q1 and Q2 for both the authentic handand banana conditions as follows: (Q1-banana: p = 0.0022 <0.01/4, Q1-hand: p = 2.0 × 10−5 < 0.001/4, Q2-banana:p = 0.0079 < 0.05/4, Q2-hand: p = 0.0053 < 0.05/4).
As shown in Fig. 2, irrespective of the delay, the Q2 scoresfor the authentic hand condition were greater than those forthe banana condition (minimum delay condition: p = 0.025 <0.05/2, 0.5-s delay condition: p = 0.013 < 0.05/2). Agency
was felt more strongly when the realistic hand was displayed.
The two-way ANOVA did not ﬁnd a signiﬁcant main
effect of the CG on agency (F (1, 28) = 1.55, p = 0.22),
whereas the effect of delay was evident (F (1, 28) = 17.96,
p = 2.2 × 10−4 ). The interaction between the CG and delay
was not conﬁrmed (F (1, 28) = 0.016, p = 0.90). There wasno difference in the way delay lowered agency between theauthentic hand and banana conditions.
IV. D ISCUSSION
The two-way ANOVA showed no interaction between delay
and type of CG. Therefore, it was suggested that the delay

reduced agency to the same extent in the authentic handand banana conditions. This result supports the independence
model. In contrast, as shown in Fig. 2, the scores for agency
were greater with a sense of body ownership, suggestingthat the presence of ownership affected agency. This result
indicates that body ownership and agency are not entirelyindependent. Nonetheless, the main effect of CG was notobserved by ANOVA, and the effect of CG or ownership onagency was inconclusive in the current setting.
This study had some limitations. We compared two extreme
conditions in which agency was strongly felt (minimum delay)or hardly felt (500 ms delay). Moderate delay levels, suchas 250 ms, should also be tested for generalization. The
adaptation task could be improved further for better control
of experimental tasks. Participants continuously moved theirleft hand to trace a circle parallel to the ground for one minute,during which they gazed at the left hand or the banana. The
task was simple, and some participants may have been moreengaged in the experiment with a more difﬁcult task.
V. C ONCLUSION
Between the intensive-ownership condition (authentic handcondition) and the low/null-ownership condition (banana condition), the effects of motion delay on agency were notdifferent. However, agency scores tend to be greater in theintensive-ownership condition than in the low-ownership condition, which does not endorse the independence model ofownership and agency. Our results did not largely support theindependence model.

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