Body-Ownership Illusion by Gazing
at a Blurred Fake Hand Image
Hikaru Hasegawa1(B) , Shogo Okamoto1 , Nader Rajaei1 , Masayuki Hara2 ,Noriaki Kanayama3 , Yasuhiro Akiyama1 ,and Yoji Yamada1
1

School of Engineering, Nagoya University, Nagoya, Japan
hasegawa.hikaru@c.mbox.nagoya-u.ac.jp
2
School of Science and Engineering, Saitama University, Saitama, Japan
3
Human Informatics Research Institute, National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba, Japan

Abstract. Feeling body ownership over a fake body image in videogames or virtual environments may enhance the immersion and feelingof presence in them. In these situation, the sharpness of the image may
inﬂuence the induction of the body-ownership illusion. In this study, weinvestigated the eﬀect of blurring the image on the rubber hand illusion
experience under seven levels of blur intensity. The results showed thatblurring the image within the limits of hand recognizability may induce
stronger body ownership of the fake hand but may not inﬂuence agency.
This indicates that body ownership of a body image can be controlled
by the sharpness of the presented image.
Keywords: Body ownership · Agency · Rubber hand illusion ·Blurred image

1Introduction

The induction of feeling as if a body image on a computer screen is part ofone’s own body is expected to provide greater immersion and presence in thevirtual environments or video games. The feeling of a part of one’s body (bodyownership) and the sense of authorship of body motion (body agency) [1, 2] havebeen investigated in the rubber hand illusion (RHI) paradigm [3]. The RHI is
an illusory experience one feels body ownership over a fake hand while gazing
at it being exposed to spatially and temporally congruent visuohaptic stimuli.
Many studies have attempted to induce the RHI through a computer
screen [4–7]. For instance, one study found that, when screen projection of thecamera image of the hand was delayed by more than 300 ms, the RHI becomes
weaker [4]. However, the conditions for inducing a body-ownership illusion has
not been investigated thoroughly. In this study, we focused on the sharpness ofthe body image captured by a camera and projected on a computer screen, whichc Springer Nature Singapore Pte Ltd. 2019
H. Kajimoto et al. (Eds.): AsiaHaptics 2018, LNEE 535, pp. 9–14, 2019.
https://doi.org/10.1007/978-981-13-3194-7_3

10H. Hasegawa et al.

Fig. 1. Left: top view of the apparatus for inducing an active RHI. The right hand iscovered with a black cloth. The movements of the actual and fake hands are synchronized by the rod. Right: speciﬁcation of the screen and image used in the experiment.

is often restricted by the speciﬁcation or performance of the camera, monitor,computer, and communication environment used. Determining the inﬂuence ofimage sharpness on the RHI contributes to the development of video games andvirtual reality environments that employ the body-ownership illusion.
The inﬂuence of image sharpness on the RHI has not previously been investigated. Therefore, we investigated it in an RHI experiment where participantsgaze at a fake hand image. Participants moved their own hand and the fake
hand synchronously while gazing at seven images (each with a diﬀerent level ofblur) in a random order. Then, they answered to a questionnaire related to bodyownership and agency. The results of this study are beneﬁcial for controlling thebody ownership of a fake body image on a computer screen.

2Materials and Methods

2.1

Participants

Five university students (all right handed males aged between 21 and 23) who
had no experience of the RHI participated in the experiment with informedconsent. This experiment was approved by the ethical committee of the Schoolof Engineering, Nagoya University (#17–12).
2.2

Experimental Setup for Inducing the RHI Through a Screen

Rubber gloves, an acrylic rod for synchronizing the hand motion, and a black
cloth for covering the actual right hand and arm were used for inducing theRHI (Fig. 1 (left)). This apparatus was also used in [8]. Participants wore therubber gloves and moved their hands synchronously with the fake hand by softly
grasping the acrylic rod ﬁxed to the fake hand.
The motion of the actual and fake hands was captured by a camera (HD WEBCAM C270, Logicool), and the camera image (15 cm × 20 cm, 408 × 544 pixel) wasBody-Ownership Illusion by Gazing at a Blurred Fake Hand Image

11Fig. 2. Side view of the experimental setup for inducing RHI through a screen.

projected on a monitor (FlexScan EV2116W, EIZO) with a refresh rate of 59–
61 Hz (Fig. 1 (right)). The captured image was ﬂipped horizontally to give a mirror
image [2]. The camera and monitor were located in front of the participant as shownin Fig. 2, and the captured image of their hands was shown on the monitor. Themonitor height was same as the participant’s eye level. The fake and actual hands
were hidden by a frame covered with black cloth in order to prevent a direct view.

2.3

Methods

The camera images were blurred by an average ﬁlter. The blur levels werechanged by the ﬁlter size, which had seven levels that range from not blurred(ﬁlter size: 0) to blurred beyond hand recognition (ﬁlter size: 75 × 75 pixel) in ageometrical series. The seven levels of blurred image are shown in Fig. 3.
2.4

Task

Before the experiment, participants tapped or rubbed the table with them onand experienced the RHI by gazing at the fake hand as shown in Fig. 1 (left), suchthat they could familiarize themselves with the rubber gloves. Then, participants
experienced the RHI at seven levels of blurred image projected in a random
order. The ﬁrst four trials were used for practice. Then, 14 trials were conducted(each of the seven levels of blur was shown twice). Participants experienced thestimulus for one minute in each trail.
The magnitudes of body ownership and agency over the fake hand are rated
by the questionnaire [1–3]. The questionnaire shown in Table 1 was a modiﬁed
version of the one used by Botvinick and Cohen [3]. Q1 and Q2 are related
to body ownership and agency, respectively, and the others are control items.

12H. Hasegawa et al.

Fig. 3. Seven levels of blurred image used in the experiment. For the image of level1, no ﬁltering eﬀect was applied. For levels 2–7, average ﬁlters of diﬀerent sizes wereapplied.
Table 1. Questionnaire for evaluation of the RHI intensity
Q1 I felt as if the rubber hand was my own hand
Q2 It seemed like I was directly moving the rubber hand
Q3 It seemed as if I might have more than one right hand or arm
Q4 It felt as if my (real) hand was turning “rubbery”
Q5 It appeared (visually) as if the rubber hand was drifting towards the right(towards my real hand)
Q6 The rubber hand began to resemble my own (real) hand in terms of shape,size, or some other visual feature
Q7 I felt as if my (real) hand was drifting towards the left (towards the rubberhand)

Participants gave each question a score on a seven-level scale for each trial (−3 forstrongly disagree up to +3 for strongly agree). A positive answer score indicatesthat the participant agreed with the question.

3Results

The answer scores from each participant were averaged in each stimulus condition. Figure 4 shows the means and standard errors of the answer scores for eachquestion and each blur level. The answer scores against Q3–Q7 (control items)
were equally negative under all conditions, which denied the suggestibility ofthe setting. We applied a one-way ANOVA to the answer scores against Q1 andQ2, which are related to the RHI, by considering the blur levels as the factor toinvestigate the eﬀect of blur process.

Body-Ownership Illusion by Gazing at a Blurred Fake Hand Image

13Fig. 4. The means and standard errors of the RHI questionnaire scores for each blurlevel.

The answer scores against Q1 (body ownership) were positive values for blurlevels 1–4 and were negative values under stronger blurring conditions. Furthermore, they peak between blur levels 3 and 4. However, these were not signiﬁcantresults (F (6, 63) = 1.27, p = 0.29). The answer scores against Q2 (agency)
were positive values under all blurring conditions and there was no signiﬁcantdiﬀerence among any conditions (F (6, 63) = 0.35, p = 0.91).

4Discussion

Body ownership was reported in slightly blurred conditions (blur levels 3 and 4),
but not in intensive blurring conditions where the hands become unrecognizable.
Moreover, the answer scores against body ownership under blurring conditions
within the limits of hands being recognizable (i.e., blur levels 3 and 4) were higher
than under the unblurred condition. This result can be explained by the inverse
eﬀect [9], where the illusion, which is a multisensory integration of visuohapticcues, might have been strengthened by degradation in the reliability of visualcues. Also, slightly blurred conditions (blur levels 3 and 4) may masked thevisual diﬀerence between the fake and actual hands, and might have resulted inintensive illusions. However, given the limited number of participants, we could
not achieve signiﬁcant results and further investigation is required.
Although agency was reported regardless of the blurring intensity, the typesof it diﬀered between blur levels. Under blur levels 3 and 4, body agency may
be induced since body ownership was also reported. On the other hand, under14H. Hasegawa et al.

stronger blurring conditions (blur levels 5–7), external agency [1] may be inducedin the same way as when you use a tool or manipulate an object.
In this way, using an unclear image within the limits of hand recognizability
may be eﬀective for inducing body ownership on the fake hand image whileremaining agency.

5Conclusion

In this study, we investigated the eﬀect of blurring a fake hand image on theRHI experience. Camera images of participants’ hands were blurred to sevendiﬀerent extents and projected on a monitor in front of them. Participants
answered a questionnaire related to illusion after moving their actual and fakehands synchronously while gazing at the blurred image. The results showed thatblurring the image within the limits of hand recognizability induced stronger
body ownership than under unblurred conditions. However, agency was induced
equally under all blurring conditions. We suggest that the blurring process within
the limits of hand recognizability may be eﬀective for inducing body-ownership
illusion with a fake hand image on a screen.

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