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Communication Cues in Human-Robot Touch Interaction
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ABSTRACT

Haptic interaction is a key capability for social robots thatclosely interact with people in daily environments. Suchhuman communication cues as gaze behaviors make haptic
interaction look natural. Since the purpose of this study is toincrease human-robot touch interaction, we conducted anexperiment with 20 participants who interacted with a robot
with different combinations of gaze behaviors and touchstyles. The experimental results showed that both gazebehaviors and touch styles influence the changes in theperceived feelings of touch interaction with a robot.

encounter situation [14, 15], object-transfer [16, 17], andconversations [18-21]. We believe that this knowledge will
increase the understanding of communication cues inhuman-robot touch interaction.
In this paper we deal with a robot’s gaze as a major
communication cue that increases the naturalness of touch
interaction (Figure 1). We investigate which kinds oftouching styles (touching a robot, touched by a robot, andmutual touch) are important to make touch interaction morenatural.

Author Keywords

Robot touch; social robot

DESIGNACM Classification Keywords

In this section, first we describe the details of the robot usedin the experiment, because its characteristics, such asappearance and size, are important for the design ofcommunication cues. Next, we refer to related works about
experiment designs, i.e., gaze behavior and touch styles inhuman-robot interaction, and describe the detailed designsof both gaze behaviors and touch styles that will be used in
the experiment.

H.5.2. User Interfaces – Interaction styles
INTRODUCTION

Physical interaction, particularly haptic interaction, is one
promising research path in the human-agent interaction
research field. Because of its physical existence, which isan important advantage enjoyed by robots over CG agents,
robots can communicate with people through haptic
interactions that are well known to change the behaviors ofothers and facilitate their efforts in human science
literatures [1-7]. In the research field of human-robot
interaction, various research works have reported similareffects of haptic interaction between robots and people:
mental therapy [8], increasing motivation [9], and attitudechanges by touch [10-12].

Robot

Figure 2 shows Pepper, a personal humanoid robot
developed by Softbank. It has 20 DOFs: two DOFs in its
head, shoulders, elbows and waist, one DOF for wrists,hands, and knee, and three DOFs for its wheels. The robot
is 121 cm tall and is equipped with microphones, cameras, adepth sensor, touch sensors, and so on. It has five fingers on
its hands.

However, these past research works on haptic interaction
with robots mainly focused on the influences to people’s
feelings or behavior changes by physical interaction. In
other words, they focused less on the communication cuesof robots in such touch interactions. The following
communication cues have been thoroughly investigated:gaze behavior in such contexts as approaching [13],Fig. 1 Robot touches a person

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about hand-over research works, i.e., Gharb et al. whoinvestigated the effects of gaze cues in hand-over situations[16]. According to their work, we investigated two kinds ofgaze behaviors during haptic interactions: face-only andface-hand-face. We employed these two behaviors because
the face-hand-face behavior was the most preferred patternby participants in hand-over situations, and the face-only
behavior received low evaluations in the prepared
conditions, which can be used as a baseline condition.
Face-only

In this behavior, the robot looks at the interacting person’s
face during the touch (Fig. 3, left).
Face-hand-faceFig. 2 Pepper

In this behavior, the robot looks at the interacting person’s
face first (Fig. 3, left), then at the person’s hand (Fig. 3,
right), and finally at the person’s face again (Fig. 3, left).
The duration of the gaze behaviors was 2250 ms, like in
Gharb’s work [16].
Touch design

Fig. 3 Pepper’s gaze behavior: looking at participant’s face(left) and participant’s hand (right)
Gaze design

Gaze behavior is essential for natural communication
between people and such anthropomorphic agents as robots.
Many research works have investigated gaze behaviors inhuman-robot interaction [13-21]. For example, Satake et al.
developed a natural approaching model for a mobile social
robot. While approaching, the robot’s gaze behavior isimportant to share its intention to approach the target [13].
Hayashi et al. focused on encounter interactions in public
environments where people and robots walk around andmodeled the behaviors of human experts (guards) to show
the robot’s availability through gaze and roaming paths [14].
Seki et al., which also focused on a situation where a robot
initiates a conversation with a person, included gazebehaviors to make interactions look more natural [15].
These research works deal with interactions before therobot meets people; of course, studies have also focused on
the interaction after the robot meets people [18-21] in nonhaptic interaction contexts, such as approaching andproviding information.
To design gaze behaviors in touch interaction, we focusedon a situation where a robot hands an item to people,
because in such situations, the hands of the robot and theparticipants become closer like in actual touch interactions.
We employed several gaze behaviors from the literature

Past research works related to touch interaction investigatedthree touch styles: touching a robot, touched by a robot, andmutual touch between a person and a robot. Researchers
have mainly investigated the “touching a robot” style. For
example, Shibata et al. developed a seal robot named Parofor therapy with senior citizens through interaction,
including touching the robot [8]. The “touching a robot”
style is also used to recognize touch interaction [22-24],
where Cooney used inertia sensors to recognize full-bodygestures by haptic interactions toward a humanoid robot[24].
A few research works investigated the effects of being“touched by a robot.” For example, Cramer et al. argued
that a touch behavior by a robot decreases machine-likenessbut negatively affects its dependability [10, 11]. Tiffany et
al. investigated the influence on the impressions of a robot’s
touches with verbal communication cues [12].
To the best of our knowledge, only Shiomi et al.
investigated the effects of mutual touch in the context ofincreasing the motivation of the interacting person [9]. This
research work reported that mutual touch provides
facilitation effects, where people who did mutual touchwith the robot continued a simple and monotonous task
longer than people who did not touch the robot.
Based on these research works, we employed the following
three touch styles. The details of each with Pepper aredescribed below.
Touch-to-robot

In this touch style, first Pepper asked the interacting persons
to “please touch my left hand” and then extended it (Fig 4).
In this condition, Pepper did not actively touch theinteracting persons. We put a round plastic yellow sphere in
Pepper’s left hand for uniform touch feelings among theother conditions.

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Pepper touched the hand with its own right hand (Fig. 5,
right). In this condition, Pepper actively touches theinteracting person’s hand, different from the touch-robot
style.
To design Pepper’s touching behavior, we employed theknowledge of human science literature that investigates theeffects of touching speed on impressions [25]. Since thispaper reported that touching at a speed of 5 cm/s wasevaluated more positively than 0.5 or 50 cm/s, we set the
speed of the robot’s hand during the robot’s touching toabout 5 cm/s. Pepper’s touching behavior is preprogrammed, and its hand follows a fixed trajectory, basedon the human’s hand size.
Fig. 4 Pepper extends its hand

Mutual-touch

In this touch style, first Pepper asked the interacting person
to touch its left hand, as in the touch-robot style. After theinteracting person touched Pepper’s hand, Pepper touches
the hand with its own right hand, as in the touched-by-robot
style (Fig. 6). Therefore, this condition mixes both thetouch-robot and touched-by-robot styles; both Pepper andthe interacting person touch each other. We used Pepper’s
identical touching behavior in this style: touched-by-robot
style.
EXPERIMENT

We experimentally investigated the effects ofcommunication cues between the combinations of the gazeand touch designs toward people.
Fig. 5 Identical round plastic yellow sphere on the stand (left)
and Pepper touches a hand (right)

Fig. 6 Pepper touches person’s hand, which also touches
Pepper’s left handTouched-by-robot

In this touch style, a stand is placed near Pepper on which
the interacting person’s hand is put. The stand’s height isidentical to Pepper’s left hand in the touch-robot style, andthe same round plastic yellow sphere is placed on the stand(Fig. 5, left). Pepper asked the interacting persons to“please put your hand on the stand.” After they did so,

Hypotheses and Prediction

Based on human-robot interaction research, gaze behavior
is essential for more natural and acceptable interactions [1321]. For this purpose, both eye contact and telling intentions
by gaze behaviors are important. In particular, a recent
work suggested that a combination of gaze behaviors at
faces and objects produces more natural interaction feelingsin hand-over interactions [16]. Because of the similarity
between hand-over and touch, such gaze behaviors may
make touch interactions more natural.
The touch style in haptic interaction is also essential tochange the perceived feelings of people. Past researchworks commonly showed that the touched-by-robot style
created negative feelings [10, 11], but the mutual-touchstyle created more positive feelings than the touch-to-robot
style [9]. Based on these considerations, we made thefollowing predictions:
Prediction 1: A touch interaction with a face-hand-facebehavior will be perceived as more natural, more
comfortable, and create a better impression in theparticipants than a touch interaction with a face-only
behavior.
Prediction 2: A touch interaction with a mutual-touch style
will be perceived as more natural, more comfortable, andcreate a better impression in the participants than touch
interactions with both the touched-by-robot and touch-to-The final publication is available at
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robot styles. A touch interaction with the touch-to-robot
style will be perceived as more natural, more comfortable,
and create a better impression in the participants than atouch interaction with touched-by-robot style.
Environment

We conducted the experiment in a laboratory room. We
placed in front of Pepper a chair in which the subjects sat
during the experiment (Fig. 7).
Conditions

We used a within-participant experiment design to evaluate
and compare the effects of communication cues: two gazebehaviors (face and face-hand-face) and three touch styles(human touch, robot touch, and mutual touch), as describedin Section 2. An operator manually decided the timing tostart Pepper’s touching behavior.

RESULTS
Verification of Predictions

Figure 8 shows the feeling of naturalness. For it, we
conducted a two-way repeated-measure ANOVA with two
within-subject factors: gaze and touch. A significant main
effect was revealed in the touch factor (F(2, 38)=6.508,
p=.004, partial η2=.255). No significance was found in thegaze factor (F(1,19)=0.195, p=.664, partial η2=.010) andthe interaction within these factors (F(2,38)=0.108, p=.898,
partial η2=.006). Multiple comparisons with the Bonferroni
method revealed significant differences in the touch factor:
touch-to-robot > touched-by-robot (p=.007), but there wasno significance between touch-to-robot and mutual-touch (p=.110) or touched-by-robot and mutual-touch (p =.975)
Figure 9 shows the feeling of comfort. For it, we conducted
a two-way repeated-measure ANOVA with two withinsubject factors: gaze and touch. A significant main effect
was revealed in the gaze factor (F(1, 19)=5.448, p=.031,
partial η2=.223). No significance was found in the touchfactor (F(2,38)=1.893, p=.165, partial η2=.091) and in theinteraction within these factors (F(2,38)=0.437, p=.649,
partial η2=.022). Multiple comparisons with the Bonferroni
method revealed a significant difference in the gaze factor:
face-only > face-hand-face (p=.031), but there was no
significance between touch-to-robot and mutual-touch (p=.110) or touched-by-robot and mutual-touch (p =.975)
7

**

6

Fig. 7 Experimental scene

5

Participants

Twenty people (10 women and 10 men, who averaged 35.5
years old, S.D 9.7) participated in the experiment.
Procedure

Before the first session, the participants were given a brief
description of our experiment’s purpose and procedure.
Since it had a within-participant design, each participant
participated in six sessions of different conditions.
In all the conditions, after requesting touch initiation Pepper
had a short chat with the subject, such as “I’m 121 cm talland weigh about 28 kg. I’m lighter than you think, right?”
We prepared six chat contents to avoid repeating themamong the conditions. During the chat in the touched-byrobot and mutual-touch styles, Pepper patted the hand of
the participants three times. The order of the conditions andthe chat contents was counterbalanced. The participantsfilled out a questionnaire after each session.

4
3
2
1

Touch

TouchedFace-only

MutualFace-Hand-Face ** : p < .01

Fig. 8 Feeling of naturalness in touch interaction
7

*

6
5
4
3

Measurements

In this experiment, we measured three subjective items byquestionnaire: the feeling of naturalness of the touch
interaction, its feeling of comfort, and the total impressionof the robot. The questionnaire item was evaluated on a 1to-7 point scale.

2
1
Touch

TouchedFace-only

Face-Hand-Face

Mutual* : p < .05

Fig. 9 Feeling of comfort in touch interaction

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Figure 10 shows the total impression. For it, we conducted
a two-way repeated-measure ANOVA with two withinsubject factors: gaze and touch. A significant main effect
was revealed in the touch factor (F(2, 38)=4,377, p=.019,
partial η2=.187). No significance was found in the gazefactor (F(1,19)=1.197, p=.288, partial η2=.059) and in theinteraction within these factors (F(2,38)=0.481, p=.622,
partial η2=.025). Multiple comparisons with the Bonferroni
method revealed a significant trend in the touch factor:
touch-to-robot > touched-by-robot (p=.061), but there wasno significance between touch-to-robot and mutual-touch (p=.121) or touched-by-robot and mutual-touch (p =1.000).
These results show that prediction 1 was not supported; theopposite phenomenon was observed. Prediction 2 waspartially supported.
+

7
6
5
4
3

2
1
Touch

TouchedFace-only

Face-Hand-Face

Mutual+ : p < .10

Fig. 10 Total impression of robot

DISCUSSION
Why did our participants prefer face-only gaze behavior
to face-hand-face gaze behavior?

Past research work reports that a combination of eye
contact and looking at an object increased impressions thanjust eye contact during a handing interaction [16]. But our
research work showed an opposite result: people preferred
eye-contact during a touch interaction. We assumed thatboth handing and touch situations are similar and required
similar communication cues, but this assumption might beinappropriate. Future work will investigate gaze differences
between handing and touching situations and unveil
essential gaze behaviors in touch interaction.
Why didn’t they prefer mutual-touch style to other touchstyles?

In this work, the mutual-touch style did not show
significant differences about feelings from other touchstyles, unlike past research work. On the other hand, thetouched-by-robot style showed more negative impressions
than the touch-to-robot style, similar to past research work.
We thought that additional factors might influence theimpressions of touch interactions in addition to gaze andtouch style. For example, a trajectory of touching and touchfeelings is important for this purpose, and future work will
investigate them.
Limitation

Since our experiment was conducted with an existing robot,
Pepper, robot generality is limited. The effect shown in theexperiment would probably be moderated if our participantsinteracted with a robot with a different appearance, size,and so on. Pepper’s eye design is different from that ofhuman eyes. Since this design simplifies making eyecontact with the interacting person, it also influences theperceived feelings in haptic interaction.

CONCLUSION

In this research work, we focused on the effects ofcommunication cues toward perceived feelings to a robot in
haptic interaction: gaze behaviors and touch styles. We
employed two gaze behaviors during touch (looking at theface of an interacting person and looking at a face, a hand,
and the face again) and three touch styles (a person
touching a robot, a robot touching a person, and a person
and robot touching each other) by considering past related
works. To investigate the effects of these communication
cues, we conducted a within-subjects experiment in which a
robot interacts with participants through touch.
Our experimental results indicated that our participantspreferred a gaze behavior that only looks at a face to a gazebehavior that looks at a face, a hand, and the face again.
Moreover, they preferred a touch style in which a person
initiates the touch of a robot rather than when a robot
initiates the touch interaction. But unlike past research work,a touch style in which a person and robot touch each otheris not preferred over other touch styles. We believe thisknowledge will help robotics researchers who are focusing
on haptic interaction of a social robot to design their
behaviors.

ACKNOWLEDGMENTS

Omitted for anonymized review.

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