What Kinds of Robot's Touch Will Match Expressed Emotions?
Xiqian Zheng, Masahiro Shiomi, Takashi Minato, and Hiroshi Ishiguro

Abstract—This study investigated the effects of touchcharacteristics that change the strength and the naturalness ofthe emotions perceived by people in human-robot touch
interaction with an android robot that has a feminine,human-like appearance. Past studies on human-robot touch
interaction focused on understanding what kinds of humantouches conveyed emotion to robots, i.e., the robot’s touchcharacteristics that can affect people’s perceived emotions
received less focus. In this study, we concentrated on three touchcharacteristics (length, type, and part) based on arousal/valence
perspectives, and their effects on the perceived
strength/naturalness of a commonly used emotion inhuman-robot interaction, i.e., happiness, and its counterpart
emotion, (i.e., sadness), borrowing Ekman’s definitions. Our
results showed that the touch length and its type are useful tochange the perceived strengths and the naturalness of theexpressed emotions based on the arousal/valence perspective,
although the touch part did not fit such perspective assumptions.
Finally, our results suggest that a brief pat and a longer contact
by the fingers are better combinations to express happy and sademotions with our robot. Since we only used a female android,
we discussed future works with a male humanoid robot and/or arobot whose appearance is less humanoid.

I. INTRODUCTION
Touch is an essential factor in emotional communication
for human beings in conjunction with facial expressions andlanguage. Various human science literatures have revealed
how touch interactions are used between people [1-3] and how
touch shapes emotions [4-6]. The positive and negative effects
of touch interactions have also been broadly investigated, forinstance, its importance for human well-being [7-9]. To
understand the relationship between touch and conveyed
emotions, Hertenstein et al. conducted a large-scale datacollection with touch-type and touched-part and different
emotions (based on Ekman’s emotions [10, 11] and prosocial
emotions [12]) and proposed a touch-emotional map [13]. CT
afferent research perspectives focused on brain mechanisms
for processing affective touch by investigating the brain
regions involved in the perceptions of CT-supported affective
touch [14, 15].
Due to the advance in daily environments of sociallyintelligent robots that perform such services asphysical/mental health support [16-18], education [19-21],and companionship [22-24], representing the emotions ofrobots is critical for achieving more natural and acceptableinteractions with ordinary people. Based on this context and* This research work was supported in part by the JST ERATO Ishiguro
Symbiotic Human Robot Interaction Project (Grant Number: JPMJER1401),
the JST CREST Grant Number JPMJCR18A1, Japan, and JSPS KAKENHI
Grant Numbers JP17K00293.
X. Zheng, M. Shiomi, T Minato, and H Ishiguro are with ATR, Kyoto,
Japan. (e-mail: zheng.xiqian@irl.sys.es.osaka-u.ac.jp). X. Zheng and H.
Ishiguro are also with Osaka Univ., Osaka, Japan.

following the human literature related to touch in emotional
communication, robotics researchers have recently started tofocus on human-robot touch interaction [25-29]. For instance,
several studies scrutinized people’s touches and the conveyed
emotions/impressions [30-32]. Others focused on such
positive aspects of human-robot touch interaction as mental
therapy or stress-buffering effects [16, 33, 34]. Similar tohuman-human touch interaction, several studies investigatedthe effects of robot-initiated touches [35][36-38]
However, even if these studies identified the positiveeffects of a robot’s touch, appropriate touch design remains
unknown for expressing a robot’s emotions toward people. In
other words, past studies generally focused on human-robot
touch interaction from people to robots and overlooked
touches from robots to people in emotional interaction
contexts [30-32]. Other work described a tactile-emotional
map in human-human interaction [13] as well as human-robot
interaction in the context of touching from people to robots[39][40, 41] and reported that the touch situation contexts (e.g.,
emotions) complicate appropriate touch styles. Althoughseveral past studies focused on touch-speed characteristics, i.e.,
CT-optimal touch (around 3~5 cm/s) [42-44], they did notreport any detailed effects of such major touch characteristics
as length and concentrated less on the design guidelines for arobot’s touch interaction scheme.
One critical question remains: what kinds of touchcharacteristics of robots effectively match a robot's emotional
expressions? When we touch another person, depending on
the emotions we hope to convey, we often implicitly use
different characteristics: a short/long touch, contacting/patting,and/or touching by fingers/hand. For instance, a caregiver may
use touch behaviors to express emotions and/or empathy when
he is interacting with seniors. If social robots are used in suchsituations, they must choose appropriate touch characteristics
to match the emotions they are going to express.

Happy

Sad
Figure 1. ERICA’s touch interaction and happy/sad facial expressions

This study serves as a first step to understand therelationships between touch characteristics and emotions inhuman-robot touch interactions. We investigated therelationship among three kinds of touch characteristics (length,
type, and part) from an arousal/valence perspective and two
emotions (happiness and sadness, which is a typical emotion
pair in human-robot interaction contexts to express positive
and negative responses) based on the definitions of Ekman’s
six basic emotions [11]. We experimented with an android
named ERICA who has a feminine, human-like appearance(Fig. 1). Our study answers the following question:- What combinations among touch characteristics areappropriate to express happy/sad emotions?
II. TARGET EMOTIONS AND TOUCH CHARACTERISTICS
In this section, we give detailed explanations about how wedesigned our study, why we chose happy/sad emotions, andhow we selected the three kinds of touch characteristics aswell as our touch behavior implementation.
A. Robot setup
To investigate the relationships between the expressed
emotions and touch characteristics, we need a robot that can
express emotion by its voice and facial expressions. It also
needs enough degree of freedoms (DOFs) in its arms to touch
people with different touch characteristics. Based on theserequirements, we chose a robot with a human-like appearance
to express emotions by facial expressions and its voice as wellas its arms to touch people in various ways.
We used an android named ERICA [45], which has afeminine appearance and the capability to express human-likefacial expressions. She has three DOFs in her torso and ten ineach of her arms. With the two DOFs on each of her wrists andthe three on her palms, she can touch people with several
touch characteristics. She uses an open-loop movement
control system and can update each of her actuator target
positions every 50 milliseconds. Even though hersilicon-based skin appears very human-like, unfortunately, hertouch feels different from human skin. We put gloves on her
hands to avoid mismatched impressions between her
appearance and the feeling of her touch.
Figure 1 shows ERICA’s facial expressions for happy andsad emotions. We also prepared corresponding fillers andsentences to express her emotions. For instance, when she
expresses a happy emotion, she laughs and says (in Japanese)“I’m really happy.” Voice cues are critical for the cognitive
representations of the facial expressions of emotions in adults[46]. The facial expressions and voice cues are synchronizedwith the start timing of the touch behaviors. For the speechsynthesis function, we used HOYA text-to-speech software(http://voicetext.jp/) that provides her with rich, human-like
Japanese speech.
B. Touch setting
In human-human interaction, people can touch anotherperson to express emotions. A past study investigated therelationship-specific maps of body regions where social touch
is allowed and reported that touching the hands of anotherperson is acceptable regardless of their relationship [47].
Another study investigated the body locations touched byparticipants to convey different emotions and concluded thatthey touched the hands/forearms to express happiness and thehand/shoulders to express sadness [13]. From a different
perspective, another study concluded that participants mainly
touched hands/forearms for both happy/sad emotions withrobots [39, 40]. These differences remain unclear. Since allstudies suggested that touching the hand commonly expresses
happy/sad emotions and that the hand is an appropriate
location for social touching, we chose a participant’s hand as arobot’s touch target.
To guarantee that all the participants experienced the sameinteractions with identical touch behaviors, we positioned
them next to the robot (Fig. 1), allowing them to easily
observe its facial expressions as well as its touching behaviors.
We placed markers on a table to indicate where theparticipants should put their palms and forearms. We asked
them to keep their right hands and arms on those markers
when ERICA did her touch behaviors. The details of thetouch characteristics are explained in Section III.D.
C. Emotions: happiness and sadness
Emotional expression is an essential factor for social
robots to build friendly relationships with interacting people.
In fact, several past studies investigated the positive effects ofa robot’s emotional expressions, including facial expressions,body gestures, and/or speech [48-53]. In this context, showing
positive emotions (typically happiness) is one basic
interaction strategy for such robots in daily environments forfriendly interactions. For example, past studies designed
robots that expressed happiness to build relationships withpeople in long-term interaction settings [54, 55]. Based onthese reasons, we focused on the relationships among touchcharacteristics with which robots more strongly and naturally
express happy emotions.
We simultaneously focused on sad emotions because
happy and sad emotions are considered a pair of bipolar
emotions based on Russell’s circumplex model of affect [56].
In other words, these two emotions have opposite
arouse/valence aspects (happy: high arousal and valence, sad:low arousal and valence). From the perspective of HRI design,
sad emotions are typically used as negative emotions [51-53].
In fact, expressing sadness has richer application scenarios
such as showing empathy [57].
We focused on just two (instead of all six) of Ekman’s
basic emotions for two reasons: 1) comparing all six emotions
combined with all the permutations of touch characteristics
would greatly complicate our analysis, and 2) happiness andsadness are the most typical emotions used in designinghuman-robot interactions, and the scenarios that use theremaining four are situational and less frequent. For example,
anger is another candidate emotion to replace sadness basedon Russell's model; however, in the context of current HRI
applications, anger is probably less common than sadness.
Based on these reasons, we focused on happy and sad emotion
pairs in our study.
D. Touch characteristics: length, type, and partTo change the perceived strength and naturalness of theemotions expressed by a robot, we focused on touchcharacteristics that can show different levels of

arousal/valence. For example, a touch that expresses a higharousal/valence impression is more natural for expressing a
happy emotion with a stronger impression. Although manyother touch characteristics resemble the reasons for selecting
emotions (i.e., difficulties in comparing large numbers), we
investigated three kinds of related touch characteristics: length,
type, and part (Figs. 2). All three items show different
arousal/valence aspects.

their perceived strength and naturalness (i.e., happy and sad)from people will increase. Based on these considerations, wemade the following three hypotheses about the relationships
between emotions and touch characteristics.
Contact
Hand

FingerLength of touch: A past study reported that participants
perceived a longer touch as a significantly negative valence
perspective [58]. On the other hand, to the best of ourknowledge, no past studies in touch interaction directly
investigated the effects of touch characteristics on arousal.
One past study on emotional expressions for a social robot did
report that a gesture’s speed influences its perceived arousal,
e.g., a fast behavior can express higher arousal than a slow
behavior [59]. Another paper [32] reported that the touch
duration with high-arousal situations is relatively longer than
low-arousal situations, even though the touching target is not ahuman being. Therefore, based on these considerations, weprepared short and long touches that express high/low
arousal/valence feelings from the length perspective. Due to alack of published references, we conducted a small pilot study
within our laboratory and heuristically decided the actual
lengths for short and long touches that enable people to feelthe differences between them. We used 0.5- and 2-second
contact durations for the short and long touches (Fig. 3).

Part of touch: Although people use different body parts tomake contact and express meaning [27, 60], e.g., hands,
elbows, upper torso, etc., they are less focused on comparing
arousal/valence perspectives. However, a previous study
focused on the intensity of the touch stimuli [58] and reported
that a high intensity showed a greater arousal than a low
intensity without any valence significance. Another past study
reported that the intensity of the aroused pats is stronger thanrelaxed pats [32]. In this study, we changed the size of area
and the total touch pressure to show different touch intensities,i.e., using hand and finger (Fig. 2).
III. EXPERIMENT
A. Hypotheses and predictions
People can select appropriate touch characteristics thatmatch their own emotions with which they convey their
feelings more strongly and naturally and achieve smoothinteraction with others. In human-robot interaction, expressing
such typical emotions as happiness and sadness are also
important to build smooth interactions and relationships withpeople. If a robot’s touch behaviors match the emotions being
expressed (as designed from an arousal/valence perspective),

Hand

FingerFigure 2. Touch behaviors

Actuator position

Elbow actuator movement for touch behavior
long condition
short condition
C

A

C

A
B

B

Touchby-hand
B

AC

Figure 3. ERICA’s touch behavior in touch condition

Elbow actuator movement for touch behavior
long condition
short condition

Actuator position

Type of touch: A past study reported that participants felt
high arousal and valence with more pulses in the touch stimuli[58]. Another work reported that a patting gesture showed
more aroused emotion than contact without movement [32].
We assumed that a simple contact (Fig. 3) is a single-pulse
type, and a pat-like touch (Fig. 4) is a multi-pulse type.
Therefore, we prepared contact and pat touches that express
high/low arousal/valence feelings in the type perspective. We
also heuristically determined a 50-millisecond stay time forshort-pat touches and a 250-millisecond stay time for long-pat
touches (Fig. 4).

Pat

A

C

A

C

B

E
D

B

D

E

Touchby-finger
A

B

C

D

Figure 4. ERICA’s touch behavior in pat condition

E

Prediction 1: When ERICA expresses a happy emotion, ashort touch will be perceived as stronger and more naturalthan a long touch. When she expresses a sad emotion, a longtouch is perceived as stronger and more natural than a short
touch.

experiencing ERICA’s emotional expression combined with atouch, the participants completed questionnaires (Section
III.E). ERICA repeated the above procedure until theparticipant experienced both emotions. The orders of the touch
behaviors and emotions were counterbalanced.

Prediction 2: When ERICA expresses a happy emotion, a
pat type touch will be perceived as stronger and more naturalthan a contact-type touch. When she expresses a sad emotion,
the contact-type touch will be perceived as stronger and morenatural than the pat-type touch.

E. Measurements
To compare and investigate the perceived emotional
impressions from ERICA’s emotional expressions with atouch to the baseline (i.e., without touching), we asked theparticipants to compare two aspects, strength (“degree ofstrength of the perceived emotion through the android’s
behaviors”) and naturalness (“degree of naturalness of thetouch behavior to express the emotion”) on questionnaires tothe baseline condition, which we evaluated on a 1 to 7 point
scale. Directly in front of the participant, we put a computer onanother desk with a program that displayed a questionnaireeach time ERICA finished a touch behavior.

Prediction 3: When ERICA expresses a happy emotion, afinger touch will be perceived as stronger and more naturalthan a hand touch. When she expresses a sad emotion, a handtouch will be perceived as stronger and more natural than afinger touch.
B. Participants
Twenty-two native Japanese (11 females and 11 males
whose ages ranged from 19 to 39 and averaged 29.0)
participated in our experiment. They were recruited fromcommercially available lists to provide a wide range ofbackgrounds and lifestyles. None had ever interacted with anandroid that touched them.
C. Conditions
This study had a within-participant experiment design, i.e.,each participant experienced all the combinations (16 trials) of
the touch characteristics.
Emotion factor: happy and sad (Section II.C).
Length factor: short and long (Section II.D).
Type factor: contact and pat (Section II.D).
Part factor: hand and finger (Section II.D).
D. Procedure
Before the experiment, the participants were given a brief
description of its purpose and procedure. This research wasapproved by our institution’s ethics committee for studies
involving human participants. Written, informed consent was
obtained from all of them.
First, we explained that the android expresses specificemotions with a touch, facial expressions, and speech. The
participants sat next to ERICA on her left (Fig. 1). Then wecalibrated the table markers to reproduce identical touch
behaviors for all the participants. At the beginning of theexperiment, ERICA greeted them and briefly introduced its
purpose and procedure from her own perspective: thanking
them for helping her collect data to improve herinterpersonal-touching ability. Then she told them that she
would randomly select an emotion and express it withdifferent touch behaviors and asked the participants tocompare its strength and naturalness to baseline conditions.
First, ERICA randomly selected one of the two emotions,only used facial expressions and a set of fillers and sentences
to express that emotion (i.e., happiness or sadness without any
touching), and identified that this was the baseline condition.
Then she randomly selected one of the eight touch behaviors
and expressed an emotion by touch and the same facial
expression and the set of fillers and sentences. After

IV. RESULTS
A. Statistical analysis about strength impressions
We conducted a four-factor mixed ANOVA for each scaleon length, type, part, and emotion for the strength impressions.
For the sphericity of the analysis, note that since the number ofthe levels of the repeated measures is two, sphericity has notbeen violated in this setting. We identified the significant main
effects in the type factor (F(1,21)=5.143, p=.034, partialη2=.374) and in the part factor (F(1,21)=10.337, p=.004,
partial η2=.330). We also identified the simple interaction
effects between emotion and length (F(1,21)=15,717, p=.001,
partial η2=.428) and emotion and type (F(1,21)=22.066,
p=.001, partial η2=.512). No other simple main andinteraction effects were significant (Table I). To verify our
predictions, we conducted a multiple comparison of theinteraction effects.
B. Statistical analysis about naturalness impressions
We also conducted a four-factor mixed ANOVA for eachscale on length, type, part, and emotion for the naturalness
impressions. We identified the significant main effects in thepart factor (F(1,21)=49.941, p=.001, partial η2=.704) and thesimple interaction effects between emotion and length(F(1,21)=14.384, p=.001, partial η2=.407) and emotion andtype (F(1,21)=28.453, p=.001, partial η2=.575). No othersimple main and interaction effects were significant (Table II).
To verify our predictions, we conducted a multiple
comparison of the interaction effects.
C. Verification of prediction 1: touch length
In our analysis of the strength and naturalness impressions,
we found interaction effects between emotions and length.
Therefore, we conducted multiple comparisons with the
Bonferroni method of the simple main effects and identified
significant differences in both the strength and naturalness
impressions (Fig. 5). For strength, we found significant
differences in happy with short > long (p =.022) and sad withlong > short (p =.030). For naturalness, we found significant
differences in happy with short > long (p =.008) and sad withlong > short (p =.006). Therefore, prediction 1 was supported;short was appropriate for happy emotions, and long wasappropriate for sad emotions.

TABLE I.
STATISTICAL RESULTS OF STRENGTH
IMPRESSIONS (BOLD INDICATES THAT P-VALUE IS LESS THAN .05)
p

Length (L)

0.877

T*E

0.001

Type (T)

0.034

P*E

0.649

Part (P)

0.004

L*T*P

0.926

Emotion (E)

0.868

L*T*E

0.530

L*T

0.406

L*P*E

0.943

L*P

0.713

T*P*E

0.441

L*E

0.001

L*T*P*E

0.626

T*P

0.409

p

*

*

Long

Short

Long

Happy

Sad

Happy

Sad

(a)

(b)

Figure 5. Questionnaire results of strength (a) and naturalness (b) of touch
length and emotion factors

7
6
5
4
3
2
1

* : p < .05

7
6
5
4
3
2
1

* : p < .05

Contact

Pat

Contact

Pat

Happy

Sad

Happy

Sad

(a)

TABLE II.
STATISTICAL RESULTS OF NATURALNESSIMPRESSIONS (BOLD INDICATES THAT P-VALUE IS LESS THAN .05)
SourceShort*

Source* : p < .05

*

p

7
6
5
4
3
2
1

*

Source* : p < .05

*

7
6
5
4
3
2
1

*

E. Verification of prediction 3: touch partIn our analysis of the strength and naturalness impressions,
we did not find any interaction effects between emotions andpart (Fig. 7); we only found a significant difference of theeffect in the part factor. Finger touches are better than handtouches for both the happy and sad emotions based on strengthand naturalness perspectives (p=.004 and p=.001). Prediction
3 was not supported.

F. Additional analysis
In this study, we eliminated the gender factor due to a large
number of factors; however, as an additional analysis, we
conducted a five-factor ANOVA by considering it. Our results
did not show any significant effects of gender. One possible
inference about why there is no significant effect in the gendereffects is the toucher’s gender (i.e., female-appearance robot).
If we conducted the same experiment with a male-type
android, gender effects might be apparent due to gender
combinations.

*D. Verification of prediction 2: touch typeIn our analysis of the strength and naturalness impressions,
we also found interaction effects between emotions and type.
Therefore, we conducted multiple comparisons with the
Bonferroni method of the simple main effects and identified
significant differences in both the strength and naturalness
impressions (Fig. 6). For strength, we found significant
differences in happy with happy with pat > contact (p =.012)
and sad with contact > pat (p =.001). For naturalness, happy
with pat > contact (p =.030), and sad with contact > pat (p=.001). Therefore, prediction 2 was supported; pat wasappropriate for happy emotions, and contact was appropriate
for sad emotions.

Source(b)

Figure 6. Questionnaire results of strength (a) and naturalness (b) of
touch type and emotion factorspLength (L)

0.835

T*E

0.001

Type (T)

0.086

P*E

0.331

Part (P)

0.001

L*T*P

0.316

Emotion (E)

0.656

L*T*E

0.767

L*T

0.883

L*P*E

0.838

L*P

0.318

T*P*E

0.762

L*E

0.001

L*T*P*E

0.334

T*P

0.494

* : p < .05

7
6
5
4
3
2
1

* : p < .05

7
6
5
4
3
2
1

*

*

Hand

FingerHand

Finger

Happy

Sad

Happy

Sad

(a)

(b)

Figure 7. Questionnaire results of strength (a) and naturalness (b) of
touch-part and emotion factors

Our experiment results showed several significant effects
in the touch characteristics, but not all exceeded the baseline.
In other words, we did not compare the effectiveness of eachtouch characteristic with the baseline. We conducted a
two-tailed binominal test (we classified questionnaire results
into two classes: strong/natural (5 to 7) or weak/unnatural (1 to4)) to investigate whether each touch is significantly strong or
natural compared to the baseline. Our results showed similar
trends to the ANOVA results. Note that the detailed results
about the two-tailed binominal test are described in the
supplemental material.
Our analysis results showed similar trends between theperceived strength and naturalness. Therefore, we investigatedthe correlation between strength and naturalness (happy:
r=0.69, p<.001, sad: r=0.61, p<.001). Although these values
show a positive correlation in each emotion, they are notstrong (i.e., not more than .80).
V. DISCUSSION
A. Design implications
Our experimental results showed that choosing
appropriate touch characteristics is helpful for designingstronger and more natural touch behaviors for robots that are
expressing happy and sad emotions. At least for our robot,
ERICA, which has a feminine-like appearance, short patting
by fingers and longer contacting by fingers were better touch
behaviors for expressing happy and sad emotions. Even if
other kinds of robots need touch interaction to show theiremotions, the results suggest that touch length and types are
more useful than the touch part.
Based on our data analysis, at least two touchcharacteristics are important: touch length and touch type.
Even if hypothesis 3 is not supported, the statistical analysis
showed a significant main effect in the part factor for both thestrength and naturalness impressions (strength: finger > hand,
p=.004, naturalness: finger > hand, p=.001). These results
suggest that the touch part (i.e., finger or hand) might not be
explained from the aspects of arousal/valence. But an androidthat is touching with her fingers might be considered more
appropriate by the participants.
In a past study that described how people conveyed theiremotions to a robot by touch [39], we found several common
characteristics with our study. For example, that study
reported that the mean duration touch is relatively long in sad
emotions. Our study also showed that a longer touch provided
stronger and more natural impressions to express sad emotions.
Comparing the effects of touch characteristics between
touches from people and robots might provide interesting
phenomena.
B. Different touch characteristics
We focused on three kinds of touch characteristics basedon arousal/valence perspectives. However, other kinds oftouch characteristics should be considered. In our study, therobot’s touches were relatively light for safety considerations,but applying greater strength might be one essential
characteristic to express such emotions as anger. Using suchother touch behaviors as gripping or stroking is another crucial
factor for expressing emotions and should be implemented infuture robot touch designs.

Moreover, our robot only touched the hand of theparticipants. Testing people’s impressions when they are
touched on the forearms, shoulders, or even faces would
deepen our understanding of the relationships between touchcharacteristics and the emotions expressed by robots. Suchelements as contact temperature [61] or applied pressure [62]are also critical factors that affect people’s perception when
they are being touched.
From another perspective, the position relationshipbetween robots and people also has an influence. In this study,
participants sat next to the robot where they could easily see its
facial expressions and touch behaviors, but in real settings,
people can touch others under various positional relationships.
Since such relationships also limit the potential touch part andits characteristics, investigating these effects is anotherinteresting future work.
C. Expressing other emotions
In this study we only focused on happy and sad emotions,because the former is often used in HRI applications [48-53]
and the latter is an contrary emotion following Russell’s
definition [56]. Comparing the effects of touch characteristics
from arousal and valence perspectives might illuminate
human-robot touch interaction designs. We did not investigate
whether our knowledge is applicable for expressing the otherfour emotions from Ekman’s basic emotions.
Social interaction scenarios for using these four emotions
in current human-robot contexts are relatively less significant
than the happy and sad emotion pair. However, social robots
will need to express more complex feelings to interact withpeople in daily environments. Thus, investigating therelationships between touch characteristics and all the basic
emotions is an interesting future work.
D. Gender, appearance, and hand shape effects
In this study we used an android with a feminine
appearance, although human-science literature argues thatperceived gender changes the touch impressions [60, 63].
Since another study also reported that gender influences theperceived impressions of a robot’s touch [64], we should
address the effects of a robot’s gender and appearance before
applying our knowledge to other robots. A masculine-looking
robot might increase the knowledge’s usefulness.
Moreover, our android has hands that resemble humanhands. Recent social robots, like Pepper, have human-likehands, but many have a more machine-like appearance andsimple-shaped hands like Robovie [65]. Such robots might
have difficulty changing the part characteristics (i.e., hand or
finger). Moreover, we put gloves on the robot’s hands to avoid
mismatched impressions between her appearance and the
feeling of her touch, because such impressions often produce
uncanny valley effects. In touch situations, researchers must
consider the uncanny valley effects from movements or touchfeelings as well as appearances. Even if the appearances are
human-like, inappropriate movements and touch feelings
easily evoke uncanny valley effects and unnatural feelings tointeraction with the robot. At a minimum, touch feelings
should match the assumption caused by its appearance. Basedon these considerations, more knowledge about the
combinations of other touch characteristics is needed fordesigning a robot’s touch behaviors in emotional interaction.

E. Limitation and future work
We need to scrutinize our analysis results. Since we onlyused a specific android robot with a female appearance, before
generalizing our experimental results we must test different
types of android robots: different appearances, including
gender, age, as well as robots with more robotic appearancessuch as Pepper and Robovie. The knowledge from our study
may only be applicable to social robots that are designed forsuch interactions.
Moreover, how people interpret touch behaviors often
reflects their social status [66], interpersonal relationships,
culture backgrounds, and gender [67]. In this study the robot
only touched the hands of Japanese participants, i.e., the effectof other body parts and different culture backgrounds was notevaluated. Investigating these factors would be interesting tounderstand what touch characteristics are (not) commonly
accepted in haptic communication to express emotions.
Additional qualitative approaches such as interview analysis
and open-question items might also support such analysis.
Another possible future work is to conduct a CT afferent
study and/or using several common devices or measurementsin the research field, such as EEG, NIRS, and/or fMRI, toinvestigate the effects of touch characteristics for usefulknowledge about human-robot touch interaction.
VI. CONCLUSION
We investigated the relationship between an android’s
touch characteristics and its perceived strength andnaturalness in the context of expressing typical emotions to aninteracting person in human-robot interaction, happy and sad.
Although human beings often convey emotions by touching,such knowledge is sparse for designing appropriate touch
behaviors for a social robot to express emotions. Therefore,
we selected three kinds of touch characteristics byconsidering arousal/valences and investigated their effects.
We found that touch length and touch type affected theperceived strength and the naturalness of the emotions
expressed by our robot. In addition, our results suggest that a
robot that touches with its fingers is deemed more appropriate
for both strength and naturalness than using its hand. Basedon these analyses, we concluded that a short pat and a longcontact by the fingers are respectively better touch behaviors
to express happy and sad emotions, at least for our robot,
ERICA, who has a feminine appearance.
ACKNOWLEDGEMENT
We would like to thank Sayuri Yamauchi for her help inorganizing the experiment.
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