Does a Robot’s Subtle Pause in Reaction Time to People’s Touch
Contribute to Positive Influences? *Masahiro Shiomi, Kodai Shatani, Takashi Minato, and Hiroshi Ishiguro, Member, IEEE

Abstract—This paper addresses the effects of a subtle pause
in reactions during human-robot touch interactions. Based on
the human scientific literature, people's reaction times to touch
stimuli range from 150 to 400 msec. Therefore, we decided touse a subtle pause with a similar length for reactions for morenatural human-robot touch interactions. On the other hand, inthe human-robot interaction research field, a past study reportsthat people prefer reactions from a robot in touch interactionthat are as quick as possible, i.e., a 0- second reaction time isslightly preferred to 1- or 2- second reaction times. We note thatsince the resolution of the study’s time slices was every second, it
remains unknown whether a robot should take a pause ofhundreds of milliseconds for a more natural reaction time. To
investigate the effects of subtle pauses in touch interaction, we
experimentally investigated the effects of reaction time topeople’s touch with a 200-msec resolution of time slices between
0 second and 1 second: 0 second, 200, 400, 600, and 800 msec.
The number of people who preferred the reactions with subtle
pauses exceeded the number who preferred the 0- second
reactions. However, the questionnaire scores did not show any
significant differences because of individual differences, eventhough the 400-msec pause was slightly preferred to the othersfrom the preference perspective.

I. INTRODUCTION
Reaction time design for interacting systems is one
essential factor for smooth and non-frustrating communication.
For example, we usually prefer a quick response from suchcomputer applications as web browsers and integrated
development environments. In fact, past studies onhuman-computer interaction reported that people preferred areaction time less than 1 second from such computer systems[1, 2]. Guynes supported the two second rule, a well-known
guideline for designing system response times, i.e., that argues
that computer systems should be designed to respond within 2
seconds [3].
Is such a guideline for a reaction time design applicable forhuman-robot interaction? Several research works investigated
the reaction time effects in human-robot interaction andreported that social robots basically should respond as quickly
as possible, but interaction modality influences people's
preferred reaction times. For example, in conversational
interaction, a 1-second reaction time is slightly preferred (butnot significant) to a 0- second reaction time, and such reaction
times should be less than 2 seconds, similar tohuman-computer systems [4]. In touch interactions, we* This research work was supported by the JST ERATO Ishiguro
Symbiotic Human Robot Interaction Project (Grant Number: JPMJER1401),
and JSPS KAKENHI Grant Numbers JP16K12505 and JP17K00293.
M. Shiomi, K. Shatani, T. Minato, and H. Ishiguro are with ATR, Kyoto,
Japan. (e-mail: m-shiomi@atr.jp) K. Shatani and H. Ishiguro are also with
Osaka Univ., Osaka, Japan.

reported that a 0-second reaction time is slightly better than a
1-second reaction time (but also not significant) andsignificantly better than 2 seconds [5] (the solid line in Fig. 1
shows our study’s preference scores).
However, the resolution of the time slices in the above
studies was every second, even though people’s reaction times
were on the order of hundreds of milliseconds. For example, inthe literature, reaction times are about 150 to 200 msec forvisual, audio, and touch stimuli [6-9]. A past study reported
that reaction times to touch stimuli include wide variances that
range from 200 to 400 msec [10]. Therefore, it remains
unknown how subtle pauses in reaction times will influence
the preferences of people in human-robot interaction contexts.
In particular for touch interactions, we hypothesized thatpeople will prefer intervals of hundreds of milliseconds inreaction time over 0-sec reaction times (the dotted line in Fig.
1 shows our assumption), because responding too quickly tobeing touched might seem unnatural.
To investigate the effects of the intervals of hundreds ofmilliseconds in reaction time during human-robot touch
interaction, we experimented with an android named ERICA
that has a human-like appearance. Since we placed a touch
sensor on her shoulder, she reacts to being touched there by aperson. In this study, we used her and experimentallyinvestigated the effects of subtle pauses in human-robot touch
interaction and addressed the following research questions:- Do people prefer reaction times of hundreds ofmilliseconds over 0-second reaction times in human-robot
touch interaction?
- If so, does following a human-like reaction time (e.g.,
200 or 400 msec) improve preference and human-likenessfeelings about an android?
7?

6

Preference



5
4

3
2
1

0

1
2
System Reaction Time [seconds]

Figure 1. Preferences of reaction time in human-robot touch interaction
based on our previous work [5] (solid line) and an illustration of ourhypothesis (dotted line): do reaction times of hundreds of milliseconds
change impressions in human-robot touch interaction?

II. RELATED WORK
Simple reaction times of humans have been broadly
investigated by many researchers. Past studies reported thatthe reaction time to audio stimuli is basically faster than thereaction time to visual stimuli: about 140 to 160 msec to audio
stimuli and 180 to 200 msec to visual stimuli [6-8]. For touch
stimuli, a past study reported reaction times of about 150 msec[9], but another concluded that the reaction time to touch
stimuli is between 200 to 400 msec [10]. Reaction times tovibration (haptic) stimuli have been reported to be faster than
audio and visual stimuli [11], but the average response time
was about 200 msec.
In human-robot interaction, several researchersinvestigated the reaction time of robots for more naturalcommunication [12] [13]. For example, Yamamoto et al.
reported that a robot should react within 300 msec at theexchange of conversational greetings [12], and Kanda et al.
reported that a robot’s gestural reaction should be delayed for890 msec to contribute to natural feelings in a route guidanceinteraction [13]. Shiwa et al. investigated the effects ofreaction time in conversation settings and compared 0 to 3
seconds with 1-second time intervals and concluded that a
robot should respond within 2 seconds [4]. These studies
reported that basically robots should respond quickly, andsuggested that a subtle pause would contribute to makeappropriate tempos in interaction.
Several researchers identified touch interaction effects inhuman-robot interaction from various viewpoints: mental
health support in elderly care with a touchable pet-type robot[14], huggable robots for stress buffering effects [15],encouraging self-disclosure and/or prosocial behaviors[16-20], appropriate communication cues in touch interaction[21, 22], motivation improvement through mutual touch
between people and a robot [23], long-term conversational
interaction with a tele-operated huggable robot for elderly
support [24], anxiety reduction through conversation with ahuggable robot [25], and people preferred mutual contact withthe robots despite initial preference of subject to initiate touch[26]. Related to a robot’s reaction time in a touch interaction
context, a past study investigated the appropriate reaction timein human-robot touch interaction settings and reported that the
robot should also react to being touched within two seconds[5].
However, these studies on reaction time effects inhuman-robot interaction only tested limited time slices:
1-second time intervals. Subtle pause effects in responses
remain uninvestigated. In fact, the resolution of people’s
reaction times is hundreds of milliseconds, and therefore only
using 1-second resolutions is insufficient to understand how
human perceptions change due to subtle pauses in reaction
times. Therefore, in this study we focused on the effects ofintervals of hundreds of milliseconds in a robot’s reaction
time.
III. EXPERIMENT DESIGNIn this section, first we describe the details of the robot andthe sensor for our experiment. Next, we refer to related works
about reaction time in human-robot touch interaction anddescribe the details of our reaction time design for our
experiment settings.

Figure 2. ERICA and a touch sensor on her shoulder

Figure 3. Participant touches ERICA from behind, and she turns toward a
touched person

A. Robot and sensor
In this study, we used ERICA, an autonomousconversational android characterized by its female-likeappearance [27] (Fig. 2, left). She has 44 DOFs for her torso
and face as well as both network connection and voicesynthesis functions. To detect being touched, we installed onher left shoulder a touch sensor called ShokacCube by
Touchence (Fig. 2, right), which can measure the heightchanges on the top surface of a soft material with 16
measurement points. This sensor is 36 x 20 x 30 mm and sends
information with 100 Hz at maximum. We installed it on her
left shoulder that is tapped by participants in our experiment.
When the sensor detects a particular amount of pressure, thisinformation is sent to the robot system through a network.
B. Touch interaction design
To investigate the reaction time effects in human-robot
touch interaction, we modified our past study’s interaction
style [5]. In our setting, first the participants stand behind andto the left of the robot so that neither their positions nor their
touch behaviors are visible to it (Fig. 3); at the past study, theparticipants entered to the room and then touched the robot
which is chatting with other robot, but to build a simple
situation we modified the setting. Moreover, since a past study
reported that the awareness of a touch influences impressions
about reaction time effects [5], we fixed the position
relationship between the robot and the participants. The
participants assume that the robot cannot estimate the touchtiming in this position relationship.

P
7.6 m

E

E

ERICA

P

Participant

8.4 m
Figure 4. Experimental environment

C. Design of response time
As described above, several studies investigated thepreferred reaction times of robots to human actions (speech or
touch). But since the time slice resolutions were 1 second, theeffects of intervals of hundreds of milliseconds between 0 and
1 seconds in reaction time remain unknown.
In human science literature, reaction time to touch stimuli
ranges from 150 to 400 msec [6-10] and shows a wide
variance. Hence, using excessively short time resolutions (like50 msec) would require a huge number of conditions andwould not be appropriate to investigate the reaction timeeffects. Moreover, controlling the robot’s reaction behavior
with a quite precise frequency is difficult because of its
servo-valve actuator characteristics. Based on these factors,
we determined the time slice resolutions as 200 milliseconds
for this study, i.e., we investigated the reaction time effects
between 0 to less than 1 second in 200-msec intervals andcompared 0, 200, 400, 600 and 800 msec as a reaction time
factor.
IV. EXPERIMENT
We conducted a laboratory experiment to investigate thereaction time effects for a robot being touched by people basedon intervals of hundreds of milliseconds.
A. Hypotheses and predictions
Past studies in human-robot interaction argued that robots
should react to their interaction partners as quickly as possible,
regardless of such modalities as speech or touch [4, 5, 12, 13].
In particular, for touch interaction, a 0-second response time isslightly better than a 1-second interval [5]. But that study onlyused 1-second resolution intervals for comparing reaction
times and focused less on short-time intervals like hundreds ofmillisecond pauses.
In human science literature, people’s reaction time totouch stimuli is around 150 milliseconds [9]. The reaction
times to touch stimuli range from 200 to 400 msec [10]. We
believe that a robot should follow human-like reaction times
for more natural interactions in human-robot touch interaction,
similar to other interaction modalities such as conversationand gestures [12, 13]. Adding intervals of hundreds ofmilliseconds for the reaction times will increase the robot’s
human-likeness and people’s preferences, especially when theinterval values resemble human values. Based on these
considerations, we made the following hypotheses:

Prediction 1: The number of people who prefer a responsetime that includes intervals of hundreds of milliseconds will belarger than people who prefer the 0-second response time.
Prediction 2: People’s preference ratings will peak when
the response time ranges were 200 or 400 msec.
Prediction 3: The number of people who feel that the
robot seems more human-like in the response times (which
include intervals of hundreds of milliseconds) will be larger
than people who prefer the 0-second response time.
Prediction 4: People’s human-likeness ratings will peakwhen the robot’s response time ranges were 200 or 400 msec.
B. Participants
Twenty people (ten women and ten men) were paid fortheir participation in this experiment. Their average ages were23.2, SD 1.82.
C. Environment
We conducted our experiment in an 8.4 x 7.6 m room in alaboratory where we set the robot. The participants stoodbehind and to the left of the robot during the experiment (Fig.
4).
D. Conditions
Our experiment had a within-participant design with thefollowing reaction time factor. The order of the conditions wascounterbalanced.
Reaction time factor: For this factor, we prepared fiveconditions: 0 second, 200, 400, 600, and 800 msec. These time
periods indicate the duration between being touched by aparticipant and responsive speech to it. A 0-second reaction
time indicated that when the touch sensor detected a particularamount of pressure, the robot immediately spoke and turned
toward the participant. Note that the looking behavior wasdelayed about 400 msec compared to the speech timing due tothe characteristics of its servo-valve actuators and network
connections.
E. Procedure
Before the first session, the participants were given a brief
description of our experiment’s purpose and procedure. This
research was approved by our institution’s ethics committee
for studies involving human participants. Written, informed
consent was obtained from all of them.
In addition, we showed our robot and literally
demonstrated how to touch her shoulder. We asked them totouch it as lightly as if they were applying a similar touch toanother person. The participants joined five sessions based on
response time factors. After each session, they filled out
questionnaires.
F. Measurements
We investigated whether response times of hundreds ofmilliseconds changed their impressions of their preferences
and the robot’s human-likeness. We prepared twoquestionnaire items: the robot’s human-likeness and reaction
timing preferences. The items were evaluated on a 1-to-7 point
scale, where 7 is the most positive.

V. RESULTS
A. Verification of prediction 1
To measure the number of participants who preferred areaction time that included intervals of hundreds ofmilliseconds, we classified the participants into two categories
based on questionnaire scores between a 0-second reaction
time and other reaction times (200 to 800 msec) (Table 1):
preferring 0 second or non-zero second. If a participant’s
questionnaire score in any non-zero reaction time exceeds thequestionnaire score in the 0-second reaction time, theparticipant is classified in the preferring non-zero second
category. If the questionnaire score in the 0 second and the
maximum score in any non-zero reaction time is the same (e.g.,a user gave the highest rating both for 0-second and 200 msec,
or only 0-second), the participant is classified in the preferring
0-second category.

E. Summary
The experiment results showed that the number of people
who preferred non-zero second reaction times is larger than
the number who preferred 0-second reaction times from apreference viewpoint (prediction 1). But their questionnairescores about preference did not show a significant difference,even though the questionnaire values at 400 msec wereslightly higher than the others. Moreover, from ahuman-likeness viewpoint, there were no significant
differences for either the number of people or thequestionnaire scores among the conditions.

TABLE I.

We conducted a two-tailed binominal test for these values
and found a significant difference between the two categories(p=.041). Thus, prediction 1 is supported; the number ofpeople who preferred reaction times that included intervals of
hundreds of milliseconds was larger than people who
preferred the 0-second reaction time.

Number

B Verification of prediction 2
Figure 5 shows the questionnaire results of the preferences.
We conducted a one-way repeated measures ANOVA for thereaction time factor, and the results did not show significant
differences in the response time factor (F(4,76)=0.702, p=.593,
η2 = 0.036). Thus, prediction 2 was not supported; people’s
preference ratings did not peak when the reaction time ranged
from 200 or 400 milliseconds.

5

C. Verification of prediction 3
To measure the number of participants who felt morehuman-likeness to the robot in the response time that includes
intervals of hundreds of milliseconds, we classified theparticipants into two categories based on questionnaire scores
between 0-second reaction times and other reaction times (200to 800 msec) as shown in Table 2: most human-like at 0
second or most human-like at non-zero second. Thus, if aparticipant’s questionnaire score in any non-zero reaction timeexceeds the questionnaire score in the 0-second reaction time,
the participant is classified into human-like in the non-zero
second.

NUMBER OF PEOPLE IN PREFERENCE CATEGORIES
Preferring 0
secondPreferring non-zero
secondp-value

515

p < .05

7
6

4

3
2
1
0

200

400
Milliseconds

600

800

Figure 5. Questionnaire results about preferences

TABLE II.

NumberWe conducted a two-tailed binominal test for these values
and found a significant difference between the two categories(p=.503). Thus, prediction 3 was not supported; the number ofpeople who felt more human-like impressions in the response
times that included intervals of hundreds of milliseconds was
not larger than people who preferred the 0-second reaction
time.

7

D. Verification of prediction 4
Figure 6 shows the questionnaire results abouthuman-likeness impressions. We conducted a one-way
repeated measures ANOVA for the reaction time factor, andthe results did not show significant differences (F(4,76)=1.217,
p=.311, η2 = 0.060). Thus, prediction 4 was not supported;
people’s human-likeness ratings did not peak when the
response time were 200 or 400 msec.

3

NUMBER OF PEOPLE IN HUMAN-LIKENESS CATEGORIES
Most human-like
at 0 second

Most human-like at
non-zero second

p-value812

p = n. s.

6
5
4

2
1
0

200

400
Milliseconds

600

800

Figure 6. Questionnaire results of human-likeness feelings

VI. DISCUSSION
A. Implications
The experimental results showed that intervals of hundredsof milliseconds contributed to people’s preferences. Even
though such intervals did not significantly influence
impressions of human-likeness, further study of subtle pauses
will be useful for designing reaction behaviors inhuman-robot touch interaction.
We directly compared the questionnaire results and foundno significant differences between the interval times in either
the preferences or the human-likeness viewpoints, eventhough a 400-msec pause produced a slightly better
preference feeling than the other pauses1, and the correlations
between preferences and human-likeness rating showed
relationship between them: 0 second: r = 0.429, p = .059, 200
msec: r = 0.412, p = .071, 400 msec: r = 0.605, p = .005, 600
msec: r = 0.742, p < .001, and 800 msec: r = 0.303, p=.194.
The main reason for these results is probably related to theindividual differences of participants. Since appropriate
reaction times for individuals are highly subjective, robots
need to adapt to them.
Similar to this personalization concept in human-robot
interaction, a past study reported that since the preferredpersonal distance is different among individuals, robotics
researchers tried to adapt such differences during interaction
for social robots by adjusting the interaction distance between
partners [28]. Related to this approach, adaptations ofreaction time during continuous interaction might be helpful
to identify better reaction times for each interaction partner.
For example, a social robot could employ 400 msec as a basic
reaction time (because in our study this time interval showedslightly better impressions than others) and then modify its
own reaction time by observing its partner’s feelings and/or
reactions.
B. Awareness of being touched
In this study the participants touched the robot from behindand to the side to avoid awareness effects, because if the robot
can literally see a potential touch from the participants, i.e.,
when it is aware that it might be touched, the preferred
reaction times and human-likeness will change. For example,
if the participants touched from the front of the robot, theywould assume that the robot would understand the actual
timing of the touch. In such settings, people might preferfaster reaction times than those in our settings.
Reaction behaviors to being touched also change theimpressions of people. Our previous study investigated theeffects of a subtle reaction when the robot is touched andreported that it would be better to make a subtle reaction when
it wasn’t aware in advance, as humans do [5]. Such a subtle
reaction behavior would be useful to make subtle pauses more
1
For reference, if we directly compared the questionnaire results between
0 second and 400 msec with a paired t-test, it showed significant differences(t(19)=2.565, p=.019, r=.51), but a paired t-test between 0 second and 400
msec for human-likeness did not show any significant differences(t(19)=1.324, p=.201, r=.29).

naturally, e.g., because employing 400 msec as the time
length of a subtle reaction behavior will contribute to people's
preferences.
Even though such awareness effects are beyond the scopeof this study, investigating them will contribute to future
understanding of the relationship between awareness andpreferred response time.
C. Other factors related to response time design
What other factors influence appropriate reaction time
design in human-robot interaction? A robot’s appearance andsize might influence the impressions of participants in touch
interactions. In this study we only used an android with afemale appearance; if we use a robot with a moremachine-like appearance such as Pepper (SoftBank
Robotics 2 ) or a small-sized robot like Sota (Vstone 3 ), therelationships between appearances and appropriate reaction
time would be different. Investigating them is interesting
future work.
Interestingly, a past study reported that participants with agreater body mass index reacted significantly slower thanother participants [29], suggesting that people assume aslower reaction time from a large-sized robot than asmall-sized one. From another perspective, several studies
reported gender effects of reaction time, e.g., males react
more quickly than females [30, 31], perhaps caused by theaverage amount of muscles. Therefore, a robot’s perceived
gender might influence the preferences of its reaction times.
Touch strength also influences reaction time. Usuallypeople react more quickly to heavy touches than lighttouches, and therefore robots must also adhere to suchdifferent reactions to the strengths of a touch. In this study weemployed the same reaction behavior of the robot to theexperiment regardless of strength, but its reaction time andbehavior (speech and looking motion) can be modified basedon the touch’s strength.
D. Limitations
This study has several limitations. Its experiment was
conducted with our android robot and a specific situation,
where participants just touched it from behind. Moreover, therobot’s bodily response was delayed compared to the verbal
response, due to its hardware limitations. Therefore, wecannot ensure that our findings can be applied to all
human-robot touch interaction situations. To generalize thereaction time effects, we need to investigate them withdifferent situations, e.g., participants who are visible to therobot before touch interactions and with other kinds of robots.
In addition, this experiment result about preference
showed slightly different phenomenon from the past study[5]; a 0-second reaction time is not better than a 1-second
reaction time, which might be caused by the difference of
touching situation. It would indicate that touch situations
would have influences to preferred reaction time of robots.
2
https://www.ald.softbankrobotics.com/en/robots/pepper, Last accessed:
2018/03/09.
3
https://www.vstone.co.jp/english/, Last accessed: 2018/03/09.

However, we believe that our setting offers essential
knowledge for researchers who are interested in human-robot
touch interactions.
VII. CONCLUSION
In this study, we focused on the effects of reaction times
with hundreds of millisecond resolutions in a robot’s reaction
behavior when the robot is being touched by a person. For thispurpose, we used a touch sensor and an android robot toreproduce touch interaction situations; the robot canautonomously react to being touched using speech and alooking behavior. We experimentally compared thepreferences and the human-likeness impressions ofparticipants to the robot with different reaction times to being
touched.
Our experiment results showed that the number of people
who preferred a reaction time that includes intervals of
hundreds of milliseconds is more than the number of people
who preferred a 0-second reaction time. On the other hand,even though the experiment results showed that reaction
times of 400 milliseconds were slightly better than otherreaction times for the preference viewpoint, the questionnaire
results between the conditions did not show any significant
differences. Moreover, the human-likeness feeling did notshow any significant differences in either the number ofpeople who preferred the reaction time between 0 second andnon-zero second or the questionnaire results.

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