KUSUGURI: A Shared Tactile Interface
for Bidirectional TicklingMasahiro Furukawa

Hiroyuki Kajimoto

Susumu Tachi

Keio University
Graduate School of Media Design
4-1-1 Hiyoshi Kohoku-ku Yokohama
Kanagawa 223-0061+81-45-548-5688, JP

The University of ElectroCommunications
1-5-1 Chofugaoka Chofu
Tokyo 182-8585+81-42-443-5445, JP

Keio University
Graduate School of Media Design
4-1-1 Hiyoshi Kohoku-ku Yokohama
Kanagawa 223-0061+81-45-548-5688, JP

m.furukawa@tachilab.org

kajimoto@hc.uec.ac.jp

tachi@tachilab.org

ABSTRACT
Tickling, a nonverbal form of communication, can provide
entertainment. Therefore, tickling is a desirable addition ascontent as a remote communication method. However, ticklingis difficult to realize because it requires both body contact aswell as bidirectionality. In this paper, we propose a method of“Shared Tactile Interface” which allows sharing of a body partwith another user at a distance. The interface has three features:
direct contact, transfer of the tickling sensation, andbidirectionality. The first allows users to view another person’s
finger as if it is directly contacting the user’s own palm andmoving on the user’s palm. The second feature delivers avibration to the user’s palm which generates an illusion andperception of a tickling sensation. The third feature enables
bidirectional tickling because one user can also tickle the otheruser’s palm in the same manner. We built prototypes based on
this design method, and evaluated the proposed method throughtwo technical exhibitions. The users were able to tickle eachother, which confirmed that the design method “Shared Tactile
Interface” works as expected. However, we found issues
especially regarding the reliability of the tickling sensation.

Categories and Subject Descriptors
H.5.2 [Information Interfaces and Presentation]: User
Interfaces – Haptic I/O, Screen design, Theory and methods

General Terms
Design, Theory, Human Factor

Keywords
Tickling Sensation, Tactile Illusion, Haptic Communication,
Tactile Communication, Tactile Interface

1. INTRODUCTION
Tickling has an important role in fostering a sense of trust [1].
In general, we can talk to each other with voice by telephone
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Figure 1. Concept of the “Shared Tactile Interface”.
(a) Being tickled directly, (b) Being tickled virtually with
the Shared Tactile Interface, (c) Tickling each other with
our proposed method
and we can also use gestures with video chat, even if we cannot
meet person-to-person. On the other hand, it is still difficult tocultivate our relationships with physical contact at a distance.
Tickling as an informal form of communication is not easy torealize across distances because tickling is based both ontouching each other and on reciprocal touching.
For tickling devices to be effective, both users need to use themto tickle each other. However, employing these devices requires
a mechanism that tends to be quite large. In addition, ticklingworks as communication only when each person is able totouch the other; otherwise, it lacks context. In addition, onesided tickling can harass another person.
In this paper, we propose a design method that we call a“Shared Tactile Interface.” The device shown in Figure 1 (b)allows the user to share a part of the body in order to tickle asecond person at a distance. To achieve this, we focus on thefeature of tickling where a feeling of being tickled occurs whentickling is initiated by another, but not by oneself. This featuretakes into account the use of a part of body not only in order totickle another person, but also in order to be tickled. Thismethod provides the users with the experience of tickling eachother with handheld devices without losing general-purpose
properties. In other words, our proposed method is expected tobe simple but also will achieve scalability.
We found that the tickling as a form of nonverbal
communication requires the following three conditions: thetickling sensation must be transferred; bidirectional contact isneeded; and bidirectional tickling must be sensed. Of course,
the user at the other end must feel the tickling sensation.
Bidirectional contact between users is necessary for interactive

tickling. No method has yet been proposed that fulfills thesethree conditions. The contributions of this paper are thefollowing:
1. Provides a method that gives the illusion of tickling
2. Measures and transfers a tickling sensation
3. Provides a visual design method that gives the impressionof users touching each other

2. RELATED WORKThe goal of this research is for two users to tickle each otheracross a long distance. Typically, a vibration motor is attached
to a body [2][3] to achieve a sensation. Kume proposed theFoot interface, which has vibration motors on the underside ofa slipper shaped device, and provides the sensation of
movement of tickling insects moving under the slipper when
the user steps down [2]. However, this sensation is notbidirectional. Tsetserukou et al. reported a wearable humanoid
robot that included a tickling device called the “HaptiTickler”,
which allowed users to tickle each other [3]. This method,
however, requires a virtual space and avatar in addition to thetickling device.
Methods that do not use vibration motors have also been
proposed; for example, the tickling of crawling insects has beenrealized by random direct contact of small hard wires to theskin [4]. This method requires a large number of spatial
actuators although its effect has a high degree of reality.
Kitagawa determined that a sound recorded by an inner ear
microphone in a dummy head when its ear is stroked with apaintbrush delivers a tickling sensation to a listener. However,
applying this method for bidirectional tickling is difficult
because it requires visual feedback for the listener during
stroking.
Bidirectional haptic or tactile communication can be roughly
divided into two methods: a way to deliver a force [6][7], and a
way to deliver simulated gestures [8][9]. Scott has proposed therolling tube, named “inTouch,” which allows transfer of anactive gesture as a torque, but also bidirectional perception of
the gesture. However, in this case, the purpose is not for users
to touch each other. Eichhorn has reported a handheld device
that allows a user to stroke each other [8]. However, this device
is not suitable for general purposes; it is designed only for thispurpose. Hemmert has designed a mobile phone-shaped device
that allows kissing and whispering. It is difficult to achieve thisconcept with a small handheld device because it needs to be
driven by an outer actuator.

Figure 2. Visual Design of the Shared Tactile Interface(a) Two static images are placed on the display of a
handheld device, (b) Only the finger movement, as atickling gesture, is transmitted to the other side.

Pseudo-haptics are often used for haptic communication.
Tsukada proposed a way to superimpose the movement of acursor, which shows each user’s gesture inputs on the faces andshoulders in each screen during a video chat [10]. Although themethod allows the users to stroke each other, this method
differs from our proposed method in two points: the physicalbody is not used to superimpose the gestures, and exchange ofvideo streams is required.
Methods using a part of the user’s body as a human-computerinterface fall roughly into two classifications: input methods[11][12][13] and output methods [14]. Harrison et al. haveproposed the “Skinput” method, which allows the user to tap
buttons projected on the user’s own forearm. This method
appears to be difficult to apply for measuring a tickling gesturebecause it requires detection of a progressive wave propagating
through skin and bone, although the method is sufficiently
accurate for use as a tapping interface. Pranav has introduced
an input method called “Sixth Sense” [12]. With this method, it
is necessary to run a computer vision program and place anexternal camera to track the fingertip during the gesture.
Nakatsuma et al. have proposed a wrist-watched shaped device,
which can track the point where the finger taps the back of thehand [13]. It is difficult to use this method for tickling because
the display is placed on the wrist, not on the palm. This meansthat the back of the hand, as the input surface, is shared withthe display as the output surface. This method does have afeature that allows the user to feel where the touch occurs
because the user touches the back of his/her own hand. Tamaki
et al. introduced the concept of using the user’s own hand,which is actuated by an electrical stimulation, as a display forhand gestures. This differs from the method proposed here as
the hand is used only as a display.

3. SHARED TACTILE INTERFACEIn this paper, we propose a “Shared Tactile Interface,” which isachieved by sharing a part of the user’s body and which allows
users to tickle each other from a distance. This method is basedon three requirements: bidirectional contact, bidirectional
tickling, and transfer of tickling.

3.1 Bidirectional Contact and Tickling
We focused on a specific characteristic of tickling: it is wellknown that the tickling sensation is not felt if one tickles
oneself. Prediction of one’s own behavior suppresses theperception brought about by the behavior [15]. In other words,
tickling will only be sensed by a tickling action initiated byanother person. This is the point that allows one user to tickle
another even when sharing a palm.
Figure 1 (a) shows a scene during tickling. If the user tickles
his/her own palm as in Figure 1 (a), the user feels no tickle
sensation. We propose to use the user’s own palm as if it wasanother’s, in order to tickle, and to send this tickling gesture
onto the other user’s palm at the same time, as shown in Figure
1 (b). This figure shows that user on the other side is tickled by
the appearance of the tickling gesture on his/her palm. As aresult, he/her can feel the tickling sensation. Therefore, thisstrategy requires that the user makes the motion of tickling
his/her own palm while imagining that his/her own palm is
actually the palm of the other user. In addition, “bidirectional
tickling” is achieved because the same effect occurs in thereverse direction. “Bidirectional contact,” which enables bothusers to contact to each other, is also achieved.

3.2 Transfer of the Tickling Sensation
Several methods can produce the tickling sensation. First,
tickling sensations are commonly known to arise whensomething strokes the skin with a small force. For example,
Blakemore used a tickling robot that had a rod with a soft formon the tip [15]. A tickling robot, however, is not practical forbidirectional tickling due to its size.
Second, the appearance of being tickled can be utilized because
a tickling gesture alone can create a weird feeling. In otherwords, a sensation like tickling can be elicited even when nocontact is made with the skin. This phenomenon is considered
to arise because a moving visual cue is interpreted as the sameas a somatic sense [16]. This finding seems to indicate ahypothesis that transfer of the tactile sense is not essential. Our
results obtained in a previous study, however, do not support
this hypothesis [17].

3.2.1 Illusion of a Tickling Sensation
We use illusion obtained from the integration of visual andtactile sensations in order to transfer the tickling sensation. To
realize the illusion, the following three points are required: first,
the tickling gesture must be made to appear as if it is occurringon the user’s own palm. Second, the item doing the ticklingmust move on the palm. Third, a slight vibration needs to occur
at the same time as that movement.
First, as shown in Figure 2 (a), we place a static image, which
we call the “palm image,” on a handheld-sized portable device.
This gives the impression that the device is a part of the user’s
own palm. The device itself is visually transparent as long as
the position between the user’s palm and the device remains
consistent.
The palm image does not need to be generated in real time,
because it is so small that no large change will appear in thepalm unless the palm is pushed with a large force. A large
deformation could not be obtained visually from the image thatwas used in the previous study [17], which supports thepossibility that the use of a static image for the palm image isfeasible.
Second, we add another static image, which we call the “index
finger image” shown in Figure 2 (a), onto the palm image sothat the movement appears to occur on the palm. At this time,
only the movement of the tickling gesture is transferred onto
the palm image in the other device, as shown in Figure 2 (b).
This gives the visual effect that the index finger is tickling thesecond user’s palm in the other device. The possibility also
exists to shoot and show a real gesture of the index finger; forexample, bending of the finger and scratching. This image
however would require placement of an additional camera for
capturing the actual gesture, and would also consume
considerable bandwidth to maintain exchanges of the video
stream. For this reason, we use the static image and make itfollow the tickling gesture, based on the basic finding that themoving visual cue elicits a tickling sensation on its own [17].
Third, we provide a slight vibration that is synchronized withthe visual movement of the tip of the index finger because ourprevious result [17] indicates that the vibration tends toreinforce the tickling sensation. The tickling force is very small,which we mentioned before. Of course, transmission of an
actual stimulus is relatively simple. However, realization of thestimulus is not as easy, due to issues of measurement as
described in subsection 3.2.2. Therefore, we employ asinusoidal wave with a constant frequency and small amplitude
as a simulated tactile stimulus and this is modified dependingon the velocity of finger movement. The value of the constantfrequency is 250Hz, which is known as the peak of Pacini
corpuscle response [18].
One issue that determines the nature of the stimulus is how higha quality of tactile stimulation is necessary in a spatial domain.
Ideally, the point where the user feels the sensation of being
touched should move corresponding to the actual finger
movement. However, previous research has not dealt with thistype of movement because the vibration provided by thebackside of a handheld device consists of a spatially flat
vibration with a center that does not move around [17].
Therefore, contribution of movement of a tactile image to thetickling sensation has not yet been addressed.
We evaluated the contribution of moving of the tactile image
using a method called the “Tactile Brush” [19]. Although thistechnique employs a device with a number of spatially placed
vibrators, it is not appropriate for our method because the small
size of the handheld device restricts the number of vibratorsthat can be placed on it. Therefore, we use a minimum of two
vibrators to realize the movement of the tactile image, referred
to as “Phantom Sensation,” which is perceived as a tactile
sensation occurring between two vibrators [20].

3.2.2 Measurement of Tickling Gesture
Measurement of the tickling stimulus as a vibration propagating
through the skin is difficult because the amplitude is small. Thismeans that the required high-precision distance sensor, whichcosts additional space in a handheld device, will conflict withthe design for realizing bidirectional contact and tickling.
Consequently we have focused on the movement of the tickling
gesture, which can be readily measured by a capacitive sensor.
Our method can use this type of sensor because it has two
advantages: it is built sufficiently small that it can be embedded
in a handheld device and it does not require a pushing force.
The consideration of the tickling stimulus as a finger movement
can overwhelmingly cut down the amount of data that needs tobe transferred. For example, if the stimulus is measured as asound with a sampling rate of 44.1 kHz and resolution of
amplitude at 16bit, it transfers as 706kbps uncompressed. On
the other hand, if it is represented with two 16-bit integer
values describing position on a two-dimensional surface andwith a frame rate at 30fps, it can be expected to transfer as
0.96kbps, apart from transmitting the finger posture and thepushing force, which are ignored in this paper.

4. Implementation
We evaluated the Shared Tactile Interface throughimplementation. The Apple iPod Touch 3rd and 4th Generation
were used as target handheld devices due to their embedded
capacitive touch sensors.

of touch, movement, and release then trigger the sending of thedata, which are expressed as text data including the contact
position and three event states. The number of fingers isregulated to only one for simplification. Therefore, the data
format is described as “x,y,m” with three integers; where “x,y”
means the position and “m” means the three event states. Data
are never sent and the index finger image never appears unlesscontact takes place.

f

Two responses occur when the data are received at the otherside: First, transparency of the index finger image, and second,
its position are controlled. Control of transparency is effectedby having the index finger appear and disappear gradually at
the time of touch and release. The index finger image remains
to appear during the state ‘move’. The position of the finger
pad of the image corresponds to the received touch point.

Figure 3. Schematic of the outer case that provides
spatially distributed vibrations.

Therefore, the exchange of data between the two equivalentdevices accomplishes bidirectional contact and bidirectional
tickling.

.

4.2 Transfer of the Tickling Sensation
Although vibration motors embedded in the handheld device
are generally used to provide a vibration to the user, we do not
employ these for the following reasons: this type of vibration
motor uses a rotating decentering weight and cannot provide
independent control of the intensity of vibration and frequency.
In addition, rising time takes longer than for a piezoelectric andelectromagnetic drive type. A vibration motor with adescending weight is not appropriate for nonverbal
communication, as this requires a quick response andcontrollable frequency, although it is suitable for providing asymbolic vibration.

Figure 4. Vibrator attached to the outer case.
.

We employed piezoelectric and electromagnetic drive types.
The piezoelectric drive embedded piezoelectric speaker insidethe device 1 [21] is used, and the electromagnetic drive is
implemented by enclosing it in an outer case attached to thedevice. We use an audio signal with a sinusoidal wave at
250Hz to provide proportional modification of the velocity of
the index finger movement.
The prototype of the piezoelectric drive does not require anychange of hardware. The center of the tactile image provided
with the piezoelectric prototype does not move as there is only
one actuator inside.

Figure 5. Device as seen from the user’s viewpoint.

4.1 Bidirectional Contact
The palm image is taken of a male’s palm, and the index finger
image is taken of a female’s index finger using an Eos Kiss X4(Canon). The size and position of the palm image is modified toretain position consistency between the user’s hand and theimage displayed on the handheld device. Post editing is used to
add a shadow to the index finger image to make the user feel as
if the user’s palm is being touched directly. Although these two
images should appear as an actual image taken from the user’s
own palm and index finger, we have found that the ticklingsensation is felt even if the image is the same male’s palm.
Therefore, both of these images are stored in each device inadvance and are not changed.
The palm image gives the effect that the device display is partof the user’s palm. The index finger therefore looks like ittouches the palm directly when its image appears. The
rendering refresh rate is maintained at 30fps.
The two devices are connected via a Bluetooth network, which
is implemented in a GameKit framework in iOS5 SDK. Events

The two vibrators were used to produce the movement of thetactile image in only one dimension, although four vibrators
seem to be necessary to provide the effect in two dimensions.
The intensities of vibration of each vibrator, g0 and g1, aregiven by:
g0 

h
v,
H

g1 

H h
v, (0  h  H , 0  v)
H

where g0 and g1 indicate the gain of the audio signal, H is theavailable moving length in the screen, and v is the moving
velocity of the finger. The vibrators are driven by two 1W Class
D Audio Amplifiers TPA2001D1 at a gain of 12dB.
The vibrators are ForceReactor AF series L type from ALPS
Electric Co., Ltd., and are placed on the backside top and1

We did not employ the inner speaker in the iPod Touch 4th generation because it wasdifficult to make the backside vibrate with an audio output due to different acoustic
seal structures of the iPod Touch 4th generation and the iPhone. The iPod Touch
series does not contain a vibration motor.

bottom of the screen at a distance of 60mm. These vibrators,
which are attached with double-sided elastic tape 1mm thick,and the amplifier circuit, are built into this outer attachment asshown in Figures 3 and 4 on the left. The vibrators are covered
with fabric tape as shown in Figure 4 on the right and attached
1mm higher than the surface of the backside. Figure 5 shows
the appearance from the user’s viewpoint.
The prototypes are listed in Table 1. “Prototype A,” which has
one vibrator attached to the backside, is used in the preliminary
study. “Prototype B” has the piezoelectric speaker replacedwith the electromagnetic one. “Prototype C” has twoelectromagnetic vibrators.
Figure 6. User Test Using Prototype B at CEDEC2011.

Table 1. Vibrator Specifications
PrototypeVibrator Type

NumberA

Electromagnetic

1

B

Piezoelectric

1

CElectromagnetic2.

5. User Study
We conducted a user study in order to evaluate the feasibility ofthe proposed method. The evaluation points were: (1)bidirectional contact, (2) bidirectional tickling (3) transfer of
the tickling sensation. Evaluation was divided into three trial
types; scenario 1 was the user being tickled; scenario 2 was theuser tickling another user; and scenario 3 was two users ticklingeach other.
The exhibition style was a demonstration. We prepared a pairof prototypes that allowed visitors to have a free experience ofthe devices. Technical explanation was provided by a poster
and a video. The presenter stood in front of the prototypes andalso explained the technical aspects and instructed the visitors
on use of the devices. In this paper, we describe the experience
flow and the visitors’ responses.

5.1 Experience Flow
Visitors were able to experience the devices after establishing aBluetooth connection with each prototype.
At first, the presenter gave a brief summary: “This is a remotetickling technique.” He passed the prototype, which was
already displaying the palm image, instructed the visitor toplace the device on the visitor’s own left palm, and then asked
visitor to position the prototype as precisely as possible. The
presenter placed the other device on his own left palm. Only thepalm image was displayed at this time. The distance between
the visitor and presenter was approximately 50cm, and they
could face and talk to each other.
Once the devices were positioned, the presenter stroked theprototype on his own palm with his right index finger. This
triggered the appearance of the index finger image, which
moved with the same motion as the presenter’s actual finger,
and provided the previously described vibration. We observed
the visitor’s responses at this time (scenario 1).
Next, the presenter explained that the visitor was also able totickle and prompted the visitor to tickle the device on thevisitor’s palm. We also observed this as scenario 2.

Figure 7. User Test Using Prototype C
at SIGGRAPH ASIA 2011.
After that, the presenter handed over the prototype to anothervisitor and left the two visitors to experience the devices as
scenario 3.

5.2 Exhibition
The user study was conducted at two conferences. The first one
was the CESA DEVELOPERS CONFERENCE (CEDEC)
2011, which is the largest Japanese domestic conference, heldin Yokohama Japan on September 6th to 8th [22]. The attendees,
which are almost all male, range in age from their 20s to 50s
and most work as engineers or specialists in the technical field,with a few from academic fields and from the general public.
The number of visitors to our exhibit was about 50. A part ofthis exhibition was recorded as about 1.3 hours of video in total.
Only prototype B was used for this exhibition.
The second venue was the ACM SIGGRAPH ASIA 2011,
which is an international conference on computer graphics andinteractive techniques, held in Hong Kong on December 12th to15th 2011 [23]. A technical demonstration was held for threedays, form the 13th to 15th. Attendees were about 60% male,and included a large number of high school students due totours by high schools, a number of engineers working on
computer graphics and artists, specialists, and researchers. The
number of visitors was about 600. A part of this exhibition wasrecorded as about 15 hours of video in total. Two pairs ofprototype C, four devices in total, were used for this exhibition,although prototype B was used when necessary, for about 10%of the total demonstration time.

5.3 Results
The scenes of visitor experiences are shown in Figure 6 and 7.
Figure 6 is an image taken at the first exhibition that used only

prototype B, with the piezoelectric actuator. Figure 7 is animage is taken at the second exhibition with prototype B and C,
with the electromagnetic type attached. The following results
were obtained regardless of prototype B/C, unless otherwise
noted.

5.3.1 Scenario I: Being tickled
Both prototypes were placed approximately correctly based on
our observations, but matching the palm image to the visitor’spalm was difficult in the case of a small hand. To feel thetickling sensation, the user needed to feel that the palm image
in the screen was a part of the user’s own body. The
misunderstanding of direction for placement, which wassometimes observed in cases where the visitor tried to place thedevice on his/her own, caused that the device to be rotated 90
degrees. On the other hand, placement was always correct when
the presenter asked the visitor to open his/her left palm andplaced the device on the palm. Therefore, it was apparently notvery easy to understand how the device should be rotated on
the palm unless the user’s looked carefully at the image.
We found the following responses to tickling provided by thepresenter through observation of the use of the pair of devices,
regardless of prototype B or C: About half of the visitors did
not show any distinct change in their facial expression andnodded weakly in response to the presenter’s explanation. On
the other hand, the remaining half of the visitors’ faces changed
into smiles, frequently wide ones. The first set of visitors who
did not present obvious facial expressions answered “No, I
could not feel it” in response to an oral question “Do you feel atickle?” asked casually by the presenter. They often continued
to give comments such as “It’s creepy,” or “I only felt avibration.” On the other hand, most of the second group ofvisitors answered “Yes,” that they felt a tickling sensation. The
effect, however, was not so ticklish as to be startling. Only
about 1 % of the visitors, who were all female, seemed to wince
in response.
Some of the visitors who felt the tickling sensation tried totickle the device at the presenter’s side in order to make sure of
the sensation. In this case, it seems that they could not feel thetickling compared to the case where they were tickled by thepresenter.
We gave visitors who tried the prototype C the opportunity toexperience prototype B if they wished. About half of themusually commented that prototype C was better than prototype
B, although they answered that they felt the tickling even when
they evaluated the piezoelectric type prototype B.
Visitors who experienced prototype C voluntarily reported thatthey could feel as if the pointing finger was moving on theirpalm. Few visitors who tried prototype B reported this. Somevisitors, who were academic people, experienced prototype C
said “I felt a horizontal movement of the tactile image”
although the number was few. Almost all visitors turned overthe prototype C after their experience in order to see how it
works.
After giving the presenter’s question of “What do you feel?”the presenter often asked the visitors to close their eyes andthen asked them what they felt when they were not watching
their palm. The visitors who said that they had felt the ticklingsensation when viewing their palms answered that they did not
feel it when they could not see their palms.

Visitors who could not feel any vibration from the prototype B
usually could feel the vibration from the prototype C because
prototype C was able to produce a much higher intensity ofvibration than prototype B. On the other hand, there were some
visitors who could felt the tickling sensation from prototype B
but commented that the vibration provided from prototype C
was too strong to be called a tickling sensation.

5.3.2 Scenario II: Tickling Another
Almost all of the visitors were puzzled when told that theyneeded to consider their own palm as that of another person at
the beginning of the experience. The visitor also needed to
discover that the finger movement on the other’s palm wassynchronized with visitors’ own as they looked directly at theother’s, because no feedback was provided, such a visualchange or vibration for the visitor who tickled the device on
their own palm. The visitors in particular who started
voluntarily to tickle the device on their own palms before given
the brief summary, did not obtain any feedback from the device
and did not comprehend completely until the presenter
explained and tickled them to show what they should feel.
Some visitors expressed displeasure about the lack of feedbackin the case of one-side tickling. Once they found out that theirown finger movement was sent to the other side, they radicallychanged their behavior; they tried to tap or to stroke the screenof the device randomly before understanding. After that they
started to enjoy the tickling task, continued to do so, and tried
to confirm whether they were able to tickle the other side bywatching the screen at the other side.
Some visitors pointed out that the finger appearing on the otherside was not the actual visitor’s own finger.
Many visitors tried to tap the screen of the prototype. As
mentioned before, because prototypes B and C are not designed
to transfer taps, the index finger image only appeared for aquick moment and did not provide any vibration even if theytapped. The visitors seemed to figure out that they could not tap
to each other after a while.

5.3.3 Scenario III: Tickling Each Other
The concept of being able to tickle each other appeared to be
immediately understood after the demonstration. We found thatmany visitors began to smile regardless of age or gender.
Pairs of visitors continued to tickle each other. The high school
students especially tended to be enthused about tickling in thesecond exhibition at SIGGRAPH ASIA 2011. Some female
high school student groups attempted to write a letter on her
own palm in order to send it to a friend. However, it seemed
difficult to transfer letters without looking at their own palms.
On the other hand, some visitors who were typically engineers
in their 30s and 40s did not change their facial expressions atall. These visitors commented that they could not feel ticklingand likely remained in order to confirm the movement of thefinger.

5.4 Discussion
The transfer of the tickling sensation and also the comment“creepy” indicates that the palm image on the device wasconsidered as the user’s own palm, because the movement of
the finger seemed to produce the tickling sensation by a visualmeans, as described earlier.

Whether the visitors felt a tickling sensation depended on thelarge individual differences in sensitivity to tickling. The reason
for this seemed to be the sensitivity to vibration because theintensity of the vibration was not modified for different
individuals.
Too strong a vibration would not provide a tickling sensation
but just a vibration. Therefore, the intensity should be modified
carefully depending on the individual.
The feature of tickling, whereby a self-inflicted tickle stimulus
does not create a tickling sensation was observed with thedevice. The visitors evidently were able to experience a ticklingsensation. Therefore, the design method “Shared Tactile
Interface” enabled users to tickle each other.
The finding, that the movement of the tactile image producedby the prototype C enabled the visitors to feel the ticklingsensation more readily seems to indicate that this provides abetter experience. In this case, however, the number ofactuators needs to increase.
Above all, the results of the scenario 1 “Be tickled” showedthat the bidirectional contact was made and the ticklingsensation was transferred. Additionally, the result of the
scenario 2 “Tickle another” and scenario 3 “Tickle each other”
indicates that the bidirectional tickling was achieved. Therefore,
these results suggest that the proposed method “Shared Tactile
Interface” is feasible.

5.5 Limitations
The issue raised regarding that there was no feedback or an
acknowledgement given to the initiating user whether thereceiver at the other end tickled or not indicates that the current
prototype is not properly fit to be used for remotecommunication. Therefore, the prototype seems to require
additional means of communication, such as voice or visual(e.g. facial expressions). Furthermore, it is possible to replace
the static images with dynamic ones which express active
responses by showing the bending of the palm when being
tickled.
On the other hand, some visitors requested to distribute theapplication software of the prototype without the outer casewhich encloses all the actuators. There could be some factors
affecting the evaluation of the remote communication in our
experimental setting since the actual distance between two
users were not large enough to simulate a remotecommunication scenario. Even though users preferred andeager to use the proposed method, further evaluation of remotecommunication features would be desirable.
The vibration itself was not ticklish as we expected. It is,however, ideal to be used one providing tickling sensation
without any illusions. Thus, such unpredictable stimulation
which has been used by Tsetserukou et.al [3] is worth
considering to be used instead.
The prototypes used the images prepared in advance without
using pictures of visitors’ palm and index fingers. Some visitors
pointed out that they could not relate to the device’s picture, inother words, this tends to affect visitors’ identity. Therefore,
exchanging of the images taken by the user will enable theusers to preserve his/her identity.
The prototype could not transfer the tapping sensations due tofocus given to the tickling gestures. The tapping, however, canbe measured with an accelerometer and vibrator seems to beable to re-produce it properly.

6. CONCLUSION
In this paper, we proposed a “Shared Tactile Interface,” whichis achieved by sharing a part of the user’s body and allows
users to tickle each other from a distance. In order to exchange
this nonverbal cue, the proposed method must fulfill threerequirements: bidirectional contact, the bidirectional tickling,
and transfer of the tickling sensation. The tickling sensation isdefined as to feel as if tickling another user and to feel as if
being tickled oneself.
We achieved the three features: bidirectional contact,bidirectional tickling, and transfer of the tickling sensation. The
proposed method shows a part of the user’s own body in order
to measure the finger gesture and to show the movement of theindex finger at the same time.
We conducted a user study in order to evaluate the feasibility ofthe proposed method through two exhibitions as technicaldemonstrations at conferences. The proposed method enabled
us to transfer the tickling sensation between users. Therefore,
we concluded that this result indicates the feasibility of a design
method for a tactile interface that uses a part of the user’s body.
It was difficult to produce a stable tickling sensation even if thetickling stimulus was provided stably. The reason for this seems
to be the large individual differences in sensitivity to ticklingand the fact that a vibration itself is not a tickle. In addition, thegestures such as tapping and tickling with multiple fingers,which were left out of consideration for implementing theprototypes used here, should be included in order to evaluate
the proposed design more extensively. We will deal with theseissues in our next steps.

7. ACKNOWLEDGMENTS
This work was partly supported by JST-CREST Haptic Media
Project.

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