This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/TOH.2020.2970057
JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

1

Edge Vibration Improves Ability to Discriminate Roughness
Difference of Adjoining Areas
Yuki Ban, Yusuke Ujitoko, and Kouta Minamizawa,

Abstract—Researchers have studied the discrimination thresholds between different vibrotactile signals under various conditions. Humans
cannot recognize slight differences in vibrotactile stimuli that are smaller
than the perception threshold. This is a constraint in the vibrotactile
design used in practical applications. This study focuses on the vibrational
feedback at the ”edge” between multiple areas, while previous studies
have not considered this. We assume that the edge vibration not only
emphasizes the presence of the edge itself, but also has an effect on thevibrotactile perception of the adjoining areas. Specifically, we hypothesizethat the edge vibration would modify the user’s ability to discriminate
vibrotactile differences between adjoining areas. We conducted a user
study to test this hypothesis. As a result, we found that presenting edge
vibrations at the boundaries between adjacent textures makes it easier todiscriminate the frequency and amplitude differences of the vibrationsof those uneven textures. This work could increase the flexibility ofvibrotactile design, and vibrotactile designers could use these results todesign a wider variety of vibrations for adjacent areas.
Index Terms—Texture Rendering, Vibrotactile

I. I NTRODUCTION
Owing to the popularity of mobile devices with vibrotactile actuators, surface tactile technology has gained significant attention
in recent years [1]. Vibrotactile stimuli enable humans to perceive
the texture of surface materials. Considering the development ofpractical vibrotactile applications, tactile designers need to design
multiple vibrations to indicate different areas of material surfaces. A
vibrotactile recorder [2] and vibrotactile designing toolkit [3] weredeveloped recently, and now designers have the means to obtainwhatever vibrations they want. Using such tools, designers can design
a vibration for each surface area. However, this does not makesense if users cannot distinguish the differences between vibrations at
different areas. Therefore, in such cases, designers have to consider
whether users can distinguish between the different vibrotactile signals being provided. This capacity is quantified by the discriminationthreshold. Generally, it is shown that a difference of at least 2030% in amplitude or frequency is necessary for robust discrimination
between vibrotactile stimuli for practical applications [4], [5]. Thus,
it is difficult to enable users to recognize a slight difference in thevibration of different areas, and this is a constraint for vibrotactile
design. For example, the user may not feel the texture as the designer
intended, or the designer may not be able to express the difference
between delicate textures.
The focus of this study is the design of vibrations at the edges
between different areas. Few conventional implementations havefocused on the edges between areas, but they did not set specificvibrations on the edges. However, Vardar et al. attempted to separate
two textural regions in a haptic presentation on a touch screen byelectrovibration, using the effect of tactile masking [6]. The tactile
masking is a phenomenon by which presenting one stimulus might
Yuki Ban is with the Graduate School of Frontier Sciences, the Universityof Tokyo, Chiba, Japan. E-mail: ban@edu.k.u-tokyo.ac.jp.
Yusuke Ujitoko is with Research & Development Group, Hitachi, Ltd.,
Yokohama, Japan. and is a Graduate Student of the University of ElectroCommunications, Tokyo, Japan. E-mail: yusuke.ujitoko.uz@hitachi.com.
Kouta Minamizawa is with Keio University Graduate School of Media
Design, Tokyo, Japan. E-mail: kouta@kmd.keio.ac.jp.
Manuscript received April 19, 2005; revised August 26, 2015.

Failed to distinguish (Feel same textures)
Virtual Edge

Vibrotactile texture

Edge disabled

Percieve the diﬀerence

Edge enabled
Vibrotactile Edge

Fig. 1. Proposed method for improving the discrimination ability of vibrotactile stimuli on adjacent areas using edge vibration.

interfere with the perception of another stimulus. This phenomenon
can cause certain deficits in perception, such as increase in detection
thresholds and hindering of localization or identification [7], [8].
Vardar et al. revealed that the sharpness of the edge separating textural
regions can be enhanced by producing the tactile masking effect byenhancing the local frequency contrast between the background andforeground stimuli. Besides, Dallmann et. al. showed that masking
significantly reduces the precision of speed discrimination. This result
suggests that slip-induced vibrations help with the discrimination oftactile speed [9]. On the other hand, tactile masking using voicecoils [8], vibration motors [10], etc., has been studied; however, there
has been no verification of the discrimination of multiple textural
regions for touch-screen applications as in [11].
Considering these previous studies, this study focuses on attaching
additional edge vibrations using a vibrator for interactions on touch
screens with pen devices. We assumed that an edge vibration can
not only emphasize the edge itself but also affect the ability todiscriminate the vibrotactile ruggedness of areas divided by edges.
Based on the analogy of an optical illusion in which a visual edge
between two areas affects the perception of adjacent areas [12], [13],it is expected that there would be a similar effect for a vibrotactile
edge. Specifically, we hypothesize that users can recognize a slightdifference in vibrotactile texture with edge vibration, while theycannot recognize it without edge vibration (see Fig.1). In other words,
the discrimination threshold with edge vibrations would be smaller
than that without edge vibrations.
In the case of texture presentation using a touch screen, it ispreferable that the vibration is less restricted from the viewpointof practicability. In the previous approach [11], to produce theeffect of tactile masking, it was necessary to adjust the frequencyof the vibration stimulus in the region near the edge, due to thedifference in the frequency of the neighboring texture. However,in our proposed method, it is only necessary to present the edge
vibration at the boundary between textural areas. Thus, when theedges are superimposed on a content having multiple vibrational
textures, there is the advantage that we do not have to redesign thevibrations of the original textural surfaces (such as decreasing or
increasing the frequency of the textural vibration near the edges); weonly have to attach a simple edge vibration.
We conducted a user study to test this hypothesis. Through the user
study, we verified whether the vibration frequency and amplitude of
two adjacent textures could be correctly discriminated by adding edge
vibration. We chose the frequency and amplitude as the validation

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/TOH.2020.2970057
JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

2

Rectangle Area

Head Phone

input signal to vibratiorEdge

output vibration

Ampliﬁer

input signal output vibrationA

1/f

Tablet device
Force Reactor

TM

Conductivesheet

A: vibration amplitude (RMS level of accelaration)f : frequency of input square wave

Frequency Ex. ・・・ A: ﬁxed

f : variable

Amplitude Ex. ・・・ A: variable f : ﬁxed

Conductive nilb

0.005 simpulse wave

duration and amplitude
are ﬁxed

Fig. 3. Summary of parameters of vibration stimuli in this user study.

Fig. 2. Experimental system.

parameters because these two parameters significantly contribute to
the presentation of various types of rugged textures. The contributionsof this study are following two points.
• We focus on edge vibration and consider the problem of its
effect on the ability to discriminate vibrotactile stimuli of thedivided areas.
• A user study showed that the presenting edge vibrations between
adjacent textures makes it easy to discriminate the frequency andamplitude differences of the vibrations of those textures.
II. R ELATED WORK
Researchers have conducted studies on vibrotactile perception thatare useful for the design of effective vibrotactile applications. Thisknowledge can be particularly helpful to designers in choosing what
vibrotactile signals to render.
Researchers have studied the absolute detection threshold of vibrotactile stimuli [14]. This threshold is the weakest intensity ofa stimulus that allows a human to perceive with confidence, thepresence of the stimulus. As is common with tactile perception, it
depends on a number of factors such as the frequency, body site, or
contact area [15]. For example, if the contact area or stimulus duration
increases, all the thresholds of the PC channel decrease because of
the summation effect. [16], [17] reviews on the absolute detection
thresholds determined using tool-mediated stimulation conditions
were used in the user study in this paper. Delhaye et al. revealed thepresence of high-frequency vibrations in the wrists of users exploringrough surfaces, and showed that this vibration helps users detect theroughness of textural surfaces [18].
Once a stimulus is known to be perceptually recognizable, thentactile designers have to consider the ability of users to discriminate
multiple vibrotactile stimuli. This is because there are multiple
surfaces or content that exist simultaneously in a typical case. The
capacity to detect a stimulus is quantified by the discriminationthreshold. Because differential thresholds depend on the strength of
the reference stimulus, discriminability is generally represented by aWeber fraction, which shows a linear relationship between differential
threshold and stimulus intensity. Weber fractions mostly clusteraround 10%–30% for vibration intensity and around 15%–30% forvibration frequency[5]. In general, it is known that a difference ofat least 20%-30% in amplitude or frequency is necessary for robust
discrimination between vibrotactile stimuli in practical applications[4], [5]. Thus, it is difficult for users to recognize a slight difference
in roughness between multiple areas, and this is a constraint forvibrotactile design.
III. E XPERIMENTAL D ESIGN
A. Experimental system
The task in this study was to move a pen-type device on a tablet
device while receiving vibrotactile feedback via the pen (Fig.2). The
pen-device, which we handcrafted, was approximately 140 mm inlength and weighed approximately 20 g. The diameter of the pen’s

grip was approximately 10 mm. We covered the pen tip with aconductive material because the shaft of the pen was plastic anddid not conduct to the grip part. We wound a conductive sheet onto
the grip to react with a capacitance type touch screen by electricallyconnecting a hand, grip, and a pen tips. We embedded a vibrator(ALPS Inc., Force Reactor) inside the pen-type device 20 mm fromthe tip of the pen, where the participants gripped. The operating
frequency range from this vibrator was 0 - 500 Hz [19]. Its rated
supply voltage was 3.3 - 5.0 V, and we measured the peak-to-peakvoltage on the vibrator when f was 30 Hz (described below), andthe voltage was 4.67 V. The vibrator was small (35.0 mm × 5.0
mm × 7.5 mm) and light (approximately 5 g), so the participants
were not tired when moving the pen. When the participants moved
the pen on the touchpad, the vibration signal was emitted from theearphone jack of the tablet device (Apple Inc., iPad Pro 9.7 inch).
The amplifier (Lepai Inc., LP-2020A) amplified the signal, and thevibrator embedded in the pen presented the vibration to the fingersof the participants. This vibrator, which includes a voice coil andplate spring, can present the vibration in only one direction (along
one axis), but it’s no problem to present texture surfaces because
Romano et al. constructed the method to reduce the three-dimensional
acceleration signals to a perceptually equivalent one-dimensionalsignal [20].
The screen refresh rate was 60 Hz, and finger position acquisition
was performed at 100 Hz. Although no formal data were obtained on
the accuracy of the contact position acquisition, the resolution was
264 ppi and the contact position was acquired every 1 pixel; hence,
it is assumed that the acquisition accuracy is about 0.1 mm.
The participants wore headphones with noise-cancellation andheard white noise to avoid background noise from the environment
and to avoid hearing small sounds caused by the vibration of the pentype devices. We confirmed in advance that the participants could
hear no external noise or sounds emitted from the pen-device. A
web browser rendered the experimental task screen presented to theparticipants on the tablet screen, and a JavaScript Web Audio API
controlled the vibration signals. A signal was sent from the earphonejack of the tablet device, which was amplified by the amplifier andfinally output as a vibration stimulus by the pen-device.
B. Vibrotactile texture stimuli
As the first step to investigate the effect of a vibrotactile edge on
vibrotactile texture discrimination, we assumed a simple patterned
virtual surface and edge. By moving the pen-type device, the participants felt vibrations of two adjacent virtual surfaces, and felt thevibration of the edge when the pen moves over the edge (Fig.1).
Fig.3 shows a summary of the parameters of vibration stimuli for therectangular area and for the edge.
1) Vibration on virtual surfaces: We focus on the rugged texture ofrocks as the texture presented by the vibration and tactile stimulation.
To present a rugged feeling texture, we used a square waveform, thefrequency (f ) and amplitude (A) of which are easily controllable.
By controlling f , we can modify the interval length of the discrete
vibration, which presents the spacing of ruggedness. By controlling

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/TOH.2020.2970057
JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

8.0

Acceleration in the axial2
direction of the pen (m/s )Acceleration in the axial2
direction of the pen (m/s )

8.0
6.0
4.0
2.0
0.0

-2.0
-4.0
-6.0
-8.0

3

0

250

500

750

Time (ms)1000

156 mm

6.0
4.0“1” area

2.0
0.0

target area

-2.0
-4.0

Rectangle areas with vibrotactile texture

-6.0
-8.0

Texture Area

0250Time (ms)Elonging bars at 78mm/s

500

Edge

Fig. 4. Acceleration presented when the pen’s tip moves on the texture area
(Left), and that when the pen moves across the edge (Right). The left one isthe vibration of the texture area when A is 4.90 m/s2 and f is 20 Hz

A, we can present the size of the ruggedness. We generated the stimulus difference between areas by changing the vibrotactile amplitudeand frequency. These two parameters were controlled independently
in separate experiments. Hereinafter, an experiment in which theamplitude of vibrotactile stimuli is changed is called “AmplitudeEx.”, and an experiment in which the frequency of vibrotactile stimuli
is changed is called “Frequency Ex.”. The vibration frequency was
selected by a pilot test to make it easier to recall the rugged textureof rocks. Specifically, we focused on the rock surface with a space
period of several millimeter scale. As a pilot test, various vibrations
with frequencies in the range of 10 to 200 Hz under the constant pen
movement speed (78 mm/s) were tried by three participants. As aresult, the spatial period was found to range from 2.6 mm to 7.8 mm,corresponding to vibrations from 30 Hz to 10 Hz; these were judged
closer to the rugged feeling and were chosen as the experimental
parameters.
For setting the vibration amplitude, we directly measured theacceleration output from the vibrator attached to the pen-device
and set the scale of the parameters by calculating the RMS of theacceleration. We used this RMS of acceleration as the vibration
amplitude (A). We hung a bare vibrator in air by using a string. At
the flat side surface of the vibrator, a three-axis acceleration sensor(MPU-6050), working at 400 kHz, was tightly attached. We measured
the acceleration data along the main axis of the vibrator at 2 kHz. The
left part of Fig.4 shows the waveform of the textural area vibration.
Through these pilot tests, we chose the variations of f and A (Table
I, II). The gain was adjusted under each signal condition so that theRMS of acceleration could be adjusted even when the frequency ofvibration stimulation was different. We call the vibrotactile signal
variable “signal conditions” in this user study.
2) Vibration on the Edge: We generated an impulse waveform topresent the sensation of exceeding the edge. An impulse waveform
is usually used for presenting a key-click signal, and we consider
that this waveform is the simplest vibrotactile stimulus, since it isthe vibration that expresses the sensation of crossing edges. Throughthe pen-type device, the users felt the vibration of the virtual edge
when the pen crossed over the edge line. The duration of the impulse
waveform was set at 0.005 s. The RMS of the acceleration was
heuristically determined through the pilot test. We chose the vibration
duration and amplitude that were suitable for a small edge. The rightpart of Fig.4 shows the waveform of the edge vibration.
IV. P RELIMINARY S TUDY: S UBJECTIVE EVALUATION OF
VIBROTACTILE TEXTURE

In the main user study, we verified the effect of our method using
the JND (just noticeable differences) methodology. Therefore, it wasnecessary to confirm that the vibrotactile signals’ set is capable ofcovering a range of subjective ruggedness values. Besides, we also
had to confirm that the frequency of vibration represents the finenessof the rugged textural surface, and that the amplitude of vibration“100” area

Slider for answer the rate ofﬁneness or unevenness

Fig. 5. Experimental window of the preliminary study
TABLE I“S IGNAL CONDITIONS ” (f AND A) OF EACH TEXTURAL AREA
A
f

2

f

2

2

A
2

represents the unevenness of the textural surface. In other words,
we had to confirm that as the frequency (amplitude) of vibration
increases, the perceived texture surface becomes finer (more uneven).
Therefore, we conducted a preliminary experiment to confirm amonotone increase in fineness and unevenness of the rugged surface
with respect to the increase in frequency and amplitude of thepresented vibrations within the parameters we set (Table 1).
A. Participants
There were twelve participants (nine males and three females) withages ranging from 22 to 25 (23.5 ± 0.957). All of the participants
were right-handed. None of them reported a history of neurological,
psychiatric, or other diseases that could have interfered with tactile
sensitivity. The University of Tokyo Ethics Committee approved theexperiments presented in this paper (approbation number: 19-173),
and written informed consent was obtained from all participants inthe studies presented here and in the next section. The total timeof the experiment was about thirty minutes, including the time forproviding the explanation in advance and for the questions after thetask. No reward was paid to the participants.
B. Task Design
This user study was conducted according to a within-participants
design. The participants performed a task to evaluate the textural
unevenness and fineness. Fig.5 shows the experimental screen on thetablet device during the preliminary study. Three vibrotactile texturalareas were placed on the screen. The participants compared threesignal conditions with extreme differences by moving the pen-device
on the tablet. The participants moved the pen device over rectangle
areas from left to right at a constant speed (78 mm/s) with their
dominant hands. They were required to grip the pen with their fingers
at the position where the vibrator was embedded so that they felt thevibration in their fingers. On the tablet screen, three vibrotactile areas,
an elongating blue bar, and two buttons for providing the answer
were displayed. This blue bar notified the participants of the speed
and length (156 mm) of the pen-device movement. The target moving
speed was heuristically determined through a pilot test such that itwas not burdensome to the participants. We measured the speed of
the touching point in the background, and confirmed that the tracing
speed was within 77.2±3.7 mm/s in the 78 mm section of the central
zone, excluding the beginning and end of movement.
The experiment followed the magnitude estimation. For “Frequency Ex.”, the participants recognized that the textural area for

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/TOH.2020.2970057
JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

100
Subjective unevenness

100
Subjective ﬁneness

80
60
40
20
0

4

Edge line with or w/o vibrotactile stimulus

Two rectangle area with vibrotactile texture

80
60
40

78 mm

20

78 mm
Elonging bars at 78 mm/s

0

10
15
20
25
30
Frecuency of the vibration [Hz]0.0 2.5 5.0 7.5 10.0
2
RMS level of the vibration acceleration [m/s ]Fig. 6. Relationship between the subjective rating of fineness and thefrequency of vibration for “Frequency Ex.” (Left), and that of unevenness andthe amplitude of vibration for “Amplitude Ex.” (Right). The color differenceof the dots shows the difference of the participants.

which f was 10 Hz was expressed as 1, and that the texture area forwhich f was 30 Hz was expressed as 100. The former was presented
at the top textural area, and the latter was presented at the bottom
textural area. The middle textural area presented nine types of f(shown in Table I) as the target area. The participants traced thesethree areas in order from top to bottom. The participants were asked
to rate the fineness of the target area with the vibrotactile stimulus
displayed on the pen-device. The speed and length of the pen-device
movement was fixed as described, but the participants could trace
the three areas any number of times. Each participant repeated thetrial ten times for each signal condition; thus, overall there were 90
trials per participant (= 9 × 10). The texture areas were presented
in a random order and were counterbalanced across participants.
The procedure of “Amplitude Ex.” was almost the same as that of“Frequency Ex.”. The experimental conditions are shown in Table I.
The participants were asked to rate the unevenness of the target area.
The participants used the slider and the button to indicate their ratingsof fineness or unevenness. All the participants participated in boththe preliminary studies. (i.e., in both “Frequency” and “AmplitudeEx.”). The order of these two studies was counterbalanced among
the participants.
Before these trials, we preliminarily confirmed the participants feltthat the signal condition with f of 10 Hz was finer than that of 30
Hz, and that the signal condition with A of 8.82 m/s2 was more
uneven than that of 0.98 m/s2 .
C. Result and Discussion
The average data of each participant are shown in Fig.6. The
perceived fineness of the texture area increased with increase in
the frequency of vibrotactile stimulus (f ) in “Frequency Ex.”, And
the unevenness of the texture area increased with increase in theamplitude of vibrotactile stimulus (A) in “Amplitude Ex.” These
results suggest that there was no problem with asking the participants
about their perception of the difference in frequency/amplitude of
the vibration as the degree of unevenness/fineness of a texture area.
The data for each participant were fitted by Steven’s power function
R(s) = k · sn , where s represents f of the vibrotactile stimulus for“Frequency Ex.”, and A of the vibrotactile stimulus for “AmplitudeEx.”. R(s) represents the subjective rate of fineness and unevenness.
We applied Generalized linear mixed model (GLMM) to calculate
this fitting curve. The fitted curve is shown in Fig.6. We confirmedthat the subjective fineness/unevenness rate increased with increasein the vibrotactile stimulus frequency/amplitude. The obtained valuesfor (k, n) were (15.5 0.622) in “Frequency Ex.”, and (7.75, 1.16)
in “Amplitude Ex.”. These results indicate a monotone increase in
the fineness and unevenness of the rugged surface with respect tothe increase in frequency and amplitude of the presented vibrations
within the parameters we set. Thus, we conducted the main user study
with these “signal conditions.”

Answer button

Fig. 7. Experimental window. The participants move the pen-type device
from left to right on the two rectangular texture areas

V. M AIN U SER S TUDY
By moving the pen-type device, the participants felt the vibrotactile
textures of two adjacent areas (Fig.1). They compared the texture
vibration feedback presented when moving the pen-type device overthese two areas in succession. In terms of edge vibration, there
were two conditions: with edge vibration and without edge vibration.
There were twelve participants, and all of them participated in the
preliminary study. The total time of the study was about one hour,
including the time for the explanation in advance and the time forasking questions after the task. No reward was paid to participants.
A. Task Design
This user study was conducted according to a within-participants
design. The design of the experiment followed a JND methodology [21]. We investigated whether the participants felt the unevennessof the “comparison stimulus” to be finer or more uneven than thatof the “standard stimulus” with or without the edge vibration. The
participants had to move the pen-type device once from left to right tocross two adjacent rectangle areas (Fig. 7). They received vibrational
feedback from two textural areas through the pen device. The edge
was visually placed at the boundary between these two areas. The
width of this edge was 1 pixel, and it was just a line. The systemdetermined whether the nib had passed the edge by comparing thecurrent nib position with the preset edge position on the screen. In atouch screen vibrotactile application that presents multiple textures,
we assume that visual information is also presented. In this situation,
the boundary of the textural areas is clear from the visual differenceof the texture. Thus, we also presented a visual edge in this study.
We describe the procedure of one trial in the task. The participants
moved the pen over two rectangle areas from left to right at a constant
speed (78 mm/s) with their dominant hands. The total distance overwhich the pen was moved was 156 mm. The system notified theparticipants of the speed using an elongating blue bar on the screen.
We confirmed that the tracing speed was within 76.3 ± 3.4 mm/s inthe 78 mm section of the central zone. Besides, since the average
difference of moving speed in the left and right areas was less than
0.33 mm/s for a trial, the change of moving speed in a trial did notaffect the perception of the difference of texture in the left and rightareas. The participants were also instructed not to change the way ofgripping a pen during the experiment. It is possible that the strengthof gripping varied during the trials, even though the way of gripping
was the same. However, even if the strength of the vibrator held by
the participants had changed, the vibration of the voice coil inside
the vibrator was not affected by the grasping pressure. Thus, weconsidered that there would be no effect on the experimental result
even if the gripping strength was not controlled.
Under the condition with edge vibration, the participants felt thevibration from the edge when they went over it with the pendevice. After the participants finished the movement from left to

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/TOH.2020.2970057
JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

5

A
f

2

2

A
f

2

2

Edge condition (with edge)
None condition (w/o edge)

0.8
0.6
0.4
0.2
0.0
10.0

right with the pen-type device, they answered “which texture surface
did they feel was finer?” in “Frequency Ex.”, or “which texture
surface did they feel was more uneven?” in “Amplitude Ex.” Wethought that these questions could correctly measure the effect of theedge, because we confirmed the correspondence of the frequency andamplitude of the vibration with the perception of textural fineness andunevenness through the preliminary study. The participants tapped
one of the two answer buttons visualized on the screen. The following
additional instructions were given to the participants. We told the participants that they could move the pen under each condition on only
one round trip. We also asked them to select one of the two buttons
randomly if they thought that it was difficult to judge. This 2AFC (2Alternative Forced Choice Task) method is “criterion-free” in contrast
to the alternative method such as “criterion-dependent” task such as“yes/no” question [22]. Thus, 2AFC method is commonly used toevaluate various discrimination thresholds of haptic sensation [23].
Here, we describe the details of the experimental conditions. The
participants performed “Frequency Ex.” and “Amplitude Ex.”, and order of them was counterbalanced among the participants. In one trial,
the participants compared two vibrotactile areas: a standard stimulus
and a comparison stimulus. These two surfaces were presented under
the same condition except the “signal conditions” (see Table II).
In this trial, the texture area on the right side following the edge
vibration might be affected by tactile masking. To investigate thiseffect, the correspondence between the type of stimulus and theposition of the texture area was fixed so that the left side was
the “standard stimulus” and the right side was the “comparison
stimulus.” Moreover, there were two conditions of the edge at theboundary between these two areas. The “Edge condition” presented
a vibration stimulus as described above when the pen-device crossed
the edge, and the “None condition” did not present any edge vibration. The participants performed the experiments ten times for eachcombination of the edge conditions and the signal conditions of thecomparison stimulus. Thus, each participant performed 180 trails (=10 × 2 (edge conditions) × 9 (the signal conditions of the comparison
stimulus)) for both the tasks, namely “Frequency Ex.” and “AmplitudeEx.” The presentation order of these factors was randomly assigned
and counterbalanced across participants. After all the trials werecompleted, the participants answered whether there was a differencein the feeling of the two textures due to the presentation of an edge
vibration at the boundary, and if there was a difference, what kindof difference there was for each experiment.
B. Results
Fig.8 shows the response of one participant as an example of theresults. These figures show the rate at which the participant answeredthat the comparison stimulus was finer or more uneven than that of
the standard stimulus. These figures indicate that the participants felt
the comparison stimulus to be finer when its vibration frequency f
was higher than that of the standard stimulus, and felt the comparison
stimulus to be more uneven when its vibration amplitude was larger
than that of the standard stimulus.
Then, we calculated the psychometric function for each experimental condition to analyze which was the minimum noticeable difference

Rate of answers thatcomparison stimulus is more uneven

STIMULI

1.0

Rate of answers thatcomparison stimulus is finer

TABLE II“S IGNAL CONDITIONS ” (f AND A) OF STANDARD AND COMPARISON

15.0

20.0

25.0

30.0

Vibration frequency ofthe comparison stimulus [Hz]Edge condition (with edge)

1.0

None condition (w/o edge)

0.8
0.6
0.4
0.2
0.0
0.0

2.0

4.0

6.0

8.0

10.0

Vibration amplitude of 2
the comparison stimulus [m/s ]Fig. 8. Answer rate of one participant in “Frequency Ex.”(Left) and in“Amplitude Ex.”(Right).
TABLE IIIJND AND PSE FOR EACH EXPERIMENTAL CONDITION .
JND

PSE

Edge
Edge

that could be perceived by each participant. The JND results gave us
an insight into the minimum difference of the frequency or amplitude
coefficient that could be efficiently discriminated. The perceived
probability curve was obtained by fitting the psychometric curve to1). The value of vibration frequency
the data (f (x) = 1+exp(−A·(x−B))
or amplitude at the 50% point on the perceived probability curve
indicates the PSE (Point of Subjective Equality) of the perceived
fineness or unevenness. The half of the difference in vibration
frequency or amplitude between the 75% point and the 25% point on
this curve indicates the JND. We obtained the PSE and JND for eachparticipant (Table III), and applied Student’s paired t-test for eachexperimental condition. This test revealed a significant difference in
JND between “Edge condition” and “None condition” in “Frequency
Ex.” and “Amplitude Ex.”(p < 0.01). It also revealed that there
was a marginally significant difference in the PSE between the two
conditions in “Amplitude Ex.” (p < 0.10).
C. Discussion
The results of “Frequency Ex.” and “Amplitude Ex.” shows that
the JND of the “Edge condition” was significantly smaller thanthat of the “None condition.” This means that presenting an edge
vibration at the boundary of the two vibrotactile textures helped theparticipants discriminate the difference in vibration frequency andamplitude between the two adjacent textures correctly. Because there
was no significant difference in the PSE between the two conditionsof “Frequency Ex.”, the addition of the edge vibration did not affectthe feeling of vibration frequency within the texture area itself. This
result appeared in the answer to the question after the task, and 10 (6)out of 12 participants answered that it was easier to discriminate thedifference in the fineness (unevenness) of the left and right textures
when the vibration was presented at the edge. These results show thatcrossing over the vibrotactile edge evoked a sensation that the textureson the left and right sides of an edge were different in fineness oramplitude. Furthermore, in both experiments, 4 out of 12 participants
told that when the difference between the right and left vibrations was
small, they felt that the texture vibrations were gradually switchedwithout the edge vibration, and were abruptly switched with theedge vibration. Thus, it is considered that the change in vibration
at the boundary of the texture areas can be felt sharply with the edge
vibration.
However, the PSE of the “Edge condition” was marginally smaller
than that of the “None condition” in “Amplitude Ex.” This result

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
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JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

indicates that the presentation of the vibration stimulus of the edge
affected the perception of the amplitude of vibration of the texturalareas. These results also appeared in the answers to a question after
the task, and 5 out of 12 participants said that the texture on theright side, that is, the texture traced after crossing the edge, felt more
uneven when the edge vibration was presented.
We considered that these results might be caused by the masking
effect of the vibration stimulation. When a small stimulus is presented
after a large stimulus in time series, the former may mask the latter,and only a large stimulus presented previously is perceived. This iscalled a temporal masking, and it can be occured with the tactile
stimulation [10]. Moreover, in the case of simultaneous masking
in which the masking effect is generated by presenting two kindsof stimuli simultaneously, it has been confirmed that the masking
effect becomes remarkable when the target and the masking stimulus
frequencies are the same [24]. These findings indicate the possibility
that the vibration of the edge caused the tactile masking effect on theamplitude of the texture vibration of the right-side texture area traced
after passing the edge. This effect might be caused by the amplitude
of edge vibration being larger than that of the texture area’s vibration,and the masking effect may have been more likely to occur because
all the texture areas have the same f in “Amplitude Ex.”In summary, it was clarified that presenting edge vibrations between adjacent textures makes it easier for users to discriminate thefrequency and amplitude differences of those vibrations.
D. Limitation and Future WorkIn this study, as the first step towards verification of the proposed
method, we investigated the effect on the discrimination of uneventexture with a simple pattern vibration in a limited low-frequency
band. In future work, we should verify the effect of edge vibration on
the vibration texture of various frequencies, amplitudes, and patterns.
Moreover, in this experiment, we heuristically used a single type ofvibration on the edge. However, there is a possibility that the effect ofedge vibration would change according to the various parameters ofa vibrotactile signal. Thus, it is necessary to clarify which parametersof the edge vibration (e.g., waveform, amplitude) have the strongest
effect on the result. We expect that controlling the intensity of theedge vibration according to the balance of the intensity of vibrationin the texture areas may cancel the tactile masking effect. This may
improve the discrimination ability in terms of the vibration amplitude.
VI. C ONCLUSIONSIn this paper, we proposed a method for improving the ability todiscriminate the vibrotactile stimuli for different areas using edge
vibration. The user study yielded the following findings:• Edge vibrations on the borders between adjacent textures makes
it easier for users to discriminate the differences in frequency
and amplitude of the vibrations assigned to those textures.
• Edge vibration may cause the tactile masking effect, whichmakes the users feel that the vibration amplitude of the texture
traced after passing the edge is larger than it actually is.
In addition to the contributions from a scientific point of view,the findings in this study also contribute to vibrotactile design.
In the real world, there are many objects composed of multiple
materials, and to realize the experience of touching them virtually,
it is necessary to make the user correctly recognize the difference
between multiple textures distributed on the object surface. There
are various applications such as industrial design, e-commerce, andtelepresence that require the experience of touching a virtual texture.
Therefore, it is essential to have a vibrotactile design by which theuser can accurately perceive the difference between the vibration
textures adjacent to the virtual object surface.

6

Due to the human detection thresholds of vibrotactile signals, it
is difficult for users to recognize slight differences between multiple
vibrotactile textures. This has been a constraint in vibration design.
Therefore, if tactile designers adopt the use of edge vibration, thedifferential detection threshold would be decreased, and designers
could design a wider range of vibrations for adjacent areas. Our
proposed method can be applied not only to haptic experience througha tablet as verified this time, but also to a virtual environment
experience using a Head Mounted Display (HMD) and a controller
with vibrator; hence, it is expected to be widely used in vibrotactile
design. Although the edge effect depends on each specific caseaccording to the content or application, we provide promising new
options for designers.
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