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Vibrotactile Stimulation to Increase and Decrease Texture Roughness
Shuhei Asano, Shogo Okamoto, and Yoji Yamada, Member, IEEE
Abstract—We have developed a texture display system that modifies theperceived roughness of textured surfaces via a voice coil actuator wornon the finger. To increase the roughness sensations, the vibrotactile stimuli
from the actuator simulate the skin deformations that are activated when awavy surface is scanned. Conversely, to decrease the roughness sensationsof the textured surface, a high-frequency vibrotactile stimulus offsets theactivity levels of tactile mechanoreceptors. This offset suppresses the perceived roughness of the surfaces being touched, with increase in the offset
correlating with increase in the feeling of smoothness of the surfaces. We
conducted an experiment in which we tested the effects of these two typesof vibrotactile stimulation on grating roughness specimens, with subjective
responses acquired from eight participants via the magnitude estimation
method. The results obtained indicate that our method selectively increases
and decreases the roughness felt. The intention is for the technique to be
used to develop an augmented reality device for textures.
Index Terms—Augmented reality, haptic device, mechanoreceptors,
roughness, texture, vibrotactile stimulation.

I. INTRODUCTION

Tactile interfaces could include capabilities to vary the perceived mechanical properties of the objects including perceived
texture. Vibrotactile stimuli, which are generated by stimulators
worn on the finger or installed on the surfaces to be touched by
the finger pad, enhance the roughness percepts of material surfaces [1]–[3]. Such stimuli should be synchronized with handmotions to present sensations that match those experienced during natural active touch. Electrostatic forces that arise between
the human body and the surface of an object have been used tomodify surface texture [4]. Furthermore, noisy sounds associated with touch motions can effectively influence the roughness
percepts of materials [5], [6].
Prior methods exert additional sounds or mechanical energy
on humans in order to increase the roughness percepts of touchedsurfaces. However, decreasing the roughness percepts is morechallenging. Although Guest et al. [2] demonstrated that theattenuation of the sounds caused by touch suppresses the perceived roughness, methods to control the reduction of the roughness percepts are limited. In addition, because the squeeze effectcaused by ultrasonic vibrations can reduce the surface friction ofelastic bodies [7]–[11], such an effect may lead to a reduction in
the sense of surface roughness. However, this approach is used
for surfaces whose frequency response characters are carefully
designed and are not suited to changing textures such as thickcloth and wood.

Manuscript received March 3, 2014; revised July 1, 2014 and September
20, 2014; accepted November 1, 2014. This work was supported in part by
KAKENHI Shitsukan (25135717) and MIC SCOPE (142106003).
The authors are with the Department of Mechanical Science and Engineering, Nagoya University, Nagoya 464-8602 Japan (e-mail: asano.shuhei@h.mbox.nagoya-u.ac.jp; okamoto-shogo@mech.nagoya-u.ac.jp; yamada-yoji@mech.nagoya-u.ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/THMS.2014.2376519

Fig. 1.

Index finger mounted with vibrotactile stimulator.

In this paper, we discuss the development of a wearable texture display technique that can alter the perceived roughnessof surfaces when they are touched by the wearer. Since thedevice is installed on the human body, the technique does not require any modification of the materials or objects being touched.
The study presented here builds on [12], in which we verified
the ability of vibrotactile stimulus to decrease the roughness
percept. Here, we also use a vibrotactile stimulus to increase theroughness percepts and include two types of textured samples
with different roughness levels. These conditions curtailed any
risks of artifact or conditioning by experimenters’ instructions.
Consequently, our results show that vibrotactile stimuli can increase and decrease the roughness texture of materials in a single
experimental paradigm. The long-term goal is for the technique
to be used in a new type of vibrotactile texture display.
II. EXPERIMENTThe Ethical Committee of the Engineering School at Nagoya
University approved this study.
A. Apparatus

The apparatus included a recoil-type voice coil motor (Haptuator Mark II, Mr. Amir Berrezag, UPMC, France) driven bya current amplifier (ACJ-090-03, Copley Controls, MA, USA)through a fabric band placed between the DIP and PIP joints of
the index finger of the participant’s writing hand, as shown in
Fig. 1. The motor was affixed to the band using double-sided
tape and generated accelerations along the longitudinal directionof the finger with a refresh rate of 5 kHz.
Fig. 2 shows the experimental setup, in which a camera (PlayStation Eye, Sony Computer Entertainment, 30 ft/s,
320 × 240 px) measured the position of the marker on eachparticipant’s finger. The measured position was used for thevibrotactile stimulation method to increase the roughness percepts. The spatial (0.625 mm) and temporal resolutions of thecamera were not appropriate for texture display; therefore, we
calculated the finger position by integrating the finger velocity at

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frequency is equal to ẋ(t)/λ. As a result of the dynamics ofthe recoil-type voice coil motor, the acceleration values became
virtually flat above 200 Hz, but decayed beneath several tensof Hertz with the resonance at approximately 100 Hz. We used
several values of A to test some acceleration levels. The larger
accelerations evoked the larger roughness percepts.
2) High-Frequency Vibrotactile Stimuli to Reduce Roughness
Percepts: While no researchers reduced the perceived roughness

Fig. 2.

Signal flow in the experimental setup.

Fig. 3. Presentation of fine roughness using sinusoidal vibrotactile stimuli tofinger pad.

5 kHz to generate its smooth displacement without directly referring to the position measured by the camera. In this case,the position measurement accumulated absolute errors. The velocity was calculated using a Savitzky–Golay filter of length
five [13].
B. Vibrotactile Stimuli to Increase and Decrease Perceived
Roughness of Textured Surfaces
1) Virtual Wavy Stimulus Synchronized With Hand Motions to Increase Roughness Percept: To increase the roughness percepts oftextured surfaces, we used the virtual wavy surface method thatwas originally introduced for the presentation of virtual roughness [14], [15], and later applied to real surfaces [4]. In thismethod, a mechanical displacement is applied to the finger pad
to simulate or mimic the displacements experienced when scanning a wavy profile. This method allows the support of percepts
related to fine roughness as that possessed by some cloths andabrasive papers.
Fig. 3 shows the scheme utilized in this roughness presentation method. The sinusoidal vibratory stimuli are generated by
the voice coil actuator attached to the finger according to the finger position estimated using a camera. The voltage instruction
to the voice coil motor was determined by 2πx(t) (1)v(t) = A sin
λwhere A, x(t), and λ are the voltage amplitude, planar displacement of the finger on the textured surface, and the surface
wavelength of the virtual wavy profile, respectively. λ was fixed
to 1.0 mm in the latter experiment. The instantaneous vibratory

using vibrotactile stimuli related study follows.
Gescheider et al. [16] demonstrated that when the cutaneous
receptors are adapted after exposure to a strong vibration fora sustained period, the roughness experienced from dotted textures decreases. Further, this adaptation restricts the discrimination of finely textured surfaces [17]. These studies suggest that
cutaneous receptors temporarily incapacitated by strong vibrations disturb roughness percepts. However, our objective is tocontrol the decrease in roughness percepts without such priorstimuli for adaptation. The smoothness percept caused by theadapted mechanoreceptors is not our research objective.
Vibrotactile stimuli have a masking effect that gradually degrades the detectability of vibrotactile signals under noisy background vibration [18]. The greater the background noise, thegreater the impedance to perception of the vibratory signal.
Uncertainty exists as to whether these two types of phenomena, adaptation and masking effect, are based on the same neurophysiological principle; however, from these early studies, weformulated our hypothesis. The roughness percept can be regulated by adjusting the strength of the vibrotactile stimuli presented to a person’s finger while she/he is exploring the material.
We used a high-frequency background vibration (250 Hz) thathas a prominent effect on fast adaptive receptors. In this case,our method would influence the percept of fine roughness thatthe fast adaptive receptors are primarily in charge of, whereas
the percept of coarse roughness would not be affected.
C. Stimuli: Textured Surfaces and Accelerationsof Vibrotactile Stimuli

We used two types of rectangular grating scales made of ABS
plastic. The ratio of their ridge and groove widths was 1.0, andthe widths were 145 and 290 μm for the two types of scales.
The heights of the ridges were 200 and 600 μm for the former
and latter scales, respectively. The latter scales are perceived as
rougher when scanned by a finger.
We also used two types of vibrotactile stimuli to realize theincrease and decrease in roughness percepts. To adjust the effects of the vibrotactile stimuli, we prepared three levels foreach type of stimulus. The three levels for the roughness increase corresponded to ± 3.7, 4.9, and 6.2 m/s2 acceleration at
250 Hz. Those for roughness decrease corresponded to ± 27.0,
31.6, and 36.2 m/s2 acceleration at 250 Hz. These values were
the outputs of the voice coil motor, which delivered the vibratory stimuli to each participant’s finger through a fabric band.
In addition to these six vibratory conditions (three levels andtwo types of vibrotactile stimuli), a condition with no vibratory
stimuli was also tested. Hence, in total, seven types of conditions
were prepared.

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Fig. 4.

3

Procedures used for tactile exploration in the experiment.

D. Procedure

The experiment was conducted using the magnitude estimation method, in which each blindfolded and sound-masked participant scanned the textured surface, and stated the magnitudeof the perceived roughness using arbitrary positive numbers. They did not use any references (moduli) to judge theroughness.
To ensure that the finger moved smoothly on the surface, theparticipants were instructed to scan the surface by tracing a figure eight. This figure-eight scanning made it easier to control
the finger scanning speeds than the linearly reciprocating scanthat includes pausing phases to change the direction of finger
movement. Furthermore, in order to prevent extraordinary deviation of exploratory hand movements between participants andtrials, the participants synchronized their hand motions with thesounds of a metronome set to oscillate at 0.86 Hz. This soundwas played through headphones along with pink noise. Theyscanned half a figure eight for one beep sound, and completed
the figure after the second beep, as illustrated in Fig. 4.
During the experiments, to avoid significant tactile adaptation, the participants observed the following procedures. In asingle trial, the participants started their exploration immediately following a sound cue, with the vibrotactile stimuli used todecrease roughness sensations beginning at the same time. Eachtrial was restricted to two reciprocating motions. Thus, in total,
their fingers were exposed to the vibration for approximately
4–5 s in each trial. Between trials, a break of approximately
10 s in length was taken.
E. Participants

The participants were eight volunteer students from Nagoya
University, who were unaware of the objectives of the study.
They ranged in age from 19 to 23 years. Each read and signed
a written informed consent form before participating in theexperiment.
F. Experimental Design and Data Analysis

By combining the seven vibratory conditions and the two
types of textured materials, we tested 14 stimuli. With these 14
stimuli as one block, each participant performed four blocks. In
each block, the stimuli were presented in a randomized order.
The perceived roughness values for each stimulus condition
were geometrically averaged for individual participants. To fa-

Fig. 5. Individual perceived roughness values for the two types of vibratory
stimuli: GW signifies groove width.

cilitate easier comparison, individual values were then normalized by dividing them by the magnitude of the roughness the subject perceived for the rougher grating scale (290-μm ridges andgrooves) without vibratory stimuli. We tested the Spearman’srank correlation between the perceived roughness (dependent
variable) and the stimulus levels (independent variables) using
two-tailed t-tests for each combination of the two roughness
specimens and two types of vibrotactile stimuli with Bonferroni
correction of four (two specimens × two vibrotactile stimuli).
For each test, 32 values (four averaged answers per individual ×
eight participants) were used.
III. RESULTS

Fig. 5 shows the geometric averages of individual answers.
The horizontal axis indicates the types of vibrotactile stimuli andtheir levels. On the right and left sides of the figure, the answers
for the scales with vibrotactile stimuli to increase and decrease
perceived roughness are shown, respectively, with the answers
for the scales without vibration being at the center of the axis. For
both types of vibrotactile stimuli, their stimulus levels increase
the farther away from the center of the axis they are located. The
red and blue symbols correspond to the answers for the grating
scales of GW = 145 and 290 μm, respectively. Similarly, Fig. 6
shows the numerical averages of all the participants.
First, in the absence of vibrotactile stimuli, the grating scalewith the larger ridge and groove widths felt rougher than thatwith the smaller widths. Such natural responses toward grating
scales support the idea that the roughness percepts of participants were intact during the experiment.
Second, the perceived roughness of all the participants was
positively correlated with the intended roughness of the rendering. The vibrotactile stimuli that were designed to increase
roughness percepts increased the perceived roughness as theirstimulus level rose. For the vibrotactile stimuli that increase
roughness, the effects were statistically significant for the scaleof 290 μm (ρ = 0.73, p < 0.01, (1) in Fig. 6) and that of
145 μm (ρ = 0.70, p < 0.01, (2) in Fig. 6). Similarly, the vibrotactile stimuli that were designed to decrease roughness percepts
decreased the perceived roughness. The effects of vibrotactileThis article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
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Fig. 7. Perceived roughness versus impulsive rate of FA I unit (computedfrom [23]).
Fig. 6. Average perceived roughness values and standard deviations among
participants.

stimuli that decrease roughness percepts were statistically significant for the scale of 290 μm (ρ = 0.87, p < 0.01, (3) inFig. 6) and that of 145 μm (ρ = 0.63, p < 0.01, (4) in Fig. 6).
On the basis of these results and analyses, we concluded that thetwo types of vibrotactile stimuli could increase and decrease theroughness percepts of grating scales. These results agree withthose when we decreased the roughness stimuli in our priorstudy [12].
IV. DISCUSSION

The experimental results were predominantly in line with
our expectations that the constant vibrotactile stimuli applied to
the finger would decrease the roughness percepts of the grating scales. Let us now look at a potential mechanism for thiseffect. One idea is that when the mechanoreceptors’ activities
are adapted or saturated because of exposure to strong vibrotactile stimuli over a sustained period, they are unable to correctly
code the roughness sensations of the scales. This blockage ofroughness percepts by adaptation may well be consistent withthe results of earlier studies [16], [17]. However, this explanation does not necessarily hold for our experiment in which
the receptors were not fully adapted. Other studies on masking
effects address, for example, how the detection of short-term
tactile stimulus is influenced by another relatively strong or
long stimulus when they are initiated simultaneously or with aslight temporal gap (see, e.g., [19]–[22]). When the duration of
the stimuli is relatively long, such masking effects due to temporally close stimuli are not necessarily observed [19]. Hence,
those studies do not directly link to the roughness reduction
by vibrotactile stimuli investigated in this study. We surmisethat another mechanism, with which the offset of the receptors’ activities are caused by vibration, reduced the roughness
perceptions.
Yoshioka et al. [23] investigated the relationships between
cutaneous stimuli caused by asperities of roughness specimen,
roughness sensations, and the receptors’ firing rates from theneurophysiological and psychophysical aspects. They compared
the human roughness percepts with the receptors’ activities ofa monkey through the same roughness specimen. Fig. 7 showsa conceptual relationship between the firing rate of FA I units
and the roughness percepts. The curve was plotted based on
the equations reported in [23]. The point being made is that theperceptual quantity of roughness increases nonlinearly with thereceptors’ activities. The rate of roughness percepts to receptors’
activities, which is defined by the gradient of the curve, declines
as the receptors’ activities rise. Hence


dp 
dp 
>
, if av 0 > a0(2)
da a 0da a v 0
holds as shown in Fig. 7. a0 , at , av 0 , and av t are the receptors’
activity levels in the absence of any stimuli, while exploring thesurface, in the presence of vibrotactile stimuli, and while exploring the surface with vibrotactile stimuli being applied to thefinger, respectively. The suffixes 0 and t indicate nontouch andtouch, respectively. Further, v indicates the vibrotactile stimuli
being applied. p(•) associates the receptors’ activity with perceptual roughness quantity. On the basis of such connections
between roughness sensations and receptors’ activities, vibrotactile stimuli are likely to reduce the roughness experienced in
exploring textured surfaces.
We simplify the relationship and define the magnitude ofperceived roughness by the difference in perceptual quantities
before and after touching the surface. The roughness sensation
caused by touching the surface is described asp(at ) − p(a0 ).

(3)

Given that the vibrotactile stimuli are applied to the fingertip
through a vibrator worn on the finger, the receptors’ activity
level is av 0 . The finger then scans the textured surface, and thereceptor’s activity level rises to av t . The roughness experienced
in this scenario is described byp(av t ) − p(av 0 ).

(4)

As shown in Fig. 7, (3) and (4) followp(at ) − p(a0 ) > p(av t ) − p(av )(5)when at − a0 ≥ av t − av 0 . Hence, when a constant vibrationthat is independent of active hand movements is applied to thefingertip, humans perceive textured surfaces as being less roughthan they do without the vibration. In order to take advantage ofThis article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
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this roughness coding curve, we need intense vibrotactile stimuli
that lead to a large bias in the curve because it is a monotonicallyincreasing function with a marginally declining slope, as shownin Fig. 7. The strengths of the stimuli to decrease perceived
roughness were five to ten times as large as those to increase
perceived roughness. We ignored the variations of receptors’
activities during active exploration to simplify the discussion.
Nevertheless, similar principles are likely to dynamically hold,and the vibrotactile stimuli reduce the roughness percepts oftextures during active exploration of the textured surface.
In our experiment, limited types of specimen were tested.
The vibrotactile stimuli increased the perceived roughness ofwood, paper, cotton, and artificial leather, when the vibratory
source was installed on these materials [4]. Hence, the stimuli
by the actuator worn on a finger are also expected to influencethese types of materials in a similar fashion. Conversely, theeffect of vibrotactile stimuli to decrease perceived roughness on
general materials is unknown, and further experimental studies
are required before any conclusions can be drawn. Its effectis expected to be the same for materials (e.g., abrasive paper)
similar to the specimen used in this study. However, in terms ofother natural materials that include both fine and macroscopic
surface roughness, the effect of our strong vibrotactile stimuli todecrease perceived roughness still remains to be studied, especially because the strong vibrotactile stimuli in a high-frequency
range also influence the cutaneous receptors, which are primarily sensitive to low-frequency vibration [18]. Furthermore, human perception of material softness or friction may be alteredby these vibrotactile stimuli because the activities of cutaneous
receptors also mediate these percepts (see, e.g., [24], [25]).
V. CONCLUSION

In this paper, we discussed the development of a wearabletype vibrotactile display that increases and decreases the roughness percepts of surfaces touched by the wearer. Earlier studies
have shown that such wearable displays can increase texturalroughness. We were able to also increase the roughness experienced by scanning grating scales using vibrotactile stimuli
representing virtual wavy textures. However, in general, there is
no known method to decrease roughness percepts by applying
additional physical energy to the skin of the finger. We discovered that the vibrotactile offset of mechanoreceptors’ activities
decreases the perceived roughness of touched surfaces with increases in the offset correlating with increases in the feelingof smoothness of the surfaces. This approach reduced the perceived roughness of the grating scales for all eight participantsin our experiment. Thus, this study is a first report on a single
experiment that demonstrates that the vibrotactile stimuli canselectively increase and decrease roughness percepts.
Some of the users of these techniques may feel uncomfortablewith the vibration, especially the intense vibration that decreases
perceived roughness. This is similar to the problem of imperfectperceptual fusion of real and artificial stimuli, which is common
in augmented reality systems. At present, there appears to be
no effective method to eliminate the sense of vibration andachieve natural fusion. Nevertheless, the methods presented in5this paper can provide users with a wearable system that does
not require additional devices in the environment. These positive
and negative points should be properly weighed for individual
purposes.
ACKNOWLEDGMENT
The authors would like to thank A. Berrezag at ISIR in UPMC for
discussing this study.

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