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Detectability and Perceptual Consequencesof Delayed Feedback in a Vibrotactile
Texture Display
Shogo Okamoto, Student Member, IEEE, Masashi Konyo, Member, IEEE,
Satoshi Saga, Member, IEEE, and Satoshi Tadokoro, Fellow, IEEE
Abstract—This study estimated the maximum allowable system latency for haptic displays that produce tactile stimuli in response to
the hand movements of users. In Experiment 1, two types of detection thresholds were estimated for the time delay of stimuli throughpsychophysical experiments involving 13 participants. One was a threshold for the users to notice the existence of a time delay. The
other was a threshold for the users to experience changes in the perceived textures in comparison with stimuli with no time delay. The
estimated thresholds were approximately 60 and 40 ms, respectively. In interviews, the participants reported that they experienced
various types of subjective changes due to the time delay. In Experiment 2, the types of subjective sensations that might be altered by
the time delay were investigated. The time delays were controlled based on the acceleration of the hand motions of the participants.
The participants evaluated the differences in the perceived textures between the stimuli with a controlled time delay and ones with nodelay. The results indicated that the participants associated the time-delayed stimuli with changes in mechanical parameters such as
kinetic friction coefficient in addition to changes in the perceived roughness of the textures.
Index Terms—Detection threshold, tactile display, time delay.

Ç
1

INTRODUCTIONT

ACTILE displays can be used to produce appropriate
tactile stimuli according to the hand motions of users inorder to induce a natural sense of touch. In such display
methods, users experience the stimuli triggered by theirmotions just like those triggered when they touch real
objects when engaging in haptic exploration.
Numbers of tactile display systems have been developedthat present stimuli to users in response to their hand
movements. For example, we have demonstrated a methodthat presents various types of textures by means of stimuli insynchronization with the position, velocity, and accelerationof user’s hands [1]. Other researchers have also proposedsystems that use hand motions as inputs and output thecorresponding stimuli [2], [3], [4]. In tactile telepresence
based on master-slave systems, system operators maneuver
the master arms and then acquire corresponding tactile
feedbacks from the tactile sensors installed on the slave armand the tactile displays on the master arm [5], [6], [7], [8], [9].
In the above-mentioned display methods and systems,
system latency or communication time delay is inevitably. S. Okamoto, M. Konyo, and S. Tadokoro are with the Graduate School ofInformation Sciences, 980-8579 Tadokoro Laboratory, Tohoku University,
6-6-1, Aramaki-aza Aoba, Aoba-ku, Sendai, Japan.
E-mail: {okamoto, konyo, tadokoro}@rm.is.tohoku.ac.jp.
. S. Saga is with the Graduate School of Information Sciences, 980-8579
Deguchi Laboratory, Tohoku University, 6-6-1, Aramaki-aza Aoba, Aobaku, Sendai, Japan. E-mail: saga@fractal.is.tohoku.ac.jp.
Manuscript received 7 Mar. 2008; revised 19 Feb. 2009; accepted 24 Feb. 2009;
published online 27 Apr. 2009.
Recommended for acceptance by H.Z. Tan.
For information on obtaining reprints of this article, please send e-mail to:
toh@computer.org, and reference IEEECS Log Number TH-2008-03-0016.
Digital Object Identifier no. 10.1109/ToH.2009.17.
1939-1412/09/$25.00 ß 2009 IEEE

present between the detection of hand motions and the
production of stimuli. When the delay is significant, users
no longer consider the stimuli to be caused by their actions,or they feel that the presented objects move unnaturally.
The determination of the maximum allowable system
latency will be helpful for designing haptic devices.
The present study estimates tactile detection thresholds
for the time delay for haptic displays that produce stimuli
in synchronization with hand motions. Two types ofthresholds are investigated. One is a threshold at whichusers notice the existence of a delay. If the delay exceeds
this threshold, users will detect the delay in the feedback.
The other is a threshold at which users experience changes
in the perceived textures of presented objects, although they
may not be aware of the delay. When the delay exceeds thisthreshold, the users perceive the time-delayed objects to bedifferent from those with no delay. Haptic sensations arise
from spatio-temporal information such as the physicalproperties of target objects, hand movements, and neuralsignals. The delay of the stimulus is a type of temporal
information that can possibly alter the perceived textures ofobjects. Due to the time delay, users experience sensations
that are different from those that the system designers
intend to present.
Further, when the delay changes the perceived textures,
knowledge about the types of changes is helpful. This isbecause this knowledge leads to the development of new
displaying methods or an understanding of haptic information process in humans.
This study comprises two experiments. In the firstexperiment, two types of detection thresholds are estimatedfor the system latency of tactile displays. In the second
Published by the IEEE CS, RAS, & CES

74

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experiment, the types of changes in perceived texturescaused by the time delay are investigated.
This paper is organized as follows: Related work issummarized in Section 2. The tactile stimuli and apparatus
for the experiments are described in Section 3. The
estimation of the two types of thresholds is presented in
Section 4. The investigation of the effects of time-delayed
stimuli on perceived textures is described in Section 5.
Finally, the discussions and conclusions are presented in
Sections 6 and 7, respectively.

2

RELATED WORK

Many studies have investigated human perception of
temporal gaps or intervals of tactile stimuli. For example,
the amount of time that must pass between the onsets of
two successively mechanical stimuli applied to two index
fingers in order to temporally separate those stimuli has
been investigated [10]. The amount of time depends on theintensity levels of the first and second stimuli [11]. Such
temporal order judgment tasks have also been compared
between a single finger pad, two fingers on the same hand,
and two fingers on opposite hands [12]. The tasks have also
been investigated using electrotactile stimuli [13]. The
amplitudes of vibrotactile stimuli required to separate two
successive stimuli with a slight temporal gap have been
investigated [14]. These studies have focused on thetemporal factor between one tactile stimulus and another.
Several studies have addressed the temporal delay
between human motions and the resulting stimuli causedby these motions. For haptic input and auditory response,the detection threshold of a delay between a hammer orwand strike and the resulting collision noise delivered
through headphones is 24-40 ms [15], [16]. In another study
where virtual sounds were presented through headphones
in response to head movements of users, the detection
threshold for the system latency between head motions andvirtual sound sources is 45-80 ms [17], [18]. For virtual
musical instruments, the allowable time delay between aplaying action and sound generation by controlling atheremin is 70-80 ms (calculated from the report [19]). Forgraphical user interfaces of computers, the threshold of thedelay of graphical responses to mouse or typing motions is100-200 ms [20]. The threshold of the delay of image
presentation in response to eye movements is 100 ms [21].
The tolerance of asynchrony between hand movement andforce feedback in a virtual soft wall simulation using
PHANToM device is 30-35 ms [22]. The values determinedin these studies cannot be compared directly due todifferences in experimental methods. To the best of ourknowledge, the detection threshold for the delay between
haptic exploratory motions and tactile sensations has yet tobe reported.
In addition to the new thresholds reported, the present
study also investigates the effects of the time-delayed stimuli
on haptic perception. Although no study has focused
specifically on the effects of the delay between exploratory
motions and tactile stimuli on haptic sensations, a few
studies have reported that time delays impair task performance. For example, for remote haptic teaching or training,
the permissible communication delay has been investigatedFig. 1. Frequency responses of the vibrator when the peak-to-peakamplitude of voltage supply is 150 V.

[23], [24]. For collaboratively lifting or grasping objects inremote environments, the delay that decreases the quality ofservice has been reported [25]. In the presentation of soft
objects, it has been reported that a delay in force generation
alters the perceived softness or stiffness of the objects [26],[27]. The present study goes beyond threshold measurements by investigating the changes in haptic perception due
to time delays.

3

TACTILE STIMULI AND EXPERIMENTAL SETUP

3.1 Tactile Stimuli
The experimental task was the haptic exploration of virtual
textures. The presentation of textures is an importantfeature of many vibrotactile displays. The experimental
system required an apparatus to produce delayed tactile
stimuli in response to the hand motions of the participants.
When using real textures such as sandpapers or grating
scales, it is possible to simulate the time delay by moving
the textures along the movement direction of the hand. The
resultant stimuli resemble delayed haptic feedback in termsof the delayed change in the relative positions between thefinger and the texture. However, variations in the relative
velocities between the texture and finger also change thefriction and contact conditions between them.
Better stimuli control can be achieved by generating
virtual textures in response to measured finger movements
using an experimental apparatus such as the one used in thepresent study. Participants explored virtual textures bymoving a vertical rod along a linear track. The participant’s
middle finger was always in contact with the rod whichtransmitted vibrotactile stimuli in response to finger movements, with controlled time delay.
A vibrator was used as the tactile stimulator. The vibrator
was a piezo-stack-type actuator (ASB510C801P0, NEC/TOKIN, Sendai, Japan). Fig. 1 shows its frequency responses
with no load for input peak-to-peak voltage of 150 V. Fig. 2
shows the roughly linear relationship between vibratory
displacements and input voltages. The maximum displacement was approximately 55 m when the frequency was
0.1 Hz. The frequency responses were measured using afrequency analyzer (FRA5014, NF, Yokohama, Japan), a
voltage amplifier for the actuator (BA4825, NF, Yokohama,Japan) with a nominal maximum response frequency of

OKAMOTO ET AL.: DETECTABILITY AND PERCEPTUAL CONSEQUENCES OF DELAYED FEEDBACK IN A VIBROTACTILE TEXTURE DISPLAY

75

Fig. 3. Block diagram of the entire system.

Fig. 2. Voltage-displacement plot of the vibrator.

2 MHz, a laser range finder (ZS-LD50, OMRON, Kyoto,
Japan) for measuring magnitude responses, and a laser
Doppler velocimeter (MLD-301AS, NEOARK, Tokyo, Japan)
for measuring the phase characteristics.
The use of this vibrator was advantageous due to its highoutput force and roughly flat frequency response characteristics. Because the output force of the actuator was approximately 800 N, the force exerted by the finger was considered
to be negligible and did not affect the commanded
displacements. During the experiments, the finger force
was measured and monitored such that it did not exceed 2 N.
The amplitudes of the displacements depended on thefrequency of the driven voltage; however, they wererelatively flat in the range of up to 300 Hz that was used
for the tactile stimuli. The displacement reached 3 dB when
the frequency was approximately 310 Hz. The same voltage
amplitude A was supplied to the vibrator throughout theentire frequency range up to approximately 300 Hz.
The virtual texture had a sinusoidal height profile along
the finger-moving direction on its surface. The instantaneous position of the finger along the virtual texture wasdetermined by the position of the vertical rod, xðtÞ. The
electric voltage y supplied to the vibrator wasxðt  DÞy ¼ A sin 2
þ A;
ð1Þ

where A was the peak amplitude (75 V), D was thesimulated temporal delay between the hand movement andthe corresponding vibratory stimulus, xðt  DÞ was thedelayed rod position along the hand movement direction,and  (1 mm) was the spatial wavelength of the texture
sample. Since the relation between the input voltage and theoutput displacement was mostly linear, (1) could also beviewed as the displacement stimuli delivered to the finger.
The hand position data were buffered in a first-in-first-out
queue for 200 ms such that the delay can be changed
between 0 and 200 ms. When the stimuli included the timedelay D, the stimuli presented to the finger were based on
the hand position and the velocity recorded Ds ago.
Equation (1) can be rewritten into!
R tDvðÞd
0
y ¼ A sin 2
þ A;
ð2Þ


where vðtÞ is the finger velocity. The equation indicates thatthe vibratory frequency applied to the finger depended on
the finger velocity, and was given by vðtÞ=. The handvelocity was calculated from the hand positions.

3.2 Experimental Setup
Fig. 3 shows a block diagram of the entire system. The vibrator
was installed on a linear slider whose position on a linear
guide was measured by an optical linear encoder (SR-P1000,
Canon, Tokyo, Japan). The spatial resolution of the encoder is
1.6 m. A linear guide was selected (SS series, NSK, Tokyo,
Japan) for its low friction and low vibration during sliding. A
control computer received the encoder pulses and supplied
electric voltages to the vibrator through an amplifier. Theinterface board of the control computer was selected (Ritech
Interface Board IF-0145-1, Okazakisangyo, Osaka, Japan).
The update rate of the control computer was 5 kHz (RTLinux).
The effective length of the guide was approximately 420 mm.
The force applied by the finger along the z-axis was measured
by a force gauge. Its output was transmitted to the control
computer through a strain bridge and amplifier (MCD-8A,
KYOWA, Tokyo, Japan) so that the force could be recodedand monitored.
Fig. 4 shows the apparatus from the participant’s side.
The participants moved their hands along the linear guide(x-axis in the figure). The vibrator generated a displacement
along its longitudinal direction (z-axis in the figure). The
vibrator had a circular contact surface having a diameter of
11.6 mm. The participants used an elbow rest during trials
to avoid fatigue. As shown in Fig. 5, the participants placed
their right middle fingers on the vibrator. They were
instructed to avoid using any other part of their hand to

Fig. 4. Participant’s view of the experimental apparatus.

76IEEE TRANSACTIONS ON HAPTICS,

Fig. 5. Contact between the finger of the participant and the vibrator.

touch the apparatus. This ensured that the participants
would not sense vibrations with any parts of their bodies
other than the fingertips.
The participants commented that the vibrations transmitted by the equipment resembled the ones they perceived
when they explored rough specimens such as grating scales
through a stylus.
The inherent latency of the empirical system wasmeasured 10 times. The inherent delay was defined as thetime between the onset of the encoder pulse and theresultant voltage change to the vibrator. It was measured byan oscilloscope with a sampling frequency of 50 kHz and a
resolution of 20 s. The maximum and the median of themeasured values were 160 s and 140 s, respectively, witha standard deviation of 43 s. The inherent system latency
was the minimum delay that was achieved by the system.

4

EXPERIMENT 1: ESTIMATION OF TWO TYPESOF DETECTION THRESHOLDS

4.1 Procedures and Tasks
In Experiment 1, two types of detection thresholds were
estimated in Experiments 1A and 1B. The participants
compared a reference and test stimulus. The reference
stimulus had no delay and the test stimulus had asimulated time delay.
The objective of Experiment 1A was to estimate thethreshold at which the participants noticed the presence ofa time delay in the stimulus. In this experiment, theparticipants were informed that the system might display
a time-delayed stimulus, and the aim of the experiment
was to estimate the detection thresholds for the time delay.
The participants were required to answer whether the test
stimulus was delayed or not.
The objective of Experiment 1B was to estimate thethreshold at which the participants noticed a change in theperceived texture. In this experiment, the participants were
informed that the test stimulus might differ slightly fromthe reference stimulus, and the aim of the experiment was
to investigate whether the participants noticed a change.
The experimenter did not inform them about the types ofparameters being manipulated. The participants wererequired to answer whether the reference and test stimuli
felt the same.
Thirteen males in their 20s and 30s participated in theexperiments. None of the participants took part in bothVOL. 2,

NO. 2,APRIL-JUNE 2009

Experiments 1A and 1B on the same day. They did so
within a period of 11 days. Six participants conducted
Experiment 1A first followed by 1B on subsequent days.
The other seven participants conducted the experiments in
the reverse order.
The participants were provided with timing cues during
the trials in the form of beep sounds through headphones. The
participants explored the reference stimulus for 5 s between
the first and second beeps. Then, after an interval of 3 s, theparticipants explored the test stimulus for another 5 s.
No true-false feedback was given to the participants
during the experiment. The participants were informed not
to change the contact conditions between the finger and thevibrator during the trial. The pressing force of the finger wasmeasured during the trial. A trial was considered invalid if
the force exceeded 2 N, and such trials were repeated after
the completion of all trials. This was necessary because ahigher pressing force of around 4 N or more affects
roughness perception when exploring grating scales [28],[29]. The participants practiced exerting forces of 2 N or lessfor a few minutes before the experiments. The encoder values
were recorded for the analyses. The participants wereblindfolded with eye masks and heard pink noise over theheadphones. The noise level was adjusted for each participant so that he did not hear the frictional sounds of the slider
or the sounds generated by the vibrator.
Preliminary experiments were performed for eachparticipant to estimate the maximum delay needed before
it could be detected using the method of limits. Twoascending and descending series were repeated for eachindividual. The median of the values from the four series,
Dmax , ranged from 70-110 ms.
The main experiments were performed using the methodof constant stimuli. The reference stimulus was D ¼ 0 (notincluding the inherent latency of the experimental system).
For the test stimuli, delays were presented in six levels
between 0-Dmax ms that varied from participants toparticipants. Each test stimulus was presented 10 times ina random order. One experiment comprised 60 trials and it
lasted approximately 40 min, including the explanations
and the preliminary experiments.
The reference stimuli were presented in Experiment 1A
in order to equalize the experimental conditions ofExperiments 1A and 1B for comparison. Early work has
shown that by presenting reference stimuli before test
stimuli, the participants become more sensitive to the delay
than when the test stimulus was presented alone [18]. In
our pilot study, the same trends were observed. Since ourgoal was to estimate the maximum allowable time delay indesigning tactile displays, the experiments were designed
such that the participants would remain sensitive to thedelays.

4.2

Experimental Result

4.2.1 Calculation Method for Detection Thresholds
From the responses of the participants in Experiments 1Aand 1B, the proportion of trials where a delay was detected(Experiment 1A) or a change in perception was noticed(Experiment 1B) were calculated for each delay. A logistic

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77

Fig. 6. Histograms of the detection thresholds in Experiments 1A and 1B
and their expected normal distribution curves.

Fig. 7. Mean DTA and DTB values and their standard deviations by
experimental order.

function was fitted to the data for the individual participants. The logistic function wasshows the thresholds, Dmax , false alarm rates, and goodness of fit of the logistic functions (R2 ), although 10 trials
per stimulus was not adequate for calculating false alarm
rates. In the case of all 13 participants, the outcome of thet-test was not affected by P11.
In order to examine the influence of the experimental
order on the thresholds, a two-way ANOVA was applied
to the thresholds with the factors being the type ofexperiments and the order in which the participants
conducted Experiments 1A and 1B. Fig. 7 shows themean DTA and DTB with their standard deviations. The
circles indicate the mean thresholds of the six participants who performed Experiment 1A first. The diamonds indicate the mean thresholds of the other six
participants who performed Experiment 1B first. The
type of experiment was the main factor that contributed
toward the observed difference between the thresholds(F ð1; 20Þ ¼ 10:87; p < 0:01; p ¼ 0:0036). The experimental
order did not significantly affect the thresholds(F ð1; 20Þ ¼ 2:40; p ¼ 0:14). Also, the effect of interaction
between the type of experiment and the experimental
order was not significant (F ð1; 20Þ ¼ 0:082; p ¼ 0:78).
Although the number of trials per participant was small,fðDÞ ¼

1;
1 þ expða  bDÞ

ð3Þ

where a and b were characteristic parameters of the curve.
We defined D, where fðDÞ ¼ 0:5, as the detection threshold.
We use DTA and DTB to denote the thresholds estimated inExperiments 1A and 1B, respectively.

4.2.2 Detection Thresholds
Fig. 6 shows the histograms of the estimated thresholds of
12 participants in Experiments 1A and 1B with expectedcurves of the normal distributions of the thresholds. All
statistics were calculated based on 12 participants,
excluding participant P11. This is because his DTA waspossibly an outlier (Grubb’s test, N ¼ 13; p < 0:05, onetailed, p ¼ 0:037), and his false alarm rates in bothexperiments were 0.3, which were much higher thanthose of the others. The mean DTA (SD) was 59.2 (13.0)
ms and the mean DTB (SD) was 40.9 (14.4) ms. The mean
DTB was significantly less than the mean DTA (paired
t-test; n ¼ 12; p < 0:01, one-tailed, p ¼ 0:0010). Table 1

TABLE 1
Detection Thresholds, Dmax , False Alarm Rates, and R2 in Experiments 1A and 1B

78

Fig. 8. Dominant frequency of the hand motions versus the detection
thresholds.

the significance level is lower than 0.01 and thedivergence between DTA and DTB appears clear.
Table 1 indicates that false alarm rates were different
between Experiments 1A and 1B. We suggest two possibilities for this reason. One possibility is that the participants
used different criteria for judgment in the two types ofexperiments. Another possibility is that the deviations ofparticipants’ internal responses to the time-delayed textures
were different in the two experiments. The tasks of two
experiments varied, which might have affected the internal
process of participants. However, we do not pursue these
possibilities in the present study.
From the above results and analyses, the detection
thresholds for time delay were established. The thresholdto detect the presence of a delay in tactile stimuli wasapproximately 60 ms. The threshold to detect the changes
in the perceived textures was approximately 40 ms.
These values were on the same order as the thresholds
found with other sensory channels [15], [16], [17], [18],[19], [20], [21], [22].

4.2.3 Correlation between Detection Thresholds andHand Motions
We have noticed the differences in the hand velocities andreciprocating frequencies among the individual participants. Furthermore, during the interviews after the experiments, some participants reported that they detected adelay in the tactile stimuli when they did not experience
any tactile feedback at the onsets of their hand movements.
In order to investigate the relationship between themeasured thresholds and the hand motions, the correlation
coefficients between motion features and the thresholds
were computed. Three types of motion features werecomputed. The first feature was the dominant frequency ofhand motions. The power spectrum density of the handvelocity of each participant was computed, and the frequency at which the signal power was the maximum wasconsidered as a representative frequency of the participant’s
hand motions. The second feature was the average of peakhand speed. The peak speed was measured, and their mean
was computed as the second quantitative feature of the hand
speed of each participant. The third feature was the averageof peak accelerations. Maximum accelerations were observed around the onset of hand motion.

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Fig. 9. Average peak speed of the reciprocating touch motions versus
the detection thresholds.

Fig. 8 is a scatter plot of the frequencies versus thethresholds. The thresholds in the figure are normalized by themean DTA or DTB . There is a negative correlation between
the thresholds and the frequencies (r ¼ 0:54; t-test; n ¼ 24,
two-tailed, p < 0:01; p ¼ 0:0064). When the thresholds are
divided into those for Experiments 1A and 1B, DTB values arefound to be negatively correlated with the frequencies(r ¼ 0:66; t-test; n ¼ 12, two-tailed, p < 0:05; p ¼ 0:019)
while DTA values are not (r ¼ 0:36).
Fig. 9 shows a plot of the average peaks of the hand velocity
versus the normalized thresholds. There was a positive
correlation between the thresholds and the peak velocities(r ¼ 0:51; t-test; n ¼ 24, two-tailed, p < 0:05; p ¼ 0:011). Considering the DTA and DTB values separately, the latter(r ¼ 0:56) are more strongly correlated with the hand velocity
than the former (r ¼ 0:42), though neither is significant.
Statistical analysis showed that the accelerations ofhands were not significantly correlated with the detection
thresholds (r ¼ 0:14; t-test; n ¼ 24, two-tailed, p ¼ 0:67).
Collectively considering these relations, there was atendency for smaller thresholds when the participants
scanned the virtual textures with higher dominant frequencies and lower average velocities.
DTB values were more strongly correlated with thefeatures of hand movements than DTA values were. Thisdifference might suggest that the sensory information for
delay detection and for the detection of changes in theperceived texture is processed in different ways. However,the coefficients show that the correlations are not verystrong that we do not discuss this difference any further.

4.2.4 Subjective Impressions
After Experiment 1A, the participants were asked how theydetected the presence of time delay. According to theinterviews, the participants detected a time delay when
they sensed no tactile feedback at the onset of handmotions, when they still perceived the vibratory stimuli
even after they stopped hand motions, or when they feltthat the virtual texture was moving unnaturally.
After Experiment 1B, the participants were interviewed
about the changes in the perceived textures of the virtual
objects. They reported that they felt the texture to berougher or smoother as they perceived the magnitudes of

OKAMOTO ET AL.: DETECTABILITY AND PERCEPTUAL CONSEQUENCES OF DELAYED FEEDBACK IN A VIBROTACTILE TEXTURE DISPLAY

the roughness to be increased or decreased. They also
reported that the slider felt slippery or sticky and thefrictional resistance of the slider varied. Some of them
reported that they felt as if they had been moving theirhands in water. Furthermore, some participants compared
the changes they experienced to the changes in the mass ofthe slider. Among the obtained answers, some conflicting
opinions were observed, such as rough versus smooth orslippery versus sticky. Even for the same delayed stimuli,
some participants reported that the delayed stimuli wererougher while others judged them to be smoother, as
compared to the nondelayed stimuli.
The participants’ observations raised two questions andmotivated us to conduct Experiment 2. First, we investigated the types of changes in haptic sensations when
exploring delayed stimuli, such as changes in perceived
roughness or friction. Second, we investigated the reason
for why conflicting changes in haptic sensations wereobserved for delayed stimuli.

5

EXPERIMENT 2: EFFECT OF TIME DELAY ON
HAPTIC PERCEPTION

One of the findings in Experiment 1 was that theparticipants experienced a variation in the perceived
textures of the time-delayed stimuli as compared to thestimuli with no delay. Pursuing this observation may lead to
new design methods for haptic displays. The participants
reported changes not only in the perceived roughness ofvirtual textures but also changes that appeared to be
associated with the mechanical properties of virtual
textures. Tactile stimuli can be a substitute for a haptic
display for presenting virtual textures. Contrasting cases
have been reported where roughness can be displayed using
haptic devices by exerting forces rather than applying
geometric displacements to human fingers [30], [31].
There were two objectives in Experiment 2. First, thetypes of changes in haptic sensations while exploring thetime-delayed stimuli were investigated. The participants
were asked to indicate the perceived changes in a virtual
texture using terms related to its mechanical parameters inorder for us to clarify the types of perceptual changes
induced by the time-delayed stimuli. The mechanicalparameters are described in Section 5.2. Second, the reason
for the conflicting descriptions of perceived changes
induced by delayed stimuli was investigated. In order toinvestigate this, two types of delayed stimuli were used.
These stimuli are described in Section 5.1.

5.1

Delay Control for Observing Changes in
Haptic Sensations
In order to investigate the reason for the conflicting changes
in haptic perception for delayed stimuli, two types ofdelayed stimuli were used. We focused on the accelerationof the hand motions while the participants explored thedelayed stimuli. During the positive acceleration phase of
the hand, tactile stimuli were produced based on a slower
speed. Furthermore, the stimuli were not generated for awhile at the onset of the hand motion. In contrast, during
the deceleration phase of the hand, the stimuli were
produced on the basis of a faster speed. Furthermore, thestimuli lasted for some duration even after the motion

79

stopped. Therefore, it was conceivable that the delays in theacceleration and deceleration phases differentially affected
the perceived quality of the textures.
Based on this hypothesis, two types of delayed stimuli
based on the signs of acceleration were used. One was astimulus that included a delay only in the acceleration
phase. The other was a stimulus that included a delay onlyin the deceleration phase.
Under the condition that there was a delay only in theacceleration phase, the simulated delay D was given asdjvðtÞj
ð4Þ
D ¼ Ta ðconstÞ; if dt  0;0;otherwise;
where vðtÞ was the hand velocity of the participant. When
there was a delay only in the deceleration phase, D wasgiven asdjvðtÞj
D ¼ Td ðconstÞ; if dt  0;ð5Þ0;otherwise:
The discrete-time equation of the voltage supplied to thevibrator was given by
YT ¼ A sin ðT Þ þ A;
T ¼ 2

VT D
t þ T 1 ;


ð6Þ
ð7Þ

where T and VT were the phase of stimulus and the handvelocity at time t, respectively, and t ¼ 0:2 ms was thecontrol period of the system. The vibratory frequencychanged according to the hand velocity while maintaining
the continuity of the phase at the transition of D as follows.
When the sign of the acceleration switched, it took 10 ms tochange D linearly between 0 and Ta or 0 and Td . We
avoided a step change in the value of D because theresultant change in the vibratory frequency was quite
noticeable to the participants. For example, under the delayin-acceleration-phase condition, D was determined by (4).
When the acceleration of the hand became negative, D was
gradually decreased to 0 in 10 ms. When the sign ofacceleration changed from negative to positive, D was
gradually increased to Ta in 10 ms.
Ta and Td were the amount of delay required for theparticipants to detect a change in the perceived textures ascompared to the nondelayed stimuli. They may be different
from the 40 ms measured in Experiment 1B because thetime-delayed stimuli used in Experiment 2 were different
from the ones in Experiment 1B. Therefore, we estimatedthe values of Ta and Tb in a preliminary experiment using
the method of limits for each individual participant. Ta andTd were defined as the thresholds for the delay-inacceleration-phase and the delay-in-deceleration-phase conditions, respectively. In the preliminary experiment, theascending and descending series were repeated two times
for each participant. A step size of 20 ms was used in boththe ascending and descending series. During the ascending
series, the participant explored time-delayed stimuli withincreasing delays until a difference in the tactile perceptionof the nondelayed and delayed stimuli was detected. The
threshold was estimated by subtracting 10 ms from the timedelay at the time of detection. During the descending series,

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Fig. 10. Response sheet for the participants to describe the subjective
changes in haptic sensations.

Fig. 11. Physical model for the participants to imagine and describe thesubjective changes in haptic sensations.

the participant explored time-delayed stimuli with decreasing delays until the difference between the nondelayed andthe delayed stimuli could no longer be perceived. The
threshold was estimated by adding 10 ms to this time. Ta
was the median of the four thresholds determined by two
ascending and two descending series for the delay-inacceleration-phase condition. Td was the median for thedelay-in-deceleration-phase condition. Among the 11 participants, Ta and Td ranged from 60 to 120 ms.

linear guide. The reference stimulus was a stimulus with nodelay. The test stimulus was a stimulus whose delay wascontrolled by the method described earlier in Section 5.1. At
the beginning of a trial, the slider was positioned at thecenter of the guide by the experimenter. The participants
felt the two virtual textures on the left and right sides within
15 s and then marked the answer sheet.
When the slider crossed over the center of the guide, thecontrol rules for the stimuli changed. The phases of the two
stimuli were not continuous before and after the change ofcontrol rules. The participants were instructed to explore
the two stimuli alternately so that this discontinuity did notaffect their judgments.
Each experiment comprised of 25 trials. For 10 of the
25 trials, the test stimulus included a delay in the acceleration phase of hand movements. For another 10 trials, the test
stimulus included a delay in the deceleration phase. For theremaining five trials, the test stimulus was the same as thereference stimulus. The order of the trials was randomized.
For the first 12 trials, the reference stimuli were on the left
side and the test stimuli were on the right side of the linear
guide. For the last 13 trials, this arrangement was reversed.
The participants were informed of the arrangement of thestimuli.
While the participants explored the textures, they wereasked to close their eyes. After the exploration, they lookedat the answer sheet and marked their responses. Similar toExperiment 1, the participants were required to maintaincontact between their finger and the vibrator during eachtrial. The participants heard pink noise through a headphone. Trials during which the finger force exceeded 2 N
were considered invalid and were presented again at theend of all trials. The experiment required approximately
1 h per participant, including instructions, practice, and thepreliminary experiments for determining Ta and Td .

5.2

Evaluation of Perceived Changes Using
Mechanical Parameters
Mechanical parameters, such as mass or kinetic friction
coefficients, were adopted as evaluation parameters. Fig. 10shows the answer sheet used in Experiment 2. The
participants rated changes in four mechanical parameters
and the perceived roughness. The mechanical parameters
were the spring stiffness, mass, viscosity, and kinetic
friction coefficient of the model shown in Fig. 11. In the
figure, the force applied to the finger is_F ¼ m€x þ cx_ þ kx þ k W signðxÞ;ð8Þ

where m; c; k, and k are the mass, viscosity, stiffness, andkinetic friction coefficient, respectively. The displacement of
the mass from the equilibrium position of the spring is x. W
is the reaction force in the normal direction exerted by thefloor. On the response sheet, each item was marked withfive levels ranging from “confidently decreased” to “confidently increased.”
Before the experiments, the participants experienced themechanical parameters shown in Fig. 11 simulated by the
PHANToM Premium force display. At the same time, they
could view a piece of paper as shown in Fig. 11 and (8). The
participants were students and staff from an engineeringschool, and it was confirmed that they understood thegeneral meanings of each parameter. Brief tests wereconducted to check whether the participants recognized
the effects of each parameter on their sensations. During thetest, one parameter was changed when the others were set tozero. The participants indicated the parameter beingchanged and its direction of change. The ranges of theparameters were 0-0.13 kg for m, 0-15 N/m for k, 0-3 Ns/mfor c, and 0-0.3 for k . All participants passed the test. The
participants were 11 males in their 20s and 30s who hadparticipated in Experiment 1. The test took approximately
10 min for each participant.

5.3 Procedures and Tasks
During the experiment, the reference and test stimuli werearranged on the left and right sides, respectively, of the

5.4 Results
The proportions of trials in which the participants marked“increased” or “decreased” were calculated for the nondelayed, delay-in-acceleration-phase, and delay-in-deceleration-phase conditions. Fig. 12 shows the average of the
proportions and the standard deviations among the participants. The “confident” and “guess” responses werecategorized as one group. The suffixes þ and  indicatedthat the parameter “increased” and “decreased,” respectively. For example, R þ corresponds to “the perceived
roughness increased.”
The differences of proportions between each condition
were tested for every signed parameter by Tukey’s test

OKAMOTO ET AL.: DETECTABILITY AND PERCEPTUAL CONSEQUENCES OF DELAYED FEEDBACK IN A VIBROTACTILE TEXTURE DISPLAY

81

Fig. 12. Proportion of trials in which “confident” or “guess” were reported for every parameter by each delay condition. The asterisks  and   indicate the significance levels of p < 0:05 and 0.001, respectively. The error bars indicate the standard deviations among the participants.

(number of degrees of freedom: 30, number of groups: 3).
For example, the proportions of R þ were compared with
the nondelayed, delay-in-acceleration-phase, and delay-indeceleration-phase conditions. Significant differences between the nondelayed and delayed conditions were observed for k ; R þ , and R  . The proportion of k 
responses for the delay-in-acceleration-phase condition was
significantly different from that of the nondelayed condition(qð3; 30Þ ¼ 5:72; p < 0:001; p ¼ 0:00094). The R þ responses
for the delay-in-deceleration-phase condition were significantly different from that for the nondelayed condition(qð3; 30Þ ¼ 6:64; p < 0:001; p ¼ 0:00016). The R  responses
for the delay-in-acceleration-phase condition were significantly different from that for the nondelayed condition(qð3; 30Þ ¼ 6:79; p < 0:001; p ¼ 0:00012).
When the delay existed only in the acceleration phase,the participants frequently selected “the perceived roughness decreased” and “the kinetic friction coefficientsdecreased” with regard to the textures they explored. Whenthe delay existed only in the deceleration phase, theparticipants frequently selected “the perceived roughness
increased” and “the viscosity increased” (although it was
marginally significant with p ¼ 0:0503). In general, they feltthat it was easier to move the slider during the delay-in-theacceleration-phase, and it was harder to move the slider
during the delay-in-the-deceleration-phase.
The results are consistent with the conflicting subjective
reports that we obtained at the end of Experiment 1. We
now understood that the perceived textures of the timedelayed stimuli depended on the phase (acceleration versus
deceleration) on which the participants based their judgments on during the exploration. In some trials, theparticipants felt that the perceived roughness decreased
and the friction decreased because they were presumably
paying more attention to the delay-in-the-acceleration
phase. In other trials, they felt that the perceived roughness
increased because they were presumably paying more
attention to the delay-in-the-deceleration phase.
In case that the “guess” and “not changed” responses
were categorized as one group, the significance test
exhibited significant differences between the exact samepairs as the case where the “confident” and “guess” werecategorized as one group. The way of categorizing the“confident,” “guess,” and “not changed” answers did notaffect the results of significance tests.

6
6.1

DISCUSSION

Design Guidelines for Tactile Displays in
Terms of System Latency
The experimental results of the present study provide adesign guideline for tactile displays in terms of system
latency. For display systems that aim to present the textures ofmaterials, the allowable latency should be smaller than 40 ms.
This is because if the delay of the system is above thisthreshold, the users of the systems are likely to experience theperceived textures differently from the ones the designers
intend to present. For display systems that aim to acknowledge the completion of an action, the allowable latency
should be smaller than 60 ms. For example, the presentationof stimuli to inform the user that a button has been pressed
need only be recognized as a causal effect of the pressing
action by the user.

82

6.2 Why does Time Delay Affect Haptic Perception?
Two possibilities are suggested for the reason why participants experienced the changes in the perceived textures
when the tactile stimuli included a time delay. First, theparticipants possibly perceived the variations in the spatial
wavelength or the frequency of vibrations caused by thetime delay. A time delay changes the effective wavelengthof the displayed virtual textures, which can result in a
sufficiently large change for the participants to perceive. The
wavelength discrimination limen of a grating scale or a scalewith raised dots with a spatial wavelength of 1 mm isapproximately 2 percent - 8 percent of 1 mm [32], [33]. If weapproximate hand velocity with a sinusoidal profile and use
the average values from Experiment 1 for its amplitude andfrequency, we obtain vðtÞ ¼ 196 sin ð21:0tÞ mm/s for handvelocity. The ratio of wavelength change is equal to the ratioof vðtÞ and vðt  DÞ, and is given by  vðtÞ   sin ð2tÞ 



:
¼
ð9Þ
cðt; DÞ ¼ 
vðt  DÞ sin ð2ðt  DÞÞ
When the delay is 40 ms (D ¼ 0:04 s), for 80 percent of the
exploration time period, cðt; DÞ is less than 92 percent or
greater than 108 percent, which is equivalent to wavelengthchanges of more than 8 percent. Therefore, for the majorityof the exploration, the wavelength changes caused by thedelay were large enough for the participants to notice.
The changes in haptic perceptions due to time delay can befurther explained on the basis of the discrimination limen of
vibrotactile frequency. The frequency discrimination limenof sinusoidal vibratory stimuli is approximately 20 percent ofreference frequencies over a wide frequency range [34], [35].
The frequency change due to time delay can also be
expressed by cðt; DÞ. When the delay is 40 ms, for 55 percentof the exploration time period, cðt; DÞ is less than 80 percent
or greater than 120 percent. Therefore, during roughly half ofthe exploration period, the frequency changes are sufficiently
large for the participants to notice.
It now follows that the changes in the perceived texturecaused by time delay can be explained by the discrimination limen of both the spatial wavelength and thevibrotactile frequency of roughness scales.
Second, changes in the relationships between the handmotions and the stimuli possibly led to subjective changes in
the perceived textures. When a delay is simulated, the stimuli
are based on vðt  DÞ, which is different from the current
instantaneous velocity of the hand vðtÞ. Thus, the delay
results in a variation in the relationship between handmotions and tactile stimuli. With regard to the perception of
textures by haptic exploration, the contributions of temporalcues of the vibratory stimuli to texture perception have been
well established. For example, perceived roughness ofgrating scales depends on the vibratory frequency which isdetermined by the spatial wavelength and stroking velocities[36]. Availability of temporal cues has improved theperformance of roughness discrimination tasks [37]. For
discriminating fine textures, the lack of lateral movements
between texture and skin can degrade the performance of
discrimination tasks [38]. A mechanism of roughness
perception that takes into account stroking velocities has
been proposed, where neural signals of hand speeds inhibit
the magnitude of perceived roughness. Consequently,

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NO. 2,APRIL-JUNE 2009

roughness perception was independent of stroking velocity[39]. These studies suggest that texture perception is
influenced by the relationship between hand velocities andthe corresponding vibrotactile stimuli. Temporal delaychanges this relationship which in turn changes theperceived textures. However, there have also been some
reports that dispute the contribution of hand velocities totexture perception. For example, only a slight effect of hand
speed was observed on roughness perception when exploring grating scales [29]. When scanning roughness scales,
temporal variation of mechanoreceptive afferents was notcorrelated with perceived roughness [40].

6.3

Do People Associate Subjective Changes Due
to Time Delay with Mechanical Properties?
In Experiment 2, the delays in both the acceleration anddeceleration phases affected the perceived roughness ofvirtual textures. This was expected because the temporaldelay affected the output displacements of the vibrotactile
stimulator. However, the proportion of trials during which
the participants perceived a decrease in the kinetic friction
coefficient in the delay-in-acceleration-phase condition was
significantly greater than that in the nondelayed and thedelay-in-deceleration-phase conditions. In the presentationof virtual textures using force displays, it has been reported
that the velocity-dependent resistance (viscosity) increases
the perceived roughness of virtual textures on which resistiveand nonresistive fields are alternately arranged [30], [31]. The
viscosity and the kinetic friction coefficient are velocitydependent resistances in the model shown in Fig. 11. In
Experiment 2, when there was a delay in the acceleration
phase, the perceived roughness and kinetic friction coefficient decreased. When there was a delay in the deceleration
phase, the perceived roughness, kinetic friction coefficient,and viscosity increased while the increase in friction andviscosity were not statistically significant. Thus, in Experiment 2, the velocity-dependent resistances increased anddecreased with the changes in the perceived roughness. The
participants presumably interpreted the changes in velocitydependent resistance as changes in perceived roughness.

7

CONCLUSION

The present study estimated two types of tactile detection
thresholds for time delay with a tactile display that
produced tactile stimuli in response to the hand motionsof users. One was a threshold for users to notice theexistence of a delay, which was found to be approximately
60 ms. The other was a threshold for users to notice thechange in the perceived textures of the displayed objects,
which was found to be approximately 40 ms. The subjective
changes due to time delay were described using mechanicalparameters such as the kinetic friction coefficient as well as
the perceived roughness of virtual textures.

ACKNOWLEDGMENTS
This work was partially supported by a grant from the
Ministry of Internal Affairs and Communications SCOPE(082102006), Grant-in-Aid for Scientific Research B(18760179), and Grant-in-Aid for JSPS Fellows (191804).

OKAMOTO ET AL.: DETECTABILITY AND PERCEPTUAL CONSEQUENCES OF DELAYED FEEDBACK IN A VIBROTACTILE TEXTURE DISPLAY

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Shogo Okamoto received the BS degree inengineering from Kobe University in 2005, andthe MS degree in information sciences in 2007.
He has been a PhD candidate at the Graduate
School of Information Sciences, Tohoku University, and a JSPS research fellow for young
scientists since 2007. His research interests
include haptic interfaces and man-machineinterfaces. He received the Dean’s Award,
GSIS, Tohoku University, in 2007, the Best
Poster Award at WorldHaptics ’07, the IEEE RAS Japan Chapter YoungAward (ICRA ’08) in 2008, and the Award for the Best Hands on Demoat EuroHaptics ’08. He is a cochair of the IEEE RAS SAB in 2009. He isa student member of the IEEE.

84

Masashi Konyo received the BS, MS, and PhD
degrees in engineering from Kobe University in
1999, 2001, and 2004, respectively. In 2004, he
was a postdoctoral fellow at the BioRobtics
Laboratory, Keio University. Since 2005, he has
been an assistant professor at the Graduate
School of Information Sciences, Tohoku University. His current research interests include
haptic perception, tactile interfaces, and humancooperative robotics. He received the Best
Paper Award from the Virtual Reality Society of Japan in 2002 and2007, the Best Poster Award at the WorldHaptics ’07, and the Best
Hands on Demo Award at the EuroHaptics ’08. He is a member of theIEEE.
Satoshi Saga received the BS and MS degrees
in engineering from the University of Tokyo in
1998 and 2000, respectively. Since 2000 to
2004, he has been a researcher at SECOM IS
Laboratory. He received the PhD degree in
information physics and computing from the
University of Tokyo in 2007. Since 2007, hehas been an assistant professor at Tohoku
University. His research interests include human-based tactile interfaces, such as haptic
teaching, tactile sensors, and tactile displays. He received the Young
Researchers Award from the Virtual Reality Society of Japan (VRSJ) in
2005 and the Outstanding Paper Award from the Emerald Literati
Network in 2008. He is a member of the IEEE.

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Satoshi Tadokoro (M’89-SM’06-F’09) received
the BE degree in precision machinery engineering in 1982, the ME degree from the University ofTokyo in 1984, and the DE degree from Kobe
University in 1991. He has been a professor in
Graduate School of Information Sciences, Tohoku University, since 2005. He was a project
leader of the MEXT DDT Project on rescue
robotics in 2002-2007 having contribution of
more than 100 professors nationwide. He established RoboCupRescue in 1999, TC on Rescue Engineering of SICE in
2000 (the first chair), IEEE Robotics and Automation Society TC on
Safety, Security and Rescue Robotics in 2001 (the first chair), andInternational Rescue System Institute (IRS) in 2002. He was a chair of
the IEEE RAS Japan Chapter in 2003-2005. He is currently president of
IRS, trustee of The RoboCup Federation, chair of JSME Robotics
Mechatronics Division, and a member of IEEE Robotics and Automation
Society AdCom. He received the Robot Award 2008, the FDMA
Commissioner Highest Award in 2008, the JSME Funai Award in 2007,
the Best Book Author Award from AEM Society in 2006, the RMD
Academic Achievement Award in 2005, etc. His research interest is in
rescue robotics, virtual reality, and new actuators. He is a fellow of theIEEE.

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