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Design of pleasant vibrotactile stimulation
through a seat
Shogo Okamoto∗ , Atsuko Tamada∗ , Tadayoshi Sumiya† , and Seiji Yokoi†∗ Nagoya University, Nagoya, Japan† Toyota Motor Corp., Toyota, Japan

Abstract—Vibrotactile stimulation to the gluteal or back regions through seats is a promising approach to deliver supplemental information to humans. Designing a method to achieve
pleasant vibrotactile stimuli was addressed in this study, as suchstimuli tend to result in unpleasant experiences for users. In
particular, we investigated the properties of continual vibrotactile
stimulation to the gluteal region through experiments and foundthat decreases in the duration of individual stimuli and vibratory
frequency significantly increased the experienced pleasantness,whereas variations in the interval between consecutive stimuli
exhibited no effect. Such principles are expected to generally
hold in designing vibrotactile signals.

b)

Front
L1
L2
L3
L4I. I NTRODUCTION
Tactile stimuli are an effective communication channel asan alternative to audiovisual stimuli. Vibrotactile stimuli havebeen used in a number of products and services because of
their ease of implementation. In this paper, we investigate
vibrotactile stimulation through seats. Some commercial products have employed such approaches, and further possibilities
are being studied for integration with movie content [1] andautomobiles [2], [3]. Most existing products and earlier studies
have focused on how to effectively transfer information tousers through vibrotactile stimuli.
There exists a common problem for vibrotactile stimulation.
Vibrotactile stimuli easily result in unpleasant experiences,which are an unacceptable result for many products. Hence,
pleasantness of the stimuli is as important as the quality of information delivered through them. In general, stronger stimuli
lead to less pleasant experiences. When using seats, vibrotactile stimuli are presented to the gluteal or back regions, leading
to a larger target area as compared to the hands or fingers,which are typically targeted by mobile information terminals.
However, no general vibrotactile stimulation methods havebeen reported for restricting subjective unpleasantness while
reliably transferring information. In order to address this issue,
we experimentally investigated the subjective pleasantness ofvibrotactile stimuli provided to the gluteal region through aseat while using stimuli that were strong enough to be nearly
perfectly judged by the experimental participants.

20 cm

R1
R2
R3
R4RearVibrator

5 cm

Cushion withvibratory actuators

Fig. 1. Experimental setup. Cushion with eight recoil-type voice-coil actuators
on a seat. Four actuators on the each of the right and left sides. a) Photo of
the cushion and voice-coil actuators beneath a cloth sheet. b) Displacementof actuators on the cushion. c) Subject sitting on the cushion for experiment.

20 cm, respectively. We used several vibrators displaced ona seat because typical applications make use of spatiallydistributed stimuli [1], [2]. Above approximately 50 Hz, thevibrator yielded stimuli strong enough for participants toperceive in our experimental setup.
III. E XPERIMENT
A. Tasks
Individual participants sat on a seat with a cushion andvibrators. During the experiments, in order to mask visualand auditory cues, the participants closed their eyes and wore
headphones playing a pink noise. The participants wore normal
winter clothes. They responded on each trial on a 7-point
pleasantness scale. The scale ranged from -3 (unpleasant) to3 (pleasant), with 0 being neutral. Furthermore, in order tocheck how correctly the information was transferred to theparticipant, the participant answered whether the stimulation
was on the left or right side.

II. E XPERIMENTAL APPARATUS

B. Stimuli

As shown in Fig. 1, eight recoil-type voice coil actuators(Haptuator Mark II, Tactile Labs., Canada, 9 × 9 × 32 mm)
were installed on a seat cushion. The cushion and vibrators
were covered by a cloth sheet. The intervals between theneighboring actuators in a column and row were 5 cm and978-1-5090-4045-2/17/$31.00 ©2017 IEEEc)a)

In a single trial, a continual vibrotactile stimulus was presented to either the right or left side of the gluteal region. Fourvibrators in the same column were simultaneously actuated.
As shown in Fig. 2, the vibratory stimulus consisted of eight
repetitions of short-unit vibration with frequency and period542017 IEEE 6th Global Conference on Consumer Electronics (GCCE 2017)

IV. R ESULTS
From the analysis, the following model was acquired:L1
Leftstimulator

L2
L3

Pleasant = −0.012ta − 0.020f.

L4tatsThis model indicates that a 1-ms increase of unit vibration
period ta decreases the pleasantness by 0.012 points (p <0.01). Similarly, a 1-Hz increase of frequency f decreases
pleasantness by 0.02 points (p < 0.001). The interstimulus
period ts did not have a significant effect. The R2 value ofthe model was 0.51.

Eight short time sinusoidal
vibration at frequency f

R1
Rightstimulator

R2
R3V. D ISCUSSION
The results and analyses indicated that shorter stimulus
period (ta ) and lower vibratory frequency (f ) lead to a more
pleasant experience for the user. Because of the temporalsummation effect of vibratory stimuli, longer vibrations arefelt as stronger [4]. Furthermore, in the frequency range usedin the experiments (50–300 Hz), higher frequency leads tostronger perception, with the peak frequency at approximately
250 Hz [5]. Therefore, decreasing the vibration period andfrequency lessens the subjective strength of the stimuli. The
decrease of subjective strength then leads to heightened pleasantness. These results are consistent with a report that strongervibrotactile stimuli to the hands are likely to be disfavored [6].
Nonetheless, it is highly interesting that the properties ofpleasant stimuli were experimentally demonstrated for largearea body parts like the gluteal region. From a methodologicaldesign perspective, it is recommended that the frequency ofvibration remains small when the unit vibration is longer, andvice versa.

R4Time [s]Fig. 2. Example of stimulation pattern where four right-side actuators areactivated, and left-side actuators were still.

set to f Hz and ta s, respectively. The interstimulus period
was ts s. These parameters took any of the following values:
f = {50, 100, 150, 200, 250, 300} Hz, ta = {30, 45, 60} ms,and ts = {50, 100, 150, 200, 250} ms. Although these values
were arbitrarily determined by the experimenters, continualstimulation has been commonly used in other studies. Becauseof the mechanical properties of the vibrator, the accelerationof vibratory stimuli depended on the f value; however, forany f values tested in the present study, the stimuli wereeasily perceptible. The combination of the three types ofparameters constituted 90 types of stimuli. Each of thesestimuli was tested on the participant in randomized order. Aftera short break, a second session was conducted with the same
participant. Hence, in total, 180 trials were performed for eachparticipant.

VI. C ONCLUSION
In experiments presenting vibrotactile stimuli to the gluteal
region of human participants on a seat, we investigated thefactors of stimuli to limit the discomfort experienced by users.
It was found that decreasing the period and frequency ofvibratory stimuli is effective for achieving pleasant stimuli.
Contemplating the generality of the stimuli used in the present
study, this can be a guideline applicable to various applications.

C. Participants
Three university students who were unaware of the objectives of the study participated in the experiment after giving
informed consent.
D. Analysis

R EFERENCES

We used all the trials for statistical analyses because the participants correctly answered the side of all presented stimuli.
Given that there were minimal individual differences among
participants, we pooled the results of all the participants afteradjusting the mean response value of each participant to bezero.
In order to verify the linear effects of each parameter on thereported pleasantness values, we analyzed the results by using
multiple regression analysis. The tested regression model wasPleasant = a1 ta + a2 ts + a3 f + a0

[1] M. Kim, S. Lee, and S. Choi, “Saliency-driven real-time video-to-tactile
translation,” IEEE Transactions on Haptics, vol. 7, no. 3, pp. 394–404,2013.
[2] T. Nukarinen, J. Rantala, A. Farooq, and R. Raisamo, “Deliveringdirectional haptic cues through eyeglasses and a seat,” Proceedings ofIEEE World Haptics Conference, pp. 345–350, 2015.
[3] C. Ho, H. Z. Tan, and C. Spence, “Using spatial vibrotactile cues to direct
visual attention in driving scenes,” Transportation Research, vol. 8, pp.
397–412, 2005.
[4] G. A. Gescheider and J. M. Joelson, “Vibrotactile temporal summation
for threshold and suprathreshold levels of stimulation,” Perception &Psychophysics, vol. 33, no. 2, pp. 156–162, 1983.
[5] S. J. Bolanowski, G. A. Gescheider, R. T. Verrillo, and C. M. Checkosky,“Four channels mediate the mechanical aspects of touch,” Journal of
Acoustic Society of America, vol. 84, no. 5, pp. 1680–1694, 1988.
[6] K. Salminen, J. Rantala, P. Laitinen, V. Surakka, J. Lylykangas, andR. Raisamo, “Emotional responses to haptic stimuli in laboratory versus
travelling by bus contexts,” Proceedings of International Conference on
Affective Computing and Intelligent Interaction, pp. 1–7, 2009.

(1)

where Pleasant and a{0,1,2,3} are 7-point pleasantness valueand regression coefficients, respectively. For the analysis, weused the stepwisefit function provided by Matlab (Mathworks,MA).

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