SUGISAKAMASANORI

Journal of Robotics, Networking and Artificial Life
Vol. 9(1); June (2022), pp. 31–36
ISSN (Online): 2352-6386; ISSN (Print): 2405-9021
https://alife-robotics.org/jrnal.html

Research Article

Effects of Tactile Stimulation Near the Auricle on Body Sway
Masaya Tadokoro1, Taro Shibanoki2

Graduate School of Science and Engineering, Ibaraki University, 1-4-1 Nakanarusawa-cho, Hitachi, Ibaraki 316-8527, Japan
Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama, 700-8530, Japan

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ARTIC LE INFO
Article History
Received 25 November 2020
Accepted 01 June 2022

Keywordsbalance function
body sway
tactile stimulation
center of pressure

ABS TRACTThis paper describes the relationship between tactile stimulation and human body sway. We
previously proposed a tactile stimulation-based body sway stabilization system and revealed thatsimultaneous stimulation behind both auricles significantly improved human balance function,and unilateral stimulation may induce the deviation of the center of pressure (COP). In this paper,
some stimulation patterns were applied to participants and COP distribution before/after
stimulation was extracted. The results showed that COP values after stimulation could be inclined
to the same of the stimulation site. It indicates that tactile stimuli can control human balance
function.

© 2022 The Author. Published by Sugisaka Masanori at ALife Robotics Corporation Ltd
This is an open access article distributed under the CC BY-NC 4.0 license(http://creativecommons.org/licenses/by-nc/4.0/).

shown that vibration to the head is effective in improving
balance function in Parkinson’s disease (PD) patients9.
However, because these approaches may exert a physicalburden on the use, it is difficult to use in daily life.

1. Introduction
Falls account for about 80 percent of accidents inJapan1, and prevention of falls is an urgent issue. It is said
that the decline of motor function with aging increases
the risk of falls by about five times2, making fall
prevention an essential issue in Japan where thepopulation is rapidly aging3.

To overcome these problems, our research group has
been proposed the body sway mitigation system based ontactile stimulation and showed that body sway can be
reduced via the application of vibratory stimuli aroundthe pinna10. Here, acoustic stimulation can induce bodysway to the opposite direction of the stimulus6, however,previous studies have not fully clarified the relationshipbetween tactile stimulation and body sway. In order toclarify these relationships, we have previously tried toquantify the change in COP by applying unilateral
stimulation11. As a result, it can be concluded that there
is a tendency for COP distribution to be biased in the
direction contrary to the stimulated side. The results
showed that the COP distribution tended to be biased in

Various fall prevention and balance function
improvement systems have been developed4-10. Studies
focusing on improving balance function can be divided
into two categories: those that use light touch contact(LTC) 4 and those that use stimulation to the human bodyto improve vestibular and somatosensory fuctions5-9. In
LTC, it has been shown that the balance function can be
improved by touching a fixed point. It has also beenshown that it is possible to decrease and control bodysway by applying acoustic5,6, electrical7,8 and vibratory9
stimuli. For example, it has been shown that white noise
acoustic stimulation5 and galvanic vestibular stimulation
stabilizes body sway7,8. In addition, it has been also

Corresponding author’s E-mail: 21nm738y@vc.ibaraki.ac.jp, shibanoki@okayama-u.ac.jp

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Fig. 1. Overview of the proposed balance function
analysis system.
Fig. 3. Vibration stimulation pattern used in theexperiments.
subjects were asked to maintain an eye-closed tandem
limb stance for T [s]. Tactile stimulators were applied
near the left and right auricles to provide vibration
stimulation to the subjects.
3. Posture stabilization experiment by vibration
stimulation
Our research group has reported that steadystimulation near the auricles stabilizes posture10. In thispaper, we first clarify that body sway can be reduced byapplying constant vibration stimulation to the left andright sides near the auricle to reveal the mechanism ofbody sway reduction by vibration stimulation.

Fig. 2. Overview of the tactile stimulation system.
the opposite direction to the stimulus side, however, there
was only one participant, and the bias of COP was not
sufficiently examined.

3.1. Experimental conditions

In this paper, we attempt to quantify the influence ofvibration stimulation on body sway by clarifying therelationship between vibratory stimulation near theauricle and body sway deflection.

In the experiment, COP sway was measured in threeparticipants (Sub. A--C, 22.3 ± 0.471 [years]) whenconstant vibration stimuli were applied alternately to theleft and right mastoid processes. Two motor oscillators(KD18B1) were connected to a Raspberry Pi 3B, andvoltage was applied using PWM with a duty cycle of 0.01[s]. The stimulus was applied to the right side during 𝑇𝑇on[s], then switched to the left side, and switched to the left
side again after 𝑇𝑇on [s]. Therefore, the stimulus was notapplied to the mastoid region on the side (D = 0.0)
different from the stimulus side at the same time.

2. Method
Figure 1 shows the proposed vibratory stimulationbalance function analysis method based on the body sway
mitigation system using tactile stimulation8. The system
consists of a stabilometer, tactile stimulators, amicrocontroller for controlling vibratory stimulation
pattern (See Fig. 2), and a PC. Stimulation patterns canbe changed from the PC before experiments via TCP/IP
communication. Figure 3 shows an example ofstimulation pattern. The system can change the on-off
interval 𝑇𝑇𝑠𝑠 [s], stimulation interval 𝑇𝑇on [s], and nonstimulation interval 𝑇𝑇off [s] of each tactile stimulator. In
addition, the magnitude of the stimulus can be changedby adjusting the duty ratio D using PWM control. The
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The subjects were asked to maintain the tandem limb
position with the left foot back on the force plate witheyes closed for 𝑇𝑇 = 60 [s], including 5 [s] before andafter the measurement (See Fig. 4). The COP wasmeasured using the Wii Balance Board (Nintendo Co.,
Ltd.) at a sampling frequency of 100 [Hz]. The stimulus
pattern given to the subject is 𝑇𝑇s = 10 [s], the left side
left
left
stimulus duration is 𝑇𝑇on
= 5 [s], 𝑇𝑇off= 5 [s] and the
right
right
right side stimulus duration is 𝑇𝑇on = 5 [s] , 𝑇𝑇off =Fig. 4 Experimental scene.
right

right

left
left
5 [s] ( 𝑇𝑇on
+ 𝑇𝑇off= 𝑇𝑇on + 𝑇𝑇off = 𝑇𝑇s ). The voltage
was applied to the motor at a duty ratio of D = 0.8, and asteady-state stimulus was applied to the subject. To
compare and evaluate the effect of this stimulation
pattern on the postural sway, the comparison experiment
was also performed under the condition without
stimulation. The number of trials was 10 with andwithout stimulation.

Fig. 5 Examples of measured COPs.

3.2. Results and discussion
Figure 5 shows measured COPs (a) with and (b) without
stimulation. The time-series waveform COP{𝑥𝑥,𝑦𝑦} (𝑡𝑡) of
each axis of the measured center of pressure (COP) wassmoothed by a second-order Butterworth digital low-passfilter (cut-off frequency: 𝑓𝑓c = 10 [Hz]). It could befound that the COP sway decreased with stimulationcompared to that without stimulation.

Fig. 6 Evaluation indices of human body sway.
shown that a fixed-sound source provides a reference forbody orientation12 (same as the light touch contact4) andincreasing attention to acoustic stimuli effect on bodysway5. It is possible that the same phenomenon occurredin this experiment.

Here, we calculated several evaluation indices to assess
the stability of COP sway between stimulated andunstimulated conditions based on the method of
Watanabe et al.10. The stability of COP oscillation isquantitatively evaluated by calculating the postureholding time, variation of COP, and index of movementfrom the smoothed COP signal.

4. Body sway evaluation experiment by unilateral
vibration stimulation

Figure 6 shows the experimental results. Note that Fig.
6 shows the mean values of all subjects normalized sothat the mean value of each index value in the nostimulus condition is 1. The results of an unpaired t-test
are also shown simultaneously in the figure. As shown in
Fig. 6, each index tended to decrease significantly(p<0.05, posture maintaining time did not change
because no fall occurred in all trials). This indicates that
COP sway can be stabilized by alternating stimulation of
the left and right sides. Here, previous studies have

The experimental results showed that the COP swaystabilized even when the stimuli were not appliedbilaterally. To clarify how the stimulation near the auricle
affects the body sway, we evaluated the body sway
during unilateral stimulation.

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4.1. Experimental conditions
In the experiments performed, COP sway during theapplication of vibratory stimulation was measured withthree healthy males (22.7±0.471 [years]). Two motor
oscillators (KD18B1) were connected to a Raspberry Pi
3B and a voltage was applied by pulse width control witha duty cycle of 0.01 [s]. The stimulus was applied to only
one of the left and right sides.
The subject was asked to maintain tandem limb stancewith eye-closed and the left leg back for 𝑇𝑇 = 90[s] after
a preparation time for 10 [s]. The COP was measured ata sampling frequency of 100 [Hz] using a Wii Balance
Board (Nintendo Co., Ltd.). The stimulus pattern applied
to the subject was steady-state stimulus with 𝑇𝑇𝑠𝑠 = 10 [s],duty ratio 𝐷𝐷max = 1.0 during 𝑇𝑇on = 5 [s], and 𝐷𝐷min =0.32 determined in advance, which was less than orequal to the minimum stimulus amplitude perceived by
the subject, during 𝑇𝑇off = 5 [s]. This stimulus pattern
was applied to the left or right side.

(i) Left stimulus(ii) Right stimulus(a) COP in each direction

4.2. Results and discussion
The filtered COP value COP{𝑥𝑥,𝑦𝑦} (𝑡𝑡) was divided into
M every 𝑇𝑇𝑠𝑠 [s], and the error between meanvalues ave𝑚𝑚 in each interval (𝑚𝑚 = 1, 2, … , 𝑀𝑀) andmedian values medon
𝑚𝑚 for stimulation interval andmedoff𝑚𝑚 for non-stimulation interval were evaluated.
This clarifies the bias of body sway with and without
vibratory stimulation.

(i) Left stimulus(ii) Right stimulus(b) An enlargement of COPs
Fig. 7 Examples of experimental results.

Figure 7 (a) shows examples of signals measured
during the experiments: (i) for the left-side stimulus and(ii) for the right-side stimulus, respectively. Figure 7 (a)shows the results from the first trial, indicating
stimulation patterns and COPs. The shaded areas
represent times during non-stimulations. Figure 7 (b) isan enlargement of COPs from Fig. 7 (a), and it cannot be
confirmed that the stimulus affected to the balance
function.
Figure 8 (a) and (b) show histograms of COP of y axis
(frontal plane) with median and mean values for the leftside and right-side stimulus conditions, respectively.
Figure 8 (a) of the results for the left-side stimulus

Fig. 8 Examples of experimental results.
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Fig. 10 Examples of experimental results.
The results of the heteroskedasticity t-test confirmed asignificant difference at the level of 5 [%] (see Fig. 10(a)). On the other hand, when right stimulation wasapplied, there was a bias to the right side during
stimulation (p < 0.05), and a bias to the left side during
non-stimulation (p < 0.05) (see Fig. 10 (b)). These results
suggest that COP bias in the left-right direction may be
induced to the same side of the stimulated side. It is
possible that COP bias is induced to the same side of thestimulus, resulting in a larger bias toward the stimulus tomaintain body balance equilibrium.

Fig. 9 Examples of experimental results.
condition shows that the mean is smaller than the medianvalue in the stimulus interval, and conversely, the meanis larger than the mean value in the non-stimulus interval.
In Fig. 8, the positive value shows deviation to the left
side, therefore, the subject may tilt the same side of thestimulation.

These results indicate that regular unilateral steadystimulation to the auricle induces COP sway on the sameside of the stimulus, and that the sway may be deflectedto the stimulus side to resist it. In the future, we will
increase the number of subjects and investigate whether
this tendency appears or not, and also whether it ispossible to induce body sway arbitrarily by changing thevibration pattern or not.

Figure 9 shows the difference between mean values
during stimulation and non-stimulation and the median in
the interval of 10 [s] for left-side stimulation condition
and right-side stimulation condition, respectively. The
positive component represents the bias to the left side of
COP distribution, and the negative component represents
the bias to the right side of COP distribution. Note that inFig. 9, the phase of the unstimulated values is shifted by
5 [s]. In Fig. 9, we can see that the COP of the left
stimulus was positive (biased toward the left side) in thestimulus presence interval and negative (biased toward
the right side) in the stimulus off interval. In the rightstimulus, these relationships were reversed compared to
the left stimulus, and there was a tendency for many
intervals to be negative when the stimulus was On andnegative when the stimulus was Off.

5. Conclusion
In this paper, we examined the effects of regularunilateral steady-state stimulation patterns on COP bias
when they were applied to the near pinna. In the firstexperiment, the participant was asked to maintain tandem
limb positions with closed eyes, and quantitatively
evaluated whether alternating left and right stimulationinduces stability of COP sway. The results of theexperiment showed that the values of all indices related
to COP sway decreased significantly before and after the
stimulation, indicating that the COP sway tended to be35

stabilized even when bilateral short-term oscillatory
stimuli were applied alternately. In another experiment,
the subject was asked to maintain tandem limb positions
with closed eyes, and the change of COP deviation withand without stimulation in each stimulation pattern wascompared. It is confirmed that the stimulation may
induce the deviation of COP oscillation in the frontal
plane to the same side of the stimulated side. Thissuggests that the amount of deflection to the stimulus
opposite side may increase to maintain the equilibrium
against the induced COP oscillation.

9.

10.

11.
12.

In future research, we will increase the number ofsubjects and deepen the verification of the experimentconducted in this study. In addition, we plan to change
the pattern of the vibratory stimulation and investigate
the relationship with the body sway.

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Authors Introduction
Mr. Masaya Tadokoro

Acknowledgments
This work was supported by JSPS KAKENHI Grant
Number 20K20212.
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Dr. Taro Shibanoki

He received his bachelor's
degree in Engineering in 2021
fromtheCollegeofEngineering,
Ibaraki
University, Ibaraki, Japan. He iscurrently a master course
student at Ibaraki University.
His research interests focus onbalance function analysis.

He received the B.E. degree
fromtheUniversityofTokushima in 2008, and the
M.E. and D.Eng. degrees from
Hiroshima University in 2010
and 2012, respectively. He wasa Research Fellow of the Japan
Society for the Promotion of
Science since 2011, an AssistantProfessor(Special
Appointment) at Hiroshima
University since 2013, and an
Assistant Professor, Lecturer, and an Associate Professor
at Ibaraki University since 2014, respectively. He iscurrently an Associate Professor at the Graduate School of
Natural Science and Technology, Okayama University.
His current research interests focus on medical and welfare
robotics, biological signal processing, and human-machineinterfaces.

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