International Journal of Affective Engineering
J-STAGE Advance Published Date: 2024.04.17
doi: 10.5057/ijae.IJAE-D-23-00019

Original Article

Stroking Stimuli to the Ear to Enhance Pleasant andNon-arousing Feelings while Listening to Sounds
Yuta GOTO 1 and Shogo OKAMOTO 1* †1 Tokyo Metropolitan University, 6-6 Asahigaoka, Hino-shi, Tokyo 191-0065, Japan* Corresponding author, E-mail: okamotos@tmu.ac.jp† JSKE Member

Received: 2023.07.03
Accepted: 2024.02.14
Copyright: © 2024 by the authors. Submitted forpossible open access publication under the terms
and conditions of the Creative Commons Attribution(CC BY) license (https://creativecommons.org/licenses/
by/4.0/).

Abstract: We investigated the effects of stroking stimuli near the external
auditory meatus on the emotions aroused by the experience of listening tosounds. Previous studies demonstrated that vibratory stimuli to the upper bodymanipulate emotions while watching videos or listening to sounds; however,none examined the effects of stroking stimuli near the external auditory meatusthrough which the vagus nerve extends. This study investigated the effects ofstroking stimuli to the outer ear on emotions for each of six emotionallyevocative sounds that arouse pleasant or unpleasant feelings. They comprised
the sounds of rain, shampoo, insects’ buzz, and horrifying winds. When thesound stimuli were presented with stroking stimuli at the ear, participants’emotions mostly changed to be more pleasant and sleepy, that is less arousing.
These findings are expected to help in the development of haptic interfaces formore emotionally appealing content.
Keywords: Emotion, Natural sounds, Valence, Arousal

1. Introduction
Haptic stimuli to the human body affect emotions when experiencing audiovisual content (Branje et al., 2014;
Fukushima et al., 2014; Giroux et al., 2019; Hasegawa et al., 2019; Karafotias et al., 2017; Lemmens et al., 2009;
Makioka et al., 2022; Merchel & Altinsoy, 2014; Okazaki et al., 2015; Sakurai et al., 2014; Tara et al., 2023). For example,
vibrotactile stimuli delivered through chairs to the torsos are synchronized with movie soundtracks to improve
emotional impressions (Giroux et al., 2019; Merchel & Altinsoy, 2014). Furthermore, vibratory stimuli to the epigastricfossa region increase excitement and fear experienced while watching sports and horror videos, respectively (Makioka
et al., 2022; Tara et al., 2023). These techniques make audiovisual content more emotionally evocative. Most extant
studies have adopted vibratory stimuli to increase or decrease pleasant feelings. However, vibratory stimuli evoke theawake state and may not effectively induce non-arousal (i.e., sleepiness).
This study investigated the emotional effects of stroking stimuli on the external ear while listening to sounds. Most
studies selected torsos or hands for stimulation, and few have investigated the external ears. One exception was thestudy by Fukushima et al. (2014) where they presented vibratory stimuli to earlobes and demonstrated their effects toincrease arousing emotions. The vagus nerve monitors and controls visceral activity, and is located near the auditory
meatus (Murray et al., 2016; Peuker & Filler, 2002). Mechanical stimulation of the outer ear is linked to various
physiological responses (Addorisio et al., 2019; Boehmer et al., 2020; Lee et al., 2023). For example, pressure stimuli
to the cymba concha were found to decrease heartbeat ratios (Lee et al., 2023). Such physiological changes arelinked with emotions (Kreibig, 2010). Hence, ear stimulation may be an alternative to torso stimulation to induce theemotional effects. Furthermore, unlike earlier studies, we adopted stroking stimuli. Stroking stimuli cause pleasantfeelings when presented to the arms (Essick et al., 2010; Lernial et al., 2018; Triscoli et al., 2017). For example,
Triscoli et al. (2017) showed that stroking stimuli were perceived as more pleasant and relaxing and increased thevariation in heartbeat ratios compared to vibratory stimuli. Hence, we expect that stroking sensations applied to theouter ear will effectively cause pleasantness and sleepiness when listening to emotionally evocative sounds.

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In the present experiment, a plastic tactile contactor placed in the participants’ ear was rotated by a DC motor to provide
the stroking stimuli while participants listened to emotion-inducing sounds. This study is based on the authors’ earlier
study (Goto et al., 2023), in which only three types of sounds that were largely pleasant and relaxing were investigated.
This study advances the previous study by employing six types of sounds evoking a variety of emotions. Hence, this study
investigated the emotional effects of auditory stroke stimuli on various sound stimuli. The findings provide a basis fordeveloping new interfaces for emotional haptics.

2. Methods
2.1 Apparatus
The experimental setup is shown in Figure 1. A resin rod fabricated by additive manufacturing (Form 3, Formlabs, Inc.,
USA; standard white resin) was used as a tactile stimulator for stroking the external acoustic meatus. The rod tip was
spherical (12 mm in diameter) with an attached hemisphere (8 mm in diameter). The shape and size were determined suchthat the external acoustic meatus would be stimulated without damage. Additionally, the contactor tip was smoothed using
fine paper. The resin rod was connected to a DC-geared motor (TG-47G-SG, Tsukasa Denko, Japan; reduction ratio: 50)
via coupling (Capricorn MRG-20-6-8, Nabeya Bi-tech, Japan). The motor was given speed commands via a motor
controller (SyRen 10; Dimension Engineering LLC, USA).

Figure 1: Experimental apparatus: (a) Experimental scene, (b) tactile stimulator, (c) tip of the stimulator, (d) configuration of apparatus.

Figure 2 shows the configuration of the apparatus. The command value of the DC motor was provided through theearphone jack of a personal computer. It was then passed through a non-inverting amplifier circuit using an operational
amplifier and input. The motor controller applied a pulse-width modulation signal of 12 V tailored to a sound waveform,
as described in Section 2.3.

Figure 2: Voltage-speed instruction to the DC motor generated from sound waves.

In addition to tactile stimuli, participants were presented with sound stimuli using bone-conducting earphones (Aeropex,
Shokz, USA). We did not use inner ear or canal-type earphones as they would block the external acoustic meatus where
the stroking stimulus was applied.

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Stroking Stimuli to the Ear to Enhance Pleasantness

2.2 Sound stimuli
Table 1 lists the six sounds used in the study. Two were from the IADS-E database of emotional sounds (Yang et al.,2018), and four were from YouTube (Google, LLC., USA). Each sound was clipped at a sampling frequency of 44,100 Hz
for 30 s. We selected sounds that evoke specific emotions, either pleasant or unpleasant.
Table 1: Audio clips used in the study
Sound name

Description

Reference

Fear 1

Sounds recorded in a windy cave

IADS-E 0209

Fear 2

Monsters’ crying sounds

IADS-E 0777

Rain

Sounds of moderate rain

https://www.youtube.com/watch?v=NNLIC0bMdPE

Shampoo

Shampoo sounds recorded at ear positions

https://www.youtube.com/watch?v=RCT8hGGnTjw

Marker pen

Scratch sounds by a felt pen

https://www.youtube.com/watch?v=d_pWifU5hes

Mosquito

Buzzing of the mosquito

https://www.youtube.com/watch?v=mMYGkwiZkXI

2.3 Stroking stimuli to the ear
The plastic rod attached to the DC motor rotated in response to the sound volume. Voltage commands to the motor
were generated from the waveform of the audio stimuli using the process shown in Figure 2. First, the audio channel
corresponding to the ear to be stroked was extracted from stereo audio channels. Subsequently, the absolute values ofthe sound output voltage were calculated. Hence, the DC motor continually rotated in a single direction. Finally, thewaveform envelope was computed using the envelope function in MATLAB (MathWorks, Inc., USA), following the
local peaks of the sound signal at 22.7 ms intervals. This value determined envelope smoothness. That is, the DC motor
trembled only slightly when the value was small, whereas the motor rotated the rod smoothly when the interval waslarge. This study did not aim to optimize the stroking pattern; hence, the interval value was adjusted for comfort. The
same interval was used for all sounds and participants in the experiment.
2.4 Participants
Twelve healthy participants (nine males and three females) in their early 20s participated in the experiment after
providing written informed consent. None of the participants had any experience with the apparatus or knew the
purpose of the experiment.
2.5 Procedures and tasks
The experiment comprised two sessions. In the first, the emotional characteristics of each sound were investigated.
In the second, we investigated the effects of the stroking stimuli while listening to each sound.
During the first session, individual participants listened to each sound for 30 s and rated eight adjective items
representing different emotional states: joyful (yorokobi in Japanese), pleasant (kokochiyoi), relaxed (kutsurogeru),
sleepy (nemukunaru), depressed (ochikomu), unpleasant (fukaina), afraid (kowai), and awake (megasameru), referring
to Russell’s circumplex model of emotion (Russell, 1980). English and Japanese words were presented simultaneously.
The ratings were graded from 1 to 9, ranging from little to very much. The sounds were presented in random orderamong the participants. One minute of interval was set between two sounds.
In the second session, the participants experienced two successive conditions for each of the six sounds. In the first
condition, only the sound was played. In the second, both sound and tactile stimuli were provided as the participants
held the stimulator by the grip and maintained contact with the auditory meatus using the tip of the rod. The participants
set the contact load and angle between the rod and their ear for comfort, as individual differences in pain and discomfort
levels were significant. They were instructed to maintain the same contact condition throughout the experiment while
keeping their eyes closed. For a given sound, the sound was played for 30 s under one condition and repeated foranother 30 s after a 1 min interval for the second condition. The order in which the two conditions and six sounds were
presented was randomized. Participants closed their eyes during the presentation of the audio stimuli. After experiencing
an audio stimulus under the two conditions, participants selected the most applicable condition for each of the eightemotional items in a two-alternative forced-choice manner.

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2.6 Data analysis
For the 9-graded scores obtained in the first session, the average values among the participants were calculated for eachemotional item and the six sounds. From the results in the second session, we calculated the proportion of participants
who selected the audio-tactile stimulus conditions for each emotional item across the six sounds. A binomial test was thenperformed to determine whether each of the eight types of proportions was significantly different from chance, i.e., 0.5
with a Bonferroni correction of factor eight.

3. Results
Figure 3 shows the mean intensities of the emotions reported by the participants for the individual sounds. Fears 1 and 2
were largely judged as horrifying, unpleasant, and depressed. Rain was judged as pleasant, relaxed, and sleepy. The
Marker pen and Mosquito were unpleasant and awake. The shampoo did not show salience for any type of emotion.
Hence, the six sound types tested in this study exhibited varied emotional features.

Figure 3: Feelings felt for each sound with no tactile stimuli. Mean values and standard errors among the 12 participants.

Figure 4 shows the proportion of audio-tactile conditions selected as more suitable than the audio-only conditions for each of
the eight types of feelings and six sounds. Across the six sounds, similar trends were observed. For example, for Fears 1 and 2 andMosquito, the audio-tactile condition seemed more relaxing and sleepier, while the audio-only condition seemed to cause fear.
For Rain, Shampoo, and Marker pen, the audio-tactile condition seemed more relaxing and pleasant than the audio condition.
Figure 5 shows the average proportion across the six sounds for each emotional item. The audio-tactile condition was
judged as more relaxing (z = 4.01, p = 0.0005), pleasant (z = 3.77, p = 0.0013), sleepy (z = 3.06, p = 0.0175), and joyful(z = 3.30, p = 0.0077) than the audio condition. The audio condition was judged as more unpleasant (z = 3.77, p = 0.0013),
depressed (z = 3.06, p = 0.0175), and afraid (z = 4.01, p = 0.0005) than the audio-tactile condition. Apart from awake, oneof the two conditions was more frequently selected than the other condition.

4. Discussion
As shown in Figure 5, the stroking stimuli on the auditory meatus influenced the subjectively reported emotions. Figure 6
shows the six types of sounds in Russell’s emotional plane (Russell, 1980; Tseng et al., 2014). The coordinates of eachsound were determined using the centroid of the radar chart of the corresponding sound, as shown in Figure 3. All soundstimuli were placed in the awake–unpleasant or sleepy–pleasant quadrants. The arrows indicate how the stroking stimulichanged the emotional responses to the sounds. The directions and lengths of the arrows in Figure 6 were determined

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Stroking Stimuli to the Ear to Enhance Pleasantness

Figure 4: Proportions at which the audio + tactile condition was selected as more suitable to each of the eight emotional attributes.
Error bars indicate 95% confidence intervals.

Figure 5: Proportions at which the audio + tactile condition was selected as more suitable for each emotional evaluation item acrossthe six sounds. ***, **, and * indicate significant differences from the 0.5 chance level at p < 0.001, 0.01, and 0.05, respectively, witha Bonferroni correction of factor eight. Error bars indicate 95% confidence intervals.

Figure 6: Sound ratings and the amount of change in emotion caused by the presentation of tactile stimuli for each sound clip.

Advance Publicationby the results shown in Figure 4. The centroid of the octagonal radar chart, based on the proportions of the eight typesof emotions, was calculated for individual sounds. The vector connecting the origin of the chart, at which the eight
proportions were zero, to the centroid was considered the effect of the tactile stimuli.
The stroking stimuli made the four sounds in the unpleasant–awake quadrant feel more pleasant and less arousing.
Regarding the evaluation of sounds perceived as pleasant and sleepy in the audio-only condition, Shampoo and Rain sounds
were judged as more pleasant and sleepy when the stroking stimuli were applied. Thus, tactile stimulation made ordinarily
pleasant and unpleasant sounds to become more sleepy and pleasant. As shown in previous studies (Lernial et al., 2018;
Triscoli et al., 2017), soft stroking evokes pleasant and non-arousing feelings. The results of this experiment are consistent
with those reports. However, as mentioned, no previous studies have examined the effects of stroking stimuli on the ear.
We tested whether the participants’ responses were similar for all sounds in an ad hoc manner. For each sound, weapplied a goodness-of-fit test using the χ2 distribution to confirm that the number of participants who selected the audio+ tactile condition for the eight items agreed with the expected values. When testing the goodness of one sound, theexpected values were calculated based on the responses to the five remaining sounds. None of the sounds weresignificantly different from the others, suggesting that participants’ responses did not depend much on the type of sound(Fear 1 (χ2(7) = 1.59, p = 0.98), Fear 2 (χ2(7) = 2.75, p = 0.91), Rain (χ2(7) = 1.62, p = 0.98), Shampoo (χ2(7) = 3.01,
p = 0.88), Marker Pen (χ2(7) = 4.54, p = 0.72), Mosquito (χ2(7) = 1.56, p = 0.98)).
We found some limitations in the study method using Russell’s circumplex model. Some sounds could not be localized
in the emotional plane. For example, as shown in Figure 3, although Fear 1 gave rise to fear and feelings of depression,
they contradicted Russell’s emotional plane. Fear is the combination of unpleasantness and arousal whereas depression isthat of unpleasantness and sleepiness. Hence, in our experiment, the sound of Fear 1 was judged to be arousal and sleepy
that do not coexist in the Russell’s circumplex model. Such problems or limitations of the circumplex model haveoccasionally been pointed out by earlier researchers. For example, vector models may express affect systems in which theaxes of arousal and pleasantness are not perpendicular (Rubin and Talarico, 2009; Verschuere et al., 2001). In order togeneralize the effects of stroking stimuli to the ear, the experiments need to involve a variety of sounds including music
and human voices, although this study did not examine such sounds. Our experiments did not include sounds in thepleasant-arousal and unpleasant-sleepy quadrants. The effects of stroking stimuli on the sounds in such quadrants areunclear. Furthermore, the stimulation method, including the shape of the contactor rod and motor command for the DC
motor employed in this study, can be improved in the future. One challenge is the optimization of the stroking stimuli toindividual sounds and emotion types. For example, a specific arm-stroking velocity has been found to create comfort
sensations (Essick et al., 2010). For the ear, the frequency of mechanical stimulation may be similarly fine-tuned.
Nonetheless, Lee et al. (2023) reported a minor impact of frequency on the relaxation effects caused by ear stimulation.
The emotional dependencies of various stimuli remain study items for future work.

5. Conclusion
We investigated the emotional effects of stroking stimuli presented to the external auditory meatus while listening
to sounds. Six sounds evoking pleasant and unpleasant emotions were used. In a user study involving 12 participants,
the stroking stimuli were found to change the emotions evoked by the sounds to more pleasant and sleepy. However,for the generalization of the emotional effects of the stroking stimuli, a variety of sounds should be tested in the future.
Our current findings highlight new avenues for research in emotional haptics. We plan to optimize the stimulation
method to achieve more intensive effects.

Ethics Statement

This study was approved by the Institutional Review Board of the Hino Campus of Tokyo Metropolitan University(H23-11).

Author Contributions

Y.G.: conceptualization, apparatus, experiment, statistical analysis, and writing of the manuscript. S.O.: conceptualization,
supervision, statistical analysis, and review and editing of the manuscript. All authors contributed to the manuscript
revision and have read and approved the submitted version.

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Stroking Stimuli to the Ear to Enhance Pleasantness

Funding

This work was supported by the Institutional Research Fund of Tokyo Metropolitan University.

Conflict of Interest

The authors declare that they have no conflicts of interest.

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Yuta GOTO (Non-member)
Yuta Goto is a software engineer who received the B.S. degree in computer science from
Tokyo Metropolitan University in 2023. His research interest includes virtual reality andemotional haptics.

Shogo OKAMOTO (Member)
Shogo Okamoto received the M.S. and Ph.D. degrees in information science from the
Graduate School of Information Sciences, Tohoku University, in 2007 and 2010, respectively.
He is currently a Professor with the Department of Computer Science, Tokyo Metropolitan
University. His research interests include haptics, affective engineering, human-assistive
technology, and extended reality.