Stroking Stimuli to Ear Emotionally Affect Musical and Non-musicalSounds in a Different Way
Yuta Goto1 and Shogo Okamoto1
Abstract— Most extant studies that present haptic stimuli
to the human body simultaneously with audiovisual contents
for enhancing the emotional experience use vibration stimuli
to the upper body. Vibratory stimuli largely induce arousaland do not lead to relaxing feelings. In our previous study, weinvestigated the emotional effects of presenting stroking stimuli
to the external auditory meatus during listening to naturalsounds that evoke relaxation. The results of the previous study
suggested that the stroking stimuli to the external auditory
meatus increased the joyfulness and pleasantness of the sounds.
In this study, we investigated whether the emotional effects ofthe stroking stimuli could also be applied to classical music thatinduces relaxation. A user study involving twelve participants
showed that, as in the previous study, the stroking stimuli
enhanced pleasant feelings for non-musical sounds, whereas
they enhanced unpleasant feelings for music. These findings
help develop emotional haptic interfaces.

Fig. 1. Experimental apparatus. Left) Experimental scene. Middle) Tactile
stimulator for ear. Right) Tip of the stimulator.

I. INTRODUCTION

II. METHODS
A. Apparatus

Haptic stimuli enhance viewers’ emotions while experiencing emotionally evocative audiovisual contents [1], [2].
Vibration stimuli are typically used as haptic stimuli for such
purposes. They largely induce arousal and do not lead torelaxing feelings.
In this study, we used stroking stimuli to the external auditory meatus for affecting emotions. Most studies selected torsos or hands for stimulation, but few studies have investigated
the stimulation to external ears. The vagus nerves are locatednear the auditory meatus [3] and the mechanical stimulation
to the outer ear give rise to various physiological responses
including heart rate reduction [4]. Hence, ear stimulation may
be an alternative to torso stimulation to induce emotional
effects [5]. In addition, several studies have shown thatstroking stimuli evoke pleasant or low-arousal emotions [6].
The stroking stimuli may have emotional effects different
from vibratory stimuli when applied simultaneously withaudiovisual contents.
We previously demonstrated that stroking stimuli to theauditory meatus made non-musical relaxing sounds felt morepleasant and joyful [5]. This study further examines theeffects of the stroking stimuli to the ear on music. If
the music is felt more pleasant and joyful because of thestroking stimuli to the ear, then it may lead to new emotional
interfaces to enhance musical experiences.
*This work was in part supported by MEXT Kakenhi #23H04360.
1 The authors are with Department of Computer Science, Tokyo
Metropolitan University, 6-6 Asahigaoka, Hino-shi, Tokyo, 191-0065, Japan.

The experimental setup is shown in Fig. 1, which wasalso used in the author’s earlier study [5]. A resin rod wasused for stroking the external acoustic meatus. The rod tip
was spherical (12 mm in diameter) with a spherical nabof 6.6 mm. The resin rod was connected to a DC-geared
motor (reduction ratio: 50). The DC motor was given a speedcommand via a motor controller (SyRen 10; Dimension
Engineering LLC, OH). The speed command to the DC
motor was generated based on the sound waveform describedin Section C. The participants listened to sound stimuli
through bone-conducting earphones (Aeropex, Shokz, TX).
B. Sound Stimuli
Four sounds, that were the sounds of shampoo andrain, classical music, and piano solo music, collected from
Youtube (Google, LLC., CA) were used. In a preliminary
questionnaire, these four sounds elicited pleasant and nonarousing feelings. In the preliminary session, twelve participants rated the pleasantness and arousal of eight sounds
including these four sounds using nine-graded scales. Thesefour sounds were rated more pleasant and less arousing thanneutral on average.
C. Stroking Stimuli to Ear
The plastic rod attached to the DC motor rotated insynchronization with the volume of the presented soundstimulus. Voltage, that is the speed, commands to the DC
motor were generated from the waveform of the audio stimuli
in the following way. First, the audio channel correspondingto the ear to be stroked was extracted from the two stereo
audio channels. Subsequently, the absolute values of thesound output voltage were calculated. Hence, the DC motor
continually rotated in one direction. Finally, the waveform
envelope was computed using the envelope function in
MATLAB (MathWorks, Inc., MA).
D. Participants
Twelve healthy participants in their early 20s participatedin the experiment after providing written informed consent.
None of the participants knew the purpose of the experiment.
E. Procedures
The participants experienced two conditions in succession
for each of the four types of sounds. In one condition, only
the sound was played. In another condition, both the soundand stroking stimulus were played. Each sound lasted 30 s ineach condition. The two conditions were experimented with
the interval of 1 min. The order in which the two conditions
and four sounds were presented was randomized.
After the presentations of the individual sound under thetwo conditions, the participants selected the more applicablecondition for each of eight emotional adjectives adaptedfrom [7] in a two-alternative forced-choice task. The adjective items were awake, sleepy, joyful, depressed, pleasant,unpleasant, relaxed, and afraid.
F. Data Analysis
We calculated the proportions of the audio + stroking stimuli conditions selected for each of the music and non-music
sound groups. We pooled the results of the questionnaires forthe shampoo and rain sounds as non-music group, and thosefor the piano and classical music as the music group. For
each of the two sound groups, a binomial test was applied
to investigate whether the proportions at which the audio+ stroking stimuli condition was selected significantly foreach of the eight emotional items. For this purpose, theproportions were compared with the chance level, that is0.5, with Bonferroni correction of factor eight.
III. RESULTS
Fig. 2 shows the proportions, at which the audio + stroking
stimuli condition was selected for each of the eight items foreach sound groups. The non-musical sounds were judged as
more relaxing (z = 2.86, p = 0.0043 < 0.05/8) and pleasant(z = 2.86, p = 0.0043 < 0.05/8) due to the stroking stimuli.
Further, the non-musical sounds tended less unpleasant (z =−2.04, p = 0.041 > 0.05/8) and depressing (z = −2.04,p = 0.041 > 0.05/8) with the stroking stimuli. The musicalsounds tended to be more unpleasant (z = 2.45, p = 0.014 >0.05/8) with the stroking stimuli.
IV. DISCUSSIONAll sounds used in this study, the sounds of shampoo andrain as non-musical sounds and classical music and pianosolo music as the musical sounds, were felt pleasant and lowarousing. However, the effects of the stroking stimuli differedFig. 2. Proportions at which the audio + stroking condition was selected as
more suitable than the audio-only condition to each of the eight emotional
attributes. Non-music sounds include the sounds of shampoo and rain. Musicsounds include the piano and classical music. Each patterned bar withhorizontal stripes and with polka dots are significantly greater than 0.5 at
p < 0.01 and 0.05, respectively, without Bonferroni correction. * indicatessignificant difference from the chance, i.e., 0.5 at p < 0.05 with Bonferroni
correction.

between the two groups. The stroking stimuli altered the nonmusic sounds to more relaxing and pleasant. The stroking
stimuli to the external auditory meatus elicited pleasantfeelings as well as stroking at other sites, including thearm [6]. Further, similar to a previous study on mechanicalstimulation to the ear [4], a relaxation effect was observedin our setting. However, the stroking stimuli did not enhance
the relaxing and pleasant feelings of the music. Stroking
stimuli generate frictional sounds with the characteristicsof pink noise. Natural sounds such as shampoo and rain
typically share such characteristics wheres music does not.
The frictional sounds might have deterred the music and did
not lead to expected relaxing effects.
V. CONCLUSIONSWith stroking stimuli to the external auditory meatus,
pleasant feelings increased while listening to the non-musicalsounds. In contrast, the musical sounds were felt more
unpleasant because of the stroking stimuli.
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