The Shape of Water Stream Induces Differences in P300 and Alpha Oscillation

Abstract

Touching is a fundamental human behavior used to evaluate objects in the external world. Many previous studies have used tactile stimulation to conduct psychological and psychophysiological experiments. However, most of these studies used solid material, not water stream, as an experimental stimulus. To investigate water perception, or to easily control the temperature of an experimental stimulus, it is important to be able to control the water stimulus. In this study, we investigated the usability of water as an experimental stimulus for electroencephalography (EEG) experiments and report the basic EEG response to water stimulus. We developed a tactile stimulation device using a water stream to study EEG responses, with the ability to control the stimulus onset timing. As stimuli, we selected two types of water stream, normal and soft, based on a psychological experiment to confirm a difference of subjective feeling induced by these water streams. We conducted a typical oddball task using the two different water streams and recorded EEG waveforms from 64 electrodes while participants touched the water streams. We calculated P300 at the Pz electrode, alpha asymmetry at the frontal electrodes, and alpha suppression at the parietal area. As a result, we observed typical P300 differentiation based on the stimulus proportion (target 20% and standard 80%). We observed a weaker alpha suppression when participants touched the soft water stream compared to the normal shower. These results demonstrate the usability of water stream in psychophysiological studies and suggested that alpha suppression could be a candidate to evaluate comfort of water stream.

Full text

ORIGINAL RESEARCH
published: 20 January 2020
doi: 10.3389/fnhum.2019.00460
Edited by:
Micah M. Murray,
Université de Lausanne, Switzerland
Reviewed by:
Wael Bachta,
Sorbonne Universités, France
Hisato Sugata,
Oita University, Japan
*Correspondence:
Noriaki Kanayama
kanayama.n@aist.go.jp
Specialty section:
This article was submitted to Sensory
Neuroscience, a section of the journal
Frontiers in Human Neuroscience
Received: 07 August 2019
Accepted: 16 December 2019
Published: 20 January 2020
Citation:
Kanayama N, Mio S, Yaita R,
Ohashi T and Yamawaki S (2020) The
Shape of Water Stream Induces
Differences in P300 and
Alpha Oscillation.
Front. Hum. Neurosci. 13:460.
doi: 10.3389/fnhum.2019.00460
The Shape of Water Stream Induces
Differences in P300 and Alpha
Oscillation
Noriaki Kanayama1,2*, Shumpei Mio 3,Ryohei Yaita3,Takahiro Ohashi3
and Shigeto Yamawaki2
1Human Informatics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,
Japan, 2Center for Brain, Mind and KANSEI Sciences Research, Hiroshima University, Hiroshima, Japan, 3TOTO Limited,
Research Institute, Chigasaki, Japan
Touching is a fundamental human behavior used to evaluate objects in the external
world. Many previous studies have used tactile stimulation to conduct psychological and
psychophysiological experiments. However, most of these studies used solid material,
not water stream, as an experimental stimulus. To investigate water perception, or to
easily control the temperature of an experimental stimulus, it is important to be able
to control the water stimulus. In this study, we investigated the usability of water as
an experimental stimulus for electroencephalography (EEG) experiments and report the
basic EEG response to water stimulus. We developed a tactile stimulation device using
a water stream to study EEG responses, with the ability to control the stimulus onset
timing. As stimuli, we selected two types of water stream, normal and soft, based
on a psychological experiment to confirm a difference of subjective feeling induced by
these water streams. We conducted a typical oddball task using the two different water
streams and recorded EEG waveforms from 64 electrodes while participants touched
the water streams. We calculated P300 at the Pz electrode, alpha asymmetry at the
frontal electrodes, and alpha suppression at the parietal area. As a result, we observed
typical P300 differentiation based on the stimulus proportion (target 20% and standard
80%). We observed a weaker alpha suppression when participants touched the soft
water stream compared to the normal shower. These results demonstrate the usability
of water stream in psychophysiological studies and suggested that alpha suppression
could be a candidate to evaluate comfort of water stream.
Keywords: water, EEG, P300-event related potential, alpha oscillations, touch
INTRODUCTION
Recent technologies for modulation of tactile experiences demonstrated that, in the very near future,
tactile perception might be freely created to induce a specific affective response, in the same way
that is currently possible with visual and auditory perception. For example, the softness experience
could be modulated with vibrotactile stimuli (Hayward, 2008;Visell and Okamoto, 2014). These
stimuli could not only be induced with typical solid materials but possibly with ultrasonic mid-air
stimulation (Hoshi et al., 2009; Long et al., 2014).
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Kanayama et al. Water Stream Induces P300
A water stream is an ethologically important stimulus for
inducing tactile perception on the surface of the human body.
We often touch water in ordinary life, such as when washing
one’s face, hand washing, dishwashing, toothbrushing, bathing,
and drinking. Some uses of water include cleaning one’s body;
in this regard, touching water can induce a comforting feeling.
To elucidate the neural mechanisms underscoring the pleasure
of touching, water may be a useful stimulation.
Experimental stimuli should be shared across laboratories
to enhance replicability of results obtained from different
cognitive neuroscience studies. For this reason, many researchers
developed homemade devices for precise control of the
experience of touching solid materials (McGlone et al., 2012;
Muñoz et al., 2014; Kanayama et al., 2019). However, this is not
always possible, as materials to be touched in an experiment vary
greatly (Sakamoto and Watanabe, 2017).
The shape of a water stream could be altered by changing the
shape of a faucet. Water stimulation can easily be reproduced in
any location using an identical faucet shape. The different shapes
of water stream could have different impacts on the subjective
feeling of affective evaluation, including tactile comfort and
richness of the water stream. Comfortable touching experiences
induced by water stream could be modeled by affective/Kansei
engineering (Nagamachi, 1995) using subjective sense of comfort
about the water stream. Based on subjective reports of comfort
about various water streams, the shape of a water stream can
be optimized. However, subjective reports could be varied and
modified by spontaneous appraisal, which makes stable modeling
based on subjective reports challenging. Psychophysiological
measurements, as an objective index, can support the model of
subjective feelings of comfort (Balters and Steinert, 2017). To this
end, we explored electroencephalography (EEG) components as
a reflection of tactile comfort during perception of touching a
water stream. However, EEG components are also not always
more reliable than subjective reports, as they can vary depending
on mental health, age, and current arousal state. Measurement
of implicit and automatically activated evaluation of affective
experience could potentially provide further insight not obtained
from explicit reflective and subjective verbal reports.
To relate subjective comfort from touching a water stream,
we used two typical EEG components of alpha band oscillation.
Alpha oscillations are related to emotional responses, as the
generation of alpha oscillations stems from regional cerebral
blood flow in various cortical areas including emotion-related
areas, such as the amygdala, basal prefrontal cortex, and insula
(Sadato et al., 1998).
The first component we focused on in this study was
alpha suppression, which is typically observed after stimulus
presentation. Previous EEG studies using tactile stimuli have
repeatedly showed alpha/beta band suppression after stimulation
(van Ede et al., 2011; Singh et al., 2014). Singh et al.
(2014) reported that beta band mu-suppression and beta-band
oscillation showed a relationship with tactile caressing and
subjective ratings of pleasantness. Bauer et al. (2006) reported
that parieto-occipital alpha/beta suppression was modulated by
spatial attention. This component over the parieto-occipital
distribution has been observed in response to audiovisual
stimulation, suggesting that this component is generated by
cognitive processes independent of sensory modality (Schelenz
et al., 2013). Some studies demonstrated that this component was
more strongly suppressed when participants perceived a negative
emotional stimulus compared to when perceiving a neutral
stimulus (Jessen and Kotz, 2011; Swingle, 2013). Kostyunina and
Kulikov (1996) reported that a decrease in alpha power was
related to negative emotional states of fear and sorrow. These
findings spurred us to measure participants’ emotional states
using alpha suppression. We hypothesized that a softer stream
would elicit weaker alpha suppression compared to a normal
shower stream.
The second component we focused on was alpha asymmetry.
This EEG component has repeatedly been observed when
viewing emotional movie clips (Killeen and Teti, 2012; Lopez-
Duran et al., 2012; Meyer et al., 2014; Zhao et al., 2018).
Evidence that frontal asymmetry is modulated by tactile comfort
is scarce; furthermore, physical distress during sleep can have an
impact on this component (Flo et al., 2011). Thus, we aimed to
investigate whether touching a soft water stream would induce
alpha asymmetry. We hypothesized that a softer stream would
elicit greater alpha asymmetry compared to a normal shower
stream.
The difficulty of controlling a water stream has typically
precluded the use of water as a tactile stimulus, especially in
neuroscience and psychophysiology research. To our knowledge,
no psychophysiological study using a water stream has been
undertaken to date. Here, we examined using a water stream
as stimulus in a typical oddball paradigm and confirmed that
a water stream can induce a typical P300 component. By
this investigation, we demonstrate the usability of water in
psychophysiological studies, alongside solid and visual stimuli.
MATERIALS AND METHODS
Pilot Study for Stimulus Selection
For this study, it was necessary to be able to compare whether
or not pleasant touch was elicited in response to different
shapes of water streams. To select stimuli, we conducted a
pilot experiment on subjective evaluation of water streams.
Fifteen people (five females, 10 males) participated in the pilot
experiment. The mean age of participants was 31.67 years
(SD = 5.95; range = 25–46). Twelve of these people also
participated in the main experiment with EEG measurements.
The pilot experiment was performed more than a year before the
EEG experiment.
Participants evaluated five types of water streams (Table 1)
in terms of ‘‘richness’’ and ‘‘comfort’’ after touching the water
streams freely. For ‘‘richness,’’ participants rated whether they
have ‘‘touched a high-quality thing:’’ 1 for very low quality,
9 for very high quality, and 5 for average quality. For ‘‘comfort,’’
participants rated whether they ‘‘felt comfort’’ when touching
the water stream: 1 for high discomfort, 9 for high comfort, and
5 for neither. The difference between ‘‘comfort’’ and ‘‘richness’’
was the evaluation target. Comfort was the evaluation of the
participant’s state, which is induced by the touch of water.
Richness was the evaluation of the water stream itself. We could
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Kanayama et al. Water Stream Induces P300
TABLE 1 | Physical properties of water streams used in the pilot experiment.
Shape of flow Comfort Richness Amount of water Hole Hole Aperture
mean (SD) mean (SD) flowing (L/min) diameter (mm) numbers area (mm2)
Normal shower 1.73 (0.88) 1.60 (0.91) 3.0 0.6 18 5.09
Soft flow 7.47 (1.36) 7.67 (0.82) 3.0 1.5 19 33.56
Laminar 7.00 (1.07) 6.80 (1.15) 3.0 12.5 1 122.66
Modified Soft flow 7.13 (1.49) 7.60 (1.18) 3.0 1.5 18 31.81
Numerous hole shower 5.87 (1.46) 1.73 (0.88) 3.0 - 82 43.36
Flow with aerator 5.00 (0.00) 5.00 (0.00) 3.0 11.9 1 111.16
assume, for example, a case in which a participant feels richness
on touching the water stream but does not feel comfort. To
make the evaluation easier, participants were instructed to touch
a flow with an aerator before the experiment started, an item
not included in the stimulus list. Participants were instructed
to consider this as the baseline and evaluate all other stimuli
relative to the baseline score of 5. During the experiment, all
water streams were hidden by a wall to exclude visual effects
from the stimuli. The amount of water flowing was identical for
each water stream, which was monitored by a flowmeter (sensor:
FD-MH10A, display: FD-MA1A, KEYENCE, Osaka, Japan) and
controlled within a range of ±0.2 L/min. The temperature of the
water was kept within 23–25◦C. Other physical properties of the
two stimuli are listed in Table 1.
To confirm pairs of water streams with different subjective
feelings, we statistically analyzed comfort and richness scores
for all pairs of stimuli. We observed significant differences
in comfort scores between the pairs of normal shower vs.
soft flow (t(14)=−14.94, p<0.001, Cohen’s d=−3.86),
normal shower vs. laminar (t(14)=−14.19, p<0.001, Cohen’s
d=−3.66), normal shower vs. modified soft flow (t(14)=−11.34,
p<0.001, Cohen’s d=−2.93), Normal shower vs. numerous
hole shower (t(14)=−9.75, p<0.001, Cohen’s d=−2.52) and
soft flow vs. numerous hole shower (t(14)=−3.36, p<0.05,
Cohen’s d=−0.87). We also observed significant differences
in richness scores between pairs of normal shower vs. soft flow
(t(14)=−25.25, p<0.001, Cohen’s d=−6.52), normal shower
vs. laminar (t(14)=−14.38, p<0.001, Cohen’s d=−3.71),
normal shower vs. modified soft flow (t(14)=−18.33, p<0.001,
Cohen’s d=−4.73), normal shower vs. numerous hole shower
(t(14)=−10.81, p<0.001, Cohen’s d=−2.79), soft flow
vs. numerous hole shower (t(14)=−7.74, p<0.001, Cohen’s
d=−2.00), and modified soft flow vs. numerous hole shower
(t(14)=−6.17, p<0.01, Cohen’s d=−1.59). All p-values were
corrected by Bonferroni method. Based on these results, we
selected normal shower and soft flow as the paired stimuli for
the EEG experiment.
Participants
Thirty healthy individuals (15 females, 15 males) participated in
the EEG experiment. Mean age of participants was 31.28 years
(SD = 5.60; range = 24–46). All participants were employees
of TOTO Limited. None of the participants were informed
of the experimental aims before participation. Based on the
ethical guideline of TOTO Limited, all participants provided oral
informed consent before participation.
Materials
Stimulation Device
A stimulation device using a water stream was developed for
this experiment (Figure 1A). Two faucets with different shapes
were attached to an actuator (MISUMI, RS102, Tokyo, Japan).
One faucet produced a normal shower water stream, whereas
the other produced soft flow of water. The water stream from
the latter faucet was straighter and induced softer touch sense
compared to that of the normal shower (details in subsequent
section). The device could randomly switch between the two
types of faucets for water stimulation during experimentation
for each trial, which enabled us to conduct a discrimination
task, typically termed the ‘‘oddball task,’’ using water stream
as a stimulus. Water streams were continuously delivered to
the faucets in parallel. Solenoid valves (TOTO Limited, THE13,
Fukuoka, Japan) attached to the faucet controlled whether the
water stream was released or stopped. A controller (MISUMI,
EXRS-C1, Tokyo, Japan) of the actuator monitored the position
of the faucet. When the actuator moved the faucet to the
programmed position, transistor–transistor logic signal was sent
to the EEG amplifier as a trigger signal of touch onset using
a programmable logic controller (KEYENCE, CPU: KV-7300,
KV-B8XTD, Osaka, Japan). The aforementioned system was
attached to a stage made of aluminum frames. A basin with
a drain hose was attached to the stage and covered by a thin
plastic wall preventing water splashes and visualization of the
water stream.
Water Stream Stimuli
Two types of water stream stimuli (normal shower and soft
flow shower) were used in the EEG experiment. Before the
stimulation, the water stream was released by controlling
a solenoid valve. The actuator position was placed at a
distance from the finger such that the water stream or any
water splash would not contact the finger. The actuator was
controlled to move from the right side to the participant’s
left hand. When the actuator reached the intended position
where the water stream first touched the right end of the
left index finger, the transistor–transistor logic signal as
stimulus onset trigger was sent to the EEG amplifier. This
onset trigger timing was confirmed by a pilot experiment
using an acrylic rod instead of a finger. The position of the
actuator, where the water stream first touched the right end
of the acrylic rod, was searched in 0.1-mm steps using a
high-speed camera (Photron, FASTCAM Mini UX100, Tokyo,
Japan; Figure 1B). The discrepancy between timing of the
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Kanayama et al. Water Stream Induces P300
FIGURE 1 | Stimulation device and water stream used in the electroencephalography (EEG) experiment. (A) Schematic representation of the stimulation device.
(B) Pictures of touching a water stream with an acrylic rod (substitute for a finger) captured by a high-speed camera. (C) Schematic illustrations of arm and
finger fixtures.
trigger and that of water touching was kept below 1 ms
(0.1 mm).
Experimental Procedures
Participants were fitted with an EEG electrode cap and
received instructions about the task. In the experimental room,
participants sat on a comfortable chair and placed their arm and
hand on the arm-rest table (Figure 1C) of the stimulation device
in a palm-up posture. The angle of the table could be adjusted
seamlessly for each participant to feel comfortable. To constrain
incidental movement of the participants’ finger, participants
were required to place the index finger of their left hand into
a finger fixture. For adjustment of the program to control the
actuator based on individual finger shape, we simulated the
finger touch position using a scale. Trigger timing was adjusted
by the finger touch position.
Before the beginning of experimentation, participants wore
canal-type earphones (ZERO AUDIO, DX211-WB, Kyoto,
Japan) to hear white noise as a masking sound. Participants
heard the sound induced by movement of the actuator
without touching the water stream. Volume level of the
white noise was adjusted such that participants could not
hear device-induced noise. The maximum volume was 70 dB
for adjustments.
Participants were instructed to view the center of a box which
occluded sight of the water stream in front of their face and to
maintain their posture. Participants were encouraged to interfere
with movement of the index finger of the left hand. Participants
were required to count the touches of the water streams during
the experimental block as target stimuli. The order of targets was
counterbalanced across all participants. At the beginning of the
experimental block, after a 10-s blank period, one of the water
streams touched the index finger’s inner surface of the left hand.
Touching continued for 2,000 ms. After the stimulation offset,
a resting period was inserted. The interstimulus interval was
varied from 5,450 to 5,700 ms by 50-ms steps. The average
interstimulus interval was 5,575 ms. In one block, participants
touched the water stream 60 times. The standard stimulus was
presented 48 times, whereas the target stimulus was presented
12 times. After all trials in one block, participants were required
to report the number of target stimuli based on their internal
count. The next block started with a short break. Each session
comprised five blocks without changing the target stimulus.
Breaks were provided as per participants’ request. The second
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Kanayama et al. Water Stream Induces P300
session started with the altered target stimulus. Participants
perceived the water stream 600 times in total, divided into
10 blocks.
EEG Recordings
EEG waveforms were recorded using the BrainAmp DC amplifier
(Brainproducts, GmbH, Munich, Germany). The sampling rate
was 1,000 Hz. The reference electrode was placed on the nose tip,
and the grand electrode for the scalp EEG was placed on the back
of the neck. The 64-channel active electrodes were distributed
over the whole scalp according to the 10-10 international
standard position using an EEG recording cap (EasyCap, GmbH,
Herrsching, Germany). Horizontal electrooculograms (EOGs)
were recorded by bipolar surface electrodes placed on the left and
right outer canthus. The vertical EOG was recorded by bipolar
surface electrodes placed above and under the left eye (eyebrow).
The hardware filter settings were as follows: the low frequency
cutoff was 0.016 Hz (time constant, 10 s), whereas the high
frequency cutoff was 1,000 Hz. The impedance at each electrode
was maintained at least below 50 kand typically below 10 k.
The grand average of impedance across all participants and
electrodes at the beginning of recording was 6.24 k(SD = 1.85).
EEG Data Analysis
EEG waveforms were analyzed using EEGLAB eeglab14_1_1b
(Delorme and Makeig, 2004) under the Matlab R2018a
(MathWorks, Natick, MA, USA). First, the recorded waveforms
were digitally filtered by 1 Hz high-pass and 40 Hz low-pass
filters. Continuous EEG waveforms were segmented from
−600 to 1,200 ms after stimulus touch timing. Independent
component analysis (ICA) decomposition with the Infomax
method was applied to the segmented data. Based on the
component waveforms, artifact-contaminated trials were
discarded according to maximum and minimum amplitude,
mean trial probability, kurtosis value, and spectrum power.
Details are described in the Supplementary Material. Another
ICA was applied to trial-rejected datasets, and the component
waveforms were obtained anew. Dipole locations for all
components were estimated using dipfit2 (EEGLAB plug-in
using FieldTrip toolbox functions; Oostenveld et al., 2011).
For event-related potential (ERP) analysis to differentiate
P300 deflection by standard and target stimulus presentation,
we conducted cluster-based IC rejection to discard the
eye-movement related EEG deflection. After this rejection, we
selected the Pz electrode and calculated ERP waveforms. For
statistical analysis, the max value during 300–800 ms across
all trials was used for P300 amplitude for each participant.
Wilcoxon-signed rank test was conducted, and the pvalues were
corrected using the Bonferroni method.
Regardless of target/standard stimulus, we visualized brain
activation related to the difference between normal and soft
water streams, and calculated alpha power. For further analyses,
we modified the previously mentioned data set by merging all
trials into one condition of water stream shape regardless of
presentation probability (target/standard). For alpha asymmetry,
we calculated the values for the stimulated period (2 s) using
the traditional formula [ln(F4) −ln(F3) (Allen et al., 2004)]
and statistically tested the difference between the two streams
using Wilcoxon-signed rank test with Bonferroni correction of
the p-value. For alpha suppression, we calculated event-related
spectrum power (ERSP). Using the merged dataset, we conducted
ICA clustering analysis and derived 10 clusters. Based on the
hypothesis of alpha wave distribution, we focused on the parieto-
occipital cluster. ERSPs at the cluster were calculated for −100 to
800 ms and 3–30 Hz. The baseline was a period between
−300 and −50 ms. The difference of ERSPs between two water
streams was statistically tested using Monte Carlo permutation
statistics with cluster correction (channel neighbor parameters:
triangulation, clustering method: max-sum) implemented in
FieldTrip toolbox (Oostenveld et al., 2011).
Follow-Up Water Flow Evaluation Task
All participants were recruited for a follow-up evaluation task
more than 6 months after participation in the EEG experiment.
Participants sat on a comfortable chair in a light room and
adopted an identical posture to that in the EEG experiment.
Participants received identical water streams on the index finger
of their left hand. One of the two types of water streams was
alternately delivered to the participant’s finger. The order of
stimuli was counterbalanced across participants. Participants
were required to rate the water streams after each stimulation
in terms of ‘‘richness’’ and ‘‘comfort,’’ as conducted in the EEG
experiment. In total, 10 stimulations and ratings were conducted.
The averaged score across all ratings was used as an evaluation of
each participant.
RESULTS
P300 to the Water Stream Stimulus
The grand averaged ERP waveforms at Pz electrode for all
conditions are plotted in Figure 2. Based on visual inspection
of the waveforms, ERP waveforms to the target stimulus were
greater than those to standard stimuli regardless of the shape
of the water stream. The time period in which the waveforms
to target stimuli were greater than those to standard stimuli
was from about 300–800 ms. This time period is typical for
P300 to low-frequency target stimuli. Statistical analysis of the
maximum values during that time period revealed significant
differences between target and standard stimuli for both blocks
(soft target/normal standard condition, Z= 2.62, p<0.05,
r= 0.34, normal target/soft standard, Z= 3.92, p<0.01, r= 0.51).
Subjective Rating for Each Shower Shape
Participants evaluated the richness and comfort of perceived
water streams by normal and silky shower. The averaged values
of subjective ratings in the follow-up evaluation of water streams
are illustrated in Figure 3. Both subjective rating scores were
higher when participants received the soft water stream than
when they received the normal shower. The difference in scores
between two types of water streams were statistically analyzed
using a t-test with corrected p-values (Bonferroni method p-value
correction). Significant differences in both ratings were observed
(t(29)= 6.01, p<0.01, Cohen’s d= 1.09 for comfort; t(29)= 11.05,
p<0.01, Cohen’s d= 2.02 for richness).
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Kanayama et al. Water Stream Induces P300
FIGURE 2 | Waveforms and topographical maps of P300 components. (A) Grand averaged event-related potential (ERP) waveforms at the Pz electrode for each
condition and stimulation. (B) The location of Pz electrode. (C) Scalp distribution at amplitudes averaged across 300–800 ms time period.
FIGURE 3 | Subjective rating scores for water stream perception averaged
across 23 participants. Error bars indicate the standard errors for each
condition.
Alpha Asymmetry for Each Shower Shape
No significant difference between different water streams was
observed in alpha asymmetry index. The alpha asymmetry for
each water stream was almost identical (−0.05 for normal
shower vs. −0.06 for soft flow, Z= 0.61, n.s.; Figure 4, right
panel). Almost one-third of the participants showed the opposite
(positive) value to the trend (negative value), which suggests
great interindividual variability in alpha asymmetries (Figure 4,
left panel).
Alpha Suppression for Each Shower Shape
We observed alpha suppression at the parieto-occipital cluster
after touching a water stream (Figure 5). The centroid of the
parietal cluster was located at X, 26; Y,−41; Z, 49. More than
half of the ICs’ dipole positions (41/76) are in the parietal
area, including postcentral, superior parietal, precuneus, and
angular gyrus (Table 2). Statistical analysis revealed that alpha
suppression to the normal shower was significantly greater than
that to the silky shower stimulus. The significant time frequency
window was 8–17 Hz and 200–600 ms.
Correlation Analyses of Subjective Ratings
and EEG Component
We calculated the Spearman rho correlation coefficient to
analyze the relationship between EEG activity (alpha asymmetry
and alpha suppression) and subjective rating scores (comfort
and richness) obtained by follow-up experiments. No significant
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Kanayama et al. Water Stream Induces P300
FIGURE 4 | Alpha asymmetries in individuals for each water stream and averaged value for each stimulus. In the left panel, alpha asymmetry in each individual is
illustrated. The y-axis denotes participant numbers, whereas the x-axis denotes the value of alpha asymmetries (microV2). In the right panel, the alpha asymmetries
are averaged across all participants. The y-axis denotes the value of alpha asymmetries (microV2).
FIGURE 5 | (A) Event-related spectrum power (ERSP) for each condition, dipole locations, and scalp topography of the target cluster. (B) Schematic representation
of differentiated alpha suppression by touch with normal and silky water streams. The illustrated waveforms are typical waveforms at the occipital electrodes before
and after stimulation with normal and silky water streams.
correlations were observed. Scatter plots are illustrated in
Figure 6.
DISCUSSION
In this study, we aimed to test two hypotheses: first, whether
a water stream as a tactile stimulus could induce P300 and
whether amplitudes were different between target and standard
stimulation in the typical oddball task; second, whether tactile
comfort induced by a water stream could be indexed by alpha
oscillation. Significant modulation of P300 was observed in
this experiment, suggesting that the methodology used in this
experiment was effective for electrophysiological studies using
water streams. Alpha suppression was significantly modulated
by the shape of the water stream, which differentiated subjective
reports of comfort and richness, implying that alpha oscillation
could be involved in affective processing when touching
water streams.
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Kanayama et al. Water Stream Induces P300
TABLE 2 | Dipole characteristics for each independent component involved in
target cluster 3.
AAL name of the dipole position Nof ICs Average RV SD of RV
Postcentral_R 14 10.12 4.19
Precuneus_R 12 7.63 3.03
Parietal_Sup_R 9 10.15 4.55
Angular_R 6 8.97 4.18
Precentral_R 5 13.03 2.23
Parietal_Inf_R 5 5.41 2.41
Cingulum_Mid_R 4 7.82 3.99
Cingulum_Post_R 3 6.64 4.36
Precuneus_L 3 9.43 2.15
Cingulum_Mid_L 2 7.02 0.24
Cingulum_Post_L 2 3.69 0.87
Paracentral_Lobule_R 2 4.93 1.17
Occipital_Sup_R 2 5.54 0.49
Frontal_Sup_R 2 13.81 2.47
Insula_R 1 9.71 0
Temporal_Mid_R 1 6.92 0
Paracentral_Lobule_L 1 3.22 0
Frontal_Mid_R 1 10.85 0
Occipital_Mid_R 1 2.92 0
For example, at the top of the rows, the dipole positions of 14 independent components
(ICs) fall in the area named “Postcentral_R” (the right postcentral gyrus) based on the
automated anatomical labeling (AAL) atlas 3. The average residual variance of dipoles
across all ICs of “Postcentral_R” was 10.12, and the standard deviation was 4.19. AAL,
automated anatomical labeling; IC, independent component; RV, residual variance.
P300 amplitude was greater for the target stimulus regardless
of the shape of the water stream. This suggested that P300 could
be elicited by the shape of the water stream in a similar
manner as that of typical visual and/or tactile oddballs using
surface toughness. Scalp topography was also very typical of
P300 component during the oddball task. In the topographical
map of P300, positive deflections were also observed over the
frontal area. This is possibly related to eye movement-related
noise deflections, as we did not instruct participants to refrain
from eye blinking when touching the water stream. We tried
to discard eye movement-related EEG waveforms using ICA.
We successfully detected and excluded a cluster distributed over
the frontal pole before ERP calculation. However, the frontal
distributed positive deflection remained in the resulting EEG
data, suggesting that there were difficulties in ICA-based removal
of EOG-related activity from EEG waveforms recorded in free
eye movement conditions. In these results, we observed clear
boundaries between frontal and parietal positive deflections,
which supported the conclusion that the P300 difference at Pz
was not contaminated by EOG-related positive deflections.
Furthermore, we calculated the alpha asymmetry index for
each EEG waveform to different water streams; however, there
was no significant difference between them. Subjective reports
have revealed that the comfort and richness evaluation for
soft flow was significantly higher than that for normal shower,
which led to the assumption that alpha asymmetry during
perception of soft flow is greater than that during normal
shower because negative emotional states induce greater alpha
asymmetry than do normal and positive emotional stimuli.
However, many studies using alpha asymmetry have adopted
longer time periods for analysis. For example, the waveforms
when viewing video clips ranging from 6 s to several tens of
minutes were used for calculating spectrum powers (Davidson
et al., 1999; Allen et al., 2001; Meyer et al., 2014). In this study,
water stream stimulation continued for only 2 s, which may have
been insufficient for alpha asymmetry analysis. In the future,
experiments with longer stimulation periods are required to
clarify whether alpha asymmetry can be an emotional index of
water stream perception.
Finally, we assumed that alpha suppression at the parietal area
could be modulated by different stimuli. Compared to normal
shower, soft flow showed significantly lower alpha suppression
of a cluster at the parietal area. The relationship between
emotional response and alpha suppression has previously been
reported (Kostyunina and Kulikov, 1996; Swingle, 2013), which
suggests that alpha suppression could be related to emotional
responses to water stream stimulation. The cluster involved many
ICs with dipole locations in the postcentral, superior parietal,
precuneus, and angular gyrus, which suggested that the primary
FIGURE 6 | Scatter plots of subjective rating scores and EEG responses to water streams. (A) Alpha asymmetry. (B) Alpha suppression.
Frontiers in Human Neuroscience | www.frontiersin.org 8January 2020 | Volume 13 | Article 460

Kanayama et al. Water Stream Induces P300
and secondary somatosensory area and the sensory association
area could be related to the water pleasantness perception.
However, because the correlations between alpha suppression
and subjective evaluations were not significant, we were unable
to conclude that alpha suppression was directly related to the
feeling of comfort or richness when perceiving a water stream.
In addition, we have to note that nonparametric statistical tests
were used as we could not presume a normal distribution;
therefore, we could only determine that the rank (high or low
amplitude) was consistent across participants. Previous studies
have demonstrated that alpha suppression was greater when
participants paid attention to the stimulus (Siegel et al., 2008;
Wyart and Tallon-Baudry, 2008), especially using visuospatial
attention. In this study, exogenous covert attention is a candidate
for modulation of alpha suppression after touching a water
stream. Future research is required to clarify the relationships
among the amount of induced attention, alpha suppression, and
tactile comfort.
To conclude, we have demonstrated that water stream
could be used as tactile stimulation for electrophysiological
experiments. During a typical oddball task, water stream-
induced P300 components and a difference between target and
standard stimuli were observed. Furthermore, we demonstrated
that less alpha suppression was related to comfort/richness
of a water stream, although no direct correlation between
them was observed. Future research is required to clarify the
relationships among the amount of induced attention, alpha
suppression, and tactile comfort. Our findings may be used as an
electrophysiological index of affective evaluation for engineering.
DATA AVAILABILITY STATEMENT
The datasets generated for this study will not be made
publicly available. As a rule of the Ethical Committee,
we cannot disclose the raw biological data acquired from
human participants.
ETHICS STATEMENT
The studies involving human participants were reviewed
and approved by TOTO Limited, Research Institute, Ethical
Committee. Written informed consent for participation was not
required for this study in accordance with the national legislation
and the institutional requirements.
AUTHOR CONTRIBUTIONS
NK, SM, RY, TO and SY have read, discussed and approved the
manuscript for submission. NK, SM, RY, TO and SY discussed to
design the behavioral and EEG experiment. NK, SM, RY and TO
designed and developed the device. NK, SM and RY collected the
data. NK wrote the article.
FUNDING
This study was supported by the Japan Science and Technology
Agency’s Center of Innovation Program (JPMJCE1311).
ACKNOWLEDGMENTS
We thank Chikako Arashi and Yukari Ojima for their assistance
with experiments and data analysis.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online
at: https://www.frontiersin.org/articles/10.3389/fnhum.2019.004
60/full#supplementary-material.
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Conflict of Interest: SM, RY, and TO were employed by the company
TOTO Limited.
The remaining authors declare that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential
conflict of interest.
Copyright © 2020 Kanayama, Mio, Yaita, Ohashi and Yamawaki. This is an
open-access article distributed under the terms of the Creative Commons Attribution
License (CC BY). The use, distribution or reproduction in other forums is permitted,
provided the original author(s) and the copyright owner(s) are credited and that the
original publication in this journal is cited, in accordance with accepted academic
practice. No use, distribution or reproduction is permitted which does not comply
with these terms.
Frontiers in Human Neuroscience | www.frontiersin.org 10 January 2020 | Volume 13 | Article 460