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Citation: Walter, N.; Hinterberger, T.
Neurophysiological Effects of a
Singing Bowl Massage. Medicina
2022,58, 594. https://doi.org/
10.3390/medicina58050594
Academic Editor: Martin Schäfer
Received: 19 February 2022
Accepted: 21 April 2022
Published: 26 April 2022
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medicina
Article
Neurophysiological Effects of a Singing Bowl Massage
Nike Walter * and Thilo Hinterberger *
Section of Applied Consciousness Sciences, Department for Psychosomatic Medicine, University Hospital Regensburg,
93053 Regensburg, Germany
*Correspondence: nike.walter@ukr.de (N.W.); thilo.hinterberger@ukr.de (T.H.)
Abstract:
Background and Objectives: In recent years, singing bowl sound interventions have been
progressively implemented in the fields of well-being, therapy and education; however, the ef-
fectiveness has only scarcely been investigated. Therefore, this study was aimed at determining
neurophysiological effects of a singing bowl massage. Materials and Methods: In this prospective
cohort study
64-channel
EEG, ECG and respiration was recorded from 34 participants (mean age
36.03 ±13.43 years,
24 females/10 males) before, during and after a professional singing bowl mas-
sage. Further, subjective changes in well-being were assessed. EEG data were analyzed by deter-
mining the effect sizes of distinct frequency bands. Significant differences were calculated by a
two-tailed t-test corrected for multiple comparisons. Heart rate variability metrics, heart rate and
respiration rate were estimated and compared. Results: Overall EEG power decreased during the
sound condition compared to a task-free resting state (d =
−
0.30, p= 0.002). After the intervention,
global EEG power was further reduced (d =
−
0.46, p< 0.001), revealing a decrease in the beta 2
(
d = −0.15,
p= 0.002) and the gamma frequency band (d =
−
0.21, p= 0.004). The mean heart rate was
significantly lower after the intervention (75.5
±
19.8 vs. 71.5
±
17.9, p< 0.001) and the respiration
rate higher (
13.5 ±5.3 vs. 15.2 ±6.3
,p= 0.018). 91.2% of the participants felt more integrated, 97.1%
more balanced and 76.5% more vitalized. Conclusions: The neurophysiological effects of a singing
bowl sound massage may be interpreted as a shift towards a more mindful, meditative state of
consciousness. The intervention was perceived as beneficial for the wellbeing.
Keywords: mind–body intervention; complementary therapy; singing bowl; EEG; HRV
1. Introduction
Since centuries ago, the concept of sound healing has been utilized by various cultures
all over the world. During the last few years, music-based interventions have been pro-
gressively implemented in western health care [
1
]. Such nonpharmacological therapy has
been shown to alleviate a variety of symptoms including pain, anxiety and stress, as well
as to improve quality of life in pediatrics [
2
]. In addition, for adults, beneficial outcomes
of music therapy were reported, amongst others, in the context of depression [
3
–
5
] and
neurological disorders [
6
–
8
]. With increasing evidence for the therapeutical effects of the
mindfulness concept [
9
,
10
], traditional techniques using acoustic stimuli for the induction
of meditative states, such as singing bowl sound, were further investigated [
11
]. Thus, some
studies provided first insights into the physiological and psychological effects of singing
bowl sound, comparing pre- and posttreatment parameters by analysis of variance [
12
,
13
].
However, whereas existing studies applied heart rate variability (HRV) measures and
psychometric data, neurophysiological effects of singing bowl applications are unknown.
Further, most of the studies chose a study design using listening to the sound of singing
bowls as an intervention. In the meantime, the application of directly placing singing bowls
on the patient’s body, termed sound massage, has found its way in various applications in
the fields of well-being, therapy and education [
14
]. Therefore, this present study aimed to
investigate, for the first time, neurophysiological effects of a singing bowl sound massage.
For this purpose, electrophysiological correlates were identified during the intervention
Medicina 2022,58, 594. https://doi.org/10.3390/medicina58050594 https://www.mdpi.com/journal/medicina
Medicina 2022,58, 594 2 of 7
using 64-channel EEG and compared to a task-free resting state. Further, it aimed to de-
termine changes in HRV and respiration rate, as well as subjective changes in wellbeing,
associated with the singing bowl sound massage intervention.
2. Materials and Methods
2.1. Data Acquisition and Participants
In this prospective cohort study, electrophysiological data were recorded from
34 participants
(mean age 36.03
±
13.43 years, 24 females/10 males). All participants
gave their informed consent prior to the study. The study was approved by the institutional
ethics committee of the University Clinic Regensburg according to the Helsinki Convention
(file number: 20-1995-101). Data were recorded using a 72-channel QuickAmp amplifier sys-
tem (BrainProducts GmbH, Munich, Germany). EEG was measured with a 64-channel ANT
Waveguard electrode cap (ANT B.V., Enschede, The Netherlands) with active shielding and
Ag/AgCl electrodes, which were arranged according to the international 10/10 system.
In addition, electrocardiograms (ECGs) were measured with two electrodes between the
sternum and the left costal arch. The respiratory signal was recorded from a belt around the
lower part of the chest. The experimental procedure started with an initial 10 min baseline
resting, including 5 min with eyes open and 5 min with eyes closed. Then, a singing bowl
massage was conducted by professionals trained according to the Peter Hess
®
-method,
with a duration of 20 min. Afterwards, 10 min of silence were given to integrate the experi-
ence. Directly after, a second resting state took place, during which participants kept their
eyes closed for 5 min and subsequently opened their eyes for 5 min (Figure 1). During the
whole procedure participants lay comfortably on a massage table. Before the recording, the
Tellegen-Absorption-Scale (TAS) was measured. This questionnaire captures the absorption
capacity (i.e., the individual’s capacity for engaging attentional resources in sensory and
imaginative experiences) containing 34 true/false self-report items [
15
]. After the recording,
participants ensued the questionnaire CSP-14, which assesses changes in body sensation,
emotional state and mental state. The questionnaire contains 14 items, which are rated on a
scale ranging from −3 to +3 [16].
Medicina 2022, 58, x FOR PEER REVIEW 2 of 7
present study aimed to investigate, for the first time, neurophysiological effects of a sing-
ing bowl sound massage. For this purpose, electrophysiological correlates were identified
during the intervention using 64-channel EEG and compared to a task-free resting state.
Further, it aimed to determine changes in HRV and respiration rate, as well as subjective
changes in wellbeing, associated with the singing bowl sound massage intervention.
2. Materials and Methods
2.1. Data Acquisition and Participants
In this prospective cohort study, electrophysiological data were recorded from 34
participants (mean age 36.03 ± 13.43 years, 24 females/10 males). All participants gave
their informed consent prior to the study. The study was approved by the institutional
ethics committee of the University Clinic Regensburg according to the Helsinki Conven-
tion (file number: 20-1995-101). Data were recorded using a 72-channel QuickAmp ampli-
fier system (BrainProducts GmbH, Munich, Germany). EEG was measured with a 64-
channel ANT Waveguard electrode cap (ANT B.V., Enschede, The Netherlands) with ac-
tive shielding and Ag/AgCl electrodes, which were arranged according to the interna-
tional 10/10 system. In addition, electrocardiograms (ECGs) were measured with two elec-
trodes between the sternum and the left costal arch. The respiratory signal was recorded
from a belt around the lower part of the chest. The experimental procedure started with
an initial 10 min baseline resting, including 5 min with eyes open and 5 min with eyes
closed. Then, a singing bowl massage was conducted by professionals trained according
to the Peter Hess
®
-method, with a duration of 20 min. Afterwards, 10 min of silence were
given to integrate the experience. Directly after, a second resting state took place, during
which participants kept their eyes closed for 5 min and subsequently opened their eyes
for 5 min (Figure 1). During the whole procedure participants lay comfortably on a mas-
sage table. Before the recording, the Tellegen-Absorption-Scale (TAS) was measured. This
questionnaire captures the absorption capacity (i.e., the individual’s capacity for engaging
attentional resources in sensory and imaginative experiences) containing 34 true/false self-
report items [15]. After the recording, participants ensued the questionnaire CSP-14,
which assesses changes in body sensation, emotional state and mental state. The question-
naire contains 14 items, which are rated on a scale ranging from −3 to +3 [16].
Figure 1. Schematic overview of the experimental procedure.
Figure 1. Schematic overview of the experimental procedure.
2.2. Data Processing
MATLAB (MathWorks, Natrick, MA, USA) was used for data processing. Data were
sampled at 250 samples/sec in a range from DC to 70 Hz with a notch filter at 50 Hz. After
Medicina 2022,58, 594 3 of 7
detrending the 64 EEG channels, a correction for eye movement was done using a linear
correction algorithm [
17
]. A power spectrum time series was calculated using the Fast
Fourier Transform (FFT) for the following frequency bands: delta: 1–3.5 Hz, theta:
4–7.5 Hz,
alpha 1: 8–10 Hz, alpha 2: 10.5–12 Hz, beta 1: 12.5–15 Hz, beta 2: 15.5–25 Hz, gamma:
25.5–45 Hz, global: 1–45 Hz. To obtain a measure of the power spectral density (PSD) FFT
values were squared and all FFT bins within a frequency band range were averaged. EEG
PSD was calculated for each participant, task, electrode, and frequency band.
To determine the effects of the singing bowl massage, the following three phases of
the experimental course were compared:
1. sound vs. resting (eyes closed)
2. postresting (eyes closed) vs. sound
3. postresting (eyes closed) vs. resting (eyes closed)
The respiration signal was detrended, downsampled to 25 samples/s and filtered
using a third-order Butterworth bandpass filter in the range between 0.002 and 1 Hz. The
heart rate (interbeat interval) was calculated by analyzing the R-peaks of the ECG signal.
Time domain indices of heart rate variability (HRV), such as standard deviation of NN
intervals (SDNN) and root mean square of successive RR interval difference (RMSSD), as
well as frequency domain indices of HRV, such as low frequency power (0.04–0.15 Hz) and
high frequency power (0.15–0.4 Hz), were quantified.
2.3. Statistics
For a comparison of the PSD between the first resting, sound application and second
resting, effect sizes of the temporal mean of each frequency band for the respective phase
were calculated, defined as standardized mean differences (Cohen’s d) [
18
]. Then, effect
sizes of all participants were submitted to a paired two-tailed t-test calculated across
participants and measures. On a level of global field power eight frequency bands and three
comparisons were considered, resulting in 24 variables. These were corrected for multiple
comparison using false discovery rate (FDR) adjustment, which gives the proportion of
false discoveries among all discoveries [
19
]. FDR was applied on all dimensions across task
conditions, the eight frequency bands and channels. To estimate correlations between the
PSD and the psychometric data, Spearman’s rank correlation was used, after determining
that the distribution was not appropriate for parametric testing by the Shapiro-Wilk test.
For a comparison between HRV metrics and the respiration rate of the first and the second
resting state, a Wilcoxon signed-rank test was applied. Significance was set at p< 0.05.
3. Results
For a detailed comparison between the three phases, effect sizes were estimated for
each condition and frequency band. Significant differences were determined by a two-tailed
t-test corrected for multiple comparisons by the false discovery rate. Comparing the distinct
phases of the course of experimental procedure, there was significantly less overall EEG
power during the sound condition compared to the first resting state (d =
−
0.30, p= 0.002).
The decrease of EEG activity was specifically significant for the frequency bands alpha 2
(
d = −0.17,
p= 0.003), beta 1 (d =
−
0.16, p= 0.002), beta 2 (d =
−
0.24, p= 0.005), and gamma
(d =
−
0.35, p= 0.001). The comparison between the second resting state and the sound
condition revealed a decrease in the beta 2 (d =
−
0.15, p= 0.002) and the gamma frequency
band (d =
−
0.06, p= 0.004). Further reduction in global EEG power was observed during
the second resting states compared to the first resting state (d =
−
0.46, p< 0.001), also with
significant effects for alpha 2 (d =
−
0.21, p= 0.010), beta 1 (d =
−
0.14, p= 0.006), beta 2
(d = −0.40, p< 0.001) and gamma (d = −0.21 p< 0.001) (Figure 2).
Medicina 2022,58, 594 4 of 7
Medicina 2022, 58, x FOR PEER REVIEW 4 of 7
Figure 2. Color-coded differences of power spectral density shown as effect sizes (Cohen’s d) of the
task comparisons. Fields marked with a black circle were significant on a 0.05 level after FDR ad-
justment.
There was no statistically significant change regarding HRV metrics. The mean heart
rate was significantly lower after the intervention (75.5 ± 19.8 vs. 71.5 ± 17.9, p < 0.001) and
the respiration rate was higher (13.5 ± 5.3 vs. 15.2 ± 6.3, p = 0.018) (Table 1).
Table 1. Heart rate variability metrics and respiration rate for each condition.
Parameter Resting [Mean
(SD)]
Sound [Mean
(SD)]
Postresting
[Mean (SD)]
Comparison (p-
Value)
HR [bmp] 75.5 (19.8) 71.3 (17.0) 71.5 (17.9) <0.001 *
SDNN [ms] 68.5 (36.0) 68.3 (34.5) 73.0 (43.3) 0.152
RMSSD [ms] 52.4 (34.6) 54.5 (34.6) 55.7 (38.7) 0.057
LF power [%] 41.6 (19.6) 36.7 (14.1) 39.0 (18.0) 0.203
HF power [%] 21.8 (13.1) 25.0 (14.1) 21.9 (13.2) 0.717
Respiration rate 13.5 (5.3) 14.0 (6.5) 15.2 (6.3) 0.018*
SDNN = Standard deviation of NN intervals, RMSSD = root mean square of successive RR interval
differences, HR = heart rate, LF = low frequency, HF = high frequency. * p < 0.05.
Regarding the subjective effects of the singing bowl massage, 91.2% of the partici-
pants felt more integrated, 97.1% more balanced and 76.5% more vitalized. The bodily
feeling was rated as wider (85.3%), more intense (91.2%), more relaxed (91.2%), more com-
fortable (88.2%) and more powerful (70.6%). The emotional state appeared to be calmer
for 82.4% of the participants. Further, after the singing bowl application, participants re-
ported to be happier (79.4%), satisfied (88.2%), more secure (82.4%) and connected
(88.2%). Mentally, the majority of participants felt clearer (73.5%). Regarding the phenom-
enology of consciousness, participants scored highest in the dimensions of “openness”,
“memory”, “introversion”, “timelessness”, “imagination” and “cognitive clarity”. The
mean TAS score was 69.4 ± 27.5. The total TAS scores did not correlate significantly with
age (Spearman’s ρ = 0.19, p = 0.271) or sex (Spearman’s ρ = −0.144, p = 0.417). The highest
correlation was found between the gamma band power during the first resting state and
the total TAS score, however, it was not statistically significant (Spearman’s ρ = 0.299, p =
0.097). The dimensions of the CSP-14 did not significantly correlate with the PSD in any
frequency band during the three experimental phases, respectively.
4. Discussion
The intervention of a singing bowl massage found its way into various fields of ap-
plication such as prevention, therapy, wellness and education. In this study, neurophysi-
ological effects as well as subjective changes of wellbeing were evaluated. The results
Figure 2.
Color-coded differences of power spectral density shown as effect sizes (Cohen’s d)
of the task comparisons. Fields marked with a black circle were significant on a 0.05 level after
FDR adjustment.
There was no statistically significant change regarding HRV metrics. The mean heart
rate was significantly lower after the intervention (75.5
±
19.8 vs. 71.5
±
17.9, p< 0.001)
and the respiration rate was higher (13.5 ±5.3 vs. 15.2 ±6.3, p= 0.018) (Table 1).
Table 1. Heart rate variability metrics and respiration rate for each condition.
Parameter Resting
[Mean (SD)]
Sound
[Mean (SD)]
Postresting
[Mean (SD)]
Comparison
(p-Value)
HR [bmp] 75.5 (19.8) 71.3 (17.0) 71.5 (17.9) <0.001 *
SDNN [ms] 68.5 (36.0) 68.3 (34.5) 73.0 (43.3) 0.152
RMSSD [ms] 52.4 (34.6) 54.5 (34.6) 55.7 (38.7) 0.057
LF power [%] 41.6 (19.6) 36.7 (14.1) 39.0 (18.0) 0.203
HF power [%] 21.8 (13.1) 25.0 (14.1) 21.9 (13.2) 0.717
Respiration rate 13.5 (5.3) 14.0 (6.5) 15.2 (6.3) 0.018 *
SDNN = Standard deviation of NN intervals, RMSSD = root mean square of successive RR interval differences,
HR = heart rate, LF = low frequency, HF = high frequency. * p< 0.05.
Regarding the subjective effects of the singing bowl massage, 91.2% of the participants
felt more integrated, 97.1% more balanced and 76.5% more vitalized. The bodily feeling
was rated as wider (85.3%), more intense (91.2%), more relaxed (91.2%), more comfortable
(88.2%) and more powerful (70.6%). The emotional state appeared to be calmer for 82.4%
of the participants. Further, after the singing bowl application, participants reported to
be happier (79.4%), satisfied (88.2%), more secure (82.4%) and connected (88.2%). Men-
tally, the majority of participants felt clearer (73.5%). Regarding the phenomenology of
consciousness, participants scored highest in the dimensions of “openness”, “memory”,
“introversion”, “timelessness”, “imagination” and “cognitive clarity”. The mean TAS score
was 69.4
±
27.5. The total TAS scores did not correlate significantly with age (Spearman’s
ρ
= 0.19, p = 0.271) or sex (Spearman’s
ρ
=
−
0.144, p = 0.417). The highest correlation was
found between the gamma band power during the first resting state and the total TAS score,
however, it was not statistically significant (Spearman’s
ρ
= 0.299, p = 0.097). The dimensions
of the CSP-14 did not significantly correlate with the PSD in any frequency band during
the three experimental phases, respectively.
4. Discussion
The intervention of a singing bowl massage found its way into various fields of applica-
tion such as prevention, therapy, wellness and education. In this study, neurophysiological
effects as well as subjective changes of wellbeing were evaluated. The results showed an
overall decrease of EEG power during the singing bowl massage, as well as afterwards.
The effects were most pronounced in the beta 2 and gamma frequency band.
Medicina 2022,58, 594 5 of 7
The neurophysiological changes may be interpreted as a refrain from specific cognitive
processing such as mental conceptualization, which would be commensurate with the
essential aspect of mindfulness, namely non-judgmental awareness of the moment-to-
moment-experience [
20
]. This would be in line with another study by Hinterberger and
colleagues, reporting global decreased EEG activity as well as decreases in the frontal beta
and the central and parietal gamma band, when highly experienced meditators entered
a state of thoughtless emptiness [
21
]. Further, a decrease in power over all frequency
bands was detected during a meditation characterized as “sacred, unified, egoless, and
blessed” [
22
]. In addition, Dor-Ziderman et al. distinguished between a state of “narrative”
self-awareness and “minimal” self-awareness in a MEG neurophenomenological study. The
authors reported that the first involved frontal and medial prefrontal gamma band power
decrease, while the latter was related to a beta band power decrease in a network, including
ventral medial prefrontal, medial posterior and lateral parietal regions. Furthermore, the
authors linked an attenuation of beta band activity in the right inferior parietal lobule
to a state of selflessness [
23
]. The reported positive psychological effects are in line with
other findings in the literature. For instance, Goldsby et al. reported less tension, anger,
fatigue, and depressed mood (p<0.001) after a meditation with Tibetan singing bowl in
healthy participants, and the feeling of spiritual well-being was significantly higher [
13
]. In
addition, an improvement in positive affect and a reduction in negative affect, as captured
by the Positive and Negative Affect Schedule (PANAS) questionnaire [
24
], was reported
after a 40-min-long sound meditation with singing bowls [
25
]. In the presented study,
participants also reported to feel more vitalized, which is in accordance with another study
determining that subjective sleepiness was lower after a 20 min relaxation session with
singing bowl sound compared to a silent relaxation (p= 0.041) [
26
]. Further, listening
to singing bowl sound was shown to be a useful strategy to reduce anxiety in patients
waiting for urologic surgery [
27
], which is consistent with high percentages of participants
reporting to be more relaxed, calm and balanced in this study.
In line with the findings of a significantly decreased heart rate, Landry compared the
effects of a directed relaxation session with and without the use of Himalayan singing
bowl sound in a randomized controlled trial, reporting a decline in systolic blood pressure
(p= 0.044) and heart rate (p= 0.003) in the first group [
12
]. Whereas here no significant
changes were found in HRV metrics, a significant increase in heart rate variability was
observed when applying singing bowl sound during a relaxation session compared to
silent relaxation [28].
Here, no statistically significant correlation between the TAS scores and the frequency
band power was found. However, other findings suggested that absorption, as a personality
characteristic may be a predictor of outcomes in mind–body interventions [
29
]. For instance,
fibromyalgia patients with high levels of absorption reported more clinically relevant
improvements after a guided imagery intervention in a randomized, controlled trial [30].
The presented findings suggest that the application of a singing bowl massage is
beneficial on a physical and psychological level. Addressing the question whether the
effects are also therapeutical, Wepner and colleagues investigated singing bowl massage
interventions in patients with chronic unspecific pain. In their study, participants were
divided into three groups, either receiving singing bowl therapy, a placebo intervention, or
no treatment; both the placebo and the treatment group showed less pain intensities [
31
].
However, in a recent review it was concluded that more evidence is required to recommend
singing bowl therapies as numbers of studies eligible for inclusion were small (n= 4) [11].
This study shows several limitations. First, the sample size may limit the findings in
generalizability. In addition, this study was conducted solely with healthy participants and
for the translation into clinical practice, more evidence based on randomized controlled
trials is required. Further, the measurement and laboratory setting may have influence
subjective changes in wellbeing. In addition, no comparison with other interventions such
as relaxation without sound, or solely listening to singing bowl sound, were made and
thus, blinding of participants was not feasible. However, strengths of the study include the
Medicina 2022,58, 594 6 of 7
novelty of identifying singing bowl massage effects on a psychological, physiological and
neurological level for the first time.
5. Conclusions
In conclusion, neurophysiological correlates of a singing bowl sound massage were
identified. These were characterized by a decrease of overall EEG power most pronounced
in the beta 2 and gamma frequency band, which might reflect a mindful state of conscious-
ness. Subjectively, the intervention was perceived as beneficial for the wellbeing.
Author Contributions:
Conceptualization, N.W. and T.H.; methodology, T.H.; formal analysis, T.H.
and N.W.; data curation, N.W.; writing—original draft preparation, N.W.; writing—review and
editing, T.H.; visualization, N.W.; supervision, T.H. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement:
The study was conducted in accordance with the Declaration
of Helsinki, and approved by the Institutional Ethics Committee the University Clinic Regensburg
(20-1995-101, 20 August 2020).
Informed Consent Statement:
Informed consent was obtained from all subjects involved in the study.
Data Availability Statement:
The datasets generated and analyzed in the current study are available
from the corresponding author on reasonable request.
Acknowledgments:
The authors would like to thank the European Professional Association for
Sound-Massage-Therapy e.V. for supporting this research.
Conflicts of Interest: The authors declare no conflict of interest.
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