Journal of
Physiological Anthropology

Shin et al.
Journal of Physiological Anthropology(2024) 43:21
https://doi.org/10.1186/s40101-024-00369-0

Open Access

ORIGINAL ARTICLE

Effects of observing own/others hand
movement in different perspectives on murhythm suppression: an EEG study
Nakyeong Shin1,4* , Yuki Ikeda2,4, Yuki Motomura3 and Shigekazu Higuchi3

Abstract
Background Previous studies have reported that the sense of “self” is associated with specific brain regions and neural network activities. In addition, the mirror system, which functions when executing or observing an action, might
contribute to differentiating the self from others and form the basis of the sense of self as a fundamental physicalrepresentation. This study investigated whether differences in mu suppression, an indicator of mirror system activity,
reflect cognitions related to self-other discrimination.
Methods The participants were 30 of healthy college students. The participants observed short video clips of hand
movements performed by themselves or actors from two perspectives (i.e., first-person and third-person). The electroencephalogram (EEG) mu rhythm (8–13 Hz) was measured during video observation as an index of mirror neuron system activity. EEG activity related to self-detection was analyzed using participants’ hand movements as self-relevant
stimuli.
Results The results showed that mu suppression in the 8–13-Hz range exhibited perspective-dependent responses
to self/other stimuli. There was a significant self-oriented mu suppression response in the first-person perspective.
However, the study found no significant response orientation in the third-person perspective. The results suggestthat mirror system activity may involve self-other discrimination differently depending on the perspective.
Conclusions In summary, this study examined the mirror system’s activity for self and others using the EEG’s mu
suppression. As a result, it was suggested that differences in self and others or perspectives may influence mu
suppression.

*Correspondence:
Nakyeong Shinshin.nakyeong@gmail.com
1
Graduate School of Integrated Frontier Sciences, Kyushu University,
4‑9‑1 Shiobaru, Minami‑Ku, Fukuoka City, Fukuoka, Japan
2
Faculty of Health Science, Kyorin University, 6‑20‑2 Shinkawa, Mitaka‑Shi,Tokyo, Japan
3
Faculty of Design, Kyushu University, 4‑9‑1 Shiobaru, Minami‑Ku
Fukuoka City, Fukuoka, Japan
4
Research Fellow of the Japan Society for the Promotion of Science,
Kojimachi Business Center, Building, 5‑3‑1 Kojimachi, Chiyoda‑Ku, Tokyo,
Japan

Background
Understanding the distinctions between self and others,
and the intrinsic sense that our body and consciousness
are distinct from those of others, is a fundamental aspectof human cognition and adaptability. This distinction is
crucial for effective social interaction and communication, which are vital for human survival and thriving
within complex social structures. Previous studies haveindicated that disruptions in the sense of self that cause
difficulties in interacting with others and society are
related to mental illnesses [1, 2], suggesting that the senseof distinction between the self and others significantly
influences communicating with surrounding people.

© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Shin et al. Journal of Physiological Anthropology(2024) 43:21

Neuroscientific investigations into the mechanisms of
the sense of self have pointed to several key brain regions
around the cortical midline. These include the orbital andadjacent medial prefrontal cortex, the dorsomedial prefrontal cortex, and the anterior and posterior cingulate
cortex, which are thought to play vital roles in self-referential processing [3, 4]. Additionally, collaborates withthese cortical midline structures to form the neural basisof the sense of self [5].
The mirror neuron system (mirror system) is a neuralnetwork that functions when executing or observing anaction. Functional magnetic resonance imaging (fMRI)
studies have confirmed that this system consists primarily of a network of sensorimotor cortical areas, including
the inferior parietal lobule, inferior frontal gyrus, andadjacent ventral premotor areas [6]. The mirror system
is involved in understanding and imitating the actionsof others and is thought to facilitate social learning andcommunication by enabling individuals to simulate andunderstand the intentions and emotions of others [7, 8].
The mirror system’s activity can be examined by using
electroencephalography (EEG). Previous studies haveindicated that the mu rhythm, defined as the 8–13 Hz
rhythm arising around the brain’s central sulcus at rest,
is suppressed when executing an action. Other studies
confirmed that mu suppression also reflects action simulation [9–11]. Moreover, mu suppression occurs whenobserving an action. Therefore, it has often been used
as an effective mirror system activity index, although
its validity is controversial [12–14]. In addition, studies have reported a functional dissociation within thelower (8–10 Hz) and the upper (11–13 Hz) mu suppression bands. For example, one study reported that thelower mu is related to a widespread EEG pattern, nonspecific to movement types. In contrast, the upper mushows a more focused, movement-type-specific pattern,which is different for finger and foot movements [15].
Furthermore, the inequality between lower and uppermu suppression changes when observing and executing
an action [16, 17]. However, it remains unclear whether
these differences are related to the distinctions between
self and others.
While assimilation of self and others is effective in imitation and observational learning, the ability to discriminate between self and others is equally important for anindividual’s social adaptation and harmony with the environment, and the mirror system contributes to both. The
mirror system reacts to own and others’ actions, seemingly assimilating self and others in brain representations.
However, previous fMRI studies have indicated that themirror system’s component regions, mainly in the righthemisphere, selectively responds to self-related stimuli,
including face [18, 19], hands [20], and voice [21, 22].

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The researchers suggested a specific role for the mirror
neuron network in self-other discrimination. In motoror simulation theories, perception is thought to occur
through motor simulation or a mapping of the actionsof others onto one’s motor system. The mirror system
regions may contribute to the construction of communication between individuals via simulation mechanismsthat map the actions of others onto their motor repertoires. It is speculated that mirror system regions suchas the inferior parietal lobule and inferior frontal gyrus
are more strongly activated to one’s image or self-related
stimuli because of the ease with which one can map oneself onto one’s motor system when comparing the self toan external stimulus that is most like oneself. MolnarSzakacs and Uddin proposed a model of the conceptual
sense of self-other when responding to social cognitive
demands, in which there is an interaction between the
network responsible for higher-order mentalizing andthe network responsible for embodiment, including themirror system [23, 24]. This model describes the relationship between empathy as an inference of another’s mental
state and the mirror system as an embodied simulation.
However, the mirror system’s contribution to brain representations of the self and others remains to be examined.
When selecting a method to investigate the relationship between the sense of self and the mirror system, it
is critical to examine whether mu suppression reflects
brain processing of self-other discrimination. The mu
suppression when observing hand movements has
been examined in detail because mirror system activity is associated with motor action, including the effects
of movement types [25–27] and perspectives [28–30].
For instance, Angelini et al. examined brain activity
when observing hand movements from four perspectives [30]. They demonstrated that the most robust mu
suppression occurs when observing movements fromthe first-person perspective compared to other perspectives, perhaps because first-person hand movements are
more likely to generate the sensation of belonging to theself than a third-person movement. As a result, specificstudies have suggested that mu suppression in the firstperson perspective is a response to self-related stimuli.
However, only a few studies have investigated the validity of the contention that perspective differences represent differences between the self and others. Therefore,
the present study investigated the effects of differences
between the self and others on mu power fluctuations byusing the participants’ hand movements as a self-related
stimulus that produces self-other discriminations inpractice. Nagai and Tanaka demonstrated that observing
the action of the own and others’ hands produces morerobust mu suppression for the own hand [31]. However,their study was limited to the first-person perspective.

Shin et al. Journal of Physiological Anthropology(2024) 43:21

Hence, the participants in the current study conducted
self-other discrimination from first- and third-person
perspectives. We predicted that mu suppression to the
own hand would increase under the first- and thirdperson perspectives if mu suppression reactivity wereaffected by the hand’s owner regardless of the perspective. Therefore, this study investigated the extent towhich mu suppression reflects differences between self
and others.

Methods
Participants

Healthy college students (N = 30, 8 women; mean
age = 23.58 ± 1.59 years) participated in this study. All
the participants were right-handed, defined as a laterality quotient of 40 or higher in the 10-item Edinburgh
Handedness Inventory. The participants gave their written informed consent for participating in this study
before the experiment. The Ethics Committee of Kyushu
University approved the study. We excluded the data offive participants from the analysis: two because of excessive and continuous motion artifacts at the measurement
and three because the number of trials remaining afterFig. 1 Experimental protocol, conditions, and stimuli

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removing trials in the EEG processing process was lessthan the required number (≤ 20 trials per condition).
Equipment

The study was conducted in an acoustically and electrically sealed room. Stimuli were delivered with Presentation Ver. 18.2 (NBS Inc.) and an LCD (RL2460-B, BenQ
Corp.) refreshing on 60 Hz. The display was placed 60 cm
in front of participants. EEG was recorded by using a
64-channel EEG System (Net Amps 200, Electrical Geodesics Inc.) and a sensor net (Hydrocel Geodesics Sensor
Net, EGI) with acquisition software (Net Station 4.3.1,
EGI).
Stimuli

The stimuli consisted of video clips (each clip 2500 ms
in length) showing right-hand movements that we
presented from two perspectives (Fig. 1). Two actors,
a man and a woman, performed the action in half thevideo clips. The participants performed the actionin the other half. Before the experiment, the following simple hand movements of the participants wererecorded and edited as stimuli: pointing to a black dot
on a table and clenching hand. A metronome controlled

Shin et al. Journal of Physiological Anthropology(2024) 43:21

the movement’s speed. Two cameras, one facing theother at an identical height, recorded the action. Eachvideo clip comprised the following sequence: the Still
Hand (750 ms), a black dot and the hand in a resting
position; movement (1500 ms); static end (250 ms), andthe static hand in the final position. The experiment
comprised four conditions: 2 hand owners × 2 perspectives: (1) own hand in the first-person perspective(egocentric), (2) others’ hand in the first-person perspective, (3) own hand in the third-person perspective(allocentric), and (4) others’ hand in the third-person
perspective.
Experimental procedure

The participants sat facing a monitor in front of them.
We fitted an EEG cap on the participants’ heads andinstructed them to observe the video with their righthand relaxing on a mouse. The experimental procedure included 120 trials (30 for each condition × 4
conditions) presented in four blocks. As shown in
Fig. 1, each trial included (1) a gray screen (presented between 3000 and 3500 ms), (2) a central fixation cross (presented for 1000 ms), and (3) the video
clip (presented for 2500 ms). Each block contained
the clips seen from only one perspective (egocentric
or allocentric). We fully randomized the blocks andtrial sequences and asked the participants to respondto randomly presented image trials showing the still
hand resting by clicking the mouse. These image trials
appeared three to six times in the blocks we excluded
from the subsequent analysis. After each block, participants responded to a questionnaire assessing how
well they could distinguish themselves from others
using a 5-point scale (subjective evaluation), and we
also assessed the participants’ sleepiness degree using
the Karolinska Sleepiness Scale (KSS). We did not ask
the participants to discriminate between the self andothers during the video observation trials to avoid
artifacts. However, we confirmed they could discriminate between themselves and others by asking them toreview all the video clips and classify them as a self orothers discrimination task after all video observation
blocks. The participants also completed the Multidimensional Empathy Scale (MES) designed to discriminate between self and other orientations of cognitiveand emotional components. The MES consists of fivesubscales: other-oriented emotional reactivity, selforiented emotional reactivity, emotional susceptibility, perspective-taking, and fantasy. We planned to use
the MES results for a different study that will examine
the relationship between mu suppression and empathic
characteristics shown in previous study [32].

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EEG data acquisition and analysis

We recorded 64 channels of EEG at a sampling rate of
500 Hz (0.01 Hz high-pass filter) with a vertex reference.
The impedance of the electrodes was maintained below
60 kΩ. Offline analyses were conducted using MATLAB(MathWorks, Inc.) and EEGLAB toolboxes [33]. The
EEG data were filtered (1–35 Hz) and segmented into
single-trial epochs of 4500 ms. Each epoch consisted ofthree segments: (1) a gray screen presented before thefixation cross for 1000 ms, (2) the fixation cross displayedfor 1000 ms, and (3) the video clip lasting 2500 ms. We
used independent component analysis (ICA) to remove
ocular, cardiac, and muscular artifacts from the EEG
data. Time-frequency transforms were computed foreach electrode in the 5 to 32 Hz frequency range for eachcleaned epoch using a Morlet wavelet. Spectral data werebaseline-corrected to account for individual variability inoverall EEG power. Based on previous studies [26, 30],
we used a 300-ms pre-stimulus period within the grayscreen (from − 1650 to − 1350 ms before video clip onset)
as the baseline. We baseline-corrected the spectral dataobtained from each participant and condition by dividing
the value of each time-frequency point by the mean spectral power of the baseline at the same frequency. Basedon the literature [30], we used the central cluster aroundC3 and C4 for mu rhythm analyses, as shown in Fig. 2,
and selected the lower (8–10 Hz) and upper (11–13 Hz)
alpha bands. We averaged the ratio data across participants, conditions, and the left and right central clustersof electrodes (Fig. 3a) based on the evidence that murhythm subcomponents may have different functional
properties. Then, we averaged the data from the movement section of the video clip (1500 ms, Fig. 3b). The
still section was excluded from the average. Before statistical analysis, we applied a log10 transformation toeach absolute power ratio value. A zero log10 ratio value
indicated no EEG modulation, whereas a negative log10
ratio indicated EEG power suppression. We statisticallyanalyzed the mean powers of each cluster as the dependent variables using a within-subjects, three-way repeatedmeasures analysis of variance (ANOVA) with the band
(lower, upper), hand owner (self, others), and perspective(first-person, third-person) as the independent variables.
After a significant primary interaction, we conducted theSequentially Rejective Bonferroni-corrected post hoc
test.

Complementary analyses
The occipital cluster (around Oz; Fig. 2) was used tocheck for the possible influence of volume conduction from visual cortices on the alpha-band range. As aresult, there were no differences between conditions inShin et al. Journal of Physiological Anthropology(2024) 43:21

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Fig. 2 Clusters of electrodes were used for statistical analyses. The central cluster of the 64-channel array is a grouping of electrodes around C3and C4 (marked green). The occipital cluster is a grouping of electrodes adjacent to Oz (marked blue)

occipital alpha suppression, confirming the possibility
that the results of this study reflect the activity and characteristics of central regions. Please see “Occipital alphasuppression” in the Supplementary Information for further details of the occipital cluster’s results. There was no
between-condition difference.

Results
The mean correct response rate for the self-other discrimination tasks conducted after completing the observation tasks was 95%, confirming that participants could
accurately distinguish their hands from another person’s(Table 1).
Mu suppression

Results indicated a significant main effect of band
(F(1,24) = 7.099, p = 0.013, ηp2 = 0.23), showing an overall more potent suppression of lower mu. There were no
significant main effects for hand owner (F(1,24) = 0.0062,
p = 0.94, ηp2 = 0.0003) or perspective (F(1,24) = 0.062,

p = 0.80, ηp2 = 0.0026). Additionally, there were no significant interactions involving band, including the three-way
interaction between band, hand owner, and perspective(F(1,24) = 0.36, p = 0.55, ηp2 = 0.015).
There was a significant interaction between hand ownerand perspective (F(1,24) = 4.82, p = 0.038, ηp2 = 0.17). Post
hoc analyses of the interaction showed that the simple
main effect of hand owner was significant in the firstperson perspective (F(1,24) = 4.52, p = 0.044, ηp2 = 0.16),
with mu suppression for the own hand being more potent
than for the others’ hand in the first-person perspective(Fig. 3b).

Discussion
This study examined whether mu suppression, indicating the brain’s information processing about the self
and the mirror system’s activity, reflected differencesin cognitions associated with self-other discrimination.
Several studies have examined the relationship between
mu suppression and observing action, including theShin et al. Journal of Physiological Anthropology(2024) 43:21

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Fig. 3 Mu (8–13 Hz) power suppression. a The log10 power ratio’s time course for each condition. The elements within an epoch (gray backgroundas the baseline, the fixation cross, and the video clip including still and moving parts) are labeled in different colors. b Mean (-SE) log10 power
ratio when observing the moving part of video clips under different conditions. The graph represents the significant hand owner × perspective
interaction in the 2 × 2 repeated measures ANOVA. Asterisks indicate a significant difference among perspectives

Table 1 Mean accuracy (standard errors) of own/others’ handrecognition
1st-p pers

3rd-p.pers

Own hand1.00 (0.00)

0.98 (0.08)

Other hand0.95 (0.12)

0.91 (0.19)

Overall

0.95 (0.09)

first-person perspective as the self-viewpoint. However,few studies have investigated the hypothesis that perspective differences represent differences between theself and others. We investigated EEG activity related
to self-detection using participants’ hand movements
as self-relevant stimuli and examined the perspectivedependent mu suppression. The study analyzed mu
suppression in the lower- and upper-frequency bands
and examined differences based on hand ownershipand perspective. The results did not indicate a significant interaction between the three factors, band, handowner, and perspective, which precluded conclusions
about the band-specific effects of self-other discrimination. However, a significant interaction between handowner and perspective and a main effect of band wasfound. Therefore, we combined the bands and performed the post hoc test to examine the simple main
effect of hand owner and perspective on the mean mu
suppression at 8–13 Hz. The results confirmed thatmu suppression for the own hand was stronger thanfor others’ hand in the first-person perspective. However, there was no significant difference in suppression
between own and others’ hands in the third-person
perspective.
The significant self-oriented mu suppression reactivity in the first-person perspective was found, while nosignificant response orientation was found in the thirdperson perspective. The results suggest that the reactivity of mu suppression to self and others differs dependingon perspective. Nagai and Tanaka observed self andanother person’s hand movements with motor imagery
and demonstrated a more extensive suppression for theirown hand than for another person’s hand [31]. Macuga
and Frey’s fMRI study investigated brain activity contributing to self-other recognition by using videos of hands
color-coded by gloves [20]. The results showed that theright inferior frontal and supramarginal gyri, constituent
regions of the mirror system, were selectively activatedShin et al. Journal of Physiological Anthropology(2024) 43:21

when observing the own hand compared to the other’shand, which might reflect processing, indicating characteristics of self-other discrimination. Based on previous
findings, we predicted that the hand’s owner would affectmu suppression reactivity, regardless of the perspective,
such that mu suppression would increase to the own
hand in either viewpoint. However, the results showedthat perspective is relevant for mu suppression, contrary
to the prediction. Conson et al. examined self-other discrimination reaction times when own hand or anotherperson’s hand picture were presented in the two perspectives. They reported that the reaction time in the thirdperson perspective was longer than in the first-person
perspective [34]. Moreover, the relationship between thereaction time and correct answer rate differed between
the perspectives. The time for detecting the self wasshorter than that for detecting others in the first-person
perspective, whereas the time for detecting others was
shorter than that for detecting the self in the third-person perspective. However, the correct answer rate forself-identification was higher in both perspectives. The
finding that responses are faster for the others’ hand in
the third-person perspective, whereas the discrimination
accuracy is lower than that for the own hand because
the others’ hand was easily confused with the own hand,suggests the hypothesis that the self-detection system
discriminates better than the others detection system.
Based on this hypothesis, we speculated that the fast andaccurate self-detection processing in the first-person perspective would result in a pronounced self-orientated mu
suppression. However, the third-person perspective isunfamiliar because we do not usually see our hands fromthat perspective. The results can also be tentative interpreted as suggesting that the mu suppression discriminates between plausible and non-plausible movement.
This would correspond to the mirroring of observed
others actions to our motor repertoire. Further study isneeded to determine whether the self-detection process
or plausibility detection predominates in the expressionof mu suppression.
This study indicated that lower and upper mu areunrelated to self and other differences or the perspective, although previous studies have shown that lower
and upper mu have functional dissociation in the bandwidth. Focal damage to the right inferior parietal lobe, amirror system component, selectively reduces the magnitude of mu suppression but not the upper mu range[35]. Moreover, EEG-fMRI studies of upper mu suppression have demonstrated correlations with regions
engaged with the frontoparietal network associatedwith several cognitive processes [36]. Previous studies
suggest the possibility that divided mu band responses
reflect specific mirror system activities or cognitive

Page 7 of 9

activities associated with self-other discrimination.
However, this experiment, designed to combine motor
representations and self-other discrimination, might
have yet to detect band-specific response stimuli.
The following limitations constrain the results ofthe present study. The first is whether the selectionof baseline interval is appropriate. The present study
used the pre-fixation baseline instead of the fixation
baseline. The fixation cross is presented to stabilize
measured data by focusing the gaze on a fixed point.
However, in previous studies [26, 30], power suppression and rebound occurred during this fixation crossperiod, so the pre-fixation was selected as the baseline for stability. As a result, the mu suppression looks
to differ slightly among conditions already in the segment of the fixation cross. In the statistical analysis, a
two-way ANOVA with hand owner and perspective asfactors for mean mu suppression during the fixation
cross period (− 1 s to 0 s) showed no significant effects
nor interactions (p > 0.1). Although there was no difference in the statistical analysis, uneven mu suppression
and rebound were observed during the fixation presentation. Distinguishing mu suppression due to action
observation from mu suppression due to the fixation
period requires further study after considering differences in the mechanisms of their occurrence. Moreover,
we did not measure self-other discrimination responses
during stimulus observation to prevent artifacts. We
suggest that future studies measure responses while theparticipants are making observations to examine thecorrelation more concretely between mu suppression
and self-other discrimination accuracy. Furthermore,
the present study suggested that mu suppression mightreflect the distinction between the self and others.
However, a limitation of EEG studies is that they cannot clarify whether a particular reactivity intrinsic to aneural network, such as the mirror system, generates
mu suppression or whether an input external to processing generates the suppression. We expect simultaneous verification using EEG and fMRI measurements
to clarify this issue.

Conclusions
In summary, this study examined the mirror system’s
activity for self and others using the EEG’s mu suppression. As a result, it was suggested that differences
in self and others or perspectives may influence mu
suppression.
Abbreviations
ANOVA	Analysis of variance
EEG	Electroencephalogram
fMRI	Functional magnetic resonance imaging
KSS	Karolinska Sleepiness Scale

Shin et al. Journal of Physiological Anthropology(2024) 43:21

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40101-​024-​00369-0.

Page 8 of 99.
10.

Supplementary Material 1.
11.
Acknowledgements
The authors appreciate Nagi Uemura and Yumeko Uemura for recording the
data.

12.

Authors’ contributions
NS conceived the idea of this study, and YI substantially contributed to designing the experiment. NS performed the data acquisition and analyses andwrote the paper. YI, YM, and SH significantly contributed to the data analysis
and interpretation. All authors critically reviewed and revised the manuscript
and approved the final version for submission.

13.

Funding
This work was supported by JSPS KAKENHI Grant Number JP22J13177.

15.

Availability of data and materials
The datasets used and analyzed during the current study are available fromthe corresponding author upon reasonable request.

16.

Declarations
Ethics approval and consent to participate
All participants gave their written informed consent for participating in thisstudy before the experiment. The study was approved by Ethics Committee of
Kyushu University.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.

14.

17.

18.

19.
20.

Received: 11 April 2024 Accepted: 11 August 2024
21.

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