Ikeda et al. Journal of Physiological Anthropology
https://doi.org/10.1186/s40101-018-0192-8(2019) 38:1

ORIGINAL ARTICLE

Open Access

Effects of the differences in mental states on
the mirror system activities when observing
hand actions
Yuki Ikeda1,2* , Yuki Nishimura1,2 and Shigekazu Higuchi3

Abstract
Background: It is known that the activities of the mirror system are related to imitation and understanding of theintention of an action. It has been reported that the activity of the mirror system is higher for observations forimitating and understanding the intention of an action than for simple observations. However, observations thatfacilitate the mirror system’s activities, if they are observations intending to imitate an action or observations forunderstanding the intention of an action, have not been clarified to date.
Methods: The types of observations of actions that highly facilitate mirror system activities were investigated.
Participants were right-handed university students (N = 23). They observed videos showing hand actions
following three types of instructions: (1) to observe the videos intending to understand the intention of theaction (action understanding, AU), to observe the videos intending to imitate the hand action (imaginarily
imitation, II), and to observe the videos without any intention (observation, OB). Brain waves during observation were
measured, and the suppression rate of 8–10 Hz (lower mu/α) and 10–12 Hz (upper mu/α) in the central and occipitalregions of the brain was calculated. The rate of suppression was compared among the conditions using a repeatedmeasures analysis of variance for each region.
Results: There was a main effect of the condition in the central region in 10–12 Hz. The degree of suppression in the
AU condition was significantly larger than SO condition (p < 0.05) and II condition (p < 0.1). However, there were nodifferences among conditions in 8–10 Hz, the occipital region, or in either frequency band.
Conclusions: These results suggest that activities of the mirror system are enhanced when observing an action withthe purpose of understanding the intention of the action. Differences in the mirror system activities according to thechanges of inner states might be better reflected in high-frequency mu waves.
Keywords: Mental states, Mirror system, Mu suppression

BackgroundA mirror neuron is a neuron that fires when both performing and observing an action, which were discoveredin the area F5 of a macaque monkey [1]. The discoveryof this neuron suggested that the processes of performing and perceiving an action might be shared in thebrain. Studies using the transcranial magnetic stimulation [2], brain-imaging studies [3, 4], and studies using* Correspondence: yuki.ikeda.430@s.kyushu-u.ac.jp
1
Graduate School of Integrated Frontier Sciences, Kyushu University, 4-9-1
Shiobaru, Minami-ku, Fukuoka City, Fukuoka 8158540, Japan
2
Japan Society for the Promotion of Science, Kojimachi Business Center
Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo, Japan
Full list of author information is available at the end of the article

electroencephalogram (EEG) [5] have indicated that activation of the motor-related areas also occurs in humanbeings when observing another person’s action. The nervous system related to this phenomenon is called themirror neuron system or mirror system. Brain regions
such as the inferior frontal gyrus, inferior parietal lobule,and superior temporal sulcus are known to compose themirror system [6]. The mirror system has been identified
as the neural basis supporting imitation and understanding of other people’s intentions, which is a basic function
required by a society [7].
Among brain wave components, the rhythms between 8
and 13 Hz that occur around the central sulcus are called© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. 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.

Ikeda et al. Journal of Physiological Anthropology(2019) 38:1

mu waves. The mu wave is suppressed not only when performing an action but also when observing another person’s action. Therefore, it is used as an index of mirror
system activity [8–10]. Previous studies examining correlations between functional magnetic resonance imaging
and brain waves have indicated correlations between theincrease in the activity rate of the inferior parietal lobule,superior parietal lobule, and the dorsal premotor cortex
and the decreased rate of activity in the α-band power inthe central region compared between the time of observing an action and that of performing an action [11, 12].
The brain regions showing the correlations corresponded
to the areas composing mirror system, suggesting that mu
waves might reflect mirror system activities. Fox et al. discussed the validity of a mu wave as an index of the mirror
system through meta-analysis and concluded that a mu
wave is regarded as an index with a certain level of reliability [13]. On the other hand, it has been indicated thatmu waves might be affected by the α waves appearing in
the occipital region, and careful examination is required[12]. Moreover, it has been suggested that mu waves reflect different functions between the lower band (8–10Hz) and the upper band (10–12 Hz). Pfurtscheller et al. indicated that mu suppression widely occurred on the somatic sensory cortex in the lower band, regardless of thetype of movement, whereas in the upper band, the suppression occurred more limitedly, depending on the typeof arm movement [14]. Furthermore, it has been reported
that the mu wave upper band shows specific responses,
correlated with object-oriented actions [15], social interactions [16], and actions with high target orientation using
hands or tools [17].
The mirror system is said to be involved in imitationof an action and understanding of the intention of an action. Caspers et al. conducted a meta-analysis of thebrain regions that were activated when observing an action aiming to imitate it and when observing it without
any intention, indicating many of the activated brain regions were consistent [18]. This study also suggested
that mirror system activities were higher when observing
with an intention of imitation. Moreover, Iacoboni et al.
indicated the mirror system was activated in both simple
observation and observation aiming to understand theintention of an action [19]. Furthermore, Perry et al. reported that mu suppression increased when understanding a social context based on an action, compared towhen judging gender [20]. As described above, the levelof mirror system activation when observing an action isconsidered to change depending on the inner state, i.e.,
whether intending to imitate the action or intending tounderstand the intention of the action. Previous studies
compared the changes of mirror system activities withthe control condition, i.e., in observation intending toimitate an action and in simple observation, as well as inPage 2 of 7

observation intending to understand the intention of anaction and in performing the action. It has not beenclarified yet which inner state might more facilitate mirror system activities.
This study examined what type of action observation
might most facilitate mirror system activities, throughrequesting participants to observe hand actions aiming toimitate the actions, to observe aiming to understand theaction intention, and to observe without any intention.

Methods
Participants

Right-handed university students (N = 23, 15 males, and8 females, mean age = 23.1 ± 1.0) participated in the experiment. The dominant hand was confirmed using the
Edinburgh Handedness Inventory. Moreover, personality
traits were examined using the Japanese version of theInterpersonal Reactivity Index [21], and there were no
participants having extreme personality traits. Explanations about the experiment were provided in advance,
and written consent for participating in the experiment
was obtained. This experiment was conducted after
obtaining approval from the ethics committee of the
Graduate School of Design, Kyushu University, following
the Declaration of Helsinki.
Experimental conditions and procedures

Three conditions were prepared when observing the videos: (1) to actively observe the videos thinking about theintention of the action (action understanding, AU), (2)to actively observe the videos intending to imitate theaction (imaginarily imitation, II), and (3) to passively observe the videos without any intention (simple observation, SO). The following instructions were given to theparticipants before showing the videos. AU condition:“Please watch the videos with thinking about in what
context the action is performed.” II condition: “Pleasewatch the videos intending to imitate the action, without
moving your hands.” SO condition: “Please just observe
the action.” In AU condition, to make participants actively observe the video, they were encouraged to remember inferred intentions of action as many as
possible. This was ensured by telling them beforehandthat we will ask about the intentions which they cameup with at the end of the condition.
The observation was conducted three times. Prior tothe observation, one of the three instructions was given
to the participants. The video consisted of a static image
for 4 s (start frame), a moving image for 2 s, and a static
image for 2 s (last frame), referring to the presentation
method suggested by Hobson and Bishop [22] (Fig. 1).
The video was edited using Adobe Premiere Pro CC2018 and repeatedly shown 30 times. Three types of actions using the right hand and a sponge were presented:

Ikeda et al. Journal of Physiological Anthropology(2019) 38:1

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Fig. 1 An example of the video and the ways of presentation

picking up a white sponge with the index finger andmiddle finger, tapping the upper side of the sponge with
the index finger, and rubbing the desk with the sponge.
The combination of the video and instruction was randomly set depending on the participant.
Experimental procedures were as follows: after obtaining the response to IRI, electrodes for electroencephalogram (EEG) were attached to the participants’ scalp, andvideo observation was conducted under three types of
instructions. After finishing observation under each condition, participants answered a questionnaire. Question
items for subjective assessment were as follows: (1) “To
what extent could you follow the instruction?” (2) “To
what extent could you concentrate on the task?” (3)“How much did you feel sleepy?” and (4) “Have you everexperienced the action you observed?” Regarding (1),(2), and (3), the visual analog scale (VAS) was used forthe response. Under the AU condition, participants
freely described (5) “What did you think was theintention of the action?”
EEG measurement and analysis

Brain waves were measured using 64ch EEG (Net Amps
200, EGI), with the setting of sampling 500 Hz andhigh-pass filter 0.01 Hz. Cz was used as a reference electrode. Following the operation manual, impedance wasmaintained under 50 kΩ. Stimuli were presented using
Presentation Ver. 20.0 (NBS Inc.) and a 23-in. display(LG Electronics).
MATLAB 2017b (Mathworks, Inc.) and EEGLAB
v14.1.2 [23] were used for analysis. As pretreatment,
data obtained through measurement was filtered using aband-pass filter. Bad channels were excluded using
clean_rawdata plugin of EEGLAB, which is a plug-in
software of EEGLAB, and data were complemented
using other channels. The reference type was changed to
a common average. Epochs including values exceeding± 500 μV and epochs including values exceeding 6 SD at
single channel as well as including values exceeding 2
SD at all the channels were excluded. Data were analyzed using independent component analysis. Amongthe obtained components, electrooculogram, electrocardiogram, and components that obviously seem artifacts
were excluded by visual assessment.
After completing the process above, event-related
spectral perturbation (ERSP) was calculated with regard
to the 2-s static image before starting the moving image
as the baseline. The frequency band of mu waves, which
are the analysis subjects, was divided into 8–10 Hz and
10–12 Hz, referring to Pfurtscheller et al. [14]. Based on
ERSP, mean values were calculated in 8–10 Hz and 10–
12 Hz respectively. Moreover, the mean value during 2 sof presenting the moving image was calculated. Here,two participants that showed outliers were excluded.
The regions of interest were the right and left of thecentral region where mu waves were observed and theoccipital region where α waves were observed. The meanvalues of the ERSP of plural electrodes were calculated(Fig. 2). The ERSP values obtained in the right and leftof the central region were regarded as mu suppression,and those obtained in the occipital region were regarded
as α suppression.
Statistics

The results of the questionnaires were analyzed using
one-way analysis of variance, with regard to the condition as a factor depending on the question item, andcomparison was made among three conditions. Mu suppression and α suppression were examined using a
one-sample t test, regarding whether suppression ataction observation significantly occurred compared tothe baseline in the right/left side of the central region
and in the occipital region respectively. Next, theamount of suppression was compared among threeIkeda et al. Journal of Physiological Anthropology(2019) 38:1

Fig. 2 Layout of electrodes. 15, 16, 20, 21, and 22 were used in theleft central region; 41, 49, 50, 51, and 53 were used in the right central
region; and 35, 37, and 39 were used in the occipital region

conditions using analysis of variance, in each region ofinterest. Since the stimuli were actions performed by theright hand, the central region was analyzed using twoway repeated measures analysis of variance, with theposition (right/left) and the condition as factors. Regarding the occipital region, one-way repeated measures
analysis of variance was conducted with the condition asa factor. For adjusting multiple comparisons, which is a
sub-effect test, the Bonferroni correction was used.

Results
Subjective assessment using VAS

No differences were shown among three conditionsin (1) engagement in the task, (2) attention to thetask, and (3) sleepiness (Table 1). One participantthat answered he/she had no experience of an action(4. experience of an action) and one participant thatreported excessive sleepiness (3. sleepiness) were excluded from the analysis subjects.
mu suppression when observing the videos (the central
region)The results of a one-sample t test in the central region indicated significant mu (10–12 Hz) suppression compared

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to the baseline in the left side under AU and SO conditions (AU: p < 0.001; SO: p = 0.012). Under II condition,
suppression tended to be significant (p = 0.053). In theright side, mu suppression compared to the baseline wasalso significant under AU and SO conditions (AU: p <0.001; SO: p = 0.031). Under II condition, suppression
tended to be significant (p = 0.069). Next, the results of
two-way analysis of variance (conditions and right/left positions as factors) indicated a significant main effect of thecondition (F(2,36) = 3.67, p = 0.035). Main effects of theright/left positions as well as interactions between theright/left positions and conditions were not indicated.
Therefore, a sub-effect test was conducted using the meanvalues of the right/left position, indicating that the suppression amount under AU condition was larger than that
under SO condition (t(18) = 3.09, p = 0.019). Moreover,the suppression amount under AU condition tended to belarger than that under II condition (t(18) = 1.91, p = 0.072)(Fig. 3, left).
The results of a one-sample t test in the central region indicated significant mu (8–10 Hz) suppression
in the left side under all the conditions (p < 0.001).
Significant suppression was also indicated in the rightside under all the conditions (AU, SO: p < 0.001; II:
p < 0.001). Two-way analysis of variance (conditions
and right/left positions as factors) indicated that main
effects of the conditions and right/left positions aswell as interactions between the two factors were notsignificant (Fig. 3, right).
α suppression when observing the videos (the occipitalregion)The results of a one-sample t test in the occipital region
indicated significant α (10–12 Hz) suppression under all
the conditions (AU, SO: p < 0.001; II: p < 0.001). Moreover, the results of one-way analysis of variance (thecondition as a factor) did not indicate significant main
effects of the condition. The results of a one-sample ttest in the occipital region indicated significant α (8–10
Hz) suppression under all the conditions (AU: p < 0.001;
II: p = 0.001; SO: p = 0.003). Moreover, the results of
one-way analysis of variance (the condition as a factor)
did not indicate significant main effects of the condition.

Discussion
This study examined what type of action observation
would most facilitate mirror system activities using brain

Table 1 Subjective assessment using VAS (mm, mean (SD), n = 19)
AU

II

SO

F ratio, p value

1. Engagement

77.9 (16.1)

75.8 (14.2)

71.6 (16.8)

F(2,36) = 1.03, p = 0.37

2. Attention

77.4 (16.3)

73.8 (17.5)

72.5 (16.0)

F(2,36) = 1.01, p = 0.38

3. Sleepness

32.7 (24.8)

37.2 (26.3)

35.5 (26.2)

F(2,36) = 0.92, p = 0.41

Ikeda et al. Journal of Physiological Anthropology(2019) 38:1

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Fig. 3 Mu suppression in the central region when observing the videos

waves. Comparison of the suppression amount of murhythms (8–10 Hz/10–12 Hz) when observing videos
among three conditions, i.e., AU, II, and SO conditions, indicated that the suppression amount of mu(10–12 Hz) was largest in the central region under
AU condition, compared to SO condition. Moreover,the suppression amount under AU condition was larger than that under II condition. AU condition includes the perception of social information, i.e., theconjecture of another person’s intention. Perry et al.
reported mu suppression more increased when perceiving social context, compared to when judging thegender of the observation target [20]. It might bepossible that mirror system activities were facilitatedby a higher level request, i.e., perceiving social context, compared to SO or II condition.
In this study, participants observed the videos freely,
without any limitation in the number of intentions of
the action conceived under AU condition. The direct
matching hypothesis, which explains the mechanism ofaction understanding, suggests that neurons in the ventral premotor cortex encode the goal of an action, andthe goal is represented by the motor system of an observer when observing another person’s action, and thus
the action is understood [24]. Under AU condition,
more actions are represented in the brain, compared to
II condition (imitating an action) and SO condition(simple observation), and mu suppression is supposed to
be facilitated.
No significant differences were indicated between
II and SO conditions. Some previous studies reported the activation level increased when observing
with intending imitation, compared to simple observation [25, 26]. In these studies, imitation was performed during or after observing an action. In thepresent study, on the other hand, imitation was not
performed just after observation, and only an instruction to observe with intending imitation wasgiven. Since there were no differences among theconditions in the engagement in and attention to thetask, participants are supposed to have watched thevideos sufficiently following the instruction. However,
the effect of the II condition might have been decreased because imitation was not performed.
Differences among the conditions were observed in
the high-frequency band (10–12 Hz), whereas no differences were shown in the low-frequency band (8–10 Hz). The stimuli given in this study were actionsof the right hand using a sponge by the fingers.
High-frequency components of mu waves are considered to reflect finer movements, compared to
low-frequency components [14]. Therefore, differences between the conditions might have moreclearly reflected in 10–12 Hz. Moreover, there aresome reports that social cognitive abilities reflected
in mu waves are observed in the high-frequency
band. For example, Naeem et al. indicated specificpatterns in 10–12 Hz when intentional social coordination was made [16]. Furthermore, Hudac et al. reported that mu suppression patterns shown byautism spectrum disorder patients when observing
an action were different from healthy controls, especially in 10–12 Hz [27]. Since AU condition requires
social cognition, differences among conditions are
considered to have been observed in 10–12 Hz.
On the other hand, significant mu suppression occurred also in 8–10 Hz during action observation. It
might be possible that mu suppression by mirror system
activation occurred also in the low-frequency band.
Some studies suggested that mirror system activities are
more reflected in the low-frequency band [28]. Further
examination is required.
In the occipital region, no suppression was indicated in either high- or low-frequency band. Mu suppression when observing an action might be affectedby α wave suppression in the occipital region, which
fluctuates depending on the input of visual stimuli or
attention [22]. The present study did not indicate thedifferences among conditions in the occipital region,
which might increase the possibility that the resultsin the central region might be produced by mirror
system activities.

Ikeda et al. Journal of Physiological Anthropology(2019) 38:1

Conclusions
It was indicated that mu suppression when observing anaction aiming to understand the intention of the action
was larger, compared to simple observation as well as
when observing an action aiming to imitate the action.
It was suggested that mirror system activities are mostactivated when observing an action aiming to understand the intention of the action. Moreover, this tendency was shown only in 10–12 Hz and not shown in
8–10 Hz. The results above suggest that differences in
the mirror system activity levels depending on the innerstate might be shown in mu high-frequency band. In the
future mirror system studies, differences depending on
the frequency band should be examined in detail.
Abbreviations
AU: Action understanding; EEG: Electroencephalogram; ERSP: Event-related
spectral perturbation; II: Imaginarily imitation; SO: Simple observation; VAS: Visual
analog scale
Acknowledgements
We thank Mr. Fujiwara and other laboratory members for their support.
Funding
This work was supported by the Japan Society for the Promotion of Science(17J02763).
Availability of data and materials
The datasets used and/or analyzed during the current study are available fromthe corresponding author on reasonable request.
Authors’ contributions
YI conceived and designed the study, performed the experiments and thestatistical analysis, and drafted the manuscript. YN helped to carry out theexperiments and analyze the data. YN and SH participated in designing andcoordinating the experiment and helped to draft the manuscript. All authors
have read and approved the final manuscript.
Ethics approval and consent to participate
All participants gave their written informed consent after a complete explanationof this study before participating in the study. The study was performedin accordance with the guidelines of the Declaration of Helsinki and wasapproved by the ethics committee of the Kyushu University.
Consent for publication
Not applicableCompeting interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
Graduate School of Integrated Frontier Sciences, Kyushu University, 4-9-1
Shiobaru, Minami-ku, Fukuoka City, Fukuoka 8158540, Japan. 2Japan Society
for the Promotion of Science, Kojimachi Business Center Building, 5-3-1
Kojimachi, Chiyoda-ku, Tokyo, Japan. 3Department of Human Science, Facultyof Design, Kyushu University, 4-9-1 Shiobaru, Minami-ku, Fukuoka City,
Fukuoka, Japan.

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Received: 23 October 2018 Accepted: 17 December 2018

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