Table 1.

List of the studies mentioned with EEG-based hyperscanning methodologies.

Research area	Authors, year, journal	Task instructions	Number of subjects	Results
Joint action	Balconi et al., 2015, Brain and Cognition	Participants were required to observe affective pictures during EEG recording, and they should attend to them the entire time of exposition.	20	An increased theta activity for negative stimuli in the right more than in the left side was observed.
	Balconi and Vanutelli, 2016, frontiers in Psychology	Participants were required to develop a strategy to obtain a better outcome than a competitor (in term of error rate, and response time, RT).	24	A decreased left alpha activity (increased brain response) for post-feedback compared to pre-feedback condition was observed.
	Dumas et al., 2010, PLoS ONE	Participants were engaged in spontaneous imitation of hand movements.	18	Symmetrical increase in PLV was found between the right parietal regions of the model (CP6, P8) and of the imitator (CP6, P4, P8) in the alpha-mu frequency band. The central region (FC1, Cz) of the model's brain and the parieto-occipital brain region (P8, PO2, PO10) of the imitator were synchronized in the beta frequency band. A wide frontal central area (F4, FC2, Czar, C4, CP6) of the model's brain was synchronized with the parietal area (CP2, PZ, P4, P8, PO2, PO10) of the imitator's brain for the gamma frequency band.
	Kawasaki et al., 2013, Scientific Reports	Alternating speech tasks.	40	Speech rhythms were more likely to become synchronized in human–human tasks than human–machine tasks. Moreover, theta/alpha (6–12 Hz) amplitudes synchronized in the same temporal and lateral-parietal regions in each pair. Behavioral and inter-brain synchronizations were enhanced after human–machine tasks.
	Konvalinka et al., 2014, NeuroImage	A synchronized finger-tapping task.	18	The interactive condition was characterized by a stronger suppression of alpha and low-beta oscillations over motor and frontal areas in contrast to the non-interactive computer condition.
	Kuhlen et al., 2012, frontiers in Human Neuroscience	Speakers were given the task to make the stories interesting and fun for future listeners to listen to.	12	The EEG is more similar among listeners attending to the same speaker than among listeners attending to different speakers, indicating that listeners' EEG reflects content-specific information. Listeners' EEG activity correlates with the attended speakers' EEG, peaking at a time delay of about 12.5 s. This correlation takes place not only between homologous, but also between non-homologous brain areas in speakers and listeners.
	Lindenberger et al., 2009, BMC Neuroscience	Eight pairs of guitarists played a short melody together.	18	Phase synchronization both within and between brains increased significantly during the periods of preparatory metronome tempo setting and coordinated play onset. Phase alignment extracted from within-brain dynamics was related to behavioral play onset asynchrony between guitarists.
	Ménoret et al., 2014, Neuropsychologia	Participants were instructed to perform object-directed movements toward one of three different objects: a box, a saucer and a candle-holder.	40	For the observer, an observation related motor related potentials was measured in all conditions but was more negative in the interactive context over fronto-central electrodes. Moreover, this feature was specific to biological actions. Concurrently, the suppression of beta oscillations was observed in the actor's EEG and the observer's EEG rapidly after the onset of the actor's movement. Critically, this suppression was stronger in the interactive than in the non-interactive context despite the fact that movement kinematics did not differ in the two context conditions.
	Sänger et al., 2012, frontiers in Human Neuroscience	Participants sat face-to-face to each other. One participant was assigned the leading role, meaning that he or she was responsible for bringing the other in and determining the playing tempo. The follower was asked to exclusively orient himself toward the leader.	22	Phase locking as well as within-brain and between-brain phase-coherence connection strengths were enhanced at frontal and central electrodes during periods that put particularly high demands on musical coordination. Phase locking was modulated in relation to the experimentally assigned musical roles of leader and follower, corroborating the functional significance of synchronous oscillations in dyadic music performance. Graph theory analyses revealed within-brain and hyperbrain networks with small-worldness properties that were enhanced during musical coordination periods, and community structures encompassing electrodes from both brains (hyper-brain modules).
	Tognoli et al., 2007, Proceedings of the National Academy of Sciences of the United States of America	Pairs of subjects sat in front of each other while executing self-paced rhythmic finger movements during 1-min trials.	16	High-resolution spectral analysis of electrical brain activity before and during visually mediated social coordination revealed a marked depression in occipital alpha and rolandic mu rhythms during social interaction that was independent of whether behavior was coordinated or not. In contrast, a pair of oscillatory components (phi1 and phi2) located above right centro-parietal cortex distinguished effective from ineffective coordination: increase of phi1 favored independent behavior and increase of phi2 favored coordinated behavior.
	Yun et al., 2012, Scientific Reports	Participants were instructed to look at the other participant's finger while holding his own finger as stationary as possible.	20	Synchrony of both fingertip movement and neural activity between the two participants increased after cooperative interaction.
Shared attention	Dikker et al., 2017, Current Biology	Twelve high school students engaged in a semester during regular classroom activities such as group discussion.	12	Students' brain-to-brain group synchrony predicts classroom engagement and social dynamics.
	Jahng et al., 2017, NeuroImage	Prisoner's Dilemma Game.	56	The EEG hyperscanning identified temporal dynamics and inter-brain synchronization across the cortex, providing evidence for involvement of these regions in the processing of face-to-face cues to read each other's intent to cooperate. Most notably, the power of the alpha frequency band (8–13 Hz) in the right temporoparietal region immediately after seeing a round outcome significantly differed between face-to-face and face-blocked conditions and predicted whether an individual would adopt a “cooperation” or “defection” strategy. Moreover, inter-brain synchronies within this time and frequency range reflected the use of these strategies.
	Lachat et al., 2012, frontiers in Human Neuroscience	In socially driven instructions, the participants had to follow explicitly their partner's gaze, while in color-driven instructions, the objects to be looked at were designated at by their color so that no explicit gaze following was required.	29	Joint attention periods—as compared to the no-joint attention periods—were associated with a decrease of signal power between 11 and 13 Hz over a large set of left centro-parieto-occipital electrodes, encompassing the scalp regions where alpha and mu rhythms have been described. This 11–13 Hz signal power decrease was observed independently of the task instruction: it was similar when joint versus no-joint attention situations were socially driven and when they were color-driven.
	Leong et al., 2017, Proceedings of the National Academy of Sciences of the United States of America	In experiment 1, infants viewed videos of an adult who was singing nursery rhymes with direct gaze (looking forward), indirect gaze (head and eyes averted by 20°), or direct-oblique gaze (head averted but eyes orientated forward). In experiment 2, infants viewed the same adult in a live context, singing with direct or indirect gaze.	36	Across both experiments, the adult had a significant (Granger) causal influence on infants' neural activity, which was stronger during direct and direct-oblique gaze relative to indirect gaze. During live interactions, infants also influenced the adult more during direct than indirect gaze. Further, infants vocalized more frequently during live direct gaze, and individual infants who vocalized longer also elicited stronger synchronization from the adult.
	van den Heuvel et al., 2018, Clinical Neurophysiology	A lateral weight-shifting task.	24	For congruent visual feedback no significant differences in cortical activity between the two groups were present. For incongruent visual feedback, the Parkinson's disease group showed significantly higher beta modulation in primary motor cortex, and higher alpha modulation in primary visual cortex.
Interactive decision-making	Balconi and Vanutelli, 2016, frontiers in Psychology	Subjects were required to develop a strategy to obtain a better outcome than a competitor (in term of error rate, and response time, RT).	24	A decreased left alpha activity (increased brain response) for post-feedback compared to pre-feedback condition was observed.
	Hu et al., 2018, Biological Psychology	Prisoner's Dilemma Game.	30	There was a higher cooperation rate and larger theta/alpha-band inter-brain synchrony in condition human–human than in human-machine. In the condition human–human, there were larger centro-frontal theta band and centro-parietal alpha-band inter-brain synchrony in tasks set for high cooperation (higher cooperation index vs. lower cooperation index). Enhanced inter-brain synchrony covaried with increased cooperative choices observed between lower cooperation index and higher cooperation index. Furthermore, a subjective measure of perceived cooperativeness mediated the relationship between game context and inter-brain synchrony.
	Jahng et al., 2017, NeuroImage	Prisoner's Dilemma Game.	56	The EEG hyperscanning identified temporal dynamics and inter-brain synchronization across the cortex, providing evidence for involvement of these regions in the processing of face-to-face cues to read each other's intent to cooperate. Most notably, the power of the alpha frequency band (8–13 Hz) in the right temporoparietal region immediately after seeing a round outcome significantly differed between face-to-face and face-blocked conditions and predicted whether an individual would adopt a “cooperation” or “defection” strategy. Moreover, inter-brain synchronies within this time and frequency range reflected the use of these strategies.
	Kawasaki et al., 2013, Scientific Reports	Alternating speech tasks.	40	Speech rhythms were more likely to become synchronized in human–human tasks than human–machine tasks. Moreover, theta/alpha (6–12 Hz) amplitudes synchronized in the same temporal and lateral-parietal regions in each pair. Behavioral and inter-brain synchronizations were enhanced after human–machine tasks.
Affective communication	Dumas et al., 2010, PLoS ONE	Participants were engaged in spontaneous imitation of hand movements.	18	Symmetrical increase in PLV was found between the right parietal regions of the model (CP6, P8) and of the imitator (CP6, P4, P8) in the alpha-mu frequency band. The central region (FC1, Cz) of the model's brain and the parieto-occipital brain region (P8, PO2, PO10) of the imitator were synchronized in the beta frequency band. A wide frontal central area (F4, FC2, Czar, C4, CP6) of the model's brain was synchronized with the parietal area (CP2, PZ, P4, P8, PO2, PO10) of the imitator's brain for the gamma frequency band.
	Goldstein et al., 2018, Proceedings of the National Academy of Sciences of the United States of America	Romantic partners were assigned the roles of target (pain receiver) and observer (pain observer) under pain–no-pain and touch–no-touch conditions. The women were asked to rate their pain intensity 2 s before the end of each condition using the numerical pain scale. Concurrently, the male partners were instructed to rate their partners' level of pain. Both partners wrote the number on a small piece of paper not visible to the other member of the couple.	42	Hand-holding during pain administration increases brain-to-brain coupling in a network that mainly involves the central regions of the pain target and the right hemisphere of the pain observer. Moreover, brain-to-brain coupling in this network was found to correlate with analgesia magnitude and observer's empathic accuracy.
	Konvalinka et al., 2014, NeuroImage	A synchronized finger-tapping task.	18	The interactive condition was characterized by a stronger suppression of alpha and low-beta oscillations over motor and frontal areas in contrast to the non-interactive computer condition.
	Müller and Lindenberger, 2011, PLoS ONE	Participants were aligned in a predetermined position with the 11 singers facing the conductor and standing in two rows. They engaged in choir singing.	12	Phase synchronization both in respiration and heart rate variability increase significantly during singing relative to a rest condition. Phase synchronization is higher when singing in unison than when singing pieces with multiple voice parts. Directed coupling measures are consistent with the presence of causal effects of the conductor on the singers at high modulation frequencies. The different voices of the choir are reflected in network analyses of cardiac and respiratory activity based on graph theory.
	Wang et al., 2015, PLoS ONE	Participants watched emotional movies together, seated side-by-side. Participants were required to refrain from talking and making gross movements throughout the whole experiment.	78	The autonomic signals of co-present participants were idiosyncratically synchronized and that the degree of this synchronization was correlated with the convergence of their emotional responses.

This table included empirical researches related to EEG-based hyperscanning.