Table 1.

List of studies mentioned includes EEG- and fNIRS-based hyperscanning methodologies

Authors (year)	Method	Paradigm	Task	N	Frequencies recorded	Frequencies synchronized	Regions	Analysis	Findings
Hu et al. (2018) a	EEG	Cooperation	Prisoner’s Dilemma game, with high or low cooperation indexes and conditions of (believed) human-human or human-machine interaction.	30	Theta, alpha, beta, gamma	Theta, alpha	Theta: fronto-central, Alpha: centro-parietal.	PLVn	Higher cooperation rates and greater inter-brain synchrony were present when participants believed to be interacting with a human than with a machine. In human-human conditions, inter-brain coupling was higher in contexts with a high cooperation index. Participants’ reports of perceived cooperativeness mediated the relationship between game context (high or low cooperation) and alpha inter-brain synchrony.
Bezerianos et al. (2015)	EEG	Cooperation	Computer-based piloting task.	8	Delta, theta, alpha, beta, gamma	Not evaluated	Centro-parietal and frontal.	PDC	Cross-brain coupling increased as task difficulty increased.
Sinha et al. (2016)	EEG	Cooperation	Computer-based cooperation-competition game.	24	Theta, alpha, beta, gamma	Alpha, beta	Central and parietal.	PCC	Inter-brain synchrony between the subjects was significantly higher when they cooperated with each other compared to the competitive scenario.
Toppi et al. (2016)	EEG	Cooperation	Flight simulation (takeoff, cruise and landing).	12	Theta, alpha	Theta, alpha	Alpha and theta: Fronto-parietal and centro-parietal. Theta: parieto-parietal.	PDC	A denser pattern of interconnections linking the dyads’ brain activities appeared during the two cooperative flight phases (takeoff and landing) with respect to the non-cooperative cruise phase (cruise). There was a density modulation according to the degree of cooperation between the two pilots. Results sustained random pair analysis.
Astolfi et al. (2010)	EEG	Cooperation	Card Game (Scopa).	14	Theta, alpha, beta, gamma	Alpha, beta, gamma	Prefrontal cortex, anterior cingulate cortex and parietal cortex.	PDC	Strong estimated functional connectivity was found in subjects belonging to the same team but not in subjects belonging to different teams. The functional connectivity links found were directional (i.e. the signals from the second players revealed a statistically significant Granger-causality with signals of the first players of the same team).
De Vico Fallani et al. (2010)	EEG	Cooperation	Iterated Prisoner’s Dilemma game (dyads with option to defect, cooperate or choose a tit-for-tat strategy in each trial).	58	Theta, alpha, beta, gamma	Theta, alpha, beta, gamma	Frontal and pre-frontal.	PDC	Non-cooperative interactions can be predicted during the decision-making phase. Prior to making the decision, defector couples already show significantly less inter-brain connectivity than couples playing cooperative or tit-for-tat strategies.
Balconi and Vanutelli (2018)	EEG	Competition	Computer-based competition task.	30	Delta, theta, alpha, beta	Delta, theta	Prefrontal.	PCC, ANOVA	Pair’s task performance (response times and error rates) was improved during competitive tasks with respect to control condition, with further improvement after receiving reinforcing feedback. Inter-brain functional connectivity was reduced during competition.
Mu et al. (2017)	EEG	Coordination	Mental counting task (with partner or with computer) after reading a text about in-group or out-group societal threats (with in-group no-threat control).	90	Delta, theta, alpha, beta, gamma	Gamma	Frontal, central and parietal.	PLVn	Greater inter-brain synchrony between participants under an in-group threat, which correlated with greater task coordination. Synchrony not present when each individual would try to mentally coordinate counting with a computer. Inter-brain synchrony mediated the effect of ingroup threat on interpersonal coordination.
Mu et al. (2016)	EEG	Coordination	Mental counting task.	68	Alpha	Alpha	Central and posterior.	PLVn	Dyads showed smaller interpersonal time lags of counting and greater inter-brain synchrony during the coordination task compared to control task. These effects were observed in female but not male dyads.
Mu et al. (2016)	EEG	Coordination	Mental counting task (with intranasal oxytocin vs placebo administration).	60	Alpha	Alpha	Central and posterior.	PLVn	Intranasal oxytocin (vs placebo) administration in male dyads improved interpersonal behavioral synchrony in both the coordination and control tasks but specifically enhanced inter-brain neural synchrony during the coordination task.
Szymanski et al. (2017)	EEG	Joint attention	Enumeration visual search task.	52	Delta, theta, alpha, beta	Delta	Frontal, central and parietal.	PLVn, IPC	Higher phase synchrony when doing the task together (joint attention) than when doing the same task individually (individual attention). Team efficiency could be predicted by measures of synchronization during dyadic performance.
Balconi and Vanutelli (2018).	EEG	Joint attention	Sustained selective attention task (cooperation of speed and accuracy was asked of each pair of subjects).	32	Delta, theta, alpha, beta	Delta, theta	Left prefrontal.	PCC, ANOVA	Cooperation correlated positively with inter-brain synchrony. External positive feedback increased both behavioural (higher reaction time and lower error rate) and brain synchronization.
Dikker et al. (2017) a	EEG	Joint attention	High school students engaged in a semester during regular classroom activities.	12	Alpha	Alpha	Not reported.	TI	Brain-to-brain synchrony predicted classroom engagement and social dynamics (group affinity, empathy and social closeness). Joint attention, and not passive co-presence, predicted brain-to-brain synchrony.
Bevilacqua et al. (2019) a	EEG	Joint attention	High school students in class under different teaching styles (videos and lectures).	12	Alpha	Alpha	Not reported.	TI	Brain-to-brain synchrony between teachers and students varied as a function of student engagement as well as teacher likeability. Students who reported greater social closeness to the teacher showed higher brain-to-brain synchrony with the teacher. This was only the case for lectures (compared to videos)—that is, when the teacher was an integral part of the content presentation.
Sänger et al. (2012)	EEG	Joint action	Playing guitar in duets in a leader-follower fashion.	24	Delta, theta	Delta, theta	Frontal and central.	PLVn, IPC	Phase locking and both within-brain and between-brain phase-coherence connection strengths were enhanced during periods that put particularly high demands on coordination. Phase locking was modulated in relation to the assigned musical roles of leader and follower.
Dumas et al. (2010)	EEG	Joint action	Spontaneous imitation of hand movements.	18	Theta, alpha, beta, gamma	Alpha-mu, beta, gamma	Alpha-mu: right parietal, Beta: central and parieto-occipital, Gamma: fronto-central and parietal.	PLVn	Inter-brain synchronization corresponded with interactional synchrony.
Pérez et al. (2017)	EEG	Communication	Speaking and listening.	30	Delta, theta, alpha, beta	Delta, theta, alpha, beta	Alpha: fronto-central. Beta: fronto-temporal.	PLVt	Increased inter-brain synchrony present between participants while speaking and listening (compared to surrogate data). The activity of the listener would become entrained to that of the speaker. These effects were not an epiphenomenon of auditory processing.
Kinreich et al. (2017) a	EEG	Positive affect	Nonverbal social behaviour (gaze and affect) between co-habiting romantic couples or strangers.	104	Delta, theta, alpha, beta, gamma	Gamma	Temporo-parietal.	SPC	Inter-brain synchrony found in couples, but not in strangers. Among couples, neural synchrony was anchored in episodes of gaze and positive affect. Strangers’ synchrony did not show significant differences between moments of gaze-no gaze or positive affect-no affect, but a correlation was observed between the amount of time strangers spent in social gaze or positive affect and their gamma synchrony. In couples, synchrony was negatively correlated with attachment anxiety reports. For both couples and strangers, brain-to-brain synchrony was unrelated to episodes of speech-no-speech, speech duration, or general content of conversation.
Goldstein et al. (2018) a	EEG	Pain perception	Dyads (with a target and observer) under pain-no-pain and touch-no-touch conditions.	44	Alpha, beta	Alpha-mu	Target: central. Observer: right hemisphere.	Ccorr	Handholding during pain administration increased brain-to-brain synchrony, which correlated negatively with the target's pain perception and positively with the observer’s empathetic accuracy.
Pan et al. (2017) a	fNIRS	Cooperation	Computer-based cooperation game.	98	N/A	N/A	Right superior frontal cortex.	WTC	Cooperative behaviour was higher in lover dyads compared to friend and stranger dyads. Lover dyads demonstrated increased inter-brain coherence, which also covaried with task performance.
Cui et al. (2012)	fNIRS	Cooperation	Computer-based cooperation-competition game.	22	N/A	N/A	Superior frontal cortex.	WTC	Coherence between brain activation patterns increased significantly during cooperation, but not during competition. This increase accompanied an increase in cooperation performance in the cooperation game.
Funane et al. (2011)	fNIRS	Cooperation	Cooperative button-press task (with prior individual mental count after auditory cue).	12	N/A	N/A	Pre-frontal cortex.	Covariance	Spatiotemporal coherence of inter-brain signals of paired participants associated with cooperative performance. When the brain-activity patterns during the counting period were more synchronized, the interval between their button-press times was shorter, a result not explained in terms of a motion artefact.
Osaka et al. (2015)	fNIRS	Coordination	Singing/humming.	58	N/A	N/A	Left inferior frontal cortex.	WTC	Significant increase in neural synchronization for cooperative singing or humming (regardless of being face-to-face or face-to-wall) compared to singing or humming alone or listening to the other participant sing or hum.
Fishburn et al. (2018)	fNIRS	Joint attention	Completing a puzzle (together or individually).	57	N/A	N/A	Prefrontal cortex.	PCC, WTC	Interpersonal neural synchronization was greater when completing the puzzle together than when completing the same puzzle individually, watching a pair complete the puzzle or watching a movie of other people completing the puzzle.

^aThe studies that took into account the participants’ subjective experience or their relationship nature (e.g. lover dyads vs strangers).

PLVn, phase locking value (trial averaged); PLVt, phase locking value (time averaged); IPC, interbrain phase coherence; PCC, Pearson correlation coefficient; ANOVA, analysis of variance; PDC, partial directed coherence; TI, total interdependence; SPC, spearman correlation coefficient; CCorr, circular correlation; WTC, wavelet transform coherence; N/a, non-applicable.