Rocking together: dynamics of intentional and unintentional interpersonal coordination

Places for reasoning

In this article, we explore behaviour settings that enable reasoning and the diversity of constraints that not only limit but also make these behaviour settings possible. We focus specifically on reasoning and surveying how behaviour settings allow for the generation of norms of action that are nevertheless differentiated by geographies and sociocultural systems. These geographies and sociocultural systems involve diverse trajectories for reasoning even within similar behaviour settings. We will touch on places for reasoning like Twitter, social movements, traditional knowledge and laboratories set up for experimentation on our reasoning abilities. We will show how these places and the behaviour settings that emerge in them can be studied in terms of the complexity of the interactions between their participants and in terms of enabling constraints. This article is part of the theme issue 'People, places, things, and communities: expanding behaviour settings theory in the twenty-first century'.

Group Coordination Catalyzes Individual and Cultural Intelligence

A large program of research has aimed to ground large-scale cultural phenomena in processes taking place within individual minds. For example, investigating whether individual agents equipped with the right social learning strategies can enable cumulative cultural evolution given long enough time horizons. However, this approach often omits the critical group-level processes that mediate between individual agents and multi-generational societies. Here, we argue that interacting groups are a necessary and explanatory level of analysis, linking individual and collective intelligence through two characteristic feedback loops. In the first loop, more sophisticated individual-level social learning mechanisms based on Theory of Mind facilitate group-level complementarity, allowing distributed knowledge to be compositionally recombined in groups; these group-level innovations, in turn, ease the cognitive load on individuals. In the second loop, societal-level processes of cumulative culture provide groups with new cognitive technologies, including shared language and conceptual abstractions, which set in motion new group-level processes to further coordinate, recombine, and innovate. Taken together, these cycles establish group-level interaction as a dual engine of intelligence, catalyzing both individual cognition and cumulative culture.

Higher or lower? Interpersonal behavioral and neural synchronization of movement imitation in autistic children

How well autistic children can imitate movements and how their brain activity synchronizes with the person they are imitating have been understudied. The current study adopted functional near-infrared spectroscopy (fNIRS) hyperscanning and employed a task involving real interactions involving meaningful and meaningless movement imitation to explore the fundamental nature of imitation as a dynamic and interactive process. Experiment 1 explored meaningful and meaningless gesture imitation. The results revealed that autistic children exhibited lower imitation accuracy and behavioral synchrony than non-autistic children when imitating both meaningful and meaningless gestures. Specifically, compared to non-autistic children, autistic children displayed significantly higher interpersonal neural synchronization (INS) in the right inferior parietal lobule (r-IPL) (channel 12) when imitating meaningful gestures but lower INS when imitating meaningless gestures. Experiment 2 further investigated the imitation of four types of meaningless movements (orofacial movements, transitive movements, limb movements, and gestures). The results revealed that across all four movement types, autistic children exhibited significantly lower imitation accuracy, behavioral synchrony, and INS in the r-IPL (channel 12) than non-autistic children. This study is the first to identify INS as a biomarker of movement imitation difficulties in autistic individuals. Furthermore, an intra- and interindividual imitation mechanism model was proposed to explain the underlying causes of movement imitation difficulties in autistic individuals.

Perceptual coupling in human dyads: Kinematics does not affect interpersonal synchronization

The minimal, essential condition for individuals to interact is that they exchange information via at least one sensory channel. Once informational coupling is established, it enables basic forms of coordinated behavior to spontaneously emerge from the interaction. Our previous study revealed different coordination dynamics in dyads engaged in a joint finger-tapping task based on visual versus auditory coupling. This observation led us to propose the 'modality-dependent hypothesis', which posits that coordination dynamics are influenced by the sensory modality mediating informational coupling. However, recognizing that different modalities have inherent differences in accessing spatiotemporal features of perceived movement, we formulated the alternative 'kinematic hypothesis'. This hypothesis posits that differences in dynamics would vanish given equivalent kinematic information across modalities. The study involved forty (N = 40) participants, grouped into twenty (N = 20) dyads, who engaged in a joint finger-tapping task. This task was conducted under varying conditions of visual and auditory coupling, with manipulations in the access to kinematic information, categorized as discrete and continuous. Contrary to our initial predictions, the results strongly supported the 'modality-dependent hypothesis'. We observed that visual and auditory coupling consistently yielded distinct attractor dynamics, regardless of the access to kinematic information. Furthermore, all conditions of auditory coupling resulted in higher levels of synchronization than their visual counterparts. These findings suggest that the differences in interpersonal synchronization are predominantly influenced by the sensory modality, rather than the continuity of kinematic information. Our study highlights the significance of sensorimotor interactions in interpersonal synchronization and addresses the potential of sonification strategies in supporting motor training and rehabilitation.

Partner gaze shapes the relationship between symptoms of psychopathology and interpersonal coordination

Interpersonal coordination is a key determinant of successful social interaction but can be disrupted when people experience symptoms related to social anxiety or autism. Effective coordination rests on individuals directing their attention towards interaction partners. Yet little is known about the impact of the attentional behaviours of the partner themselves. As the gaze of others has heightened salience for those experiencing social anxiety or autism, addressing this gap can provide insight into how symptoms of these disorders impact coordination. Using a novel virtual reality task, we investigated whether partner gaze (i.e., direct vs. averted) influenced the emergence of interpersonal coordination. Results revealed: (i) spontaneous coordination was diminished in the averted (cf. direct) gaze condition; (ii) spontaneous coordination was positively related to symptoms of social anxiety, but only when partner gaze was averted. This latter finding contrasts the extant literature and points to the importance of social context in shaping the relationship between symptoms of psychopathology and interpersonal coordination.

The effect of inherent and incidental constraints on bimanual force control in simulated Martian gravity

The ability to coordinate actions between the limbs is important for many operationally relevant tasks associated with space exploration. A future milestone in space exploration is sending humans to Mars. Therefore, an experiment was designed to examine the influence of inherent and incidental constraints on the stability characteristics associated with the bimanual control of force in simulated Martian gravity. A head-up tilt (HUT)/head-down tilt (HDT) paradigm was used to simulate gravity on Mars (22.3° HUT). Right limb dominant participants (N = 11) were required to rhythmically coordinate patterns of isometric forces in 1:1 in-phase and 1:2 multifrequency patterns by exerting force with their right and left limbs. Lissajous displays were provided to guide task performance. Participants performed 14 twenty-second practice trials at 90° HUT (Earth). Following a 30-min rest period, participants performed 2 test trials for each coordination pattern in both Earth and Mars conditions. Performance during the test trials were compared. Results indicated very effective temporal performance of the goal coordination tasks in both gravity conditions. However, results indicated differences associated with the production of force between Earth and Mars. In general, participants produced less force in simulated Martian gravity than in the Earth condition. In addition, force production was more harmonic in Martian gravity than Earth gravity for both limbs, indicating that less force distortions (adjustments, hesitations, and/or perturbations) occurred in the Mars condition than in the Earth condition. The force coherence analysis indicated significantly higher coherence in the 1:1 task than in the 1:2 task for all force frequency bands, with the highest level of coherence in the 1-4 Hz frequency band for both gravity conditions. High coherence in the 1-4 Hz frequency band is associated with a common neural drive that activates the two arms simultaneously and is consistent with the requirements of the two tasks. The results also support the notion that neural crosstalk stabilizes the performance of the 1:1 in-phase task. In addition, significantly higher coherence in the 8-12 Hz frequency bands were observed for the Earth condition than the Mars condition. Force coherence in the 8-12 Hz bands is associated with the processing of sensorimotor information, suggesting that participants were better at integrating visual, proprioceptive, and/or tactile feedback in Earth than for the Mars condition. Overall, the results indicate less neural interference in Martian gravity; however, participants appear to be more effective at using the Lissajous displays to guide performance under Earth's gravity.

Inter-brain entrainment (IBE) during interoception. A multimodal EEG-fNIRS coherence-based hyperscanning approach

This work examined the impact of interoceptive manipulation and the presence of a shared goal on inter-brain entrainment (IBE) during a motor synchronization task. A multimodal functional Near Infrared Spectroscopy - Electroencephalogram (fNIRS-EEG) system-based hyperscanning approach was applied to 13 dyads performing the motor synchrony task during an interoceptive (focus on the breath) and control condition. Additionally, two version of the motor task-one with and one without a clearly defined common goal-were presented to participants to emphasize the task's collaborative purpose. The multimodal approach was exploited to record the electrophysiological (EEG) cortical oscillation and hemodynamic (oxy-Hb and deoxy-Hb) levels. Results revealed significant correlations between EEG delta, theta, and alpha band and hemodynamic oxy-Hb in the left compared to right hemisphere for the interoceptive confronted with the control condition. This significant EEG/fNIRS IBE correlation was also found for delta and theta band whereas the task was presented with an explicit shared goal confronted with the no-social version. In addition to separate functional connectivity EEG and fNIRS analysis, this study proposed a novel analysis pipeline including statistical tests for examining the coherence between functional connectivity EEG-fNIRS signals within couples. Besides proposing methodological advancements on EEG-fNIRS signals hyperscanning analysis, this research demonstrated that, in dyads undertaking a motor synchronization task, both the interoceptive attention to respiration and an explicit joint intention activate left anterior regions.

A review of psychological and neuroscientific research on musical groove

When listening to music, we naturally move our bodies rhythmically to the beat, which can be pleasurable and difficult to resist. This pleasurable sensation of wanting to move the body to music has been called "groove." Following pioneering humanities research, psychological and neuroscientific studies have provided insights on associated musical features, behavioral responses, phenomenological aspects, and brain structural and functional correlates of the groove experience. Groove research has advanced the field of music science and more generally informed our understanding of bidirectional links between perception and action, and the role of the motor system in prediction. Activity in motor and reward-related brain networks during music listening is associated with the groove experience, and this neural activity is linked to temporal prediction and learning. This article reviews research on groove as a psychological phenomenon with neurophysiological correlates that link musical rhythm perception, sensorimotor prediction, and reward processing. Promising future research directions range from elucidating specific neural mechanisms to exploring clinical applications and socio-cultural implications of groove.

From Cognitive Agents to Cognitive Systems: Theoretical, Methodological, and Empirical Developments of van Gelder's (1998) "Dynamical Hypothesis"

Over two decades have passed since the publication of van Gelder's (1998) "dynamical hypothesis." In that paper, van Gelder proposed that cognitive agents were not digital computers-per the representational computational approach-but dynamical systems. The evolution of the dynamical hypothesis was driven by parallel advances in three areas. Theoretically, a deeper understanding of genetics, biology, neuroscience, and cognitive science inspired questions about how systems within each domain dynamically interact and extend their effects across spatiotemporal scales. Methodologically, more sophisticated and domain-general tools allowed researchers to discover, model, and quantify system dynamics, structure, and patterns across multiple scales to generate a more comprehensive system-level understanding of behaviors. Empirically, we can analyze a system's behavior while preserving its natural dynamics, revealing evidence that the reductionist approach leads to an incomplete understanding of the components and the overall system. Researchers have traditionally reduced a complex system into its component processes and assumed that the parts can be recombined to explain the whole. These three advances fundamentally altered our understanding of a "cognitive agent:" How their behaviors are driven by long-range coordination across multiple processes, how the interdependent and nested structure of interacting variables produces behaviors that are greater than the sum of its parts, and how environmental constraints shape adaptive yet stable behavioral patterns.

Behavioural Synchronisation between Dogs and Humans: Unveiling Interspecific Motor Resonance?

Dogs' behavioural synchronisation with humans is of growing scientific interest. However, studies lack a comprehensive exploration of the neurocognitive foundations of this social cognitive ability. Drawing parallels from the mechanisms underlying behavioural synchronisation in humans, specifically motor resonance and the recruitment of mirror neurons, we hypothesise that dogs' behavioural synchronisation with humans is underpinned by a similar mechanism, namely interspecific motor resonance. Based on a literature review, we argue that dogs possess the prerequisites for motor resonance, and we suggest that interspecific behavioural synchronisation relies on the activation of both human and canine mirror neurons. Furthermore, interspecific behavioural studies highlight certain characteristics of motor resonance, including motor contagion and its social modulators. While these findings strongly suggest the potential existence of interspecific motor resonance, direct proof remains to be established. Our analysis thus paves the way for future research to confirm the existence of interspecific motor resonance as the neurocognitive foundation for interspecific behavioural synchronisation. Unravelling the neurocognitive mechanisms underlying this behavioural adjustment holds profound implications for understanding the evolutionary dynamics of dogs alongside humans and improving the day-to-day management of dog-human interactions.

Putting it Together, Together

People are not as fast or as strong as many other creatures that evolved around us. What gives us an evolutionary advantage is working together to achieve common aims. Coordinating joint action begins at a tender age with such cooperative activities as alternating babbling and clapping games. Adult joint activities are far more complex and use multiple means of coordination. Joint action has attracted qualitative analyses by sociolinguists, cognitive scientists, and philosophers as well as empirical analyses and theories by cognitive scientists. Here, we analyze how joint action is spontaneously coordinated from start to finish in a novel complex real-life joint activity, assembling a piece of furniture, a task that captures the essentials of joint action, collaborators, things in the world, and communicative devices. Pairs of strangers assembled a TV cart from a stack of parts and a photo of the completed cart. Coordination prior to each assembly action was coded as explicit, using speech or gesture, or implicit, actions that both advanced the task and communicated the next step. Initial planning relied on explicit communication about structure, but not action nor division of labor, which were improvised. That served to establish a joint representation of the goal that informed actions and monitored progress. As assembly progressed, coordination was increasingly implicit, through actions alone. Joint action is a dynamic interplay of explicit and implicit signaling with respect to things in the world to coordinate ongoing progress, guided by a shared representation of the goal.

Impaired perception of a partner's synchronizing behavior reduces positive attitude toward humanoid robot in schizophrenia patients

As interpersonal synchrony plays a key role in building rapport, the perception of another agent's synchronizing behavior could be an important feature to assess, especially with patients with social deficits such as in schizophrenia. Twenty-four schizophrenia patients and twenty-four matched healthy controls performed jointly fitness movements with another agent embodied by a humanoid robot which was programmed to either synchronize with the participants or move at a fixed frequency with them. Self-report of participants' perception of the robot's synchronizing behavior was collected after each interaction. Results indicated that patients were impaired in their ability to accurately perceive the robot's synchronizing behavior. Patients' subjective perception of the robot's synchronizing behavior was associated with positive attitude toward it, suggesting that the belief to be synchronized with others could have similar impact on affiliation than real interpersonal synchrony. It leads to new perspectives for understanding social deficits in people with severe mental illness.

Spontaneity competes with intention to influence the coordination dynamics of interpersonal performance tendencies

Research has shown that spontaneous visual coupling supports frequency entrainment, phase attraction, and intermittent interpersonal coordination when co-actors are switched from a no-vision (NV) to vision (V) context. In two experiments, co-actors started in a NV context while producing the same or different amplitude movements. The same amplitude resulted in similar self-paced frequencies, while different amplitudes resulted in disparate frequencies. In experiment 1, co-actors were instructed to maintain amplitude while receiving no instructions to coordinate their actions. Frequency and phase entrainment was limited in the V context even when co-actors started the NV context with the same amplitude. In experiment 2, co-actors were instructed to maintain amplitude and intentionally coordinate together, but not at a specific pattern. Significant frequency modulations occurred to maintain amplitude as the co-actors sought to coordinate their actions. With the open-ended instructions, co-actors produced in-phase and anti-phase coordination along with intermittent performance exhibited by shifts between a variety of stable relative phase patterns. The proposed hypotheses and findings are discussed within the context of a shared manifold representation for joint action contexts, with the coordination dynamics expressed by the HKB model of relative phase serving to conceptualization the representations in the shared manifold.

Dynamics in interbrain synchronization while playing a piano duet

Humans interact with each other through actions that are implemented by sensory and motor processes. To investigate the role of interbrain synchronization emerging during interpersonal action coordination, electroencephalography data from 13 pairs of pianists were recorded simultaneously while they performed a duet together. The study aimed to investigate whether interbrain phase couplings can be reduced to similar bottom-up driven processes during synchronous play, or rather represent cognitive top-down control required during periods of higher coordination demands. To induce such periods, one of the musicians acted as a confederate who deliberately desynchronized the play. As intended, on the behavioral level, the perturbation caused a breakdown in the synchronization of the musicians' play and in its stability across trials. On the brain level, interbrain synchrony, as measured by the interbrain phase coherence (IPC), increased in the delta and theta frequency bands during perturbation as compared to non-perturbed trials. Interestingly, this increase in IPC in the delta band was accompanied by the shift of the phase difference angle from in-phase toward anti-phase synchrony. In conclusion, the current study demonstrates that interbrain synchronization is based on the interpersonal temporal alignment of different brain mechanisms and is not simply reducible to similar sensory or motor responses.

The microstructure of intra- and interpersonal coordination

Movements are naturally composed of submovements, i.e. recurrent speed pulses (2-3 Hz), possibly reflecting intermittent feedback-based motor adjustments. In visuomotor (unimanual) synchronization tasks, partners alternate submovements over time, indicating mutual coregulation. However, it is unclear whether submovement coordination is organized differently between and within individuals. Indeed, different types of information may be variably exploited for intrapersonal and interpersonal coordination. Participants performed a series of bimanual tasks alone or in pairs, with or without visual feedback (solo task only). We analysed the relative timing of submovements between their own hands or between their own hands and those of their partner. Distinct coordinative structures emerged at the submovement level depending on the relevance of visual feedback. Specifically, the relative timing of submovements (between partners/effectors) shifts from alternation to simultaneity and a mixture of both when coordination is achieved using vision (interpersonal), proprioception/efference-copy only (intrapersonal, without vision) or all information sources (intrapersonal, with vision), respectively. These results suggest that submovement coordination represents a behavioural proxy for the adaptive weighting of different sources of information within action-perception loops. In sum, the microstructure of movement reveals common principles governing the dynamics of sensorimotor control to achieve both intra- and interpersonal coordination.

Stage 2: Visual information and communication context as modulators of interpersonal coordination in face-to-face and videoconference-based interactions

Interpersonal coordination of body movement-or similarity in patterning and timing of body movement between interaction partners-is well documented in face-to-face (FTF) conversation. Here, we investigated the degree to which interpersonal coordination is impacted by the amount of visual information available and the type of interaction conversation partners are having. To do so within a naturalistic context, we took advantage of the increased familiarity with videoconferencing (VC) platforms and with limited visual information in FTF conversation due to the COVID-19 pandemic. Pairs of participants communicated in one of three ways: FTF in a laboratory setting while socially distanced and wearing face masks; VC in a laboratory setting with a view of one another's full movements; or VC in a remote setting with a view of one another's face and shoulders. Each pair held three conversations: affiliative, argumentative, and cooperative task-based. We quantified interpersonal coordination as the relationship between the two participants' overall body movement using nonlinear time series analyses. Coordination changed as a function of the contextual constraints, and these constraints interacted with coordination patterns to affect subjective conversation outcomes. Importantly, we found patterns of results that were distinct from previous research; we hypothesize that these differences may be due to changes in the broader social context from COVID-19. Taken together, our results are consistent with a dynamical systems view of social phenomena, with interpersonal coordination emerging from the interaction between components, constraints, and history of the system.

Motor Decision-Making as a Common Denominator in Motor Pathology and a Possible Rehabilitation Target

Despite the substantial progress in motor rehabilitation, patient involvement and motivation remain major challenges. They are typically addressed with communicational and environmental strategies, as well as with improved goal-setting procedures. Here we suggest a new research direction and framework involving Neuroeconomics principles to investigate the role of Motor Decision-Making (MDM) parameters in motivational component and motor performance in rehabilitation. We argue that investigating NE principles could bring new approaches aimed at increasing active patient engagement in the rehabilitation process by introducing more movement choice, and adapting existing goal-setting procedures. We discuss possible MDM implementation strategies and illustrate possible research directions using examples of stroke and psychiatric disorders.

Autonomic synchrony induced by hyperscanning interoception during interpersonal synchronization tasks

According to previous research, people influence each other's emotional states during social interactions via resonance mechanisms and coordinated autonomic rhythms. However, no previous studies tested if the manipulation of the interoceptive focus (focused attention on the breath for a given time interval) in hyperscanning during synchronized tasks may have an impact on autonomic synchrony. Thus, this study aims to assess the psychophysiological synchrony through autonomic measures recording during dyadic linguistic and motor synchronization tasks performed in two distinct interoceptive conditions: the focus and no focus on the breath condition. 26 participants coupled in 13 dyads were recruited. Individuals' autonomic measures [electrodermal: skin conductance level and response (SCL, SCR); cardiovascular indices: heart rate (HR) and HR variability (HRV)] was continuously monitored during the experiment and correlational coefficients were computed to analyze dyads physiological synchrony. Inter-subject analysis revealed higher synchrony for HR, HRV, SCL, and SCR values in the focus compared to no focus condition during the motor synchronization task and in general more for motor than linguistic task. Higher synchrony was also found for HR, SCL, and SCR values during focus than no focus condition in linguistic task. Overall, evidence suggests that the manipulation of the interoceptive focus has an impact on the autonomic synchrony during distinct synchronization tasks and for different autonomic measures. Such findings encourage the use of hyperscanning paradigms to assess the effect of breath awareness practices on autonomic synchrony in ecological and real-time conditions involving synchronization.

Spontaneous dyadic behaviour predicts the emergence of interpersonal neural synchrony

Synchronization of neural activity across brains - interpersonal neural synchrony (INS) - is emerging as a powerful marker of social interaction that predicts success of multi-person coordination, communication, and cooperation. As the origins of INS are poorly understood, we tested whether and how INS might emerge from spontaneous dyadic behavior. We recorded neural activity (EEG) and human behavior (full-body kinematics, eye movements and facial expressions) while dyads of participants were instructed to look at each other without speaking or making co-verbal gestures. We made four fundamental observations. First, despite the absence of a structured social task, INS emerged spontaneously only when participants were able to see each other. Second, we show that such spontaneous INS, comprising specific spectral and topographic profiles, did not merely reflect intra-personal modulations of neural activity, but it rather reflected real-time and dyad-specific coupling of neural activities. Third, using state-of-art video-image processing and deep learning, we extracted the temporal unfolding of three notable social behavioral cues - body movement, eye contact, and smiling - and demonstrated that these behaviors also spontaneously synchronized within dyads. Fourth, we probed the correlates of INS in such synchronized social behaviors. Using cross-correlation and Granger causality analyses, we show that synchronized social behaviors anticipate and in fact Granger cause INS. These results provide proof-of-concept evidence for studying interpersonal neural and behavioral synchrony under natural and unconstrained conditions. Most importantly, the results suggest that INS could be conceptualized as an emergent property of two coupled neural systems: an entrainment phenomenon, promoted by real-time dyadic behavior.

Motor performance in joint action tasks: The impact of dyadic motive fit

In many daily situations, two or more individuals need to coordinate their actions to achieve a common goal and perform successfully. Past research on joint action has predominantly focused on the question of how such interactions are accomplished. Here we focus on the impact of inter-individual, trait-like differences to predict joint action performance. More specifically, we examined whether performance in a joint action task is moderated by the (in)congruence of individuals' motive dispositions. To this end, 27 dyads performed a joint action task in which they had to navigate a ball through a maze with each partner using a joystick and each being responsible for either moving the ball along the x-axis or the y-axis. As dependent measures, we analyzed dyads' performance (times and errors). As trait-like predictors, we assessed implicit and explicit motives by means of the Picture Story Exercise and the Unified Motive Scale, respectively. Linear regression modeling revealed that congruent explicit affiliation motives predict faster best times and that higher congruent implicit achievement motives are associated with reduced errors. Exploratory Response Surface Analyses yielded identical results for the affiliation motive. These findings provide initial evidence to suggest that interindividual differences and in motives as well as their fit are related to joint action performance. Future directions of this new paradigm and novel ways to analyze dyadic motive fits and their relation to joint action performance are discussed.

Beyond speaking: neurocognitive perspectives on language production in social interaction

The human faculty to speak has evolved, so has been argued, for communicating with others and for engaging in social interactions. Hence the human cognitive system should be equipped to address the demands that social interaction places on the language production system. These demands include the need to coordinate speaking with listening, the need to integrate own (verbal) actions with the interlocutor's actions, and the need to adapt language flexibly to the interlocutor and the social context. In order to meet these demands, core processes of language production are supported by cognitive processes that enable interpersonal coordination and social cognition. To fully understand the cognitive architecture and its neural implementation enabling humans to speak in social interaction, our understanding of how humans produce language needs to be connected to our understanding of how humans gain insights into other people's mental states and coordinate in social interaction. This article reviews theories and neurocognitive experiments that make this connection and can contribute to advancing our understanding of speaking in social interaction. This article is part of a discussion meeting issue 'Face2face: advancing the science of social interaction'.

Paradoxical decrease of imitation performance with age in children

Imitation development was studied in a cross-sectional design involving 174 primary-school children (aged 6-10), focusing on the effect of actions' complexity and error analysis to infer the underlying cognitive processes. Participants had to imitate the model's actions as if they were in front of a mirror ('specularly'). Complexity varied across three levels: movements of a single limb; arm and leg of the same body side; or arm and leg of opposite body sides. While the overall error rate decreased with age, this was not true of all error categories. The rate of 'side' errors (using a limb of the wrong body side) paradoxically increased with age (from 9 years). However, with increasing age, the error rate also became less sensitive to the complexity of the action. This pattern is consistent with the hypothesis that older children have the working memory (WM) resources and the body knowledge necessary to imitate 'anatomically', which leads to additional side errors. Younger children might be paradoxically free from such interference because their WM and/or body knowledge are insufficient for anatomical imitation. Yet, their limited WM resources would prevent them from successfully managing the conflict between spatial codes involved in complex actions (e.g. moving the left arm and the right leg). We also found evidence that action side and content might be stored in separate short-term memory (STM) systems: increasing the number of sides to be encoded only affected side retrieval, but not content retrieval; symmetrically, increasing the content (number of movements) of the action only affected content retrieval, but not side retrieval. In conclusion, results suggest that anatomical imitation might interfere with specular imitation at age 9 and that STM storages for side and content of actions are separate.

Emergent and planned interpersonal synchronization are both sensitive to 'tempo aftereffect contagion'

Interpersonal synchronization is fundamental for motor coordination during social interactions. Discerning emergent (entrainment) from planned synchronization represents a non-trivial issue in visually bonded individuals acting together, as well as assessing whether inter-individual differences, e.g., in autistic traits, modulate both types of synchronization. In a visuomotor finger-tapping task, two participants replicated a target tempo either synchronizing ('joint' condition) or not ('non-interactive' condition, 'non-int') with each other. One participant was exposed ('induced') to tempo aftereffect (a medium tempo seems faster or slower after exposure to slower or faster inducing tempi), but not the other participant ('not induced'); thus they had different timing perceptions of the same target. We assessed to what degree emergent and/or planned synchronization affected dyads by analyzing inter-tap-intervals, synchronization indexes, and cross-correlation coefficients. Results revealed a 'tempo aftereffect contagion': inter-tap-intervals of both induced and not-induced participants showed aftereffect in both the joint and non-int conditions. Moreover, aftereffects did not correlate across conditions suggesting they might be due to (at least in part) different processes, but the propensity for tempo aftereffect contagion correlated with individuals' autistic traits only in the non-int condition. Finally, participants co-adjusted their tapping more in the joint condition than in the non-int one, as confirmed by higher synchronization indexes and the mutual adaptation pattern of cross-correlation coefficients. Altogether, these results show the subtle interplay between emergent and planned interpersonal synchronization mechanisms that act on a millisecond timescale independently from synching or not with the partner.

The contextual cueing effect disappears during joint search in preschool children

During preschool years, children's interacting with others increases. One of the involved developmental skills is task co-representation, through which children aged 5 years and older represent a partner's task in a similar way to their own task. In adults, task co-representation makes participants attend to and form memories of objects relevant to both their own task and their partner's task; however, it is unclear whether children can also form such memories. In Experiment 1, we examined the memory facilitation of joint search using a contextual cueing effect paradigm. Children were presented with search displays repeatedly with the same or random layouts and searched and responded to the target either alone (the single group; n = 32; M_age = 73.6 months, range = 61-80) or with their parent (the joint group; n = 32; M_age = 74.3 months, range = 64-81). Results showed that the search with the same layouts was faster than that with the random layouts for the single group, indicating that children form associative memories of target and distractors relevant to their own task. For the joint group, this effect was not statistically different from that of the single group, with exploratory analysis suggesting that it was disrupted. In Experiment 2, children performed the search with a peer (n = 32; M_age = 72.7 months, range = 67-79) and the effect was also not found. Our findings suggest that the self's and partner's tasks are represented but might not be incorporated into associative memory in 5- and 6-year-old children.

Interpersonal synchronization of spontaneously generated body movements

Interpersonal movement synchrony (IMS) is central to social behavior in several species. In humans, IMS is typically studied using structured tasks requiring participants to produce specific body movements. Instead, spontaneously generated (i.e., not instructed) movements have received less attention. To test whether spontaneous movements synchronize interpersonally, we recorded full-body kinematics from dyads of participants who were only asked to sit face-to-face and to look at each other. We manipulated interpersonal (i) visual contact and (ii) spatial proximity. We found that spontaneous movements synchronized across participants only when they could see each other and regardless of interpersonal spatial proximity. This synchronization emerged very rapidly and did not selectively entail homologous body parts (as in mimicry); rather, the synchrony generalized to nearly all possible combinations of body parts. Hence, spontaneous behavior alone can lead to IMS. More generally, our results highlight that IMS can be studied under natural and unconstrained conditions.

How frequent is the spontaneous occurrence of synchronized walking in daily life?

Experimental work has suggested that individuals walking side by side may frequently synchronize their steps. The present study created video records of pedestrian activity on pedestrian pathways in order to estimate the frequency of continuous synchronization among pairs of walkers going about their daily lives. About 6% of 498 coded pairs were continuously synchronized. Analysis and modeling of the distributions of frequency differences suggested that while different walkers will tend to have different preferred frequencies for a given speed (i.e., a preferred ratio of step length to step frequency, or walk ratio), they may tend to adjust their walk ratios slightly toward one another's even when they are not synchronizing their steps.

The role of reciprocity in dynamic interpersonal coordination of physiological rhythms

A central question in social cognition research is how people coordinate their bodily rhythms, and how important reciprocity of interaction is for interpersonal coordination. Previous research has primarily focused on interpersonal action coordination, which has been shown to be facilitated by mechanisms of prediction and mutual adaption. Recent research is beginning to show that people also coordinate their physiological rhythms (i.e., respiration, heart rhythms) when they engage in natural forms of social interaction, such as conversation, choir singing, and rituals. However, the mechanisms underlying interpersonal physiological synchronization remain obscure, and could provide insight into the dynamic mechanisms that underlie continuous and regulatory, rather than instrumental, joint actions. Using real-time biofeedback, we investigated whether people synchronize their respiration rhythms by forming a joint dynamical system through reciprocity of interaction, or by producing more predictable respiration rhythms. Our results show that people are more in-phase synchronized but less phase-locked when interacting bidirectionally versus unidirectionally (online), but there is no difference in synchronization during reciprocal interaction and when adapting unidirectionally (offline) to recordings of respiration signals that emerged during the reciprocal interaction. Moreover, the strength of synchronization is driven by the predictability of the respiration rhythms that emerge in the bidirectional interaction - specifically, the slowing of breathing rhythms and stability of breathing frequencies - rather than the online mutual adaptation itself. These results suggest that coordination is facilitated by the emergence of predictable breathing patterns, rather than reciprocity itself.

An examination of the potential benefits of expert guided physical activity for supporting recovery from extreme social withdrawal: Two case reports focused on the treatment of Hikikomori

Extreme and long-term social withdrawal, first described in Japan as Hikikomori, has now become a globally recognized mental health problem. Intervention studies severely lag behind epidemiological and phenomenological research. We present two descriptive case reports of Japanese university students with Hikikomori who participated in an early phase test of a structured intervention involving physical activities that was developed and facilitated by clinicians and physical education specialists-Human Movement Consultation (HMC). The two recipients (19- and 29-years old at the start of treatment) completed approximately 40 consultation sessions delivered over 3 years consisting of a combination of outdoor workouts (i.e., walking, running, and cycling) and interpersonal sports (e.g., table tennis, badminton, and tennis). Changes in social withdrawal behavior were independently rated from clinical health records using a structured scale (the Glasgow Hikikomori Scale; GHS). Behavioral observations and scale data for both cases indicated improvements from pre-treatment levels of social withdrawal. At the end of the intervention, both had returned to normative levels of functioning. Case A returned to university and Case B secured a new job upon the completion of HMC. To help advance our understanding of treatment options, these case descriptions analyze potential change mechanisms in order to understand how HMC can support recovery from extreme social withdrawal. One key observation is that both outdoor workouts and interpersonal sports offer a non-threatening method of enabling Hikikomori to engage in interpersonal interactions. Such connections via structured activities may allow the reinstatement of social skills in a graded manner. In addition, an initial focus on physical experiences may help promote psychological and social connectedness without triggering the social fears and challenges that underlie the Hikikomori state. The findings from these two cases offer a framework to guide further research and the development of exercise-based interventions for this hidden and often neglected group.

Analysis of group behavior based on sharing heterogeneous roles in a triad using a coordinated drawing task

Humans often share roles and aim to achieve a group goal based on sociality, which is the tendency to spontaneously involve oneself with others. Cognitive science, psychology, and neuroscience studies suggest that in such planned coordination, adjusting one’s own actions based on other roles is crucial for high task performance. However, the mechanisms of complex and dynamically planned coordination, such as non-verbal group behavior with three or more members, remain to be fully investigated. This study introduced a coordinated drawing task in a triad, quantitatively analyzed non-verbal group behavior based on sharing heterogeneous roles, and investigated an important role. Participant triads engaged in the task repeatedly by operating reels to change thread tensions and moving a pen connected to the three threads to draw an equilateral triangle. Then, the three roles (pulling, relaxing, and adjusting) had to be shared. The pulling and relaxing roles served to move the pen as if an operator pulled it closer to the hand and to support the pen’s movement, respectively. However, these roles alone could not draw a side considering the task specification. The adjusting role needed to change the tension flexibly and maintain an overall balance. In the experiment, we measured the pen positions and tensions, and established statistical models to fit the analyzed data. The results estimated that the action in the adjusting role was related to the improved performance of faster drawing on a side. This role may moderately intervene in the actions by the other roles and fine-tune without disturbing the pen’s smooth movement while avoiding great pen deviation. Our findings may suggest the crucial role as a facilitator that handles resiliently in non-verbal coordinated behavior of a triad, and contribute to our understanding of social interactions.

Interpersonal sensorimotor communication shapes intrapersonal coordination in a musical ensemble

Social behaviors rely on the coordination of multiple effectors within one's own body as well as between the interacting bodies. However, little is known about how coupling at the interpersonal level impacts coordination among body parts at the intrapersonal level, especially in ecological, complex, situations. Here, we perturbed interpersonal sensorimotor communication in violin players of an orchestra and investigated how this impacted musicians' intrapersonal movements coordination. More precisely, first section violinists were asked to turn their back to the conductor and to face the second section of violinists, who still faced the conductor. Motion capture of head and bow kinematics showed that altering the usual interpersonal coupling scheme increased intrapersonal coordination. Our perturbation also induced smaller yet more complex head movements, which spanned multiple, faster timescales that closely matched the metrical levels of the musical score. Importantly, perturbation differentially increased intrapersonal coordination across these timescales. We interpret this behavioral shift as a sensorimotor strategy that exploits periodical movements to effectively tune sensory processing in time and allows coping with the disruption in the interpersonal coupling scheme. As such, head movements, which are usually deemed to fulfill communicative functions, may possibly be adapted to help regulate own performance in time.

Endogenous sources of interbrain synchrony in duetting pianists

When people interact with each other, their brains synchronize. However, it remains unclear whether interbrain synchrony (IBS) is functionally relevant for social interaction or stems from exposure of individual brains to identical sensorimotor information. To disentangle these views, the current dual-EEG study investigated amplitude-based IBS in pianists jointly performing duets containing a silent pause followed by a tempo change. First, we manipulated the similarity of the anticipated tempo change and measured IBS during the pause, hence, capturing the alignment of purely endogenous, temporal plans without sound or movement. Notably, right posterior gamma IBS was higher when partners planned similar tempi, it predicted whether partners' tempi matched after the pause, and it was modulated only in real, not in surrogate pairs. Second, we manipulated the familiarity with the partner's actions and measured IBS during joint performance with sound. Although sensorimotor information was similar across conditions, gamma IBS was higher when partners were unfamiliar with each other's part and had to attend more closely to the sound of the performance. These combined findings demonstrate that IBS is not merely an epiphenomenon of shared sensorimotor information but can also hinge on endogenous, cognitive processes crucial for behavioral synchrony and successful social interaction.

Positive emotions foster spontaneous synchronisation in a group movement improvisation task

Emotions are a natural vector for acting together with others and are witnessed in human behaviour, perception and body functions. For this reason, studies of human-to-human interaction, such as multi-person motor synchronisation, are a perfect setting to disentangle the linkage of emotion with socio-motor interaction. And yet, the majority of joint action studies aiming at understanding the impact of emotions on multi-person performance resort to enacted emotions, the ones that are emulated based on the previous experience of such emotions, and almost exclusively focus on dyadic interaction. In addition, tasks chosen to study emotion in joint action are frequently characterised by a reduced number of physical dimensions to gain experimental control and subsequent facilitation in data analysis. Therefore, it is not clear how naturalistically induced emotions diffuse in more ecological interactions with other people and how emotions affect the process of interpersonal synchronisation. Here, we show that positive and negative emotions differently alter spontaneous human synchronous behaviour during a multi-person improvisation task. The study involved 39 participants organised in triads who self-reported liking improvisational activities (e.g., dancing). The task involved producing improvisational movements with the right hand. Participants were emotionally induced by manipulated social feedback involving a personal ranking score. Three-dimensional spatio-temporal data and cardiac activity were extracted and transformed into oscillatory signals (phases) to compute behavioural and physiological synchrony. Our results demonstrate that individuals induced with positive emotions, as opposed to negative emotions or a neutral state, maintained behavioural synchrony with other group members for a longer period of time. These findings contribute to the emerging shift of neuroscience of emotion and affective sciences towards the environment  of social significance where emotions appear the most—in interaction with others. Our study showcases a method of quantification of synchrony in an improvisational and interactive task based on a well-established Kuramoto model.

The benefits of learning movement sequences in social interactions

Although we frequently acquire knowledge and skills through social interactions, the focus of most research on learning is on individual learning. Here we characterize Interaction Based Learning (IBL), which represents the acquisition of knowledge or skill through social interactions, and compare it to Observational Learning (OL)—learning by observation. To that end, we designed a movement synchronization paradigm whereby participants learned Tai-Chi inspired movement sequences from trained teachers in two separated sessions. We used a motion capture system to track the movement of 40 dyads comprised of a teacher and learner, who were randomly divided into OL or IBL groups, and calculated time-varying synchrony of three-dimensional movement velocity. While in the IBL group both the learner and the teacher could see each other through a transparent glass, in the OL group dyads interacted through a one-way mirror, such that the learners observed the teacher, but the teacher could not see the learners. Results show that although the number of movements recalled was not different between groups, we found improved movement smoothness in the IBL compared to the OL group, indicating movement acquisition was better in the IBL group. In addition, we found that motor synchronization levels in dyads improved over time, indicating that movement synchronization can be learned and retained. In the first session, the IBL group, but not the OL group, showed a significant improvement in synchronization. This suggests that dyadic interaction is important for learning movement sequences, and that bidirectional communication of signals and mutual feedback are essential for the consolidation of motor learning.

Coordination dynamics of thoracic and abdominal movements during voluntary breathing

Thoracic and abdominal movements can be tightly coupled during voluntary breathing, such as when singing and playing wind instruments. The present study investigated the coordination of thoracic and abdominal movements during voluntary breathing using a dynamical systems approach. We examined whether there are two stable coordination patterns, and if the coordination pattern would abruptly change when the breathing frequency increased, which is known as phase transition. The participants inhaled and exhaled repeatedly at 7.5, 15, 30, 60, or 120 breaths per minute. At the beginning and end of the experiment, the participants performed breathing at their preferred frequency. As a result, the coordination pattern at the lower and preferred frequencies exhibited an in-phase pattern. When breathing frequency increased, participants showed deviated coordination patterns from the in-phase pattern to either a thoracic-leading pattern, an abdominal-leading pattern, or an anti-phase pattern depending on the individual. These deviations occurred gradually; thus, phase transition was not observed. Our findings suggest that thoracic and abdominal movements are tightly coupled at lower frequencies, but their patterns vary depending on the breathing frequency and individuals. Therefore, the present study suggests the importance of viewing breath control in terms of coordination of thoracic and abdominal movements.

Endogenous rhythms influence musicians' and non-musicians' interpersonal synchrony

Individuals display considerable rate differences in the spontaneous production of rhythmic behaviors (such as speech, gait, dance). Temporal precision in rhythmic behavior tends to be highest at individuals’ spontaneous production rates; musically trained partners with similar spontaneous rates show increased synchrony in joint tasks, consistent with predictions based on intrinsic frequencies of coupled oscillators. We address whether partner-specific influences of intrinsic frequencies are evidenced in musically trained and untrained individuals who tapped a familiar melody at a spontaneous (uncued) rate individually. Each individual then synchronized with a partner from the same musicianship group at an initially cued rate that matched the partners’ spontaneous rates. Musically trained partners showed greater synchrony in joint tapping than musically untrained partners. Asynchrony increased in both groups as the partners’ difference in individual spontaneous rates increased, with greater impact for musically untrained pairs. Recurrence quantification analysis confirmed that musically untrained individuals demonstrated greater determinism (less flexibility) in their tapping than musically trained individuals. Furthermore, individuals with greater determinism in solo performances demonstrated reduced synchrony in joint performances. These findings suggest that musicians’ increased temporal flexibility is associated with decreased endogenous constraints on production rate and greater interpersonal synchrony in musical tasks.

Synchrony in triadic jumping performance under the constraints of virtual reality

The use of an immersive virtual reality system as a work space for sports and physical education can help maintain physical communication from separate places. In this study, we verified the possibility of constructing a movement synchrony system by reproducing the mathematical ordered pattern of “triadic jumping” in a virtual space. Three jumpers were asked to move together in a space that was cramped and insufficient for them to pass each other. Within this restricted space, the ordered pattern of the jumpers’ synchrony systematically transited to another state depending on the geometrical configuration of the work space. Although the temporal rigidity of the synchrony was partially lost, the ordered pattern of the “triadic jumping” synchrony that emerged in the virtual space was qualitatively equivalent to that emerging in real space. We believe the idea of expanding the work space for physical education to a virtual one could turn into reality if the sensory feedback of the collision successfully improves the spatial-temporal rigidity of the joint action ordered pattern.

The Role of a Mechanical Coupling in (Spontaneous) Interpersonal Synchronization: a Human Version of Huygens’ Clock Experiments

Interpersonal musical interaction typically relies on the mutual exchange of auditory and visual information. Inspired by the finding of Christiaan Huygens that two pendulum clocks spontaneously synchronize when hanging from a common, movable wooden beam, we explored the possible use of mechanical coupling as an alternative coupling modality between people to strengthen (spontaneous and instructed) joint (musical) synchronization. From a coupled oscillator viewpoint, we hypothesized that dyads standing on a common movable platform would cause bidirectional passive body motion (and corresponding proprioceptive, vestibular and somatosensory sensations), leading to enhanced interpersonal coordination and mutual entrainment. To test this hypothesis, we asked dyads to perform a musical synchronization–continuation task, while standing on a movable platform. Their rhythmic movements were compared under different conditions: mechanically coupled/decoupled platforms, and spontaneous/instructed synchronization. Additionally, we investigated the effects of performing an additional collaborative conversation task, and of initial tempo and phase differences in the instructed rhythms. The analysis was based on cross wavelet and synchrosqueezed transforms. The overall conclusion was that a mechanical coupling was effective in support of interpersonal synchronization, specifically when dyads were explicitly instructed to synchronize using the movable platform (instructed synchronization). On the other hand, results showed that mechanical coupling led only minimally to spontaneous interpersonal synchronization. The collaborative task and the initial phase and tempo have no strong effect. Although more research is required, possible applications can be found in the domains of music education, dance and music performance, sports, and well-being.

Reaction time coupling in a joint stimulus-response task: A matter of functional actions or likable agents?

Shaping one owns actions by observing others’ actions is driven by the deep-rooted mechanism of perception-action coupling. It typically occurs automatically, expressed as for example the unintentional synchronization of reaction times in interactive games. Theories on perception-action coupling highlight its benefits such as the joint coordination of actions to cooperatively perform tasks properly, the learning of novel actions from others, and the bonding with likable others. However, such functional aspects and how they shape perception-action coupling have never been compared quantitatively. Here we tested a total of hundred-fifteen participants that played a stimulus-response task while, in parallel, they observed videos of agents that played the exact same task several milliseconds in advance. We compared to what degree the reaction times of actions of agents, who varied their behavior in terms of functionality and likability in preceding prisoner dilemma games and quizzes, shape the reaction times of human test participants. To manipulate functionality and likability, we varied the predictability of cooperative behavior and correctness of actions of agents, respectively, resulting in likable (cooperative), dislikable (uncooperative), functional (correct actions), and dysfunctional (incorrect actions) agents. The results of three experiments showed that the participants’ reaction times correlated most with the reaction times of agents that expressed functional behavior. However, the likability of agents had no effects on reaction time correlations. These findings suggest that, at least in the current computer task, participants are more likely to adopt the timing of actions from people that perform correct actions than from people that they like.

Synchrony During Online Encounters Affects Social Affiliation and Theory of Mind but Not Empathy

Moving together in time affects human social affiliation and cognition. However, it is unclear whether these effects hold for on-line video meetings and whether they extend to empathy (understanding or sharing others' emotions) and theory of mind (ToM; attribution of mental states to others). 126 young adult participants met through online video in unacquainted pairs. Participants either performed 3 min of synchronous arm movements paced by sounds (n = 40), asynchronous movements (n = 46) or a small talk condition (n = 40). In a subsequent empathy task, participants engaged in a conversation. A video recording of this conversation was played back, and each participant rated, at predetermined time points, how they felt and how they thought their partner felt. From this we calculated empathic accuracy (accuracy of the estimation of the other's emotions) and emotional congruence (emotion sharing). ToM was measured by showing videos of geometrical shapes interacting and asking the participants to describe what happened, measuring the amount of intentionality. We found that participants in the synchrony condition rated feeling greater closeness and similarity to their partners relative to the asynchronous condition. Further, participants in the synchrony group tended to ascribe more intentionality to the abstract shapes than participants in asynchrony condition, suggesting greater ToM. Synchrony and asynchrony groups did not reliably differ in empathic accuracy nor emotional congruence. These results suggest that moving in synchrony has effects on social affiliation measures even in online encounters. These effects extend to ToM tendencies but not empathic accuracy or emotion sharing. These results highlight the potential of synchronous movement in online encounters to affect a subset of social cognition and affiliation measures.

Collective Attention and Collective Intelligence: The Role of Hierarchy and Team Gender Composition

Collective intelligence (CI) captures a team’s ability to work together across a wide range of tasks and can vary significantly between teams. Extant work demonstrates that the level of collective attention a team develops has an important influence on its level of CI. An important question, then, is what enhances collective attention? Prior work demonstrates an association with team composition; here, we additionally examine the influence of team hierarchy and its interaction with team gender composition. To do so, we conduct an experiment with 584 individuals working in 146 teams in which we randomly assign each team to work in a stable, unstable, or unspecified hierarchical team structure and vary team gender composition. We examine how team structure leads to different behavioral manifestations of collective attention as evidenced in team speaking patterns. We find that a stable hierarchical structure increases more cooperative, synchronous speaking patterns but that unstable hierarchical structure and a lack of specified hierarchical structure both increase competitive, interruptive speaking patterns. Moreover, the effect of cooperative versus competitive speaking patterns on collective intelligence is moderated by the teams’ gender composition; majority female teams exhibit higher CI when their speaking patterns are more cooperative and synchronous, whereas all male teams exhibit higher CI when their speaking involves more competitive interruptions. We discuss the theoretical and practical implications of our findings for enhancing collective intelligence in organizational teams.

The Components of Interpersonal Synchrony in the Typical Population and in Autism: A Conceptual Analysis

Interpersonal synchrony - the tendency for social partners to temporally co-ordinate their behaviour when interacting - is a ubiquitous feature of social interactions. Synchronous interactions play a key role in development, and promote social bonding and a range of pro-social behavioural outcomes across the lifespan. The process of achieving and maintaining interpersonal synchrony is highly complex, with inputs required from across perceptual, temporal, motor, and socio-cognitive domains. In this conceptual analysis, we synthesise evidence from across these domains to establish the key components underpinning successful non-verbal interpersonal synchrony, how such processes interact, and factors that may moderate their operation. We also consider emerging evidence that interpersonal synchrony is reduced in autistic populations. We use our account of the components contributing to interpersonal synchrony in the typical population to identify potential points of divergence in interpersonal synchrony in autism. The relationship between interpersonal synchrony and broader aspects of social communication in autism are also considered, together with implications for future research.

Mutual beta power modulation in dyadic entrainment

Across a broad spectrum of interactions, humans exhibit a prominent tendency to synchronize their movements with one another. Traditionally, this phenomenon has been explained from the perspectives of predictive coding or dynamical systems theory. While these theories diverge with respect to whether individuals hold internal models of each other, they both assume a predictive or anticipatory mechanism enabling rhythmic interactions. However, the neural bases underpinning interpersonal synchronization are still a subject under active investigation. Here we provide evidence that the brain relies on a common oscillatory mechanism to pace self-generated rhythmic movements and to track the movements produced by a partner. By performing dual-electroencephalography recordings during a joint finger-tapping task, we identified an oscillatory component in the beta range (∼ 20 Hz), which was significantly modulated by both self-generated and other-generated movement. In conditions where the partners perceived each other, we observed periodic fluctuations of beta power as a function of the reciprocal movement cycles. Crucially, this modulation occurred both in visually and in auditorily coupled conditions, and was accompanied by recurrent periods of dyadic synchronized behavior. Our results show that periodic beta power modulations may be a critical mechanism underlying interpersonal synchronization, possibly enabling mutual predictions between coupled individuals, leading to co-regulation of timing and overt mutual adaptation. Our findings thus provide a potential bridge between influential theories attempting to explain interpersonal coordination, and a concrete connection to its neurophysiological bases.

The relationship between stability of interpersonal coordination and inter-brain EEG synchronization during anti-phase tapping

Inter-brain synchronization is enhanced when individuals perform rhythmic interpersonal coordination tasks, such as playing instruments in music ensembles. Experimentally, synchronization has been shown to correlate with the performance of joint tapping tasks. However, it is unclear whether inter-brain synchronization is related to the stability of interpersonal coordination represented as the standard deviation of relative phase (SDRP). In this study, we simultaneously recorded electroencephalograms of two paired individuals during anti-phase tapping in three interactive tapping conditions: slow (reference inter-tap interval [ITI]: 0.5 s), fast (reference ITI: 0.25 s), and free (preferred ITI), and pseudo tapping where each participant tapped according to the metronome sounds without interaction. We calculated the inter-brain synchronization between pairs of six regions of interest (ROI): frontal, central, left/right temporal, parietal, and occipital regions. During the fast tapping, the inter-brain synchronization significantly increased in multiple ROI pairs including temporoparietal junction in comparison to pseudo tapping. Synchronization between the central and left-temporal regions was positively correlated with SDRP in the theta in the fast condition. These results demonstrate that inter-brain synchronization occurs when task requirements are high and increases with the instability of the coordination.

The intrapersonal and interpersonal consequences of interpersonal synchrony

Interpersonal synchrony, the time-matching behaviors, is pervasive in human interactions. This resonation of movements or other forms was generally considered as one of critical survival skills for humans, as the important consequences of synchronizing with other persons in review of the empirical data in this article. These include positive affects towards and between interacting partners, but also include complex effects on the individual level. The intrapersonal effects of interpersonal synchrony are varied with positive or negative ones, including cognitive style, attitude bias, mood state, self-regulatory ability, and academic performance. At the interpersonal level, synchronized movement consistently affects the interaction with the partner and his/her affiliations, but they can be eliminated or magnified by several moderators, such as physiological arousal, shared intentionality, group bias, and musical rhythm. Finally, the research discussed the possible mechanisms underlying the effects of interpersonal synchrony in psychological and biological aspects.

Stepping in time: Alpha-mu and beta oscillations during a walking synchronization task

Interpersonal behavioral synchrony is referred to as temporal coordination of action between two or more individuals. Humans tend to synchronize their movements during repetitive movement tasks such as walking. Mobile EEG technology now allows us to examine how this happens during gait. 18 participants equipped with foot accelerometers and mobile EEG walked with an experimenter in three conditions: With their view of the experimenter blocked, walking naturally, and trying to synchronize their steps with the experimenter. The experimenter walked following a headphone metronome to keep their steps consistent for all conditions. Step behavior and synchronization between the experimenter and participant were compared between conditions. Additionally, event-related spectral perturbations (ERSPs) were time-warped to the gait cycle in order to analyze alpha-mu (7.5-12.5 Hz) and beta (16-32 Hz) rhythms over the whole gait cycle. Step synchronization was significantly higher in the synchrony condition than in the natural condition. Likewise regarding ERSPs, right parietal channel (C4, C6, CP4, CP6) alpha-mu and central channel (C1, Cz, C2) beta power were suppressed from baseline in the walking synchrony condition compared to the natural walking condition. The natural and blocked conditions were not found to be significantly different in behavioral or spectral comparisons. Our results are compatible with the view that intentional synchronization employs systems associated with social interaction as well as the central motor system.

Altered activation in the action observation system during synchronization in high loneliness individuals

Lonely people tend to evaluate social exchanges negatively and to display difficulties in interactions. Interpersonal synchronization is crucial for achieving positive interactions, promoting affinity, closeness, and satisfaction. However, little is known about lonely individuals' ability to synchronize and about their brain activity while synchronizing. Following the screening of 303 participants, we recruited 32 low and 32 high loneliness participants. They were scanned while engaged in movement synchronization, using a novel dyadic interaction paradigm. Results showed that high loneliness individuals exhibited a reduced ability to adapt their movement to their partner's movement. Intriguingly, during movement adaptation periods, high loneliness individuals showed increased activation in the action observation (AO) system, specifically in the inferior frontal gyrus and the inferior parietal lobule. They did not show increased activation in the dorsomedial prefrontal cortex, which in the context of synchronization was suggested to be related to gap-monitoring. Based on these findings, we propose a model according to which lonely people may require stronger activation of their AO system for alignment, to compensate for some deficiency in their synchronization ability. Despite this hyperactivation, they still suffer from reduced synchronization capacity. Consequently, synchronization may be a relevant intervention area for the amelioration of loneliness.