Neural and Physiological Synchrony During Shared Physical Activity: A Cross-Species Review

1. Introduction: The Synchronized Brain in Action

1.1. Defining the Phenomenon: Interpersonal Neural Synchrony (INS) / Inter-Brain Synchrony (IBS) / Neural Coupling

Social interaction is a cornerstone of life for many species, enabling complex behaviors ranging from cooperation and communication to collective movement and social bonding. Underlying these interactions, a fascinating neurobiological phenomenon has emerged: the tendency for the brain activity of interacting individuals to become temporally correlated or aligned. This phenomenon, broadly termed Interpersonal Neural Synchrony (INS) or Inter-Brain Synchrony (IBS), refers to the dynamic, time-varying similarity between the spatio-temporal neural fluctuations of multiple individuals engaged in a social exchange.¹ It represents a convergence and coupling of distinct neurocognitive systems, reflecting a fundamental mechanism of bi-directional attunement between partners.¹

While INS/IBS is the most common terminology, related concepts such as brain-to-brain coupling, inter-subject correlation (ISC), between-brain connectivity, and neural coupling are frequently used in the literature, often denoting the same core idea of synchronized neural activity between individuals.² It is crucial, however, to distinguish this inter-brain phenomenon from intra-brain synchrony, which refers to the coordinated activity across different regions within a single individual's brain.² The study of INS/IBS is central to the "second-person neuroscience" perspective, which advocates for simultaneously measuring brain activity from multiple interacting individuals (a technique known as hyperscanning) to capture the dynamic neural processes that underpin social cognition—processes potentially missed by traditional single-brain studies.²

This synchrony is not limited to the neural level. Social interactions often involve alignment across multiple modalities simultaneously. Behavioral synchrony is readily observable when individuals unconsciously mirror each other's postures, facial expressions (like smiles or laughter), or movements.⁵ Physiological synchrony manifests as coordinated changes in autonomic nervous system activity (e.g., heart rate, breathing rhythm, electrodermal activity) and hormonal states between partners.⁵ These different levels of synchrony—behavioral, physiological, and neural—are often interconnected, influencing one another during social engagement.²⁰

While the terms are often used interchangeably, a subtle distinction sometimes exists between "synchrony" and "coupling." Neural synchrony typically emphasizes the temporal alignment or similarity of oscillatory patterns between brains.² Neural coupling, however, can sometimes represent a broader concept, encompassing any lawful relationship between brain activities, including those that are not perfectly mirrored or simultaneous, such as time-lagged dependencies or asymmetric relationships where activity in one brain region predicts activity in a different region in the partner's brain.¹⁰ This distinction is relevant because interactions, especially complex ones involving complementary roles, may rely on coordinated but non-identical neural dynamics across individuals.¹⁰ Focusing solely on identical, time-locked activity might overlook these more complex forms of inter-brain coordination. Acknowledging this potential nuance, this report will primarily use the terms interchangeably while recognizing that the underlying inter-brain relationship can encompass more than simple mirroring.

1.2. The Significance of Synchrony in Social Interaction and Shared Physical Activity

Interpersonal neural synchrony is increasingly recognized not just as a correlate of social interaction but as a functionally significant neural substrate that actively facilitates a wide range of social processes. Research consistently links higher levels of INS/IBS to enhanced empathy, more effective communication, increased cooperation, greater rapport and social bonding, and improved prosocial behavior.³ It is considered a key biological marker reflecting the quality and success of social connection and shared understanding.³

This phenomenon holds particular relevance for understanding shared physical activities. Many human endeavors, from the coordinated movements of musicians in an ensemble and dancers performing choreography to the collaborative efforts of athletes in team sports and partners performing cooperative motor tasks, fundamentally rely on precise temporal coordination of actions.⁴ INS/IBS may provide the essential neural scaffolding that enables individuals to align their movements, anticipate their partners' actions, and achieve shared goals requiring physical coordination.

However, interpreting INS findings requires careful consideration. While correlations between INS and behavioral outcomes are robust, establishing direct causality remains a challenge.⁴⁷ Critiques within the field highlight that observed synchrony could, in some cases, be partly driven by factors other than genuine interpersonal interaction, such as both individuals processing the same external stimuli (e.g., a shared visual scene or auditory cue) or performing similar motor actions in parallel, rather than actively coupling their neural dynamics through interaction.¹⁷ Furthermore, the lack of standardized definitions and methodological flexibility across studies has led some to question the theoretical robustness of INS as a construct.⁴⁸ Therefore, evaluating the significance of reported INS necessitates critical assessment of experimental designs, particularly the use of appropriate control conditions (e.g., pseudo-pairing, individual task performance, non-interactive social presence) and analytical techniques aimed at isolating interaction-specific neural coupling from shared stimulus processing or motor output.¹⁷

1.3. Scope: Exploring Human-Human, Animal-Animal, and Human-Animal Dynamics

This report aims to synthesize the current state of research on neural and physiological synchrony specifically observed during shared physical activities involving two interacting entities. Recognizing that the capacity for synchronized action exists beyond human interactions, the scope encompasses three distinct dyad types:

1. Human-Human Dyads: This category represents the most extensively studied area, providing a foundational understanding of INS mechanisms during cooperative motor tasks, rhythmic activities like music and dance, and sports.
2. Animal-Animal Dyads: Investigating synchrony in non-human animals offers insights into the evolutionary origins of coordinated action. This includes examining the widespread phenomenon of behavioral synchrony in collective movement (e.g., flocking, schooling) and exploring emerging research on potential neural synchrony during social interactions in species like bats and rodents.
3. Human-Animal Dyads: This burgeoning area explores the fascinating possibility of interspecies synchrony, particularly relevant given the deep social bonds humans form with certain companion animals, notably dogs and horses, often involving shared physical activities like play, walking, or riding.

By examining research across these three categories, this review seeks to provide a comprehensive overview of how brains and bodies synchronize during shared physical engagement, highlighting common principles, species-specific variations, and the underlying mechanisms driving these phenomena.

2. Underlying Mechanisms of Interpersonal Neural Synchrony

The emergence of synchronized neural activity between interacting individuals is thought to be supported by several fundamental neurocognitive mechanisms. These mechanisms allow individuals to perceive, interpret, predict, and adapt to each other's actions in real-time, creating the conditions for both behavioral and neural alignment.

2.1. Action-Perception Coupling and Motor Resonance

A core mechanism believed to underpin interpersonal coordination is action-perception coupling. This principle posits that observing an action activates neural representations in the observer's brain that overlap significantly with the representations used when executing that same action.¹¹ This creates a "common coding" framework where perception and action share a neural substrate, primarily within sensorimotor and parietal brain regions.¹² This coupling is not merely passive; it facilitates the understanding of observed actions and the prediction of their outcomes and intentions.⁵¹

During social interactions involving physical activity, this mechanism is crucial. When one individual observes their partner's movements, their own motor system is primed or resonated with, facilitating behavioral synchrony and coordinated joint action.¹¹ This shared neural activation provides a direct pathway for brain activity to become coupled across individuals. The perceptual system of one brain becomes linked to the motor system of the other, mediated by environmental signals such as the sight or sound of the partner's actions.¹¹ Sensory feedback plays a critical role in modulating this process; visual, auditory, and tactile cues from the partner, as well as proprioceptive and kinesthetic feedback from one's own movements, continuously inform and refine the coordination process.²¹ Notably, the auditory system often exerts a dominant influence on temporal processing and synchronization accuracy.²¹

Importantly, this coupling mechanism extends beyond simple imitation or mirroring of identical actions. It also supports the coordination of complementary actions, where partners perform different but interdependent movements to achieve a shared goal.¹⁰ For example, studies comparing joint tasks requiring identical versus complementary movements have found distinct patterns of inter-brain synchronization, suggesting that the underlying neural mechanisms support differentiated roles and coordinated non-identical actions, not just mimicry.⁴³ This implies that INS reflects a sophisticated coordination process capable of handling the complexities of real-world joint actions where partners often need to do different things in a coordinated manner.

2.2. Predictive Coding and Mutual Prediction Frameworks

Another influential theoretical framework for understanding INS is predictive coding.⁵⁵ This theory proposes that the brain operates as a prediction machine, constantly generating internal models to anticipate upcoming sensory input and minimizing the difference (prediction error) between the prediction and the actual input.⁵⁵ In the context of social interaction, this extends to predicting the actions, intentions, goals, and even mental states of interaction partners.⁵⁵ Effective social coordination relies on minimizing "social prediction errors" – the discrepancies between expected and observed social behavior.⁵⁶

Interpersonal neural synchrony is hypothesized to emerge naturally when two such predictive systems become coupled during interaction.⁵⁵ By aligning their neural activity, individuals can more effectively model and predict each other's behavior, thereby reducing uncertainty and facilitating smoother coordination.²¹ This mutual prediction operates across a hierarchy, involving high-level representations of shared goals and intentions (potentially involving prefrontal regions like the LPFC) and lower-level sensorimotor monitoring and prediction of specific movements (involving sensorimotor networks and potentially the TPJ for intention understanding).⁵⁵ Studies have indeed found correlations between INS and the predictability of interaction or successful coordination, with specific brain regions like the right dorsolateral prefrontal cortex (DLPFC) showing increased synchrony linked to mutual prediction during competitive tasks.⁵⁵

However, the relationship between prediction, engagement, and synchrony might be more complex than a simple linear increase. Recent theoretical proposals, such as irruption theory, suggest that while synchrony (IBS) facilitates prediction and integration, situations demanding high levels of subjective involvement, agency, or adaptation might lead to an increase in neural entropy within an individual's brain.⁵⁷ This increased entropy, reflecting greater internal processing complexity or exploration of behavioral options, could manifest as inter-brain desynchronization (IBD).⁵⁷ This perspective challenges the notion that more synchrony always equates to better interaction. Instead, it suggests a dynamic interplay between IBS, supporting mutual prediction and alignment, and IBD, reflecting individual agency and adaptation. Optimal social interaction might involve flexible transitions between these states depending on the specific demands of the task – sometimes requiring tight coupling, other times requiring individual divergence or initiative. This interplay could potentially explain some of the variability observed in IBS findings across different studies and contexts.

2.3. The Role of Shared Attention, Goals, and Intentionality

Successful joint physical activity inherently requires more than just movement coordination; it necessitates alignment at cognitive levels, including shared attention, shared goals, and shared intentionality. Joint actions are typically defined by the presence of a collective goal pursued through coordinated individual roles.³⁷ During such actions, individuals must represent not only their own task and goal but also those of their partner and the relationship between them.⁴³

Shared attention, often operationalized through mutual gaze, serves as a potent social trigger for establishing and modulating INS.⁵ The synchronization of neural activity, particularly in attentional networks, may reflect the shared attentional state required to monitor the partner and the joint task, facilitating coordinated action.³

Shared intentionality – the mutual commitment to engage in a joint activity with coordinated plans and roles – is also strongly linked to INS.⁴ Studies comparing cooperative tasks (implying shared intentionality) with individual or competitive tasks consistently find enhanced INS during cooperation.³ Specific frequency bands, such as gamma oscillations, have been proposed as potential neural markers specifically reflecting this state of shared intentionality during cooperative efforts.⁴⁰ Furthermore, research has begun to disentangle the neural correlates of different components of joint action, finding, for instance, that inter-brain synchrony in the right inferior frontal cortex (IFC) was more strongly associated with representing the shared goal, whereas left-IFC synchrony was more related to the action coordination component itself.⁴³

The observation that specific brain networks and frequency bands are differentially engaged depending on the precise nature of the coordination (e.g., goal vs. motor execution) and the social context (e.g., cooperation vs. competition, familiar vs. unfamiliar partner) underscores that INS is not a monolithic phenomenon.⁴⁰ Rather, it appears to be a dynamic and context-sensitive reflection of the specific cognitive, attentional, and motor processes recruited by the demands of the ongoing interaction. Different tasks engage different neural resources, and the resulting patterns of inter-brain synchrony reflect this differential engagement.

2.4. Contributions of the Mirror Neuron System (MNS)

The Mirror Neuron System (MNS), typically encompassing regions such as the inferior frontal gyrus (IFG) and inferior parietal lobule (IPL), is characterized by neurons that fire both when an individual performs an action and when they observe the same action performed by another.²⁷ This system is widely implicated in action understanding, imitation, and potentially empathy.²⁷

Given its role in linking action observation and execution, the MNS is considered a likely contributor to the action-perception coupling mechanisms discussed earlier, thereby providing a potential foundation for motor resonance and behavioral mimicry relevant to INS.⁵⁰ Some studies investigating joint action and INS have reported activation or synchronization involving MNS-related areas.²⁷ Theoretical accounts suggest the MNS may help individuals not only mirror their partner's actions but also process the complementary or even opposing actions required for successful coordination in complex tasks.⁵⁴

However, while the MNS likely plays a foundational role in processing observed actions and enabling basic motor resonance, the patterns of INS observed during complex social interactions often extend beyond MNS regions. Synchrony frequently involves higher-order brain areas associated with more complex cognitive functions, such as the prefrontal cortex (PFC) for planning, goal representation, and executive control, and the temporoparietal junction (TPJ) for mentalizing, perspective-taking, and intention understanding.³ Many studies explicitly note that observed INS effects cannot be fully explained by shared motor activity alone and implicate attentional and mentalizing networks.¹³ This suggests that while the MNS may provide crucial input regarding the partner's actions, the resulting inter-brain synchrony during sophisticated joint action likely arises from the integration of this information within broader neural networks responsible for top-down cognitive control, social prediction, and the representation of shared goals. The MNS may be necessary for the basic coupling, but it is likely insufficient to explain the full complexity of INS observed during rich social interactions.

3. Human Dyads in Motion: Neural Synchrony During Shared Physical Activities

Research using hyperscanning techniques has provided substantial evidence for interpersonal neural synchrony during a variety of shared physical activities in human dyads. These studies typically involve simultaneously recording brain activity (most often using EEG or fNIRS) from two individuals while they engage in tasks requiring motor coordination, rhythmic alignment, or collaborative physical problem-solving.

3.1. Cooperative Motor Tasks and Problem Solving

A large body of hyperscanning research has focused on relatively controlled cooperative motor tasks. Common paradigms include synchronized finger tapping ⁴, coordinated key pressing ³⁰, joint drawing tasks where partners control different dimensions of movement ⁴², cooperative games like Jenga ⁴⁰ or digital puzzles ⁴, custom computer games requiring collaboration ³⁴, and cooperative construction or building tasks.⁴¹

Across these diverse tasks, a consistent finding is the enhancement of INS during cooperative conditions compared to control conditions, such as individuals performing the task alone, competing against each other, or simply being co-present without interaction.³ This suggests that the requirement to actively coordinate actions towards a shared goal drives the alignment of neural activity between partners.

The brain regions most frequently implicated in this cooperative INS include frontal areas (specifically the prefrontal cortex (PFC), encompassing dorsolateral PFC (DLPFC), inferior frontal cortex (IFC), middle frontal gyrus, and frontopolar cortex), parietal regions (such as the temporoparietal junction (TPJ) and centroparietal areas), and parts of the temporal lobe.⁴ These regions are known to be involved in executive functions, action planning, attention, social cognition (mentalizing, theory of mind), and sensorimotor integration, highlighting the complex cognitive and motor processes underlying cooperative action.

Analyses across different frequency bands reveal that INS is not restricted to a single rhythm but occurs across multiple bands, potentially reflecting distinct aspects of the interaction. Theta band (4-7 Hz) synchrony has been linked to motor coordination processes, mutual adaptation, and potentially reinforcement learning during tasks like finger pointing, tapping, and ensemble playing.⁴⁰ Alpha/Mu band (8-12 Hz) synchrony, particularly over sensorimotor and parietal areas, is often observed during motor imitation, turn-taking, and tasks requiring shared attention or representing the partner's actions.¹² Beta band (13-30 Hz) synchrony has also been reported, sometimes differentiating cooperation from competition.²⁰ Gamma band (>30 Hz) synchrony has been associated with higher task performance, successful coordination, and potentially the representation of shared intentionality.²⁰

The predominant neuroimaging methods used in these studies are electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS).³ These techniques offer advantages for hyperscanning during physical tasks due to their relatively high temporal resolution (especially EEG), tolerance for participant movement compared to fMRI, and lower cost.² Functional magnetic resonance imaging (fMRI) hyperscanning is less common due to movement constraints and cost but provides superior spatial resolution and the ability to image deeper brain structures.²

Crucially, the observed INS during these tasks is often functionally relevant. Higher levels of synchrony frequently correlate with better objective task performance (e.g., faster completion times, higher accuracy in coordination) and more positive subjective experiences (e.g., higher ratings of cooperation, connectedness, or rapport).³ In some studies involving team problem-solving, INS was found to be a better predictor of collective performance than participants' self-reported identification with the group, suggesting that inter-brain dynamics capture aspects of effective collaboration not accessible through subjective reports alone.¹³

However, the specific patterns of INS are highly sensitive to the nuances of the interaction. Tasks involving turn-taking, for instance, elicit specific synchrony patterns, sometimes showing asymmetries related to leader-follower roles, such as theta synchrony between the leader's frontal region and the follower's right TPJ during tapping.¹² Similarly, tasks designed to manipulate shared intentionality ⁴ or require mutual prediction under competitive pressure ⁵⁵ engage distinct INS profiles compared to simple cooperation. For example, cooperation and competition within the same game elicited different spatial patterns and directions (positive vs. negative correlation) of INS across multiple frequency bands.⁶¹ Acute stress has also been shown to modulate INS, enhancing synchrony in the right TPJ during cooperative tasks, potentially reflecting increased reliance on mentalizing under pressure.⁴⁹ This context-dependency demonstrates that INS is not merely a passive reflection of being engaged together, but actively mirrors the specific cognitive and interpersonal challenges posed by the interaction.

3.2. Rhythmic Coordination: Music Performance and Dance

Activities like ensemble music performance and dance represent particularly compelling examples of shared physical activity requiring high levels of precise, yet flexible, rhythmic interpersonal coordination.¹¹ These domains have provided valuable insights into the neural underpinnings of synchrony.

Studies involving musicians performing together (e.g., piano duets ¹¹, guitar ensembles ¹¹, group drumming ⁶⁶, cooperative singing ⁴) consistently demonstrate the presence of INS, often measured using EEG hyperscanning.³² This neural synchrony is frequently linked to behavioral synchrony, such as the precision of timing between musicians' actions (e.g., tone onset asynchronies).³⁸ Intriguingly, research shows that differences in musicians' natural individual tempos (spontaneous rates when playing alone) predict how well they synchronize during duet performance, providing strong support for models viewing interpersonal synchrony as arising from the coupling of underlying biological oscillators.³⁸ This suggests that the physical act of coordinating rhythmic movements directly influences the entrainment of neural oscillators between individuals. Furthermore, INS associated with musical interaction can persist even after the interaction ceases, implying the existence of active neural mechanisms for maintaining synchrony or a "neural echo" of the shared experience.²⁵ External rhythmic structures, like a musical meter, can also enhance the observed IBS during coordinated tapping tasks.⁶⁶ Research indicates synchronization occurs not only between the brains of the musicians but also between their brains and the sounds produced by the instruments, highlighting the interplay between action, perception, and inter-brain coupling.³⁶ Key brain areas involved include frontal, central, parietal, and temporal lobes ⁷³, with frontal theta synchrony specifically noted during ensemble guitar playing.⁴¹

While direct hyperscanning studies during dance are less common, likely due to motion artifacts, dance is theoretically proposed as a powerful activity for enhancing both within-brain and between-brain synchrony.²⁹ The multi-faceted nature of dance, engaging sensory, motor, cognitive, social, emotional, rhythmic, and creative processes, provides a rich context for synchronization.³⁹ Behavioral studies confirm that synchronized movement in activities like dance or marching promotes positive social outcomes such as rapport, empathy, and cooperation.¹⁴ Neuroimaging studies of individual dancers or dance observation implicate brain regions similar to those involved in INS, including prefrontal, motor, somatosensory, and temporal areas.³⁹ The inherent rhythmicity and embodied nature of both music and dance strongly point towards the mutual entrainment of neural oscillators, driven by the shared physical actions, as a key mechanism contributing to the observed INS in these contexts.

3.3. Team Sports and Competitive Interactions

Team sports provide a naturalistic setting where cooperation, competition, and highly skilled physical coordination intertwine.¹¹ While challenging to study with neuroimaging due to intense physical movement, some studies have begun to explore INS in sports-related contexts or using participants with sports expertise.

Using a motion-sensing tennis video game paradigm with EEG hyperscanning, researchers found that INS patterns clearly differentiated between cooperative and competitive modes of play within the same physical task.⁴⁰ Cooperation was associated with positive inter-brain amplitude correlations, particularly in lower frequency bands (delta and theta) across widespread brain regions, whereas competition was characterized by negative correlations, especially in the beta band over occipital areas.⁶¹ Further analysis suggested theta band IBS might relate to the motor coordination common to both conditions, while gamma band IBS emerged specifically during cooperation, possibly reflecting shared intentionality.⁴⁰

Another study used fNIRS hyperscanning to compare dyads of experienced basketball players with dyads of college students performing a cooperative joint-drawing task.⁴² The basketball players not only completed the task significantly faster and reported higher subjective cooperativeness but also exhibited significant INS in the dorsolateral prefrontal cortex (DLPFC) during the cooperative task, a pattern absent in the control student group.⁴² This finding strongly suggests that extensive experience and training in team-based sports, which inherently demand high levels of interpersonal coordination and shared strategy, can lead to neuroplastic changes that enhance the capacity for inter-brain synchronization during cooperative physical actions. This highlights that the ability to synchronize neurally with a partner during joint physical tasks is not fixed but can be shaped by long-term experience and skill acquisition in relevant domains.

4. Echoes in the Animal Kingdom: Behavioral and Neural Synchrony in Non-Human Dyads

Understanding the evolutionary roots and fundamental mechanisms of interpersonal synchrony requires looking beyond human interactions. Studies of non-human animals provide crucial insights into both the widespread nature of behavioral coordination and the potential neural underpinnings shared across species.

4.1. Collective Behavior as Behavioral Synchrony

Spectacular examples of behavioral synchrony are ubiquitous in the animal kingdom. The coordinated movements of bird flocks, fish schools, insect swarms, and mammal herds represent highly synchronized group actions.²¹ These collective behaviors typically emerge in a self-organized manner from relatively simple, local interaction rules followed by individual members: maintain cohesion by moving towards distant neighbors, avoid collisions by moving away from close neighbors, and align movement direction with nearby individuals.⁷⁴ This coordination relies on the real-time processing of sensory information about neighbors, primarily visual cues in many species like fish and birds, but also potentially auditory or chemical signals.⁷⁴

This remarkable behavioral synchrony serves critical adaptive functions, enhancing survival and foraging success through mechanisms like improved predator detection (the "many eyes effect"), collective defense (e.g., confusion effect), more efficient exploration of the environment, and potentially energy savings during locomotion.⁷⁴ From a mechanistic perspective, these collective movements demonstrate that sophisticated group-level coordination can arise from decentralized control and local interactions, representing a fundamental form of behavioral synchrony essential for group living in many species.⁸⁰

The prevalence and effectiveness of such behavioral synchrony across a vast range of animal taxa, from simple multicellular organisms to vertebrates ⁷⁶, suggest that the underlying algorithms for coordinating actions based on social cues are evolutionarily ancient and deeply ingrained. This widespread behavioral foundation for mutual adjustment and coordinated action likely predates, and perhaps provides the necessary scaffold for, the emergence of the more complex physiological and neural synchrony observed during social interactions in mammals, including humans. The basic principles of sensing, predicting, and reacting to neighbors' movements are fundamental building blocks upon which more sophisticated forms of inter-individual coupling may have evolved.

4.2. Investigating Neural Correlates in Animal Models

To delve deeper into the cellular and circuit mechanisms underlying social interaction and potential synchrony, researchers turn to animal models such as rodents, bats, and non-human primates. These models allow for the use of more invasive recording techniques (e.g., electrode implants for single-unit or LFP recordings) and causal manipulation methods (e.g., optogenetics, pharmacogenetics, targeted lesions) that are not feasible in humans.⁸²

Pioneering studies have begun to apply hyperscanning-like approaches to animals. Notably, research using freely interacting Egyptian fruit bats, a highly social mammal, employed simultaneous wireless neural recordings from pairs of individuals.²² These studies revealed strong inter-brain correlations (synchrony) in local field potential (LFP) power, particularly in the 30-150 Hz frequency range, when bats were interacting within the same physical space. This synchrony was significantly reduced or absent when the bats were physically separated, indicating it was contingent on social interaction.²² Behavioral analyses annotated various social actions like grooming, fighting, and mating, allowing correlation with neural data.²²

A significant contribution of this bat research was the analytical approach of decomposing the inter-brain relationship into two components: the mean activity across the pair (representing similarity or synchrony) and the difference in activity between them.²² This analysis revealed distinct temporal dynamics: the mean component, reflecting shared activity patterns, evolved relatively slowly, while the difference component fluctuated much more rapidly.²² This suggests that the difference signal might capture fast, moment-to-moment adjustments, negotiations, or divergences in neural states during dynamic social exchanges – information potentially obscured by measures focusing solely on similarity or correlation. It points towards a more complex interplay between alignment and differentiation in the neural dynamics of interacting partners.

In rodents, research has focused on dissecting the neural circuits involved in social behaviors like aggression, mating, and social recognition, often implicating pathways involving the amygdala, prefrontal cortex, and hypothalamus.⁸² While direct inter-brain synchrony studies are fewer, technological advancements are paving the way. For example, the CBRAIN platform integrates mobile edge computing (MEC) and LEDs onto a wireless headstage, allowing researchers to detect specific neural events (e.g., gamma oscillations in the amygdala) in real-time and visualize their occurrence via LED flashes on the heads of multiple freely interacting mice.⁸⁵ This technology enables the spatiotemporal mapping of specific neural activities onto observable group behaviors. Some studies have also reported evidence suggestive of direct brain-to-brain communication or synchrony in rats, potentially mediated by weak electromagnetic fields generated by synchronized neural activity.⁸⁶

Studies in non-human primates leverage their complex social structures and cognitive abilities, which more closely resemble humans.⁸² Research often focuses on the neural basis of social perception (e.g., face processing), social decision-making, and understanding social hierarchy, frequently examining activity in prefrontal cortex, amygdala, and temporal regions.⁸² There is a growing emphasis on moving towards more naturalistic, neuroethological paradigms that allow primates to engage in species-typical social interactions, rather than highly constrained laboratory tasks, to better understand the relevant neural mechanisms.⁸³

These advances in animal research are critically dependent on technological innovation. The development of wireless, multi-channel recording systems ²² and sophisticated real-time analysis and reporting tools like CBRAIN ⁸⁵ are essential for overcoming the limitations of traditional tethered recordings and highly controlled environments. They allow researchers to probe brain activity during the kinds of spontaneous, dynamic, and physically active social interactions that are most relevant to understanding the natural basis of behavioral and potentially neural synchrony in non-human species.

5. Bridging the Species Gap: Exploring Human-Animal Synchrony

The deep and often complex relationships between humans and certain animal species, particularly domesticated companions like dogs and horses, raise the intriguing question of whether the phenomenon of neural and physiological synchrony extends across species boundaries. Research in this area investigates the biological underpinnings of the human-animal bond, focusing on shared activities and interactions.

5.1. Neural Synchrony: Human-Dog Interactions

Dogs, with their long history of co-evolution and domestication alongside humans, have developed remarkable abilities to communicate and interact socially with people, making them a prime species for investigating interspecies synchrony.²¹ Recent groundbreaking research utilized non-invasive wireless EEG hyperscanning to simultaneously measure brain activity in humans and dogs (specifically, research beagles in the key study) while they engaged in social interactions.⁵⁹

The interactions studied involved periods of mutual gaze and petting, compared against control conditions with no interaction or only partial interaction (gaze alone or petting alone).⁵⁹ The results provided compelling evidence for interspecies neural coupling. Significant interbrain synchronization was observed between the human and dog brains specifically during the periods of active social interaction.⁵⁹

Interestingly, the synchronization appeared to be localized and activity-dependent. Mutual gazing was primarily associated with increased synchrony in frontal brain regions in both species, areas linked to attention and social cognition.⁵⁹ Petting, on the other hand, was primarily linked to increased synchrony in parietal regions, areas involved in processing tactile and somatosensory information.⁵⁹ This dissociation suggests that the interspecies neural coupling is not a diffuse effect reflecting general arousal or bonding, but rather a specific alignment of neural activity within brain networks relevant to the particular type of sensory and cognitive processing demanded by the ongoing interaction (e.g., visual attention for gaze, sensorimotor processing for touch). Furthermore, the combined interaction involving both gaze and petting resulted in significantly stronger interbrain coupling than either activity alone, indicating a synergistic effect where multiple modalities of positive social interaction enhance neural synchrony.⁵⁹

The study also revealed that this interspecies synchrony is influenced by familiarity. Over five consecutive days of interaction sessions, the intensity of the human-dog interbrain coupling progressively increased, eventually reaching a plateau around the seventh day in an extended experiment.⁵⁹ This dynamic mirrors findings in human dyads, where synchrony often increases as partners become more familiar or comfortable with each other.⁸⁹ Control analyses, comparing data from the same dyad across different sessions, showed much lower correlations, confirming that the observed synchrony was specific to the real-time, ongoing interaction.⁵⁹ Directionality analysis (Granger causality) suggested a pattern where the human tended to lead the interaction neurally, with the dog's brain activity following.⁵⁹

Further highlighting the potential significance of this interspecies synchrony, the researchers examined dogs with genetic mutations linked to autism-like social deficits. These dogs failed to show the typical interbrain coupling with human partners during interaction.⁵⁹ Remarkably, administration of a low dose of LSD, a compound known to affect serotonergic systems implicated in social behavior, was able to restore the interbrain coupling in these dogs.⁵⁹ This finding positions interspecies INS not only as a measure of the human-dog bond but also as a potential translational biomarker. The human-dog dyad could serve as a valuable model system for studying the neural basis of social impairments found in disorders like Autism Spectrum Disorder (ASD) and for testing the efficacy of potential therapeutic interventions aimed at improving social function.

Complementing these direct neural findings, theoretical work suggests that dogs possess the necessary prerequisites for motor resonance, potentially involving mirror neuron systems. This implies that the behavioral synchrony often observed between humans and dogs might be underpinned by a form of interspecific motor resonance, where observing human actions activates corresponding motor representations in the dog's brain, and vice versa.⁵⁰

5.2. Physiological Synchrony: Heart Rate Variability (HRV) and Hormones

Beyond direct neural measures, physiological signals offer another valuable window into interspecies synchrony, potentially being easier to measure during more naturalistic movements and reflecting fundamental emotional and autonomic states.⁹¹ Heart Rate Variability (HRV), an index of the balance between sympathetic (arousal) and parasympathetic (relaxation) activity in the autonomic nervous system (ANS), and levels of hormones like oxytocin and cortisol have been investigated in both human-dog and human-horse interactions.

In human-dog dyads, studies have demonstrated significant correlations, or co-modulation, of HRV between owners and their dogs, particularly during periods of calm interaction or rest.⁹² In one study, the dog's overall HRV level was predictive of the owner's overall HRV level across different tasks.⁹² During more active tasks like playing or walking, the correlation shifted towards physical activity levels, though HRV remained influenced by the shared context.⁹² This physiological linkage appears specific to the established bond, as co-modulation was not observed when dogs were paired with unfamiliar humans.⁹³ Factors such as the dog's size, the duration of ownership, and the owner's personality traits (e.g., negative affectivity) were found to influence the dog's HRV responses during interaction.⁹² General interactions with dogs, such as petting, are often associated with beneficial physiological effects in humans, including lowered heart rate and blood pressure ⁹⁵, and increased markers of relaxation (e.g., alpha power in EEG).⁹⁰

The hormone oxytocin (OT), often dubbed the "love hormone" or "bonding hormone," plays a well-established role in social attachment, trust, and prosocial behavior within species.⁹⁴ Research suggests OT is also involved in the human-animal bond. Positive interactions involving touch (petting, stroking) and mutual gaze can lead to increased OT levels in both the human and the dog.⁹⁰ Activation of the oxytocinergic system has been proposed as a key underlying mechanism mediating many of the positive psychological (e.g., mood improvement, reduced anxiety) and physiological (e.g., stress reduction) effects of human-animal interaction.⁹⁵ However, findings regarding OT release during human-dog interaction are not entirely consistent. While some studies report significant increases ⁹⁴, others have failed to find such effects.⁹⁶ This inconsistency suggests that the role of oxytocin might be more complex than simply being released as a direct consequence of positive interaction in all circumstances. Factors like the specific type of interaction, the familiarity between partners (owner vs. stranger), the quality of the relationship, and even baseline hormone levels might modulate OT release.⁹⁶ For instance, one study found that owners with lower baseline OT levels tended to touch their dogs more, and dogs with lower OT levels received more stroking, suggesting OT might modulate the propensity for interaction rather than just being an outcome.⁹⁴ Thus, OT may function more as a modulator of social motivation and behavior within the interaction, influenced by context and individual history, rather than a simple, reliable biomarker of the interaction's positivity or synchrony itself. Cortisol, a stress hormone, generally decreases in humans during positive interactions with dogs ⁹⁰, while results in dogs are mixed, sometimes showing increases, which could reflect positive excitement or mild stress related to the experimental context.⁹⁴

In human-horse interactions, the concept of "emotional transfer" suggests a mutual coordination and coupling of emotional states, potentially underpinning the therapeutic benefits of equine-assisted interventions (EAIs).¹⁰⁰ Horses are known to be sensitive to human emotions.¹⁰⁰ Studies measuring physiological synchrony have found evidence for bidirectional coupling in HRV between humans and horses.¹⁰⁹ The directionality of this coupling (who influences whom more) appears sensitive to the type of activity (e.g., horse-led exploration vs. human-led grooming) and the level of familiarity between the human and horse.¹⁰⁹ Another study observed HRV synchrony between humans and horses simply walking side-by-side, although interestingly, the horse's HRV was slightly higher during periods of behaviorally synchronous walking, perhaps indicating greater attentiveness towards the human partner.¹⁰¹ While direct neural hyperscanning between riders and horses has not yet been reported, research using horseback riding simulators monitors rider brain activity (EEG), showing prominent frontal lobe activation related to concentration and motor control.¹¹⁰ The development of wearable EEG systems for active horses holds promise for future investigations into direct rider-horse neural coupling.¹¹²

The consistent findings of physiological synchrony, particularly in HRV, across both human-dog and human-horse dyads suggest that coupling through the autonomic nervous system might represent a fundamental and perhaps more readily measurable form of interspecies synchrony compared to direct neural (EEG) synchrony. The ANS regulates basic arousal and emotional states, systems highly conserved across mammals. This physiological linkage could provide a robust channel for emotional co-regulation and connection during shared physical presence and activity, potentially forming a basis upon which more specific neural synchrony might occur in species capable of it, like dogs.

6. Synthesis and Comparative Analysis

Bringing together findings from human-human, animal-animal, and human-animal interactions during shared physical activity allows for a comparative analysis of synchrony patterns, underlying mechanisms, and the influence of context.

6.1. Comparing Synchrony Patterns Across Dyad Types

The investigation of synchrony manifests differently across the dyad types, largely influenced by methodological feasibility and research focus:

• Human-Human: Research predominantly focuses on neural synchrony (INS/IBS) measured via EEG and fNIRS during structured cooperative motor tasks, rhythmic activities (music, dance), and simulated sports scenarios. Behavioral synchrony is also frequently assessed.
• Animal-Animal: Studies primarily document behavioral synchrony, particularly the highly coordinated collective movements of flocks and schools. Neural investigations are emerging, using implanted electrodes (LFP, single units) in species like bats and rodents to explore inter-brain correlations during social interactions in more controlled or semi-naturalistic settings.
• Human-Animal: This area shows emerging evidence for neural synchrony (EEG) specifically in human-dog dyads during defined interactions like gaze and petting. There is stronger and more established evidence for physiological synchrony, particularly in HRV and hormonal changes (oxytocin, cortisol), in both human-dog and human-horse interactions. Behavioral synchrony (e.g., movement coordination) is also noted.⁵⁰

Despite these differences, some similarities emerge. Across all dyad types where measured or inferred, shared physical activity often involves temporal coordination and mutual adjustment. When neural correlates are measured (humans, dogs), frontal and parietal/temporal brain regions appear consistently involved in coordinating attention, action planning, and processing social or sensory cues relevant to the interaction. Factors like familiarity and the degree of cooperation seem to enhance synchrony where investigated (humans, human-dog).

Key differences lie in the methods employed and the complexity of interactions studied. Animal models allow for invasive techniques offering higher resolution circuit-level insights, while human and human-animal studies rely on non-invasive methods. Human studies often involve complex, goal-directed tasks requiring sophisticated cognitive coordination (planning, prediction, mentalizing), whereas animal studies might focus on more fundamental social behaviors (grooming, aggression, collective movement), and human-animal studies often examine specific interaction components (petting, gaze, walking, grooming). The relative prominence of different synchrony mechanisms may also vary; for example, explicit shared intentionality and language heavily shape human INS, while hormonal coupling appears particularly salient in the study of human-animal physiological bonds.

6.2. Common vs. Distinct Mechanisms and Markers

Certain underlying mechanisms likely contribute to synchrony across different species and contexts. Action-perception coupling, the linking of observed actions to one's own motor representations, is a fundamental process likely present in any species capable of observing and adaptively responding to the movements of others, providing a basis for motor resonance and behavioral alignment.¹¹ The principles of coupled oscillators, where interacting rhythmic systems mutually entrain, seem applicable to rhythmic coordination in both human musical performance and potentially coordinated limb movements during locomotion in animals.³⁸ Predictive processes, where individuals anticipate upcoming events or partner actions, are also likely fundamental to any form of successful coordination, from predicting a neighbor's turn in a flock to anticipating a partner's move in a cooperative task.⁵⁵

However, the emphasis and complexity of certain mechanisms likely differ. Higher-level cognitive functions, such as abstract goal representation, complex shared intentionality, theory of mind (mentalizing), and language-based communication, play a significant role in shaping human INS, particularly engaging prefrontal and temporoparietal networks.³ These cognitive layers are presumably less developed or operate differently in most non-human animal interactions. In the human-animal domain, hormonal mechanisms, especially the oxytocinergic system, receive considerable attention as potential mediators of the bond and associated physiological responses, perhaps reflecting the strong affective component of these relationships.⁹⁴

The primary markers used to quantify synchrony also vary. In human hyperscanning, direct measures of neural synchrony (e.g., EEG/fNIRS coherence, phase-locking value, correlation of BOLD signals) are central. In human-animal studies, while neural measures are emerging, physiological markers like HRV correlation and hormone levels are prominent. In studies of animal collective behavior, behavioral synchrony (e.g., alignment, coordinated turning) is often the primary outcome measure.

6.3. The Role of Physical Activity Complexity and Shared Goals

The specific nature of the shared physical activity significantly influences the observed synchrony patterns. Simple, highly rhythmic tasks, such as synchronized tapping or joint music performance, tend to elicit strong synchrony, likely driven significantly by bottom-up sensorimotor entrainment and coupled oscillator dynamics.⁴ The predictability and regularity of the movements facilitate tight temporal coupling.

In contrast, more complex, goal-directed joint actions, like collaborative problem-solving, building tasks, or strategic games, engage higher-order cognitive processes. The resulting INS patterns reflect not just motor coordination but also shared attention, goal representation, action planning, mutual prediction, and potentially mentalizing.¹⁶ Synchrony in these tasks often involves prefrontal and parietal networks associated with these cognitive functions. Furthermore, the social context matters: competitive interactions elicit different INS patterns compared to cooperative ones, likely reflecting differing goals and predictive strategies.⁴⁰

Comparing across dyads, human joint actions frequently involve explicit negotiation and representation of complex, shared goals. In animal collective behavior, the "goals" are often more implicit and related to immediate survival needs like safety or foraging, achieved through local interaction rules. Human-animal interactions during physical activities like walking, grooming, or play may involve simpler, more affectively driven shared goals related to companionship, task completion, or mutual enjoyment. The level of cognitive complexity inherent in the shared goal and the required coordination strategy likely shapes the specific neural networks engaged and the nature of the resulting inter-individual synchrony.

This variation across tasks and species suggests a potential hierarchy or spectrum of synchrony mechanisms. At a fundamental level, basic behavioral coupling (mimicry, coordinated movement) and physiological coupling (shared autonomic states reflected in HRV, hormonal influences) might be relatively widespread across species and interaction types, providing a foundation for social connection and coordination. More fine-grained neural synchrony, particularly involving specific frequency bands (like gamma) and higher-order cortical networks (like PFC and TPJ), may represent a more sophisticated layer built upon this foundation. This neural layer appears especially prominent in humans and potentially other highly social mammals, enabling the complex cognitive coordination, precise mutual prediction, and representation of shared intentionality required for elaborate joint actions and social understanding.

6.4. Proposed Comparative Table

To provide a structured overview of the diverse findings discussed, the following table summarizes key details from representative studies across the different dyad types and activities.

Note: This table provides illustrative examples and is not exhaustive. INS = Interpersonal Neural Synchrony; IBS = Inter-Brain Synchrony; EEG = Electroencephalography; fNIRS = Functional Near-Infrared Spectroscopy; LFP = Local Field Potential; MUA = Multi-Unit Activity; SU = Single Unit Activity; HRV = Heart Rate Variability; AEC = Amplitude Envelope Correlation.

7. Conclusion: Shared Rhythms, Shared Minds

7.1. Recap of Key Findings

The research reviewed herein provides compelling evidence that the brains and bodies of interacting individuals can synchronize during shared physical activities. This phenomenon, broadly termed interpersonal neural or physiological synchrony, is not confined to human interactions but extends, at least in some forms, to animal-animal and human-animal dyads.

In humans, neural synchrony (INS/IBS), typically measured with EEG or fNIRS, is consistently observed during cooperative motor tasks, rhythmic activities like music and dance, and team sports. This synchrony often involves frontal, parietal, and temporal brain regions across various frequency bands (theta, alpha, beta, gamma) and frequently correlates with improved coordination, task performance, and positive social outcomes.

In the animal kingdom, highly synchronized collective movements (flocking, schooling) represent a fundamental form of behavioral synchrony. Emerging neural studies in bats and rodents are beginning to uncover inter-brain correlations during naturalistic social interactions, aided by technological advances in wireless recording and real-time analysis.

Across the species gap, recent studies demonstrate direct neural synchrony (EEG) between humans and dogs during specific social interactions like mutual gaze and petting, with patterns influenced by familiarity and the type of interaction. Furthermore, robust evidence exists for physiological synchrony, particularly in heart rate variability (HRV), between humans and both dogs and horses during shared activities and rest, potentially mediated by autonomic nervous system coupling and hormonal systems like oxytocin.

Underlying these phenomena are likely mechanisms involving action-perception coupling and motor resonance, which create shared neural representations of actions, and predictive coding processes, where brains mutually anticipate and adapt to each other's behavior to minimize prediction errors and facilitate coordination. The specific expression of synchrony appears highly context-dependent, shaped by the complexity of the physical activity, the nature of the shared goals (cooperative vs. competitive), individual experience, and the relationship between partners.

7.2. Implications for Understanding Social Bonds, Communication, and Coordination

The study of interpersonal synchrony offers profound implications for understanding the biological basis of social life. It suggests that our ability to connect, communicate, and coordinate effectively with others is deeply rooted in the capacity of our nervous systems to dynamically couple with one another during interaction.

• Social Bonds and Empathy: INS provides a potential neural mechanism for empathy, rapport, and the feeling of "being on the same wavelength" with others.⁵ The finding that synchrony can occur even across species ⁵⁹ sheds light on the deep biological roots of the human-animal bond and interspecies communication.
• Communication and Coordination: Synchrony appears crucial for non-verbal communication and the seamless coordination required for complex joint actions, from musical performance to team sports.¹¹ Understanding the factors that promote or hinder synchrony could inform strategies for enhancing teamwork and collaborative performance.¹³
• Clinical Relevance: Atypical patterns of synchrony have been observed in disorders characterized by social interaction difficulties, such as Autism Spectrum Disorder (ASD).⁵ INS (including interspecies INS ⁵⁹) holds potential as a biomarker for diagnosing social deficits and evaluating the effectiveness of interventions, including behavioral therapies, neurofeedback, or pharmacological treatments aimed at improving social function.⁶ Similarly, understanding the role of synchrony in human-animal interaction could help optimize animal-assisted therapies.⁹⁰

7.3. Future Directions and Open Questions

Despite significant progress, the field of interpersonal synchrony during shared physical activity presents numerous avenues for future research:

• Expanding Species Scope: More research is needed to investigate neural synchrony during coordinated physical activity in non-human animal dyads beyond the initial studies in bats and rodents. Applying techniques like wearable EEG to study rider-horse neural coupling during equestrian activities is a promising direction.¹¹²
• Causality and Manipulation: While correlations between synchrony and behavior are well-established, further research using manipulation techniques like dual-brain stimulation or hyperscanning neurofeedback is needed to clarify the causal role of INS in driving behavioral outcomes.³
• Multi-Level Integration: The interplay between behavioral, physiological (ANS, hormonal), and neural levels of synchrony requires deeper investigation. How do these different levels influence each other during dynamic interactions?.⁵
• Context and Individual Differences: The influence of factors like task complexity, social context (cooperation vs. competition), relationship history, familiarity, personality traits, and cultural background on synchrony patterns needs more systematic exploration.⁵
• Methodological and Theoretical Refinement: Continued development of sophisticated analytical methods is crucial to move beyond simple correlations and capture the complex, potentially asymmetric, and time-varying nature of inter-brain coupling.¹⁰ Addressing theoretical critiques regarding the definition and interpretation of INS will strengthen the field's foundations.¹⁷
• Naturalistic Paradigms: Pushing towards more ecologically valid experimental designs that allow for spontaneous, less constrained physical interactions will be key to understanding how synchrony operates in real-world settings.¹³

In conclusion, the study of interpersonal synchrony during shared physical activity reveals a fundamental aspect of social connection across species. From the coordinated flight of birds to the intricate neural coupling between collaborating humans or interacting humans and dogs, shared rhythms appear to facilitate shared experiences and actions. Continued research into the mechanisms, functions, and modulators of this phenomenon promises to deepen our understanding of the biological basis of social behavior, communication, and the powerful bonds that link interacting individuals.