Neural synchrony is the correlation of brain activity across two or more people over time. In social and affective neuroscience, neural synchrony specifically refers to the degree of similarity between the spatio-temporal neural fluctuations of multiple people. This phenomenon represents the convergence and coupling of different people's neurocognitive systems, and it is thought to be the neural substrate for many forms of interpersonal dynamics and shared experiences. A hypothesis of natural neurostimulation explains the correlation of brain activity across two or more organisms as an evolutionary mechanism of the nervous system growth (see section "Origin").Val Danilov, I. (2024). "Child Cognitive Development with the Maternal Heartbeat: A Mother-Fetus Neurocognitive Model and Architecture for Bioengineering Systems". In: Ben Ahmed, M.; Boudhir, A.A.; Abd Elhamid Attia, H.F.; Eštoková, A.; Zelenáková, M. (eds) Information Systems and Technological Advances for Sustainable Development. DATA 2024. Lecture Notes in Information Systems and Organisation, vol 71. Springer, Cham. https://doi.org/10.1007/978-3-031-75329-9_24 Some research also refers to neural synchrony as inter-brain synchrony, brain-to-brain coupling, inter-subject correlation, between-brain connectivity, or neural coupling. In the current literature, neural synchrony is notably distinct from intra-brain synchrony—sometimes also called neural synchrony—which denotes the coupling of activity across regions of a single individual's brain.

Neural synchrony approaches represent an important theoretical and methodological contribution to the field. Since its conception, studies of neural synchrony have helped elucidate the mechanisms underlying social phenomena, including communication, narrative processing, coordination, and cooperation. By emphasizing the social dynamics of the brain, this area of research has played a critical role in making neuroscience more attuned to people's social proclivities—a perspective that is often lost on individual-level approaches to understanding the brain.

History

Motivation Driven by the desire to understand the social nature of the human brain, the study of neural synchrony stems from social cognition, a subfield of psychology that explores how we understand and interact with other people through processes like mentalization or theory of mind. Given that it relies on measuring brain activity, neural synchrony also has its roots in cognitive neuroscience.

Despite the growth of social cognition and cognitive neuroscience prior to the early 2000s, research into the brain neglected interpersonal processes, focusing mostly on the neural mechanisms of individuals' behaviors. Furthermore, neuroscience research that did probe social questions only investigated how social processes affect neural dynamics in a single brain. Considering that researchers clearly recognized how interpersonal interaction was fundamental to human cognition, the paucity of social and multi-brain neuroscience research represented a tension in the field. In response to the discrepancy between the complexity of social interaction and the single-brain focus of cognitive neuroscience, researchers called for a multi-person, interaction-oriented approach to understanding the brain.

Early history In 2002, the American neuroscientist P. Read Montague articulated the need to examine the neural activity of multiple individuals at one time. To this point, Montague and his colleagues wrote, "Studying social interactions by scanning the brain of just one person is analogous to studying synapses while observing either the presynaptic neuron or the postsynaptic neuron, but never both simultaneously." They performed the first brain scan of more than one person by using functional magnetic resonance imaging (fMRI) to take simultaneous recordings of two people engaged in a simple deception game. While this study marked the first example of multi-brain neuroimaging, in 2005, King-Casas and others combined neuroimaging with an economic exchange game to conduct the first study that directly compared neural activity between pairs of subjects. Since then, multi-brain imaging studies have grown in popularity, leading to the formation of preliminary neural synchrony frameworks.

Early conceptualizations of neural synchrony, largely shaped by the work of Uri Hasson at Princeton University, were motivated by models of stimulus-to-brain coupling. In these models, aspects of the physical environment emit mechanical, chemical, and electromagnetic signals, which the brain receives and translates into electrical impulses that guide our actions and allow us to understand the world. Researchers presumed that the synchronization of neural activity between two brains should leverage the same system that binds one's neural activity to environmental stimuli. If the stimulus is another person, then the perceptual system of one brain may couple with the behaviors or emotions of the other person, causing "vicarious activations" that manifest as synchronized neural responses across perceiver and agent. According to the theory, this process also occurs through more complex, synergistic interactions, especially when people communicate and convey meaning.

Further development Over the last two decades, neural synchrony has become an increasingly common topic of study in social and affective neuroscience research, spurring conceptual and methodological development. Along with an emphasis on ecologically valid, naturalistic experimental designs, the focus on multi-brain neuroscience studies has increased researchers' ability to explore neural synchrony in social contexts. As a result, conceptualizations of neural synchrony have been expanded to incorporate a wider range of ideas, though it is often viewed as a neural correlate for two or more people's shared experiences. Studies now involve a variety of social processes, with applications spanning simple motor synchronization to classroom learning.

Notable methodological advancements have come from the evolution of multi-brain imaging techniques beyond fMRI, especially magnetoencephalography/electroencephalography (MEG/EEG) and functional near-infrared spectroscopy (fNIRS)—methods which afford more socially interactive experimental designs. These technologies are also complemented by comprehensive data processing techniques that are useful in multi-brain analyses, such as Granger causality or Phase Locking Value (PLV).

As a progressively paradigmatic approach in social and affective neuroscience, neural synchrony undergirds the field's search for the brain basis of social interaction.

A 2022 study by the University of Helsinki measured brain synchronization among players during cooperative online video gaming.

In 2024, latvian scientist Igor Val Danilov from Riga Nordic University introduced the mother-fetus neurocognitive model. It explains cognitive development of the naive organism (fetus) where neural synchrony provides the mature (mother) and naive organisms cooperative interactions with the environment that ensure gaining positive/negative valence of stimuli by the naive organism. The significance of this model lies in explaining how the naive organism, with no prior experience, begins to categorize the world through positive/negative valence of stimuli.

Origin

A hypothesis of natural neurostimulation explains the correlation of brain activity across two or more organisms as an evolutionarily inherited mechanism of nervous system growth. Inspired by recent findings in ontogeny, Latvian professor Igor Val Danilov argues that proper nervous system development in a child during gestation is facilitated by physical interactions with the mother, where the mother's heart plays a central role.Val Danilov, I. (2023). "Shared Intentionality Before Birth: Emulating a Model of Mother-Fetus Communication for Developing Human-Machine Systems". In: Arai, K. (eds) Intelligent Systems and Applications. Intelli-Sys 2023. Lecture Notes in Networks and Systems, vol 824. Springer, Cham. https://doi.org/10.1007/978-3-031-47715-7_5 Indeed, the most potent physical forces in the human body are the low-frequency pulsed electromagnetic field and the complex acoustic wave of the heart; research shows evidence of the heart-brain interplay.Val Danilov I. (2023). "Theoretical Grounds of Shared Intentionality for Neuroscience in Developing Bioengineering Systems". OBM Neurobiology 2023; 7(1): 156; doi:10.21926/obm.neurobiol.2301156. https://www.lidsen.com/journals/neurobiology/neurobiology-07-01-156 During pregnancy, the mother's heart oscillations consolidate neuronal activity in both organisms indwelling in the same ecological context, i.e. voice stimuli from the mother's environment. The environment of the fetus includes both physicochemical interactions with the mother's body and sounds from the mother's environment that can reach the auditory system of the fetus. In this manner, physical interactions between the mother and fetus stimulate the fetal sentience. Adequate sentience (the ability of neurons to transmit electrochemical signals to other cells and to respond appropriately to electrochemical signals received from other cells) is the proper response to specific input stimuli through the sensory system. Due to neural synchrony, particular reactions of the mature nervous system become a template for the naive organism in the early stages of perception development, and subsequently, cognition. Research shows that the maternal electromagnetic field, complex acoustic wave, and the mother's ecology are fundamental factors in the proper development of the child's nervous system. The fetal nervous system learns to react in the same way as the mother's nervous system responds to stimuli. The coupling of the two nervous systems in perceiving environmental stimuli contributes to the initiation of cognition and the development of emotions through the association of affective cues with stimuli that activate neural pathways for simple reflexes. This explanation of cognitive development as a function of the mature and naive organisms cooperative interaction with the environment is called the mother-fetus neurocognitive model. The model argues that cognitive development is inherently a social process that begins months before the first breath. The mother does not just sustain the fetus physically; she acts as a biological "proxy" that interprets the complexity of the world into a binary language of safety and danger, effectively sculpting the architecture of the developing mind. This hypothesis contributes to understanding ontogenesis, particularly the binding problem and of morphogenesis in the early stages of development.Val Danilov, I.; Mihailova, S. (2025). "Reflexes and Shared Intentionality in the Origins of Emotions Development: A Scoping Review of Studies on Blinking in Infants." OBM Neurobiology 2025; 9(1): 263; https://doi:10.21926/obm.neurobiol.2501263

Methods

Hyperscanning

The study of neural synchrony is predicated on advanced neuroimaging methods, particularly hyperscanning. Coined in 2002 by Montague et al., hyperscanning refers to the method of simultaneously measuring the hemodynamic or neuroelectric responses of two or more brains as they engage with the same task or stimulus. The ability to record time-locked activity from multiple brains makes hyperscanning conducive to exploring the variation in activity across brains. It also allows experimenters to examine various aspects of neural recordings in naturalistic scenarios, from low-level stimulus processing to high-level social cognition. For these reasons, hyperscanning has helped foster a systematic investigation of interpersonal dynamics at the level of the brain.