Computational methods of brain-to-brain coupling during human interaction

Hyperscanning—the simultaneous recording of hemodynamic or neuroelectric activity of the brain——is the new technology to directly observe the brain communication between individuals. Since the publication of the first hyperscanning study in 2002, it has developed rapidly in the past decade. Brain-to-brain coupling (BBC) technology of human interaction was widely used in the research of action coordination, cooperative game, classroom teaching and other interactive tasks, which involved the use of functional magnetic resonance imaging (fMRI), Electroencephalogram (EEG), magnetoencephalography (MEG), functional near-infrared spectroscopy (fNIRS) and other neuroimaging technologies. In addition, hyperscanning research experienced a leap from pseudo-interaction to real interaction, a leap from a strictly controlled laboratory paradigm to the "field research" paradigm in nature situations. Based on the differentiation and analysis of the core concepts such as coupling and synchronization, taking the development of time and technology as the main line, this paper reviewed the common BBC indicators and their application conditions, introduced the related theories and forecasted the future research direction of BBC. In hyperscanning study, BBC refers to the interaction and influence of two or more brains, or between two or more brain regions within a single brain, while the inter-brain synchronization (IBS) refers to the consistency (relative stability) in frequency, phase, and other characteristics of signals from different brain or different regions of the brain. Thus, the similarities and differences between hyperscanning, BBC and IBS can be summarized as follows: Hyperscanning is the technique of recording the neural activity of two or more brains simultaneously. BBC refers to the interaction or interaction between two or more brains. BBC sometimes manifests as IBS, the degree of IBS can represent the strength of BBC in this case, but BBC is not the same as IBS: (1) In addition to IBS, BBC can also be manifested as functional connections and interactions between the brains; (2) due to the possibility of pseudo-synchronization, the occurrence of IBS does not mean that there must be BBC. In the field of physics and electronic engineering, the characteristics of a signal can be described by multiple indicators such as amplitude, period, frequency and phase. Accordingly, neurologic studies have found that brain activity induced by external events may take the form of phase response, frequency response, amplitude response, and production of new components. Therefore, it is not surprising that many different indicators are used to characterize the strength of BBC in existing hyperscanning studies. These indicators including the circular correlation coefficient (CCorr), wavelet transform coherence (WTC), directed coherence, phase-locking value (PLV), the coupling indexes based on the event-related potentials (ERP) such as the latency difference and the amplitude difference, the coupling indexes based on the Graph Theory such as the shortest path length, global efficiency, local efficiency, divisibility, modularity, small-word network. Through the review, we found the following three issues deserve the attention of the future hyperscanning research. Firstly, as different coupling indicators have different characteristics and applicable conditions, we suggest that the selection of indicators should be based on specific research methods, technical equipment and research problems. There are no best indicators, only the most suitable ones. Secondly, the existing theory of BBC is not complete, integrating existing theories and proposing theories with stronger explanatory power should be the focus of future hyperscanning theory research. Thirdly, there are many forms of coupling in neural processing, and the meaning of coupling is different in different forms. Therefore, it is necessary to take into account the comprehensive consideration of the specific research task type, interactive mode and other factors when discussing the significance of coupling.