## 1 Introduction
### Review Objectives  
The overarching objective of this study was to conduct a comprehensive review on hyperscanning research, with the goal of systematically identifying, critically appraising, and synthesizing all relevant studies on inter-brain neural synchrony during social interactions. An explicit systematic method, the preferred reporting items for systematic reviews and meta-analyses (PRISMA) was used to address  
\begin{table}
\begin{tabular}{c l l} \hline \hline  & **Category** & **Studies** \\ \hline \multirow{4}{*}{Modality} & fMRI family & [48] \\ \cline{2-3}  & fNIRS family & [52] \\ \cline{2-3}  & EEG family & [49; 50; 51; 54; 67] \\ \cline{2-3}  & Multimodality & [54; 55; 12; 56] \\ \hline \multirow{4}{*}{Application} & Joint action & [57; 68] \\ \cline{2-3}  & Social neuroscience & [59; 60] \\ \cline{2-3}  & Social interaction & [61; 62; 11; 63; 64] \\ \cline{2-3}  & Interpersonalcoordination & [65; 66] \\ \hline Other & General hyperscanning & [46; 47] \\ \hline \hline \end{tabular}
\end{table}
Table 1: A summary of 24 review papers on hyperscanning research published since 2002.  
three specific research questions (RQs) regarding the neuroimaging method (RQ1), the application domain (RQ2), the and experiment paradigm (RQ3), which articulate the current-state-of research being conducted with hyperscanning. PRISMA is known to minimize bias and thus provide reliable findings from which conclusions can be drawn [69]. To the best of our knowledge, this is the first systematic review study that used PRISMA to compile all relevant and cutting-edge hyperscanning research to address the current state-of-the-art hyperscanning research since the first publication in 2002.  
RQ1. What is the frequency of the four major neuroimaging methods being used in hyperscanning studies? To answer RQ1, this paper calculated the frequency of research conducted by different neuro-imaging methods being used in the selected 126 hyperscanning studies. To effectively organize and draw data from these papers, we first sorted them by the four major neuroimaging methods: magnetoencephalography (MEG) and electroencephalography (EEG); near-infrared spectroscopy (NIRS) and functional near-infrared spectroscopy (fNIRS); magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI); and others. It is important to know which neuroimaging methods are most frequently used, because that information can show us which hyperscanning methodologies researchers should focus on improving first to further neural synchrony research.  
RQ2. In what areas has past and current hyperscanning research been conducted within each neuroimaging method? To address RQ2, this paper further categorized the papers in each neuroimaging method by application domain (e.g., cognition, decision-making, motor synchronization, and education) to report the number of studies performed in each application.  
RQ3. How have past and current hyperscanning research studies been experimentally designed within each neuroimaging method and application area? To address RQ3, this paper looked into the frequency of different experimental methods (setting, orientation, and group size) within each application. This information is able to tell us what current research is using for hyperscanning experiments and how that changes based on application. From here, we are able to discuss why certain applications prefer one neuroimaging method, setting, orientation, or group size over another, and whether research should be conducted using other experimental methods to advance research in that application.