## 1 Introduction
### Research Motivation  
Despite its relatively short history, the field of hyperscanning has made significant progress in the past years on (1) applications, (2) neuroimaging methods, (3) experiment paradigms, and (4) inter-brain synchrony methodologies. First, plenty of successful applications have been demonstrated in many areas, such as common vision [20], temporal motor synchronization [9; 21; 22], music production [23; 24; 25], speech [26; 27], shared attention [28], decision-making [3; 29; 30], classroom learning [31], and cooperative piloting [6]. Second, through technological advancements in neuroimaging methods, NIRS hyperscanning could be conducted using one NIRS scanner to measure simultaneous movements in two subjects [9]. [PERSON] et al. [32] developed a processing pipeline which incorporated independent component analysis (ICA) as a precursor to calculating hyperconnectivity with fNIRS in source space; previous fNIRS hyperscanning studies had done calculations in sensor space without ICA. Third, recent hyperscanning studies have focused on increasing ecological validity through face-to-face interaction and naturalistic environments [33; 34; 35; 36]. In this past year, numerous discoveries have been made including EEG hyperscanning while monitoring in real-time [19] and using fNIRS hyperscanning to simultaneously scan musicians in an ecological setting [37]. Finally, various methods that can quantify inter-brain synchrony have been developed in fNIRS [37], fMRI [38], MEG [18], and EEG [6] hyperscanning studies. For example, Granger causality was first used in fMRI hyperscanning in 2010  
Figure 1: Timelines of hyperscanning advancements. by [PERSON] et al. [39]. This effective connectivity measure was first adopted in fNIRS hyperscanning two years later by [PERSON] et al. [40]. [PERSON] et al. [41] used Phase Locking Value (PLV) as a functional connectivity measure, in an EEG hyperscanning experiment. In 2008, [PERSON] and colleagues [42] conducted the first functional connectivity-based EEG hyperscanning study with the link patterns of intra- and inter-brain. However, it is also true that contemporary hyperscanning research identifies many gaps in the literature that warrant further investigation. A rapidly-increasing number of published hyperscanning studies occasions various problems, including (i) how to highlight methodological concerns in research studies [43] that can be used to improve future work in the hyperscanning area and (ii) how to identify questions for which the available evidence provides clear answers and thus for which further research is not necessary [44]. Systematic reviews address both of these issues. A systematic review, a \"review of the evidence on a clearly formulated question that uses systematic and explicit methods to identify,\" selects and critically appraises relevant primary research, and extracts and analyzes the data from the studies that are included in the review [45]. However, the literature on hyperscanning has mostly been narratively reviewed, which simply records and assesses the state of knowledge on a particular topic in hyperscanning. For example, we identified 24 review papers on hyperscanning research published since 2002 (see Table 1). We found these review papers shared a couple of significant limitations in their methods and approaches, which indicates a need for a more systematic review study on hyperscanning. A number of features distinguish our review study from most other review papers. First, in this paper we reviewed state-of-the-art hyperscanning research by neuroimaging methods, research paradigms and application types, as well as various methods or indices for quantifying inter-brain synchrony, which other review papers have failed to address specifically. Out of the 24 review papers, two review papers [46; 47] gave a general timeline of hyperscanning research but failed to dive into the research paradigms and indices in hyperscanning.