# Near-Infrared Spectroscopy Applied to Complex Systems and Human Hyperscanning Networking
## 5 The Hyperscanning Paradigm  
In order to properly perform similar integrated analysis of the structure of recorded signals, it is important to collect them concurrently and simultaneously. Technical and computational advances occurred in the last few years have allowed researchers to devise, test, and share novel methodological and analytical approaches that have allowed them to overcome previous limitations and to accurately sample, compare, and integrate different information flows related even to the brain activity of two (or more) individuals [20; 21]. Such an approach has been defined as _hyperscanning_ and is based on the simultaneous recording of the bodily activities of interacting agents in real-time, i.e., while social dynamics develop. The hyperscanning approach has been applied via functional Magnetic Resonance Imaging (fMRI) [22; 23; 24], electroencephalography (EEG) [18; 20; 25], magnetoencephalography (MEG) [26; 27; 28], and functional Near-Infrared Spectroscopy (fNIRS) [4; 19; 29; 30; 31; 32; 33]. Independently from the data recording technique that is actually used, one of the methodological issues that is mostly stressed and discussed in hyperscanning research is the temporal synchronization of data series. Given the primary aim of such an experimental approach and the intrinsic meaning of the metrics that are consistently computed, the time course of biosignals recorded from different participants critically needs to be consistently and accurately tracked. Since the individual contribution of co-agents to complex social dynamics occurs concurrently and since--notably--it is modulated in real-time depending on the continuous flow of information between subjects and on behavior co-regulation, any attempt to sample physiological processes supporting such phenomena has to be comparably precise and sensitive to quick fluctuations. That would foster the creation of a sufficiently realistic picture of the correlates of social exchanges. Nonetheless, as noted even by [PERSON] and colleagues [24], the advantages of precise overlapping and synchronicity of the time courses of participants' biosignals may be partially reduced by the limited time resolution of specific techniques, such as fMRI, due to sampling constraints and to the slow time course of the physiological processes they track (e.g., the rise and fall of the blood oxygen level-dependent (BOLD) response). Figure 1: Schematic representation of hyperscanning recording settings. (**a**) Example of functional Near-Infrared Spectroscopy (fNIRS)-based side-by-side hyperscanning during experimentally-controlled tasks, such as cooperative or competitive computerized games; (**b**) Example of fNIRS-based face-to-face hyperscanning during naturalistic interactions, such as real-life verbal and non-verbal communication exchanges; (**c**) Examples of single-brain 3D topographical activation maps based on local oxygenated hemoglobin (O\({}_{2}\)Hb) concentration during a cooperative task and turn-based interaction. Images have been created based on data recorded at our laboratory. Pairs of reconstructed heads refer to different couples of co-agents. Left: modulation of O\({}_{2}\)Hb concentration during a shared cognitive task. Right: modulation of O\({}_{2}\)Hb concentration during turn-based face-to-face conversation. Green-to-blue colors mark a decrease in oxygenated hemoglobin concentration; green-to-red colors mark an increase in oxygenated hemoglobin concentration. Lines connecting pairs of 3D activation maps exemplify graphical representation of hyperlinks between co-agents. Yellow-to-red colors mirror the increasing strength of the hyperlink between different cortical areas as measured by correlation indices. Because of that, electrophysiological techniques--i.e., EEG-based investigations--are thought to be peculiarly apt and promising. Indeed, they are characterized by fine-grained sensitivity to subtle changes in brain states and by high time resolution [34].