# Near-Infrared Spectroscopy Applied to Complex Systems and Human Hyperscanning Networking
## 6 Optical Imaging and Methods for fNIRS Hyperscanning Applications
Moreover, spatial localization is commonly deemed as a weak point of the EEG technique, because scalp-recorded data are the outcome of electric activity potentially generated in various points of the brain, and thus potentially mirroring the summation of the contribution of different systematically-active structures. fNIRS applications, instead, take advantage of the circumscribed distribution of hemodynamic modulations due to neurovascular coupling and of the specific sensitivity of optical measurement to even the slightest changes in both oxygenated and deoxygenated hemoglobin in the capillary components of cortical tissues. Thus, while the actual spatial resolution of an fNIRS recording partially depends on specific aspects of data recording and on the optode montage that is used, the technique allows for the sketching of finer-graded pictures of cortical activations with respect to the majority of EEG applications, even if the two techniques share the coordinate systems used to place sensors. In addition, similar to EEG but differently from other neuroimaging methods, fNIRS is both easy to use and always more cost-effective, thanks to ongoing progresses in the development and miniaturization of recording systems. Again, when compared to fMRI-based investigations, fNIRS offers the unique opportunity to track changes of oxygenated, deoxygenated, and total hemoglobin, and not only of a single or compound measure [37; 44]. Future investigations might capitalize on this richness of information to better define and differentiate between physiological, non-specific, and task-driven changes of cortical hemodynamics during interaction via optical imaging. Moreover, with respect to fMRI, PET, and SPECT, fNIRS measurement is robust against environmental exogenous noise and has a greater temporal resolution [44]. Indeed, while still poorer than electrophysiological techniques, the time resolution of optical imaging in terms of sampling rate is greater than that of other traditional imaging techniques (such as fMRI and PET). By increasing the sampling rate, by implementing specific methodological expedients, and by aptly designing experimental tasks, it seems indeed possible to track fast modulations of brain activity via optical techniques even in the sub-second range, even though such advanced applications are still a matter of debate and experimental testing. Finally, it is worth noting that fNIRS is deemed as a promising technique in the field of hyperscanning research due to the advantage of being easily complemented by other neuroscientific measures, to its remarkable usability, and to the opportunity to make recordings more ecological [36; 37; 44; 50]. Indeed, if compared to alternative techniques such as EEG and fMRI, optical imaging allows for the measurement of brain activity in more natural real-life situations, since it imposes considerably milder physical constraints (e.g., staying still) and psychological-physiological burdens (e.g., injection of radioactive agents) than other imaging techniques. fNIRS considerably reduces limitations concerning participants' posture and bodily movements, since they are not requested to lie down and remain still in a scanner and since sources and detectors are placed on a recording cap, as in EEG recordings. Freedom to act and the opportunity to implement face-to-face settings allow for the investigation of realistic interaction dynamics and prevent misrepresentations of the complexity of interacting systems. In addition, fNIRS recordings are thought to be more robust against muscular and head motion artefacts and against external electromagnetic noise when compared with EEG and fMRI [44; 51]; this is a critical characteristic, in particular, for formal investigation of naturalistic social dynamics and unconstrained interactions. Nonetheless, as underlined by [PERSON] and colleagues [38], some methodological and technical limitations of fNIRS recording (with regard to both single-brain and hyperscanning applications) still have to be properly tackled.