# Near-Infrared Spectroscopy Applied to Complex Systems and Human Hyperscanning Networking
## 6 Optical Imaging and Methods for fNIRS Hyperscanning Applications  
Similar to fMRI, Positron Emission Tomography (PET), and Single-Photon Emission Computed Tomography (SPECT), optical imaging is a form of indirect hemodynamic imaging techniques. The measurement of neural activity indeed grounds on fluctuations of oxygenated and deoxygenated hemoglobin in the cerebral blood flow instead of being directly based on electromagnetic properties of active neurons. In particular, the physiological phenomenon at the basis of hemodynamic techniques is neurovascular coupling [39; 40], which describes the link between functional activity, energetic and metabolic demands, and cerebral blood flow. Getting down to specifics, when a neural population activates to process information in response to external or internal events and stimuli, it consumes oxygen molecules extracted from local blood supplies. This initial consumption of local resources leads to an increased concentration of deoxygenated hemoglobin (HHb) and a relative decrease of the concentration of oxygenated hemoglobin (O\({}_{2}\)Hb). In order to compensate for such physiological changes, afferent blood flow is increased following the localized dilation of cerebral blood vessels. This increase of the blood supply exceeds the needs of neuronal oxygen consumption, and then leads to another localized unbalance between the concentration of oxygenated/deoxygenated hemoglobin molecules, with a notable increment of the first and a decrement of the latter. Besides being the constituents of blood oxygenation level-dependent (BOLD) signal, such physiological modulations are tracked via optical imaging by taking advantage of differences in optical and light absorption properties of chromophores within molecules constituting different biological tissues. Functional Near-Infrared Spectroscopy (fNIRS), in particular, uses specific near-infrared light radiations (range: 650-950 nm) to quantify the concentration of different biological elements--such as hemoglobin--in tissues such as skin, muscles, and cerebral cortex. Those wavelengths do indeed define radiations that are primarily absorbed by hemoglobin while skin, bone, and other tissues are mostly transparent to them. Furthermore, by using multiple single wavelengths it is possible to independently obtain specific pieces of information on the amount of hemoglobin in its two states (O\({}_{2}\)Hb vs. HHb) that is present within the transilluminated area of tissue [41; 42; 43]. In applications of fNIRS to investigate brain functioning, scalp skin is usually illuminated by laser diodes or light emitting diodes (LED), in order to grant accuracy of stimulation and control over the amount of emitted energy. Light then travels through surface and cortical tissues and is then recaptured by highly-sensitive detectors, usually constituted by glass fibers. Pairs of light sources and detectors constitute fNIRS recording channels. They are commonly placed at fixed distances depending on technical requirements of the used system and on methodological factors linked to the intended recording resolution. It is worth noting that near-infrared light follows a curve trajectory when travelling through biological tissues and is then commonly deemed to shows a penetration depth of about one half of the distance between the source-detector pair. Given that this distance is usually approximately 3-4 cm (also depending on the system), fNIRS is thought to show a penetration power of about 2 cm, sufficient to reach cortical tissues layers. For further in-depth discussion of the technical details of fNIRS recordings, we suggest the following reviews: [36; 37; 43; 44; 45]. The position of sources and detectors is usually reported by referring to the 10-10 or the 5-10 International Systems for electrode placements [46; 47].