What is fNIRS?
Functional near-infrared spectroscopy (fNIRS) is a non-invasive, portable neuroimaging technique that measures cerebral hemodynamic changes associated with neuronal activity. It quantifies concentration changes in oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HbR) in the cortical microvasculature by utilizing the differential absorption properties of near-infrared light (typically 650–950 nm) through biological tissues. As an optical brain monitoring technology, fNIRS offers a balance between spatial resolution (on the order of centimeters) and temporal resolution(on the order of seconds), making it suitable for studying brain function in naturalistic settings, clinical environments, and populations where other modalities (e.g., fMRI) are impractical.

How does fNIRS work?
fNIRS operates on the principle of neurovascular coupling: increased neuronal activity triggers a localized increase in cerebral blood flow and oxygen metabolism. The technique involves the following process:

Light Emission & Propagation: Optical sources (emitters) placed on the scalp emit near-infrared light at two wavelengths. This light penetrates the scalp, skull, and cerebrospinal fluid, scattering through the brain tissue.

Differential Absorption:?Oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HbR) have distinct absorption spectra in the near-infrared range. As light travels through the cortex, it is absorbed differently depending on the relative concentrations of HbO and HbR.

Light Detection:?A portion of the scattered light exits the head and is captured by detectors (photodiodes, typically APDs) placed at known distances (e.g., 3 cm for standard channels, 1 cm for short-distance channels) from the sources. The detected light intensity is attenuated based on the optical pathlength and the absorption properties of the underlying tissue.

Signal Reconstruction:?Using the modified Beer-Lambert law (MBLL), the attenuation changes at multiple wavelengths are converted into relative concentration changes of HbO and HbR over time. These hemodynamic responses—specifically, the characteristic increase in HbO and decrease (or slight increase followed by decrease) in HbR—serve as a proxy for regional neural activation.

Spatial Mapping:?By arranging multiple source-detector pairs (forming measurement channels) in a grid or cap over the scalp, two-dimensional topographic maps or three-dimensional tomographic reconstructions (in diffuse optical tomography, DOT) of cortical activation can be generated. In summary, fNIRS non-invasively infers brain activity by measuring the optically accessible hemodynamic consequences of neuronal firing, providing a flexible and robust window into human brain function.