Functional Near-Infrared Spectroscopy

Functional Near-Infrared Spectroscopy (fNIRS) is a non-invasive optical imaging modality that monitors changes in cerebral oxygenation by measuring the absorption of near-infrared light (650–900 nm) by hemoglobin. Surface-mounted optode arrays emit and detect light through the scalp and skull, capturing variations in the attenuation coefficient due to hemodynamic responses associated with neural activity. With a spatial resolution on the order of 10 mm and temporal resolution around 100 ms, fNIRS bridges the gap between high-temporal electroencephalography and high-spatial functional MRI.

fNIRS systems commonly use continuous-wave, frequency-domain, or time-domain measurements to disentangle absorption and scattering contributions. Source-detector separations (typically 2–3 cm) are optimized for cortical sensitivity while minimizing contamination from superficial tissues. Advances such as high-density diffuse optical tomography and improvements in signal-processing algorithms have enhanced depth sensitivity, image reconstruction, and quantification of hemoglobin concentration changes.