Neuronal activity is known to be associated with then changes in the blood oxygenation and deoxygenation due to functional brain activities. These changes are referred to cerebral hemodynamics.
The difference in the near-infrared absorption spectra of oxyhemoglobin (HbO2) and deoxyhemoglobin (HbR) enables the concentrations of the two molecules to be separately identified. NIRS employs wavelengths of 600?900 nm, which experience very low levels of absorption in tissues, allowing deeper enetration into a brain tissue.
The light emitted by a laser source pointed into a brain can be modeled by a spherical diffusive wave. The photons that reach the detector propagate along a banana-shaped trajectory through the brain channel. The variation of detected light power is caused mainly by hemodynamic changes in the activated cerebral cortex.