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Week 10: Functional NIRS (fNIRS)

Functional Near-Infrared Spectroscopy uses optical wavelengths (650–900 nm) to monitor cortical hemodynamics non-invasively. This week covers the optical window of biological tissue, light scattering, the Modified Beer-Lambert Law, and naturalistic neuroimaging in mobile populations.

Interactive Demos & Activities

• fNIRS Modified Beer–Lambert Unmixer

  Primary Week Simulator · Extinction Spectra, MBLL & 2×2 Chromophore Separation

  Work through the full optical chain: pick two laser wavelengths against the measured HbO2 and HHb extinction spectra, build the Modified Beer-Lambert matrix from the source-detector separation and differential pathlength factors, then push a simulated hyperemic response forward into two attenuation traces and solve the 2x2 system back to concentrations. Choosing both wavelengths on the same side of the isosbestic point makes the matrix ill-conditioned and reproduces the classic HbO2/HHb crosstalk artifact.

• Interactive fNIRS 'Banana Path' & Hemodynamic Simulator

  Optical Diffusion & Hemodynamic Simulator

  Watch animated photons diffuse along the characteristic curved 'banana path' between emitter and detector optodes. Trigger cortical stimulation to observe concurrent tracking of oxygenated hemoglobin ($\text{HbO}$) surge and deoxygenated hemoglobin ($\text{HbR}$) decrease.

Topics Covered

• The Near-Infrared Optical Window: Low absorption by water and lipid between 650–900 nm, allowing photons to penetrate several centimeters through scalp and skull into the cortex.
• The Modified Beer-Lambert Law: Separating absorption from high scattering using differential pathlength factors (DPF) across at least two wavelengths (e.g., 760 nm and 850 nm) to quantify $\Delta [\text{HbO}]$ and $\Delta [\text{HbR}]$.
• Advantages & Limitations: Immunity to magnetic fields, motion tolerance, portable wearable caps for infants and social interaction studies vs. limitation to superficial cortical layers.