← Back to Course Home

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 Simulator: From Photons to Haemoglobin

  Full Recording Simulator · Banana Sensitivity, Beer-Lambert, Confounds, Short-Channel Regression, GLM & HD-DOT

  Describe a recording in a small language (sources, detectors and short-separation channels over a 10-20 position, wavelengths, sampling rate and a block or event-related timeline with conditions) and record it from a five-layer head with an adjustable cortical activation, a connected pair, an infant, a thick skull or dark hair. Watch photons form the banana and trade depth against light as the separation grows, pick wavelengths against the HbO, HbR and water spectra, and follow intensity to optical density to concentration through the modified Beer-Lambert law. Switch the heartbeat, breathing, Mayer waves, a task-evoked scalp response, motion, a loose optode and room light on and off, then check quality, correct motion, filter, regress out the short channels, and watch the response emerge in the block average and the GLM while a false scalp activation disappears. Also compares fNIRS with BOLD fMRI and shows how a high-density array becomes diffuse optical tomography.

• 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.

• fNIRS Photon Transport & Short-Channel Scalp Regression

  Diffuse Optics · Monte Carlo Transport, Banana Paths & Scalp Mayer Wave Decontamination

  Simulate photon packet scattering random walks through layered scalp, skull, and brain cortex. Test dual-distance optodes to see how short-channel (8 mm) regression strips away confounding systemic blood pressure Mayer waves (0.1 Hz) to isolate true cortical activation.

• 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.

• fNIRS Modality Card Game

  Educational Card Game · Optodes & Haemoglobin

  Match source, detector and short-separation optodes, the chromophores that absorb near-infrared light, the modified Beer-Lambert law through block averaging, and applications from infants to hyperscanning.

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.