Table of Contents
What is fNIRS?
fNIRS is a brain monitoring technology.
When neurons fire an electrical signal is created that can be measured by EEG devices. But 5 seconds after neuronal activity oxygen is taken from the blood for energy. fNIRS measures this change in oxy/deoxy levels to determine which region is more active at a given time.
This five second delay makes fNIRS a poor choice for BCI applications. Whilst the temporal resolution is poor, the spatial resolution is good. fNIRS can accurately measure the location of blood flow.
How does fNIRS work?
Infrared light is shone from an emitter on the scalp into the skull. This infra-red light naturally occurs in sunlight so is not harmful. The power intensity used is low so can only penetrate a small depth. But the frequencies selected can pass through both skin and scalp, but are absorbed by hemoglobin.
Based on the absorption, reflection and scattering of the infra-red light detected by sensors placed on the scalp near the emitter, complex calculations can determine relative oxy/dexoy levels.
Your brain is powered by oxygen and glucose from blood flow. Arteries deliver oxygenated blood and through the capillary bed neurons remove oxygen. Deoxygenated blood is then removed via veins.
There is a constant baseline metabolic process feeding the brain oxygen. When a region becomes more active it can require more oxygenated blood. It will also then produce more deoxygenated blood.
How hard a brain has to work to complete a task is unique to the individual. If you ask me to calculate 17 x 34 I will get an answer but I will have to concentrate to do so. In doing so a region of my prefrontal cortex will become more active. fNIRS can detect this increased activation.
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