Brain activity can be measured optically because haemoglobin absorbs near-infrared light differently when oxygenated.

Evidence

The absorption spectra of oxygenated and de-oxygenated haemoglobin (HbO and HbR respectively) for NIR light differ in that HbO maximally absorbs light of longer wavelength (900–1000 nm) whereas HbR maximally absorbs light of shorter wavelength. These differential absorption spectra make it possible to separate out changes in the concentration of HbO and HbR. In response to neural activity in the brain, an increase in oxygenated blood is directed to the region, resulting in a drop in de-oxygenated blood. Thus HbO and HbR concentrations change in opposite directions in response to neural activity (Shah and Seghouane 2014 ). — Dijk, Pim van, Physiology, Psychoacoustics and…, 4281

Categorization

• DEFINE: Explains the physical basis of fNIRS: differential HbO/HbR absorption spectra are what make the whole technique possible.