Our brain needs a constant flow of oxygen, the supply of which occurs through the blood. This process is called oxygenation, it occurs with the help of hemoglobin cells, which are responsible for the transfer of oxygen in the blood. Two types of hemoglobin are distinguished: oxyhemoglobin (HbO2) – with oxygen and deoxyhemoglobin (HHb) – without oxygen. Near Infrared Spectroscopy (NIRS) is a modern, non-invasive technology for measuring the concentration of oxy-, deoxyhemoglobin and total hemoglobin. The technology is based on two main principles: – Human tissues are relatively transparent to light in the near infrared range. – Hemoglobin is the heaviest light absorbent in the near infrared range. In this range, HbO2 and HHb show oxygen-dependent absorption, while abortion differs for waves of different lengths. For NIRS, you need at least one receiver and one transmitter to form a channel. The transmitter emits light with two different wavelengths. Measurements take place in the capillaries, where oxygen exchange occurs. The data represent the relative concentration. Thus, you do not know the initial value, but measure the change in concentration. Functional NIRS (fNIRS) – The use of NIRS for functional neuroimaging. Using fNIRS, brain activity is measured through hemodynamic reactions associated with neuroactivity. Tissue Saturation Index (TSI), a parameter that is a direct main indicator of the percentage of oxygenated Hb in the tissue immediately below the sensor. This is a reliable and fast parameter that allows you to measure the oxygenation of brain tissue in subjects.