Wireless fNIRS sensor. a) Top-view: schematic of light sources (L1, L2, L3, and L4) and detectors (D1, D2, D3, and D4) on the sensor. b) Wireless fNIRS sensor with casing; (red) light sources, (blue) detectors, (1) analog and wireless communications and power-supply electronics, (2) optical probe . The centre of the sensor was positioned presumably covering position F3 according to the 10-20 system . Three channels were considered for analysis. D1-L1 was positioned in cranial direction, D4-L4 in caudal direction.