A continuous wave fOT system (ETG-4000, Hitachi Medical Co., Tokyo, Japan) was used to record brain response in this study. The fOT measurements were conducted with multiplexing channels (52 channels), which can allow 2-D tomographic functional maps of brain activity. The near-infrared sources at dual-wavelength 695 and 830 nm were used and the sampling rate was set to 10 Hz. According to the modified Beer-Lambert Law [29], the concentration changes in oxygenated hemoglobin (ΔHbO2) and deoxygenated hemoglobin (ΔHb) could be obtained. Then the total hemoglobin (ΔtHb) can be calculated by the components of ΔHbO2 and ΔHb (the sum of ΔHbO2 and ΔHb). In this system, the ΔHbO2 and ΔHb values include differential path-length factor (DPF) which is wavelength and age dependent [30]. In the multi-channel fOT system, 17 light emitters (the red circles of Fig. 1) and 16 photo-detectors (the blue circles of Fig. 1) are plugged into a probing holder and arranged as 3 × 11 array, resulting in a total of 52 channels detection with 3 cm source-detector separation, which permitted to measure the regions at the approximate
