Basically, the functional connectivity of brain is a statistical concept. It can be considered as temporal correlation between regions that are defined naturally as detection channels of fOT measurement [36–39]. In this study, brain functional connectivity can be explored with time-series optical signals through the following four steps (Fig. 2(a)-2(e)-2(f)-2(g) and 2(h)). First, the network modes that correspond to detection channels were defined as optode (optical-to-detector) array of fOT (Fig. 2(a)). Then the fOT signal of each channel was processed as low-frequency time-series by using band-pass filter within 0.01 to 0.08 Hz (Fig. 2(e)). The low-frequency change of hemoglobin concentration was analyzed by calculating the Pearson’s correlation coefficient rXY:
