Since the mid-1970s, functional near infrared spectroscopy (fNIRS) has been developed as a non-invasive technique to investigate brain cerebral hemodynamic levels associated with brain activity under different stimuli by measuring the change of absorption coefficient of the near-infrared light between 650 nm and 950 nm (Huppert et al. 2009; Jöbsis-vander Vliet 1977; Yodh and Chance 1995; Yuan 2013a, b; Yuan and Ye 2013; Yuan et al. 2010, 2014). Compared to other available functional neuroimaging modalities, such as functional magnetic resonance (fMRI) and positron emission tomography (PET), fNIRS has the advantages of portability, convenience and low cost. And more importantly, it offers unsurpassed high temporal resolution and quantitative information for both oxy-hemoglobin (HbO) and deoxy-hemoglobin (HbR), which are essential for identifying rapid changes in dynamic patterns of brain activities including changes in blood oxygen, blood volume and blood flow.