Initial dip detection based on both HbO and HbR vector-based phase analysis

A new threshold circle to minimize possible misclassification of initial dips in the functional near-infrared spectroscopy (fNIRS) signals using the vector-based phase analysis is investigated. In contrast to the work in [20] (i.e., the square root of the sum of the squares of oxy- and deoxy-hemoglobins), the peak value of oxy- or deoxy-hemoglobin during the resting state is used. The experiment was performed on five healthy subjects. The activity was measured using a frequency domain fNIRS system during the mental arithmetic task from the prefrontal cortex. With the new criterion, the radius of the circle becomes smaller than that in [20], and earlier detection of initial dips are possible. For the given arithmetic task, the channels of detecting the initial dips were not the same over the subjects, which reflects that the activated brain region in association with the performed task spreads in the prefrontal cortex. Also, the new method can further reduce the misinterpretation of large variations in the resting state and even during the task period.