“…Facial expression is one of the most powerful, natural, and universal signals for human beings to convey their emotional states and intentions regardless of national borders, race, and gender [1,2], and there were multitudinous related applications such as the health management [3], aided driving [4,5], and others [6][7][8][9]. In the earlier researches on facial expressions of emotion, Ekman and Friesen argued that human beings perceive certain basic emotions in the same way regardless of their cultural background, and they defined the typical facial expressions into 6 categories: anger, disgust, fear, happiness, sadness, and surprise [10,11].…”