I. Introduction
Human emotion has a great impact on our daily activities, which is concerned with various actions, such as relaxation, work, and entertainment. There are increasing research interests in the relationship between emotions and physiological functions [1]. It has been found that positive emotions could reflect pleasurable engagement, and are beneficial for human health and attitude [2]. However, accompanied with complaints of physical symptoms, negative emotions may adversely influence mental health and even cause serious psychological problems [3]. As the information explosion through social channels, it is quite challenging to reveal one’s emotional clues. Recently, affective computing (AC) has emerged under the demand of deep knowledge and reasonable utilization for emotion [4], [5]. It is a promising area of research that has attracted increasing attentions from numerous cross-curricular fields, ranging from neuroscience to computer engineering. Emotion would be subtly influenced by multiple external and psychological factors, and is a combination of time, space, experience, and cultural background [6]. This aggravates the difficulties for emotion recognition research. Although great efforts have been made to explore the mechanisms and methods for emotion recognition [7], due to the intricate external patterns, effective emotion recognition methods are still in high demand for many technological applications.