## 4 Affective States Recognition through Thermal IR Imaging
Emotion identification through thermal IR imaging was also employed in studies on children. Such applications may receive the greatest benefit from the technology thanks to the ecological nature of this technique and the difficulties associated with measuring children with skin-located sensing. [PERSON] et al. proposed an experimental design to identify five emotional states (disgust, fear, happiness, sadness, and surprise) evoked in children between 7 and 11 years old [57]. The forehead, the tip of nose, the cheeks, the chin, the periodistal and the perinasal regions were chosen to extract the affective information. Each thermal frame was processed by segmenting the ROIs and evaluating the ROIs' temperature mean, variance, and median values. Then, a linear discriminant analysis was used as a classifier. High accuracy (higher than 85%) was obtained for the classification of the five emotions, thus resulting in a robust method to identify quantitative patterns for emotion recognition in children. Temperature decrease was detected in the forehead region during disgust and surprise; in the periodistal region during happiness, sadness, and surprise; in the perinasal region during disgust and happiness; in the chin during surprise, happiness, and sadness; and finally, in the nose during disgust, fear, and happiness. The temperature increase was detected in the left cheek region for all emotions and in the nose tip during surprise. Beyond the basic emotions, thermal IR imaging has been used to characterize the two dimensions of emotions, such as valence (pleasant versus unpleasant) and arousal (low versus high). Emotions dimensions are a crucial aspect in the affective research field, on which most of the studies on emotions recognition are based. The most commonly used models representing emotions dimension in HRI are the Pleasure, Arousal, Dominance (PAD) emotional state model [58] and the circumplex model of affect [59]. The PA dimensions of PAD were developed into the circumplex model, which indeed assume that any emotion might be described with two continuous dimensions of valence and arousal [60]. The valence dimension indicates whether the subject's current emotional state is positive or negative. Arousal, on the other hand, indicates whether the subject is responsive or not, at that given moment and for that given stimulus, and how active he/she is. In particular, the theory of the dimension of the emotions proposes that the emotional states are not discrete categories but rather a result of varying  
Figure 2: Thermal IR images with marked ROIs (black rectangles). degrees of their dimensions. A graphical representation of the circumplex model of affect developed by [PERSON] is reported in Figure 3. For example, joy is characterized as the product of strong activation in the neural systems associated with positive valence or pleasure together with moderate activation in the neural systems associated with arousal (i.e., low arousal). Emotions other than joy likewise arise from the same two-dimensional systems but differ in the degree or extent of activation. This allows characterizing also complex emotions such as love or happiness other than the basic ones. The analysis of the emotion recognition solutions reveals that there is no one commonly accepted standard model for emotion representation. The dimensional adaptation of [PERSON]'s six basic emotions and the circumplex or PAD model are the ones widely adopted in emotion recognition solutions [61]. Recent studies explored temperature changes associated with different degrees of valence and arousal. For instance, [PERSON] et al. studied the relation between changes in temperature of the subject's face and valence or arousal dimensions [53]. They used pictures from the International Affective Picture System (IAPS), which is widely used in studies of emotion recognition and characterized along the two dimensions [62]. The analyzed ROIs were the forehead (left and right sides), the tip of the nose, the cheeks, the eyes, and the mouth regions.