## 4 Affective States Recognition through Thermal IR Imaging
Significant differences in temperature were found only on the tip of the nose. The results showed that high arousal images elicited temperature increases on the tip of the nose, while low arousal images led to temperature increases for pleasant images (i.e., positive valence) and decreases for unpleasant ones (i.e., negative valence). Contrasting results were indeed found by [PERSON] et al. [63]. The authors found no significant temperature differences along the valence dimension of the emotions (i.e., pleasant and unpleasant emotions). Besides, an activation effect of emotional pictures was found on the amplitude and latency of nasal thermal responses: the more arousing the pictures, the faster and larger the thermal responses. The only evaluated region was the tip of the nose. The relevance of the emotional arousal in causing changes in the nose tip temperature is supported and demonstrated in different studies, such as in [PERSON] et al., where a direct relationship was found between reduced levels of arousal and nasal skin temperature [64], and confirmed in [PERSON] et al. [65]. Moreover, [PERSON] et al. proposed quantifying stress through thermal IR imaging [54]. Stress in the valence-arousal space is identified as negative valence and high arousal [66]. [PERSON] et al. found that high stressful situations resulting in the cooling of the area around the nose tip [54]. Parallel results were found in children, [PERSON] et al. obtained, in their study of guilt in children (age 39-42 months), in which the higher the distress signs, the higher the decrease in nose temperature [67]. The sense of guilt  
Figure 3: Graphical representation of the circumplex model of affect with the horizontal axis representing the valence dimension and the vertical axis representing the arousal dimension. Adapted from [60]. was induced through the \"mishap paradigm\" in which children were led to believe they had broken the experimenter's favorite toy (Figure 4). The temperature of the nasal area decreased following the \"mishap\" condition, suggesting a sympathetic activation and peripheral nasal vasoconstriction, and it increased after soothing due to a parasympathetic activation. Vasomotor process, subcutaneous blood flow, and perspiration patterns are not the only physiological measures that can be detected through thermal IR imaging. In fact, the physiological signature of the autonomic system activation such as heart and breathing rate variation can be also monitored [69; 70; 71; 72]. These two parameters are measurable with thermal IR imaging by positioning a ROI over a superficial vessel or over the nostrils respectively and by monitoring the ROI average temperature over time. Whereas the latter is easily detectable, because the thermal difference between the expired and inspired air is easily appreciable (approximately 0.5 \({}^{\circ}\)C), the modulation of temperature caused by the pulsation of blood in the vessel is not. Therefore, a series of algorithms have been developed for the estimation of the heart rate through thermal imaging [69]. [PERSON] et al. used thermal IR imaging to detect physiological indicators of stress in the adult population by analyzing the respiration and heart rate variation during the performance of mental and physical tasks [26]. Temperature variation over time was recorded on the nose tip and regions near superficial arteries to detect respiration and heart rates, respectively. The results showed that the accuracy in the physical versus psychological stressors classification was greater than 90%, and the heart and respiration rate were accurately detected by thermal IR imaging. Whereas the evaluation of the heart rate through thermal IR imaging is not common in the literature, the respiration rate assessment through thermal IR imaging has been tested in different settings, including sleep monitoring [73], neonatal care [74], and driver's drowsiness monitoring [75].