## 4 Affective States Recognition through Thermal IR Imaging  
A considerable number of studies have explored the use of thermal IR imaging for classifying affective states and human emotions. Those studies were based on measuring a person's physiological cues that are considered related to affective states. Indeed, the observations of affective nature derive primarily from muscular activity, subcutaneous blood flow, perspiration patterns in specific body parts, as well as from changing in breathing or heart rate, which are all phenomena that are controlled by our ANS. Measuring facial cutaneous temperature and assessing both its topographic and temporal distribution can provide insights about the person's autonomic activity. This depends on the ANS's role in the human body's thermal homeostasis and in the regulation of physiological responses to emotional stimuli [29]. Alert, anxiety, frustration responses, and other affective states determine the redistribution of the blood in the vessels through vasodilation or vasoconstriction phenomena which are regulated by ANS. These phenomena can be captured by an IR thermal camera through changes in the IR emissivity of the skin. Vasoconstriction implies a decrease in temperatures of the Region Of Interest (ROI). On the other hand, vasodilatation is the cause of heating. Vasomotor processes can be identified and monitored over time because they produce a thermal variation of the skin and they can be characterized by simple metrics such as temperature difference between data at two temporal points. For this reason, most researchers have focused on studying the relationship between thermal directional changes (i.e., temperature drop and rise) of specific skin areas in relation to psychophysiological states [28; 31; 52; 53]. As a body area of interest, the human face is considered of particular importance since it can be easily recorded and it is naturally exposed to social stimuli. Regions of the face extensively characterized are the nose or nose tip, the glabella (area associated with the corrugator muscle), the periorbital area, forehead, and the orbicularis oculi (surrounding the eyes), as well as the maxillary area or the upper lip (perinasal) [24; 28; 54]. Partially evaluated regions were cheeks, carotid, eyes, fingers, and lips. An exhaustive review on this topic by [PERSON] et al. summarized the emotions, the observed regions, and the direction of the average temperature changes in those regions (Table 1) [55]. Similar results were found in a recent study by [PERSON] et al., where thermal IR imaging was used during the emotions induction process to quantify temperature changes that occurred on different ROIs of the face [56]. The authors were able to classify the emotions relying on regional temperatures with an accuracy of 89.9%. The induced emotions were joy, dis \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Stress** & **Fear** & **Startle** & **Sexual Arousal** & **Anxiety** & **Joy** & **Pain** & **Guilt** \\ \hline
**Nose** & & & & & & & \\ \hline
**Cheeks** & & & & & & & \\ \hline
**Periorbital** & & & & & & & \\ \hline
**Supraorbital** & & & & & & & \\ \hline
**Forehead** & & & & & & & \\ \hline
**Maxillary** & & & & & & & \\ \hline
**Neck-carotid** & & & & & & & \\ \hline
**Finger/palm** & & & & & & & \\ \hline
**Lips/mouth** & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the temperature variations direction, as estimated through thermal IR imaging, for each emotion in the different regions considered. The table is adapted from [55]. sadness. The examined ROIs were nose, cheeks, forehead, and maxillary area, which are depicted in Figure 2. Among all the ROIs, the nose and maxillary area were the most responsive to emotional stimuli, as they showed a significant change in temperature in all the induced emotions. The forehead temperature changed during sadness, anger, and fear, while the temperature of the cheeks changed during disgust and sadness.