Thermography also has application in psychology to understand mental and emotional processes.

Emotions are psychological processes considered as responses to stimuli involving brain processes, bodily reactions and behavior. The emotional response comprises multidimensional processes of short duration, is subjective, has a high affective charge and seeks balance or homeostasis.

Emotions can affect skin temperature, especially on the face. Although it may sound strange, several studies have shown that mood can cause variations in skin temperature, allowing us to identify different human feelings.

Thermal response of emotions:

Human body temperature is not only a vital biological function, but also plays a crucial role in the psychological realm. In both biological and emotional situations, temperature control is essential for the maintenance of internal balance and the body's adaptive response. For example, when faced with environmental changes or fighting an illness, such as fever during a viral infection, the body activates physiological mechanisms to regulate skin temperature and keep it within normal limits.

However, temperature regulation in response to emotional events is much more complex. In these cases, body temperature not only reflects environmental changes, but is also influenced by autonomic nervous system activity and emotional response mechanisms. When we experience intense emotions, such as fear or anger, the body activates a series of physiological responses that include changes in skin temperature.

The autonomic nervous system plays a key role in regulating these responses. For example, in situations of perceived threat, the sympathetic nervous system is activated, leading to subcutaneous vasoconstriction and the release of emotional sweating. These mechanisms help the body prepare to fight or flee, increasing alertness and responsiveness to danger.

In addition to its role in temperature regulation, the autonomic nervous system also influences facial expression. Facial muscles are controlled by the autonomic nervous system and can reflect internal emotional states. For example, during stressful or anxious situations, it is common to experience changes in facial expression, such as frowning or tensing facial muscles (Ioannou et al., 2014).

According to some authors such as Posner et al., (2005), there are three main approaches in the study of emotions:

• The modular approach considers that emotions can be classified into six basic categories: happiness, surprise, disgust, anger, fear and sadness.
• The dimensional approach considers that emotions can be classified according to valence dimensions, which identify whether the emotion is pleasant-unpleasant, and activation, which measures low-high emotion intensity or activation-deactivation.
• The third approach is based on the combination of a dimensional approach and a modular approach.

Classification of valence and arousal in emotions. Yellow shows the classification of the modular approach. Circumplex model of emotions, extracted from Posner et al. (2005).

Use of Thermography:

Infrared thermography is a technology that allows the measurement of the energy radiation emitted by a body in a noninvasive manner and without the limitations that arise from the use of invasive sensors. Thermography, among others, has been used to study the response to startle, empathy, guilt, shame, sexual arousal, stress, fear, anxiety, pain and joy.

Some emotions have been studied using thermography, because among others, when an emotion occurs, there is a change in facial temperature due to the blood flow that the body emits through the blood vessels in the subcutaneous area, this change can be graded and quantified using thermography.

Although results are sometimes inconsistent, correlations have been established between facial thermal changes and emotional experiences, for example:

• It has been found that increased temperature in the nose is associated with pleasant mental states, while decreased temperature may indicate negative emotions.
• In addition, it has been observed that warming of certain facial areas, such as the forehead and cheek, may be related to less positive feelings, while warming in the eye area may indicate positive self-feelings.

These correlations have been significant not only in laboratory settings, but also in everyday situations, such as driving.

ROI and the study of emotions.

To obtain information of an affective nature, Regions of Interest(ROIs) are used. The regions on which most studies have been based are the nose or tip of the nose, the periorbital and supraorbital vessels of the face generally associated with the corrugator muscle, the forehead and orbicularis oculi (surrounding the eyes), and the maxillary area or upper lip (perinasal). The regions that have been used less frequently are the cheeks, carotid, eyes, fingers and lips.

Depending on the subjects' response to the emotional stimulus, as well as to the ROI, the temperature increases or decreases. We can find a summary of emotions and ROI in the study by Ioannou et al. (2014):

General description of the direction of temperature variation in the regions of interest considered according to emotions. Extracted from Ioannou et al. (2014).

That is why some researches such as Cruz-Albarran et al. (2017), have analyzed infrared images to try to identify basic emotions such as: joy, disgust, anger, fear and sadness. According to the authors, these emotions show different thermal facial behavior due to the blood flow that radiates through the blood vessels when an emotion occurs. In this same study, the change in facial temperature was quantified in the different ROIs to study the response to these emotions.

Another study by Salazar-López,et al (2015) explored the use of thermography as a tool to better understand emotions and empathy. In particular, they focused on emotional valence (positive or negative) and arousal level, demonstrating that these factors influence face temperature, especially at the tip of the nose and forehead. The authors argued that thermography can be an effective tool for distinguishing between different emotional states and levels of empathy. For example, they observed that during situations of emotional contagion, the temperature of the nose tends to decrease, which may be related to a lower emotional activation to avoid discomfort. On the other hand, during experiences of love or empathy, facial temperature tends to increase, indicating greater emotional involvement. These results suggest that thermography can provide valuable information about how we experience and process emotions.

Applications::

As we have seen above, the use of thermography for emotion detection is presented as an effective tool and its applications can be diverse.

Spanish researchers (Moliné et al., 2018) have studied the "Pinocchio effect". They found that the change in temperature of the nose and forehead can allow us to detect when people lie about facts (Pinocchio effect markers) with high accuracy.

Example of lying, producing a decrease in the temperature of the forehead and nose. Extracted from (Moliné et al., 2018).

Other researchers such as Arcangelo Merla and his research group have studied the use of thermography for classification of workload levels (Cardone et al., 2022), synchrony of the state between mother and child (Ebisch et al., 2012). We also found applications in the study in people with severe motor disabilities (Memarian et al., 2011).

Some companies, such as Next2U, show applications in areas such as neuromarketing, video game behavior, and different interactions with machines.

ThermoHuman is already working on new algorithms for recognizing facial regions. This functionality will be aimed at different applications such as temperature identification linked to lactate threshold, also in applications for dentistry and even to issues related to emotion recognition as we have exposed in this post. We will announce you when this functionality will be available, which is sure to be very soon.

References:

Aristizabal-Tique, V. H., Henao-Pérez, M., López-Medina, D. C., Zambrano-Cruz, R., & Díaz-Londoño, G. (2023). Facial thermal and blood perfusion patterns of human emotions: Proof-of-Concept. Journal of Thermal Biology, 112, 103464.

Cardone, D., Perpetuini, D., Filippini, C., Mancini, L., Nocco, S., Tritto, M., & Merla, A. (2022). Classification of Drivers’ Mental Workload Levels: Comparison of Machine Learning Methods Based on ECG and Infrared Thermal Signals. Sensors, 22(19), 7300.

Cruz-Albarran, I. A., Benitez-Rangel, J. P., Osornio-Rios, R. A., & Morales-Hernandez, L. A. (2017). Human emotions detection based on a smart-thermal system of thermographic images. Infrared Physics & Technology, 81, 250-261.

Ioannou, S., Gallese, V., & Merla, A. (2014). Thermal infrared imaging in psychophysiology: potentialities and limits. Psychophysiology, 51(10), 951-963.

Moliné, A., Dominguez, E., Salazar‐López, E., Gálvez‐García, G., Fernández‐Gómez, J., De la Fuente, J. & Gómez Milán, E. (2018). The mental nose and the Pinocchio effect: Thermography, planning, anxiety, and lies. Journal of Investigative Psychology and Offender Profiling, 15(2), 234-248.

Posner, J., Russell, J. A., & Peterson, B. S. (2005). The circumplex model of affect: An integrative approach to affective neuroscience, cognitive development, and psychopathology. Development and psychopathology, 17(3), 715-734.

Salazar-López, E., Domínguez, E., Ramos, V. J., De la Fuente, J., Meins, A., Iborra, O., & Gómez-Milán, E. (2015). The mental and subjective skin: Emotion, empathy, feelings and thermography. Consciousness and cognition, 34, 149-162.