According to the Merriam-Webster and the Oxford dictionaries, the term “emotion” was first used in 1579 and comes from the Old French term “esmovoir.” This French term means “to stir up.” Emotion is defined as “the affective aspect of consciousness”; “a conscious mental reaction (such as anger or fear) which is subjectively experienced as a strong feeling usually deriving from one's circumstances, mood, or relationships with others.” “This feeling is typically accompanied by physiological and behavioral changes in the body.” “This mental state is an instinctive or intuitive feeling which arises spontaneously as distinguished from reasoning or knowledge.”

In his book entitled The Expression of Emotions in Man and Animals, published in 1872, Charles Darwin1 claimed that a specific set of spontaneous facial expressions is universal across countries and cultures. According to Paul Ekman,2 another body of research has established different patterns of bodily changes generated by autonomic nervous system activity and brain activity coinciding with the appearance of facial expressions. Indeed, distinct facial expressions may be related to distinct physiological responses such as changes in cortisol, oxytocin, or dopamine levels as well as in the cytokine response (related to the immune system). This means that facial expressions are not only informative about the individual's feelings, but also about neurophysiological activation in their bodies.

Emotions have been characterized by both dimensional and categorical theoretical frameworks. The dimensional theory includes two dimensions: arousal (ie, emotional strength) and valence (eg, pleasantness vs unpleasantness). The categorical view includes discrete categories such as fear and anger and refers to the basic emotions theory. The dimensional description of emotions can characterize key aspects of emotional reactions in terms of arousal and valence of a given emotion; whereas the basic emotion categorization may rather capture somatic states, facial expressions, and regional brain activations. According to Hamann,3 both theoretical frameworks are supported by neuroimaging studies. However, there is also evidence for complex network-based representations of emotion.

Emotions are largely altered in many psychiatric disorders, such as manic and depressive states, which are associated with changes in emotional processing, or in schizophrenia, which is associated with dysfunctions in emotion expression, experience, and recognition. The consequences of dysfunctions in emotional processing on social functioning, as well as the direct effects of psychotropic drugs such as antidepressants on emotions (independently of depression), needs to be better analyzed.