Neurophysiology of Emotions

Abstract

Emotions are automatic and primary patterns of purposeful cognitive-behavioral organizations. They have three main functions: coordination, signaling, and information. First, emotions coordinate organs and tissues, thus predisposing the body to peculiar responses. Scholars have not reached a consensus on the plausibility of emotion-specific response patterns yet. Despite the limitations, data support the hypothesis of specific response patterns for distinct subtypes of emotions. Second, emotional episodes signal the current state of the individual. Humans display their state with verbal behaviors, nonverbal actions (e.g., facial movements), and neurovegetative signals. Third, emotions inform the brain for interpretative and evaluative purposes. Emotional experiences include mental representations of arousal, relations, and situations. Every emotional episode begins with exposure to stimuli with distinctive features (i.e., elicitor). These inputs can arise from learning, expressions, empathy, and be inherited, or rely on limited aspects of the environment (i.e., sign stimuli). The existence of the latter ones in humans is unclear; however, emotions influence several processes, such as perception, attention, learning, memory, decision-making, attitudes, and mental schemes. Overall, the literature suggests the nonlinearity of the emotional process. Each section outlines the neurophysiological basis of elements of emotion.

1. Introduction

The study of emotions has fascinated scholars from all over the world for millennia. Socrates and Plato dealt with it about two thousand five hundred years ago, and they probably were not even the first [1]. Although there have been considerable advances since then, our knowledge is far from complete.

In this chapter, the term emotion refers to “automatic and primary patterns of purposeful cognitive-behavioral organizations” [2]. Although there is no consensus, data suggest that emotions are “automatic models” since each subtype probably has specific neurophysiological layouts [3]. Furthermore, every emotional process is “primary” because, in certain situations, it coordinates the activity of the nervous system (e.g., perception, attention, and memory) [4]. The expression “cognitive-behavioral organizations” describes the coordinating nature of emotion and its ability to facilitate distinctive behavioral responses [5], while maintaining central control. In particular, brain processing makes it possible to learn (see Section 5.1), inhibit, and regulate emotional responses [6]. Emotions are then “purposeful” because they aim to prepare the body to respond to situations that have occurred repeatedly throughout evolution [4].

On the whole, these features reveal the essential functions of emotions, namely [7]:

• Coordination: Emotions coordinate organs and tissues, thus predisposing the body to peculiar responses (see Section 2).

• Signaling: Although the central nervous system (CNS) maintains the faculty of control (e.g., inhibition), coordination activities facilitate the production of distinctive behavioral responses and expressive signals of the individual’s current emotional state (see Section 3).

• Information: The CNS interprets and evaluates emotional episodes. That allows individuals to partly consciously perceive emotions, learn from them, and direct behaviors (see Section 4).

2. Coordination

One of the first scientists to define the nature of emotions was probably William James. Until then, the prevalent idea was that situations evoke emotions that, in turn, trigger bodily changes. James instead claimed that “bodily changes follow directly the PERCEPTION of the exciting fact, and that our feeling of the same changes as they occur IS the emotion” (pp. 189–190) [8]. The Danish psychologist Lange developed similar concepts in the same period. Therefore, today, scholars refer to this idea as the James-Lange theory. Sensory systems send data about the current situation to the central nervous system. Subsequently, the CNS induces physiological changes (e.g., heartbeat and muscle tone). The following feeling of these changes is the emotion. In other words, there is no emotion without physiological changes. It seemed a counterintuitive thesis even then. Nevertheless, several scholars accepted the James-Lange theory [9].

Some years later, Walter Bradford Cannon falsified James’s idea. He considered that stopping sensory sensitivity would impair the central perception of physiological changes, thereby eliminating emotions. Thus, Cannon resected the animals’ spinal cords. Results suggested that surgically operated individuals still felt emotions, though. Furthermore, it seemed the same physiological changes accompanied various emotions. Cannon concluded that emotions disturb the activity of the autonomic nervous system (ANS) [10], and Philip Bard enriched this view. In brief, this hypothesis (i.e., the Cannon-Bard theory) [9] is an “all-or-none” approach with only two autonomic patterns: non-activated versus diffusely activated [5].

Neither theory has disappeared over the years. Indeed, they have led to two contrasting approaches.

First, nonstate theories and general arousal models suggest the inexistence of specific internal states of emotions [11]. The two-factor theory [12], the component process model [13], and other hypotheses [14] reach the same conclusion. Empirical evidence does not support the idea that emotions have specific ANS patterns. Part of the data instead suggests that undifferentiated arousal accompanies emotional experiences [3]. Something in the body happens, but are the people who label it as an emotional experience (e.g., fear, joy, and sadness) [15]. Therefore, proponents of these theories (i.e., cognitive models) claim that emotions result from brain activity [5].

The second approach supports the existence of discrete emotions, each one characterized by specific neurophysiological and behavioral routines. In this case, scholars usually view emotions as an adaptive mechanism, a product of evolution [4]. Charles Darwin was probably the first to search for the cause of expressions [16]. Subsequently, several scientists focused on the link between emotions, autonomic activity, and behavioral responses (e.g., facial expressions) [17]. Proponents of this functional-evolutionary approach claim the existence of different emotions associated with biobehavioral layouts [5].

There is a lively debate still today. In particular, scholars did not reach a consensus on the existence of emotion-specific response patterns. One explanation for this diatribe lies in the methodological challenges the study of emotions entails. Individual differences (e.g., emotion recognition skills), the choice of elicitor (e.g., there is no certainty that a given stimulus elicits a given emotion in people), indicators (e.g., a continuous recording of different physiological and behavioral measures) [5], and statistical methods are among these [18].

Despite the issues, data support the hypothesis of specific response patterns for distinct subtypes of emotions [3]. For example, in rodents, different types of fear correspond to independent neural substrates [19]. Indeed, emotional families are sets of states that share elicitors (see Section 5), autonomic patterns, expressions, and behavioral reactions [17]. The neural substrates of emotional subtypes facilitate different behavioral responses. As an illustration, consider the Fight Flight Freeze System [20]:

• The flight depends on norepinephrine activity from the locus coeruleus. Moreover, the amygdaloid complex (Amg) activates periaqueductal gray and brainstem autonomic nuclei.

• The fight has its neurobiological basis in the hypothalamic-pituitary-adrenal axis (HPA). In particular, the release of cortisol stimulates gluconeogenesis (i.e., the conversion of substrates into glucose) and glycogenesis (i.e., glycogen synthesis). That provides fuel for metabolism and activates the sympathetic division of the autonomic nervous system [21].

• Freezing also relies on a specific neural network. In brief, the central nucleus of the amygdala has connections to the lateral hypothalamus (i.e., which mediates autonomic sympathetic responses), medullar nuclei (i.e., that control parasympathetic response), and the ventrolateral periaqueductal gray [22].

3. Signaling

The bodily activity that occurs with emotions has a high signaling value. For example, organs and tissues, as well as the nervous system, can signal emotional states [20].

Furthermore, emotions and motor activation often correlate. That can affect the striated muscles of the neck, back, arms, or the smooth muscles of the blood vessels and alimentary tract. Similarly, facial muscles can also be part of the emotion [23]. All these activities can be expressions, namely, distinctive signals of emotional episodes [6].

However, emotions are not the only determinants of bodily signals. In particular, contextual and cognitive factors make it challenging to distinguish expressions from cues attributable to other causes. Individual differences (e.g., age, gender and learning) are often decisive in expression regulation. Indeed, the nervous system (e.g., premotor cortex and primary motor cortex) has the flexibility to adjust actions already planned to the current situation [9]. That means individuals can generally inhibit, mask, or even simulate expressions [6].

In brief, emotional signals can belong to three macro categories. First, verbal behavior refers to emotional expression through natural-historical languages. The second category, nonverbal behavior (NVB), concerns any type of action except the use of words. Gestures, gait, and posture are examples of NVB. Although facial expressions also fall into this category, they will be examined separately, given their significance to humans. Finally, autonomic activity can produce external manifestations (neurovegetative signals) interpretable as expressions (e.g., pupil diameter, heart rate, and breathing).

3.1 Verbal behavior

Voice and speech are the two components of the act of speaking. The voice features are pitch, volume, intensity, and rhythm. Instead, speech is the content of discourses. It includes vocabulary, grammar style, and structure [24].

Humans can use verbal behavior to express emotions [25]. Speaking is a faculty that recruits several anatomical structures: cerebral regions (e.g., the frontal lobe) [9] and the digestive and respiratory systems (e.g., lungs, larynx, sinus cavities of the vocal tract, palate, tongue, and teeth) [26]. Noteworthy, dysfunction of the frontal lobe is one of the determinants of alexithymia, a condition that involves among other things, difficulty or inability to verbalize emotions [27]. However, a vast cerebral network underpins verbal expression of emotion. The right inferior frontal cortex, the right posterior superior temporal cortex, the left mid-fusiform gyrus, the right inferior prefrontal and bilateral fusiform cortices, and the amygdaloid complex are part of this network [25].

3.2 Nonverbal behavior

Behavioral responses can be emotional clues. For example, gait (e.g., arm swing, length, and speed of stride) can reveal whether an individual is happy, sad, or angry [28]. Furthermore, the emotional state can influence posture (i.e., the position of the body or its parts) [29], produce acoustic signals, such as laughter [30], and alter the sound of voice (e.g., pitch, intensity, and tension) [31].

However, humans can voluntarily signal and fake emotional states through their bodies (e.g., facial expressions, gestures, and posture) [30]. Birds also have this ability. For example, the wild fork-tailed drongos (Dicrurus adsimilis) produce false mimicked alarm calls that scare meerkats (Suricata suricatta). Thereby, these birds steal meerkats’ food [32].

It is unclear whether emotional expression management relies on a single neurocognitive system. The intentional inhibition of human motor responses depends, at least in part, on the activation of the right inferior frontal cortex (rIFC). Indeed, the activity of the rIFC is often associated with the deactivation of other brain regions important for emotions, such as the amygdaloid complex [33].

3.3 Facial expressions

The face is informative in several ways. For example, humans get clues about people’s health through skin color [34]. Nonetheless, the main source of information is the activity of the facial muscles. Their contraction, in specific combinations, produces skin movements, namely, facial expressions. Moreover, they assume complex patterns according to the movement of the head and eyes.

The muscles of the face include two large groups. First, the mastication muscles (i.e., temporalis, masseter, and pterygoid muscles) have the primary task of moving the jaws and chewing. However, they can even participate in emotional expression. It is the trigeminal nerve that innervates these muscles (Figure 1).

Figure 1.

Schematic representation of the motor pathways of mastication.

The expressive or mimetic muscles are the second group. The facial nerve innervates these muscles. Indeed, their function is to configure the expression of the face. The temporofacial division of this nerve connects the muscles of the upper part of the face to both cerebral hemispheres. Instead, the cervicofacial facial nerve links the lower face only to the contralateral hemisphere (Figure 2).

Figure 2.

Schematic representation of the pathways of human facial expressive muscles.

The cerebral cortex controls voluntary movements through the corticospinal (or pyramidal) tract [23]. Two-thirds of this tract receives input from the motor cortex and the rest from the somatosensory areas, such as the parietal lobe [9]. For these reasons, emotional facial expressions seem to depend on the other trait, the extrapyramidal one.

The right side of the face could be dominant for emotional expressions. That is the idea of some scholars, based on some clinical evidence. For example, several people show a left bias during posed expressions [23]. Nevertheless, the empirical results are ambiguous, and academic speculations are divergent [35]. For instance, the approach-withdrawal model hypothesizes that emotions of “approach” (e.g., joy) coincide with more activity of the left frontal brain, and the “withdrawal” ones (e.g., fear) activate the right frontal brain to a superior extent [36].

Moreover, humans can exhibit brief, local contractions (i.e., microexpressions). Their duration varies from about 40 to 335 ms [37]. Microexpressions mainly involve the upper face muscles (e.g., the frontalis) and occur unconsciously, at least in part. Indeed, it is the extrapyramidal tract that mediates their production [23]. However, their alleged unintentional nature has stretched their informative potential. In particular, several scholars believe that microexpressions are reliable signals of spontaneous emotions and lies. For example, law enforcement and airport security use microexpressions as lie-detecting clues. All this despite the experimental data being inconclusive and practical applications ineffective [38]. However, microexpressions could be functional in other fields (e.g., to survey the quality of the patient-therapist relationship) [39]. Noteworthy, only a few microexpressions seem unmanageable. For example, the eyebrow flash and contempt expression are more controllable [38].

Although the prototypical patterns are well known, there is a low coherence between facial displays and emotions. Specifically, the likelihood of a person showing an expression (e.g., the Duchenne smile) when feeling the corresponding emotion (e.g., joy) is often lower than chance, in the both laboratory [40] and naturalistic settings [41]. One of the determinants of this low emotion-expression coherence lies in display rules. In brief, they are laws of expression management based on various factors (e.g., context, roles, gender, and age). Learning these rules usually takes place in the first years of life. Thereby, humans learn to repeat, amplify, and inhibit the expression of emotions [42]. It is the cerebral cortex that mediates the voluntary inhibition of facial movements [23].

3.4 Neurovegetative signals

Despite limitations and still open questions, there are enough data to state that physiological changes accompany emotions (see Section 2).

Activities of the autonomic nervous system can induce appearance variations. For example, vasodilation can cause blood vessels to bulge and alter the color of the skin. Blushing (i.e., in embarrassment) and reddering (e.g., in anger) are two typical neurovegetative signals of an increase in the caliber of blood vessels. Conversely, vasoconstriction (e.g., in fear) produces blanching.

The body can also secrete various substances. For example, tear glands provoke crying [43] related to some types of sadness [44]. Similarly, the sweat glands produce sweat (e.g., in fear), and the salivary ones are responsible for the secretion of saliva, which is typical of certain emotional states, such as disgust and anger.

Other neurovegetative signals are piloerection and the change in pupil diameter. They can be cues to emotions (e.g., anger and fear) or other states (e.g., sexual appetite) [43].

At the central level, these ANS activities are outcomes of a neural network that involves the hypothalamus (Hy), which is essential for homeostasis. The Hy links with periaqueductal gray, the reticular formation, parabrachial nucleus, ventral tegmental area (VTA), and the raphe nuclei [45].

4. Information

Rather than describing the whole emotional process, the James-Lange theory focuses on the emotional experience, namely, what the individual perceives of emotion [11].

In particular, emotional experiences consist of mental representations that can relate to different aspects of emotions. First, the individual can perceive, even if only partially, arousal [46]. The central nervous system processes the information it retrieves from the body’s activity. For example, reactions, such as wrinkles, blushing, and tearing are all signals, that potentially influence the perception of emotions [47]. Indeed, autonomic feedback (e.g., from sweating and respiration) is another essential feature of the emotional experience [48]. The second aspect is about the relational content (e.g., mental representations of dominance and submission). Third, the situational content is an integral part of the emotional experience. For example, people often link psychological situations to their emotions.

The elements of the emotional experience concern the appraisal, at least in part [46]. In brief, it consists of the cognitive assessments (e.g., of valence) accompanying emotions (e.g., positive or negative) [48]. According to the cognitive approach, the appraisal is necessary to elicit emotions, and it can also occur quickly and involuntarily [49].

The neurophysiological basis of emotional experience may rely on two distinct networks. A first circuit seems to allow value-based representation. The essential brain regions of this network are the basolateral amygdala (BLA), the anterior insula, and the orbitofrontal cortex (OFC). In particular, the BLA provides an initial assessment of a stimulus. The anterior insula instead allows the representation of interoceptive inputs. Finally, the OFC makes processing more flexible by including information regarding the context. BLA and OFC are interconnected with each other, as well as have connections with the cortical regions responsible for sensory representations. The second circuit of emotional experience seems to be a sort of affective working memory that holds and processes emotive information for short periods. Its neurophysiological basis lies in the amygdaloid complex and the reciprocal connections between the prefrontal cortex (PFC) and the anterior cingulate cortex [46].

5. Elicitors of emotions

An emotional episode begins through exposure to stimuli with specific features. For example, loss causes sadness [50]. In this sense, emotion is a process, and these stimuli (i.e., elicitors) are the inputs that initiate it.

Animals learn to feel certain emotions in specific situations. However, it is not just environmental cues that trigger an emotional episode. The state of the organism, behavior, and other complex faculties (e.g., thoughts and empathy) can be elicitors too.

5.1 Learning

Stimuli internal and external to the body can become elicitors of emotion through classical and operant conditioning. In particular, animals learn to associate a stimulus (S) with a specific emotional response (R). With an S-R association established, exposure to S may be sufficient to elicit the emotional response.

Yet, S-R associations can take complex forms. For instance, emotional reactions can even be self-reinforcing. An individual may experience a pleasant state that elicits behavior, which fuels repetition in a sort of loop [51]. However, the S-R associability is not absolute. For example, it seems impossible to teach a hungry pigeon to fly away by presenting it with food [52].

At the basis of emotional learning is a vast neural network that includes the reward system. Its architecture is complex and involves circuits for the cost/benefit assessment of specific reward values, reward expectations, and action selection (Figure 3). The amygdaloid complex and ventral striatum (vStr) underpin the appetitive processing of reward expectations. Moreover, dopamine (DA) pathways modulate motivation and behavior by connecting the ventral tegmental area, the substantia nigra pars compacta (SNc), and the striatum. The reward system is also capable of inhibiting the behavior. Specifically, five sub-circuits (or sub-loops) of the basal ganglia (BG) are essential for various functions, including action cancelation [53].

Figure 3.

Schematic representation of reward system. One: cost/benefit assessment of specific reward value. Two: cortical loop. Three: limbic loop. Four: reward expectation. Five: action selection. Six: go and stop processes. Abbreviations: Amg, Amygdaloid complex; DA, dopamine; dlPFC, dorsolateral prefrontal cortex; dStr, dorsal striatum; GP, globus pallidus; Hip, hippocampus; OFC, orbitofrontal cortex; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata; STN, subthalamic nucleus; vStr, ventral striatum; VTA, ventral tegmental area. This article was published in [53] Copyright Elsevier (2020).

5.2 Expressions

Even emotional signals (see Section 3) can be elicitors. For example, breathing [54], vocalizations [55], and postures [56] may elicit emotions. Indeed, some experiments suggest that an expansive pose lowers cortisol and increases testosterone levels. That would be enough to produce feelings of power and increase risk tolerance [57]. However, attempts to replicate such data have failed [58].

In general, peripheral feedback theories propose the idea that emotional expressions can become elicitors [56]. For example, facial expressions could trigger emotions. According to this idea (i.e., the facial feedback hypothesis), movements of the face influence the release of some neurotransmitters, thus acting as an elicitor [59]. Therefore, feedback theories follow the path traced by the James-Lange theory (see Section 2).

Observing signals and behaviors in others can also initiate the emotional process. In this sense, indirect experience (e.g., imitation and emotional contagion) is a potential elicitor of emotion. The anterior insula and the rostral cingulate cortex are part of the neural network responsible for these mechanisms [60].

5.3 Empathy

Empathy consists of the emotional states produced by observation of individuals and situations. Thus, it is an elicitor of emotion per se. Scholars usually distinguish between cognitive empathy (CE) and emotional empathy (EE). The CE deals with mental perspective-taking, mentalizing, and the theory of mind. Instead, EE consists of the vicarious sharing of emotions [61].

The resulting emotion may be the same as that of the observed individual, but not necessarily. Indeed, the emotional experience can be so intense as to produce an empathic overarousal, which often induces disengagement [62]. That happens, for example, to paramedics who, being exposed to traumatic events, adopt coping strategies, such as emotional detachment [63]. In other cases, emotions felt may be diverse from those observed. That is the case of Schadenfreude (i.e., the pleasure caused by the misfortune of others) [64].

Given the complexity of empathy as a faculty, it seems clear that its neurobiological basis is equally complex. For example, its neural substrate includes the insula, cingulate cortex, and the interoceptive network [65]. Furthermore, empathic responses probably depend on various processes (e.g., emotional contagion and affective mentalizing) that underpin distinct neural mechanisms. The temporoparietal junction, medial temporal lobe, prefrontal cortex, and dopamine pathways are part of the circuits of cognitive empathy. Instead, the neural substrates of emotional empathy include the frontal gyrus, insula, anterior cingulated cortex, and oxytocin paths [66].

5.4 Sign stimuli

Animals can feel emotions even when exposed to stimuli they have never encountered. For instance, the smell of predators they have never seen before scares rats [67]. Some scholars claim that humans also have innate fears (e.g., snakes) [68]. However, experimental results are controversial [69].

Several animals can respond to limited aspects of the environment (i.e., sign stimuli), ignoring the rest [70]. The common toad (Bufo bufo) produces defensive responses (e.g., stiff-legged) when faced with relatively simple perceptual patterns (dummies) with specific configurational features reminiscent of their predators (i.e., snakes) [71]. Moreover, newborn babies prefer and imitate human face schematizations (i.e., facelike patterns) [72].

From a neurobiological point of view, it is unlikely that there are cells in the nervous system specialized in innately identifying specific stimuli. The determinants of any sign stimuli as elicitors could lie, then, in the biological predisposition [52].

6. Outcomes

Emotions influence several processes, including [4]:

• Perception: Emotional states can magnify some perceptual aspects to the detriment of others. For instance, afraid-of-falling people overestimate the steepness of hills [73].

• Attention: According to a theoretical approach, the central nervous system allocates more cognitive resources to selective attention and vigilance under the influence of negative emotions. Instead, positive ones spread these resources. That hypothesis (i.e., broaden-and-build theory) [74] is subject to debate, however [75]; in this sense, each emotion may have selective effects on attentional performance [76].

• Learning: Emotions heighten some mnemonic aspects at the expense of others. In particular, several factors (e.g., personality and age) define the enhancement and impairment of learning due to emotional influences [77].

• Memory: Considering their relationship with attention and learning, it seems logical that emotions influence memory. The flashbulb memory (i.e., the vivid remembering of details of when a person learned about a specific dramatic event) is an excellent example of this [78].

• Decision making: Emotions impact the evaluation and interpretation processes. For example, sadness may lead people to overestimate difficulties [2].

• Attitudes and mental schemes: Some emotional episodes can shape attitudes, mentalities, and values. For instance, awe produces overwhelming and elevating experiences that strengthen the sense of unity. In particular, those who experience emotional experiences of this type usually develop a new vision of life and the universe [79].

The neurophysiological basis of the relationship between emotions and cognitive processes is not yet fully understood. In brief, neurophysiological superimposition of emotions and other mechanisms may underpin emotional outcomes. For example, amygdala, hippocampus, and orbitofrontal cortex have a role in several neural functions [80].

7. Emotional and mood disorders

Some emotion-based symptoms may appear in various conditions, such as schizophrenia (e.g., anhedonia), borderline personality disorder (e.g., emotional instability), addiction (e.g., euphoria and dysphoria), and so on. These conditions can be related to several features of emotions, such as intensity, frequency, adaptivity, physiology, expression, and experience [81]. However, some scholars disagree with placing these disorders in the same category. Indeed there is the risk of generalizing qualitatively diverse states (e.g., emotions, moods, and affect) [7].

Several scholars use the term “emotional disturbance” to refer to psychopathologies that include emotion-related symptoms, such as regulation problems, phobias, specific deficits (e.g., lack of empathy), and so on [81]. It is challenging to briefly delineate the etiology of these emotion-related psychopathologies. In brief, there are hereditary, epigenetic, developmental, environmental, and behavioral determinants. From a neurophysiological point of view, emotion disturbances usually result from cerebral peculiar functioning. Indeed, the bases of such conditions often include inefficient reuptake [82], irregularities in synaptic proteins, and neuronal density [83]. Even social activities (e.g., play) can influence the development of these brain mechanisms and shape cerebral maturation [84]. These factors can influence the functioning of a vast neural network that includes the prefrontal cortex, limbic system, striatum, thalamus, and brainstem [83].

8. Conclusions

Albeit limitations [5], the literature suggests that emotions predispose the body to timely recognition and response to specific circumstances. Situations identify ancestral problems, and the responses illustrate the solutions that have proved most profitable for evolutionary success [4]. In this sense, the emotional process has as its central themes the body’s coordination [5], the signaling of the individual’s state [17], and the processing by the central nervous system of both endogenous and exogenous information [46].

The literature does not allow a conclusive illustration of the neurophysiology of emotions. Nevertheless, each emotional subtype likely has its patterns [3]. It seems then better to speak of families rather than single emotions [17].

Due to several factors, the emotional process affects performance in different domains (e.g., perception and attention). These factors include the partial overlap of the neural basis of emotion and other faculties and the numerous brain interconnections. Furthermore, elicitors are heterogeneous and even include the essential elements of emotion (e.g., expressions). That suggests the nonlinearity of the emotional process. In other words, emotions could have stochastic or aleatory progress: In probabilistic terms, each element can initiate, be a part of, or be their outcome [7].

Conflict of interest

The authors declare no conflict of interest.

Thanks

Thanks to every enthusiast, scholar, and researcher who came before me. Thanks to those who deal with emotions today, and to those who will do so in the future. Everyone’s contribution is a step forward in the path of knowledge. Thanks to every reader, without whom the effort of writing this chapter would have been futile. Ideologies separate us, and emotions bring us together.

Abbreviations

Amg	amygdaloid complex
ANS	autonomic nervous system
BG	basal ganglia
BLA	basolateral amygdala
CE	cognitive empathy
CNS	central nervous system
DA	dopamine
dlPFC	dorsolateral prefrontal cortex
dStr	dorsal striatum
EE	emotional empathy
GP	globus pallidus
Hip	hippocampus
Hy	hypothalamus
HPA	hypothalamic-pituitary-adrenal axis
NVB	nonverbal behavior
OFC	orbitofrontal cortex
PFC	prefrontal cortex
rIFC	right inferior frontal cortex
SNc	substania nigra pars compacta
STN	subthalamic nucleus
vStr	ventral striatum
VTA	ventral tegmental area