**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].
