**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**).

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**).

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 approachwithdrawal 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

**Figure 1.** *Schematic representation of the motor pathways of mastication*.

#### **Figure 2.**

*Schematic representation of the pathways of human facial expressive muscles.*

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].
