Neuroanatomical Basis of Facial Expression in Monkeys, Apes, and Humans

Cosmetic appreciation and emotional processing in patients with a peripheral facial palsy: A systematic review

Background; The goal of this review is 1) to summarize the studies assessing PFP by casual observers, patients themselves and the cosmetic appreciation of the PFP and 2) to summarize the studies assessing whether there is a difference in emotional recognition/processing of facial emotions and/or cognitive tasks in patients with a PFP. Materials and Methods; A multi-database systematic literature search was performed using the following databases: Pubmed, Embase, Medline, and The Cochrane Library from the earliest date of each database up to December 2019. Population of interest consisted of patients with a PFP and studies that investigated cosmetic appreciation and/or emotional recognition and/or emotional processing in these patients. Two authors rated the methodological quality of the included studies independently using the 'Newcastle - Ottawa Quality Assessment Scale' for nonrandomised studies' (NOS). Two authors extracted the outcome data regarding cosmetic appreciation and/or emotional recognition/processing from the included studies. Results; Twelve hundred and thirty-two studies were found of which eleven studies met the inclusion criteria. Most studies were assessed to be of 'fair' to 'good' methodological quality. The Cohen's kappa (between author RL and SP) was 0.68. Two studies investigated emotional processing and/or emotional recognition. Nine studies investigated cosmetic appreciation in both patients and casual observers. Important findings of this systematic review are that there is a correlation between the perceived severity of the PFP of the patients and the ratings by casual observers. Secondly there seems to be a laterality difference in cosmetic appreciation and thirdly there might to be a decreased emotional recognition and processing in patients with a PFP. Conclusion; Emotional recognition and cosmetic appreciation in patients with a PFP is an under investigated area, in which further studies are needed to substantiate the findings in current literature.

A closer look at the paralyzed face; a narrative review of the neurobiological basis for functional and aesthetic appreciation between patients with a left and a right peripheral facial palsy

BACKGROUND

The facial nerve or n. facialis (NVII) is the seventh cranial nerve and it is responsible for the innervation of the mimic muscles, the gustatory organ, and the secretomotor function to the salivary, lacrimal, nasal and palatine glands. Clinical presentation of Facial Palsy (FP) is characterized by unilateral facial asymmetry and may present with a change in taste, decreased saliva production, and dysarthria. A facial palsy has a notable effect on the facial appreciation by both the patient and the environment and also affects quality of life and emotional processing. There appear to be differences in the appreciation of people with a left and right facial palsy.

PURPOSE OF THIS REVIEW

The purpose of the review is to give an overview of the anatomy of the facial nerve, neuro-anatomy of face processing, and hemispheric specialization and lateralization. Further,an overview is given of the clinical studies that translated the neuro-anatomical and neurobiological basis of these concepts into clinical studies. What this review adds: This review emphasizes the neurobiological evidence of differences in face processing between the left and right cerebral hemisphere, wherein it seems that the right hemisphere is superior in emotional processing. Several theories are proposed; 1) a familiarity hypothesis and 2) a left-right hemispheric specialization hypothesis. In clinical studies, promising evidence might indicate that, in patients with FP, there is indeed a difference in how left and right FP are perceived. This might give differences in decreased quality of life and finally in occurrence of depression. Further research must aim to substantiate these findings and determine the need for altering the standard therapeutic advice given to patients.

The neural basis of understanding the expression of the emotions in man and animals

Humans cannot help but attribute human emotions to non-human animals. Although such attributions are often regarded as gratuitous anthropomorphisms and held apart from the attributions humans make about each other's internal states, they may be the product of a general mechanism for flexibly interpreting adaptive behavior. To examine this, we used functional magnetic resonance imaging (fMRI) in humans to compare the neural mechanisms associated with attributing emotions to humans and non-human animal behavior. Although undergoing fMRI, participants first passively observed the facial displays of human, non-human primate and domestic dogs, and subsequently judged the acceptability of emotional (e.g. 'annoyed') and facial descriptions (e.g. 'baring teeth') for the same images. For all targets, emotion attributions selectively activated regions in prefrontal and anterior temporal cortices associated with causal explanation in prior studies. These regions were similarly activated by both human and non-human targets even during the passive observation task; moreover, the degree of neural similarity was dependent on participants' self-reported beliefs in the mental capacities of non-human animals. These results encourage a non-anthropocentric view of emotion understanding, one that treats the idea that animals have emotions as no more gratuitous than the idea that humans other than ourselves do.

What makes a frontal area of primate brain the frontal eye field?

The frontal eye field region (FEF) of the oculomotor pathways has been intensely studied. The primary goal of this review is to illustrate the phylogenetic displacement of the FEF locus in primate species. The locus is arrayed along the arcuate sulcus in monkeys and abuts into the primary motor strip region in humans. The strengths and limitations of the various functional, anatomical and histological methodologies used to identify such regions are also discussed.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Conical expansion of the outer subventricular zone and the role of neocortical folding in evolution and development

THERE IS A BASIC RULE TO MAMMALIAN NEOCORTICAL EXPANSION

as it expands, so does it fold. The degree to which it folds, however, cannot strictly be attributed to its expansion. Across species, cortical volume does not keep pace with cortical surface area, but rather folds appear more rapidly than expected. As a result, larger brains quickly become disproportionately more convoluted than smaller brains. Both the absence (lissencephaly) and presence (gyrencephaly) of cortical folds is observed in all mammalian orders and, while there is likely some phylogenetic signature to the evolutionary appearance of gyri and sulci, there are undoubtedly universal trends to the acquisition of folds in an expanding neocortex. Whether these trends are governed by conical expansion of neocortical germinal zones, the distribution of cortical connectivity, or a combination of growth- and connectivity-driven forces remains an open question. But the importance of cortical folding for evolution of the uniquely mammalian neocortex, as well as for the incidence of neuropathologies in humans, is undisputed. In this hypothesis and theory article, we will summarize the development of cortical folds in the neocortex, consider the relative influence of growth- vs. connectivity-driven forces for the acquisition of cortical folds between and within species, assess the genetic, cell-biological, and mechanistic implications for neocortical expansion, and discuss the significance of these implications for human evolution, development, and disease. We will argue that evolutionary increases in the density of neuron production, achieved via maintenance of a basal proliferative niche in the neocortical germinal zones, drive the conical migration of neurons toward the cortical surface and ultimately lead to the establishment of cortical folds in large-brained mammal species.

Cell and neuron densities in the primary motor cortex of primates

Cell and neuron densities vary across the cortical sheet in a predictable manner across different primate species (Collins et al., 2010b). Primary motor cortex, M1, is characterized by lower neuron densities relative to other cortical areas. M1 contains a motor representation map of contralateral body parts from tail to tongue in a mediolateral sequence. Different functional movement representations within M1 likely require specialized microcircuitry for control of different body parts, and these differences in circuitry may be reflected by variation in cell and neuron densities. Here we determined cell and neuron densities for multiple sub-regions of M1 in six primate species, using the semi-automated flow fractionator method. The results verify previous reports of lower overall neuron densities in M1 compared to other parts of cortex in the six primate species examined. The most lateral regions of M1 that correspond to face and hand movement representations, are more neuron dense relative to medial locations in M1, which suggests differences in cortical circuitry within movement zones.

Contrast of hemispheric lateralization for oro-facial movements between learned attention-getting sounds and species-typical vocalizations in chimpanzees: extension in a second colony

Studies involving oro-facial asymmetries in nonhuman primates have largely demonstrated a right hemispheric dominance for communicative signals and conveyance of emotional information. A recent study on chimpanzee reported the first evidence of significant left-hemispheric dominance when using attention-getting sounds and rightward bias for species-typical vocalizations (Losin, Russell, Freeman, Meguerditchian, Hopkins & Fitch, 2008). The current study sought to extend the findings from Losin et al. (2008) with additional oro-facial assessment in a new colony of chimpanzees. When combining the two populations, the results indicated a consistent leftward bias for attention-getting sounds and a right lateralization for species-typical vocalizations. Collectively, the results suggest that both voluntary-controlled oro-facial and gestural communication might share the same left-hemispheric specialization and might have coevolved into a single integrated system present in a common hominid ancestor.

Stereological analysis of the rat and monkey amygdala

The amygdala is part of a neural network that contributes to the regulation of emotional behaviors. Rodents, especially rats, are used extensively as model organisms to decipher the functions of specific amygdala nuclei, in particular in relation to fear and emotional learning. Analysis of the role of the nonhuman primate amygdala in these functions has lagged work in the rodent but provides evidence for conservation of basic functions across species. Here we provide quantitative information regarding the morphological characteristics of the main amygdala nuclei in rats and monkeys, including neuron and glial cell numbers, neuronal soma size, and individual nuclei volumes. The volumes of the lateral, basal, and accessory basal nuclei were, respectively, 32, 39, and 39 times larger in monkeys than in rats. In contrast, the central and medial nuclei were only 8 and 4 times larger in monkeys than in rats. The numbers of neurons in the lateral, basal, and accessory basal nuclei were 14, 11, and 16 times greater in monkeys than in rats, whereas the numbers of neurons in the central and medial nuclei were only 2.3 and 1.5 times greater in monkeys than in rats. Neuron density was between 2.4 and 3.7 times lower in monkeys than in rats, whereas glial density was only between 1.1 and 1.7 times lower in monkeys than in rats. We compare our data in rats and monkeys with those previously published in humans and discuss the theoretical and functional implications that derive from our quantitative structural findings.

On the origin, homologies and evolution of primate facial muscles, with a particular focus on hominoids and a suggested unifying nomenclature for the facial muscles of the Mammalia

The mammalian facial muscles are a subgroup of hyoid muscles (i.e. muscles innervated by cranial nerve VII). They are usually attached to freely movable skin and are responsible for facial expressions. In this study we provide an account of the origin, homologies and evolution of the primate facial muscles, based on dissections of various primate and non-primate taxa and a review of the literature. We provide data not previously reported, including photographs showing in detail the facial muscles of primates such as gibbons and orangutans. We show that the facial muscles usually present in strepsirhines are basically the same muscles that are present in non-primate mammals such as tree-shrews. The exceptions are that strepsirhines often have a muscle that is usually not differentiated in tree-shrews, the depressor supercilii, and lack two muscles that are usually differentiated in these mammals, the zygomatico-orbicularis and sphincter colli superficialis. Monkeys such as macaques usually lack two muscles that are often present in strepsirhines, the sphincter colli profundus and mandibulo-auricularis, but have some muscles that are usually absent as distinct structures in non-anthropoid primates, e.g. the levator labii superioris alaeque nasi, levator labii superioris, nasalis, depressor septi nasi, depressor anguli oris and depressor labii inferioris. In turn, macaques typically lack a risorius, auricularis anterior and temporoparietalis, which are found in hominoids such as humans, but have muscles that are usually not differentiated in members of some hominoid taxa, e.g. the platysma cervicale (usually not differentiated in orangutans, panins and humans) and auricularis posterior (usually not differentiated in orangutans). Based on our observations, comparisons and review of the literature, we propose a unifying, coherent nomenclature for the facial muscles of the Mammalia as a whole and provide a list of more than 300 synonyms that have been used in the literature to designate the facial muscles of primates and other mammals. A main advantage of this nomenclature is that it combines, and thus creates a bridge between, those names used by human anatomists and the names often employed in the literature dealing with non-human primates and non-primate mammals.

Facial musculature in the rhesus macaque (Macaca mulatta): evolutionary and functional contexts with comparisons to chimpanzees and humans

Facial expression is a common mode of visual communication in mammals but especially so in primates. Rhesus macaques (Macaca mulatta) have a well-documented facial expression repertoire that is controlled by the facial/mimetic musculature as in all mammals. However, little is known about the musculature itself and how it compares with those of other primates. Here we present a detailed description of the facial musculature in rhesus macaques in behavioral, evolutionary and comparative contexts. Formalin-fixed faces from six adult male specimens were dissected using a novel technique. The morphology, attachments, three-dimensional relationships and variability of muscles were noted and compared with chimpanzees (Pan troglodytes) and with humans. The results showed that there was a greater number of facial muscles in rhesus macaques than previously described (24 muscles), including variably present (and previously unmentioned) zygomaticus minor, levator labii superioris alaeque nasi, depressor septi, anterior auricularis, inferior auricularis and depressor supercilii muscles. The facial muscles of the rhesus macaque were very similar to those in chimpanzees and humans but M. mulatta did not possess a risorius muscle. These results support previous studies that describe a highly graded and intricate facial expression repertoire in rhesus macaques. Furthermore, these results indicate that phylogenetic position is not the primary factor governing the structure of primate facial musculature and that other factors such as social behavior are probably more important. The results from the present study may provide valuable input to both biomedical studies that use rhesus macaques as a model for human disease and disorder that includes assessment of facial movement and studies into the evolution of primate societies and communication.

Left hemisphere specialization for oro-facial movements of learned vocal signals by captive chimpanzees

BACKGROUND

The left hemisphere of the human brain is dominant in the production of speech and signed language. Whether similar lateralization of function for communicative signal production is present in other primates remains a topic of considerable debate. In the current study, we examined whether oro-facial movements associated with the production of learned attention-getting sounds are differentially lateralized compared to facial expressions associated with the production of species-typical emotional vocalizations in chimpanzees.

METHODOLOGY/PRINCIPAL FINDINGS

Still images captured from digital video were used to quantify oro-facial asymmetries in the production of two attention-getting sounds and two species-typical vocalizations in a sample of captive chimpanzees. Comparisons of mouth asymmetries during production of these sounds revealed significant rightward biased asymmetries for the attention-getting sounds and significant leftward biased asymmetries for the species-typical sounds.

CONCLUSIONS/SIGNIFICANCE

These results suggest that the motor control of oro-facial movements associated with the production of learned sounds is lateralized to the left hemisphere in chimpanzees. Furthermore, the findings suggest that the antecedents for lateralization of human speech may have been present in the common ancestor of chimpanzees and humans approximately 5 mya and are not unique to the human lineage.

The neuroanatomic basis of facial perception and variable facial discrimination ability: Implications for orthodontics

INTRODUCTION

Because of familial, ethnic-racial, cultural, and emotional preferences, achieving common facial understanding among orthodontist, patient, parents, and other health care professionals is a daunting communication challenge. Research into the neuroanatomic basis of human facial perception, including the roles of visual short-term memory and long-term memory, might apply to orthodontic facial learning.

METHODS

In this article, we review findings from functional magnetic resonance imaging and electrophysiology studies of the brain during visual perception and mental imaging of faces, and integrate these findings with facial learning needs in orthodontics.

RESULTS

Research distinguishes specialized brain areas for whole face and face feature perception, the spatial relationship of face features, and facial memory stores. The right anterior temporal lobe's fusiform face area helps recognize facial identity, whereas the bilateral superior temporal sulcus assists in perception of facial expression. The amygdala, hippocampus, and bilateral inferior occipital gyrus help process familiar, unfamiliar, and famous faces. Because visual perceptual experience and processing are individually variable, along with visual short-term memory and long-term memory capacities, it is likely that facial discrimination ability is variable.

CONCLUSIONS

Neuroanatomic research shows that each person's brain is as unique as his or her face. Due to variable neural hard-wiring, what the clinician sees facially might not be what the patient or parent sees, and vice versa. Enhanced facial learning is related to creation of a distinctive mental context associated with a facial stimulus and rich mixing between memory and visual perception. This context can be formed by information from clinical examinations, patient databases, patient-parent facial preference questionnaires, and functional face viewing. The more extensive the long-term memory facial links, the better the person knows the face. Facial discrimination exercises with electronic and hard-copy tools might improve facial learning and should be based on defined facial learning objectives. Tools should use facial prototypes and facial-feature spatial-relationship information, and emphasize categorization of whole faces and facial components. These are proven methods of expert recognition of objects having prototypical spatial configuration.

Attractiveness of natural faces compared to computer constructed perfectly symmetrical faces

Attractiveness of natural faces was compared to perfectly symmetrical faces constructed on the computer from digitized photographs, in order to assess the role of left-right symmetry in beauty assessment. Three different groups of participants viewed separate series of sequentially presented faces (natural faces, left-left, and right-right) and provided attractiveness ratings on a 5-point Likert scale. The results revealed statistically significant lower ratings for the computer constructed left-left and right-right compared to the natural faces. The discussion is in the context of a biological trend away from perfect symmetry in primates consequent to adaptive evolutionary alteration favoring functional asymmetry in the brain, perception, and face.

Muscles of facial expression in the chimpanzee (Pan troglodytes): descriptive, comparative and phylogenetic contexts

Facial expressions are a critical mode of non-vocal communication for many mammals, particularly non-human primates. Although chimpanzees (Pan troglodytes) have an elaborate repertoire of facial signals, little is known about the facial expression (i.e. mimetic) musculature underlying these movements, especially when compared with some other catarrhines. Here we present a detailed description of the facial muscles of the chimpanzee, framed in comparative and phylogenetic contexts, through the dissection of preserved faces using a novel approach. The arrangement and appearance of muscles were noted and compared with previous studies of chimpanzees and with prosimians, cercopithecoids and humans. The results showed 23 mimetic muscles in P. troglodytes, including a thin sphincter colli muscle, reported previously only in adult prosimians, a bi-layered zygomaticus major muscle and a distinct risorius muscle. The presence of these muscles in such definition supports previous studies that describe an elaborate and highly graded facial communication system in this species that remains qualitatively different from that reported for other non-human primate species. In addition, there are minimal anatomical differences between chimpanzees and humans, contrary to conclusions from previous studies. These results amplify the importance of understanding facial musculature in primate taxa, which may hold great taxonomic value.

Evolution of the brainstem orofacial motor system in primates: a comparative study of trigeminal, facial, and hypoglossal nuclei

The trigeminal motor (Vmo), facial (VII), and hypoglossal (XII) nuclei of the brainstem comprise the final common output for neural control of most orofacial muscles. Hence, these cranial motor nuclei are involved in the production of adaptive behaviors such as feeding, facial expression, and vocalization. We measured the volume and Grey Level Index (GLI) of Vmo, VII, and XII in 47 species of primates and examined these nuclei for scaling patterns and phylogenetic specializations. Allometric regression, using medulla volume as an independent variable, did not reveal a significant difference between strepsirrhines and haplorhines in the scaling of Vmo volume. In addition, correlation analysis using independent contrasts did not find a relationship between Vmo size or GLI and the percent of leaves in the diet. The scaling trajectory of VII volume, in contrast, differed significantly between suborders. Great ape and human VII volumes, furthermore, were significantly larger than predicted by the haplorhine regression. Enlargement of VII in these taxa may reflect increased differentiation of the facial muscles of expression and greater utilization of the visual channel in social communication. The independent contrasts of VII volume and GLI, however, were not correlated with social group size. To examine whether the human hypoglossal motor system is specialized to control the tongue for speech, we tested human XII volume and GLI for departures from nonhuman haplorhine prediction lines. Although human XII volumes were observed above the regression line, they did not exceed prediction intervals. Of note, orang-utan XII volumes had greater residuals than humans. Human XII GLI values also did not differ from allometric prediction. In sum, these findings indicate that the cranial orofacial motor nuclei evince a mosaic of phylogenetic specializations for innervation of the facial muscles of expression in the context of a generally conservative scaling relationship with respect to medulla size.