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

Crossing fibers may underlie the dynamic pulling forces of muscles that attach to cartilage at the tip of the nose

The present study used microdissection, histology, and microcomputed tomography (micro-CT) with the aims of determining the prevalence and patterns of the depressor septi nasi (DSN) and orbicularis oris (OOr) muscles attached to the footplate of the medial crus (fMC) of the major alar cartilage, focusing on their crossing fibers. The DSN and OOr attached to the fMC of the major alar cartilage were investigated in 76 samples from 38 embalmed Korean adult cadavers (20 males, 18 females; mean age 70 years). The DSN, OOr, or both were attached to the fMC. When the DSN ran unilaterally or was absent, some OOr fibers ascended to attach to the fMC instead of the DSN in 20.6% of the samples. Crossing fibers of the DSN or OOr attached to the fMC were found in 82.4% of the samples. Bilateral and unilateral crossing fibers were found in 32.4% and 50.0%, respectively, and no crossing fibers were found in 17.6%. The DSN and OOr that attached to the fMC could be categorized into six types according to presence of the DSN and the crossing patterns of the DSN and OOr. Anatomical findings of the DSN and OOr that attached to the fMC were confirmed in histology and micro-CT images. These findings offer insights on anatomical mechanisms that may underlie the dynamic pulling forces generated by muscles that attach to the fMCs and on evolutionary variation observed in human facial expressions. They can also provide useful information for guiding rhinoplasty of the nasal tip.

Advancing methods for the biodemography of aging within social contexts

Several social dimensions including social integration, status, early-life adversity, and their interactions across the life course can predict health, reproduction, and mortality in humans. Accordingly, the social environment plays a fundamental role in the emergence of phenotypes driving the evolution of aging. Recent work placing human social gradients on a biological continuum with other species provides a useful evolutionary context for aging questions, but there is still a need for a unified evolutionary framework linking health and aging within social contexts. Here, we summarize current challenges to understand the role of the social environment in human life courses. Next, we review recent advances in comparative biodemography and propose a biodemographic perspective to address socially driven health phenotype distributions and their evolutionary consequences using a nonhuman primate population. This new comparative approach uses evolutionary demography to address the joint dynamics of populations, social dimensions, phenotypes, and life history parameters. The long-term goal is to advance our understanding of the link between individual social environments, population-level outcomes, and the evolution of aging.

Histomorphological analysis of the superficial musculoaponeurotic system in Macaca mulatta species

INTRODUCTION

The superficial musculoaponeurotic system (SMAS) is a well described facial functional unit in humans. SMAS connects mimic musculature to the skin having many implication in facial mimic expression. One of the various morphological and physiological analogies in human and Macaca mulatta species is the facial mimic. The present study analyzed Macaca mulatta species SMAS morphology and its facial topographical differences and compared this with human SMAS tissue morphology.

MATERIAL AND METHODS

Macaca mulatta full-graft tissue blocks of skin, subcutaneous tissue and mimic muscles from five topographical different facial regions (Regio Temporalis, Regio Buccalis, Regio Infraorbitalis, Regio Angulus Oris and Regio Mandibularis) were collected postmortem from eight individuals (n = 8) at the German Primate Center, Leibniz Institute for Primate Research in Göttingen (DPZ) and studied histologically. Haematoxylin-eosin and azan stained histological serial sections of full-graft tissue blocks were analyzed and SMAS topographical differences evaluated.

RESULTS

SMAS typical tissue morphology was recognized in all Macaca mulatta histological serial sections (n = 780). Regio Infraorbitalis Macaca mulatta SMAS (MmSMAS) morphology was similar to human infraorbital SMAS morphology (type I SMAS). Suborbicularis oculi fat pad was recognized in Macaca mulatta samples. Human type I similar SMAS morphology was demonstrated over Macaca mulatta Regio Temporalis and Regio Buccalis. Regio Angulus Oris and the cranial area of the Regio Mandibularis presented human type II similar SMAS morphology. Type IV MmSMAS was closely related to the parotid gland tissue presence. The cervical area of the Regio Mandibularis presented human type V similar SMAS morphology.

CONCLUSIONS

SMAS is a complex fibro-musculo-adipose tissue network and probably an important pivot in Macaca mulatta facial system supporting mimic expression. This study provided insights into MmSMAS typology and similarity with human SMAS tissue morphology.

"Untying the knot": The primitive orofacial muscle architecture in the gorilla (Gorilla gorilla) as a key to the evolution of hominin facial movement

The anatomical underpinnings of primate facial expressions are essential to exploring their evolution. Traditionally, it has been accepted that the primate face exhibits a "scala natura" morphocline, ranging from primitive to derived characteristics. At the primitive end, the face consists of undifferentiated muscular sheets, while at the derived end there is greater complexity with more muscles and insertion points. Among these, the role of the human modiolus ("knoten" in German) has been emphasized. Recent studies have challenged this view by revealing significant complexity in the faces of several non-human primates, thereby rejecting the linear notion of facial evolution. However, our knowledge of the facial architecture in gorillas, the second closest living relatives to modern humans, remains a significant gap in the literature. Here, we present new findings based on dissection and histological analysis of one gorilla craniofacial specimen, alongside 30 human hemifaces. Our results indicate that while the number and overall arrangement of facial muscles in the gorilla are comparable to those of chimpanzees and modern humans, several orofacial features distinguish the gorilla's anatomy from that of hominins. Among these are the absence of a modiolus, the continuity of muscular fibers over the region of the mouth corner, the flat (uncurving) sheet of the orbicularis oris muscle, and the insertion of direct labial tractors both anterior and posterior to it. Collectively, the anatomical characteristics observed in the gorilla suggest that the complex anatomy of the hominin face should be considered synapomorphic (shared-derived) within the Pan-Homo clade.

CalliFACS: The common marmoset Facial Action Coding System

Facial expressions are subtle cues, central for communication and conveying emotions in mammals. Traditionally, facial expressions have been classified as a whole (e.g. happy, angry, bared-teeth), due to automatic face processing in the human brain, i.e., humans categorise emotions globally, but are not aware of subtle or isolated cues such as an eyebrow raise. Moreover, the same facial configuration (e.g. lip corners pulled backwards exposing teeth) can convey widely different information depending on the species (e.g. humans: happiness; chimpanzees: fear). The Facial Action Coding System (FACS) is considered the gold standard for investigating human facial behaviour and avoids subjective interpretations of meaning by objectively measuring independent movements linked to facial muscles, called Action Units (AUs). Following a similar methodology, we developed the CalliFACS for the common marmoset. First, we determined the facial muscular plan of the common marmoset by examining dissections from the literature. Second, we recorded common marmosets in a variety of contexts (e.g. grooming, feeding, play, human interaction, veterinary procedures), and selected clips from online databases (e.g. YouTube) to identify their facial movements. Individual facial movements were classified according to appearance changes produced by the corresponding underlying musculature. A diverse repertoire of 33 facial movements was identified in the common marmoset (15 Action Units, 15 Action Descriptors and 3 Ear Action Descriptors). Although we observed a reduced range of facial movement when compared to the HumanFACS, the common marmoset's range of facial movements was larger than predicted according to their socio-ecology and facial morphology, which indicates their importance for social interactions. CalliFACS is a scientific tool to measure facial movements, and thus, allows us to better understand the common marmoset's expressions and communication. As common marmosets have become increasingly popular laboratory animal models, from neuroscience to cognition, CalliFACS can be used as an important tool to evaluate their welfare, particularly in captivity.

Anatomical connections among the depressor supercilii, levator labii superioris alaeque nasi, and inferior fibers of orbicularis oculi: Implications for variation in human facial expressions

The aim of this study was to determine how the depressor supercilii (DS) connects to the levator labii superioris alaeque nasi (LLSAN) and inferior fibers of the orbicularis oculi (OOc INF) in the human midface. While grimacing, contraction of the DS with fibers connecting to the LLSAN and OOc INF can assist in pulling the medial eyebrow downward more than when these connecting fibers are not present. Contraction of these distinct connecting fibers between the DS and the LLSAN can also slightly elevate the nasal ala and upper lip. The DS was examined in 44 specimens of embalmed adult Korean cadavers. We found that the DS connected to the LLSAN or the OOc INF by muscle fibers or thin aponeuroses in 33 (75.0%) of the 44 specimens. The DS was connected to both the LLSAN and OOc INF by muscle fibers or aponeuroses and had no connection to either in 5 (11.4%) and 11 (25.0%) specimens, respectively. The DS was connected to the LLSAN by the muscle fibers and thin aponeuroses in 6 (13.6%) and 4 (9.1%) specimens, respectively. The DS was connected to the OOc INF by the muscle fibers and thin aponeuroses in 5 (11.4%) and 23 (52.3%) specimens, respectively. Our findings regarding the anatomical connections of the glabellar region DS to the midface LLSAN and OOc INF provide insights on the dynamic balance between the brow depressors such as the DS and brow-elevating muscle and contribute to understanding the anatomical origins of individual variation in facial expressions. These results can also improve the safety, predictability, and aesthetics of treatments for the glabellar region with botulinum toxin type A and can be helpful when performing electromyography.

Muscle spindles in the rhesus monkey platysma

The platysma of the rhesus monkey consists of two parts: a platysma myoides located similar to the human platysma, and a platysma cervicale passing the dorsal cervical region and being in contact with the cheek pouch. Our investigation showed that the muscle fiber morphology was comparable in both parts. Muscle spindles were only present in regions connected to the cheek pouch and contained only nuclear chain fibers. It is tempting to speculate that they sense the filling of the cheek pouch rather than mimic activities.

Automatic Recognition of Macaque Facial Expressions for Detection of Affective States

Internal affective states produce external manifestations such as facial expressions. In humans, the Facial Action Coding System (FACS) is widely used to objectively quantify the elemental facial action units (AUs) that build complex facial expressions. A similar system has been developed for macaque monkeys-the Macaque FACS (MaqFACS); yet, unlike the human counterpart, which is already partially replaced by automatic algorithms, this system still requires labor-intensive coding. Here, we developed and implemented the first prototype for automatic MaqFACS coding. We applied the approach to the analysis of behavioral and neural data recorded from freely interacting macaque monkeys. The method achieved high performance in the recognition of six dominant AUs, generalizing between conspecific individuals (Macaca mulatta) and even between species (Macaca fascicularis). The study lays the foundation for fully automated detection of facial expressions in animals, which is crucial for investigating the neural substrates of social and affective states.

Anatomy of the Rhesus Monkey (Macaca mulatta): The Essentials for the Biomedical Researcher

Amongst the non-human primates, the rhesus monkey (Macaca mulatta) is the most commonly investigated species in biomedical research. Its similarity to humans regarding the anatomy and physiology has resulted in an increasing number of studies in which the rhesus monkey serves as a model. This book chapter aims to fulfill the researcher’s need for easily accessible anatomical data on the rhesus monkey by presenting the essentials of its various anatomical systems. The cadavers of several rhesus monkeys of either gender were dissected for gross anatomical study of the muscular, digestive, respiratory and urogenital systems. The circulatory system was studied after injection of latex into the blood vessels. Not only did this technique allow for better visualization of the blood vessels, but it was also valuable during the description of the peripheral nerves. In addition, methyl methacrylate casts were prepared to gain insight into the organization of the arterial system. The arthrology of the rhesus monkey was studied during the maceration of several cadavers, which ultimately revealed the individual bones that were described. From one such cadaver the skeleton was mounted. The results of the dissections are textually described and illustrated by means of numerous figures.

Extending the MaqFACS to measure facial movement in Japanese macaques (Macaca fuscata) reveals a wide repertoire potential

Facial expressions are complex and subtle signals, central for communication and emotion in social mammals. Traditionally, facial expressions have been classified as a whole, disregarding small but relevant differences in displays. Even with the same morphological configuration different information can be conveyed depending on the species. Due to a hardwired processing of faces in the human brain, humans are quick to attribute emotion, but have difficulty in registering facial movement units. The well-known human FACS (Facial Action Coding System) is the gold standard for objectively measuring facial expressions, and can be adapted through anatomical investigation and functional homologies for cross-species systematic comparisons. Here we aimed at developing a FACS for Japanese macaques, following established FACS methodology: first, we considered the species' muscular facial plan; second, we ascertained functional homologies with other primate species; and finally, we categorised each independent facial movement into Action Units (AUs). Due to similarities in the rhesus and Japanese macaques' facial musculature, the MaqFACS (previously developed for rhesus macaques) was used as a basis to extend the FACS tool to Japanese macaques, while highlighting the morphological and appearance changes differences between the two species. We documented 19 AUs, 15 Action Descriptors (ADs) and 3 Ear Action Units (EAUs) in Japanese macaques, with all movements of MaqFACS found in Japanese macaques. New movements were also observed, indicating a slightly larger repertoire than in rhesus or Barbary macaques. Our work reported here of the MaqFACS extension for Japanese macaques, when used together with the MaqFACS, comprises a valuable objective tool for the systematic and standardised analysis of facial expressions in Japanese macaques. The MaqFACS extension for Japanese macaques will now allow the investigation of the evolution of communication and emotion in primates, as well as contribute to improving the welfare of individuals, particularly in captivity and laboratory settings.

Visualization and quantification of mimetic musculature via DiceCT

The muscles of facial expression are of significant interest to studies of communicative behaviors. However, due to their small size and high integration with other facial tissues, the current literature is largely restricted to descriptions of the presence or absence of specific muscles. Using diffusible iodine-based contrast-enhanced computed tomography (DiceCT) to stain and digitally image the mimetic mask of Eulemur flavifrons (the blue-eyed black lemur), we demonstrate-for the first time-the ability to visualize these muscles in three-dimensional space and to measure their relative volumes. Comparing these data to earlier accounts of mimetic organization with the face of lemuroidea, we demonstrate several novel configurations within this taxon, particularly in the superior auriculolabialis and the posterior auricularis. We conclude that DiceCT facilitates the study these muscles in closer detail than has been previously possible, and offers significant potential for future studies of this anatomy.

A parameterized digital 3D model of the Rhesus macaque face for investigating the visual processing of social cues

BACKGROUND

Rhesus macaques are the most popular model species for studying the neural basis of visual face processing and social interaction using intracranial methods. However, the challenge of creating realistic, dynamic, and parametric macaque face stimuli has limited the experimental control and ethological validity of existing approaches.

NEW METHOD

We performed statistical analyses of in vivo computed tomography data to generate an anatomically accurate, three-dimensional representation of Rhesus macaque cranio-facial morphology. The surface structures were further edited, rigged and textured by a professional digital artist with careful reference to photographs of macaque facial expression, colouration and pelage.

RESULTS

The model offers precise, continuous, parametric control of craniofacial shape, emotional expression, head orientation, eye gaze direction, and many other parameters that can be adjusted to render either static or dynamic high-resolution faces. Example single-unit responses to such stimuli in macaque inferotemporal cortex demonstrate the value of parametric control over facial appearance and behaviours.

COMPARISON WITH EXISTING METHOD(S)

The generation of such a high-dimensionality and systematically controlled stimulus set of conspecific faces, with accurate craniofacial modelling and professional finalization of facial details, is currently not achievable using existing methods.

CONCLUSIONS

The results herald a new set of possibilities in adaptive sampling of a high-dimensional and socially meaningful feature space, thus opening the door to systematic testing of hypotheses about the abundant neural specialization for faces found in the primate.

The first smile: spontaneous smiles in newborn Japanese macaques (Macaca fuscata)

Spontaneous smiles are facial movements that are characterized by lip corner raises that occur during irregular sleep or drowsiness without known external or internal causes. They are shown by human infants and infant chimpanzees. These smiles are considered to be the developmental origin of smiling and laughter. There are some case studies showing that spontaneous smiles occur in Japanese macaques. The goals of this study were to investigate whether newborn Japanese macaques show a considerable number of spontaneous smiles thus to examine the mechanism of them. Seven newborn Japanese macaques were observed in a room for an average of 44 min, and incidental sleeping situations were monitored twice. All seven participants showed spontaneous smiles at least once during the observation. They showed 8.29 spontaneous smiles in average (SD = 10.89; 58 smiles in total), all found in the state of REM sleep. Thirty-nine of the 58 smiles were produced on the left side of the mouth. These characteristics were similar to those of spontaneous smiles in human infants. This is the first evidence that macaques as well as hominoids show a considerable number of spontaneous smiles. These phenomena may facilitate the development of the zygomaticus major muscle, which is implicated in smiling-like facial expressions.

Social variables exert selective pressures in the evolution and form of primate mimetic musculature

Mammals use their faces in social interactions more so than any other vertebrates. Primates are an extreme among most mammals in their complex, direct, lifelong social interactions and their frequent use of facial displays is a means of proximate visual communication with conspecifics. The available repertoire of facial displays is primarily controlled by mimetic musculature, the muscles that move the face. The form of these muscles is, in turn, limited by and influenced by phylogenetic inertia but here we use examples, both morphological and physiological, to illustrate the influence that social variables may exert on the evolution and form of mimetic musculature among primates. Ecomorphology is concerned with the adaptive responses of morphology to various ecological variables such as diet, foliage density, predation pressures, and time of day activity. We present evidence that social variables also exert selective pressures on morphology, specifically using mimetic muscles among primates as an example. Social variables include group size, dominance 'style', and mating systems. We present two case studies to illustrate the potential influence of social behavior on adaptive morphology of mimetic musculature in primates: (1) gross morphology of the mimetic muscles around the external ear in closely related species of macaque (Macaca mulatta and Macaca nigra) characterized by varying dominance styles and (2) comparative physiology of the orbicularis oris muscle among select ape species. This muscle is used in both facial displays/expressions and in vocalizations/human speech. We present qualitative observations of myosin fiber-type distribution in this muscle of siamang (Symphalangus syndactylus), chimpanzee (Pan troglodytes), and human to demonstrate the potential influence of visual and auditory communication on muscle physiology. In sum, ecomorphologists should be aware of social selective pressures as well as ecological ones, and that observed morphology might reflect a compromise between the demands of the physical and the social environments.

Error-related electromyographic activity over the corrugator supercilii is associated with neural performance monitoring

Emerging research in social and affective neuroscience has implicated a role for affect and motivation in performance monitoring and cognitive control. No study, however, has investigated whether facial electromyography (EMG) over the corrugator supercilii-a measure associated with negative affect and the exertion of effort-is related to neural performance monitoring. Here, we explored these potential relationships by simultaneously measuring the error-related negativity, error positivity (Pe), and facial EMG over the corrugator supercilii muscle during a punished, inhibitory control task. We found evidence for increased facial EMG activity over the corrugator immediately following error responses, and this activity was related to the Pe for both between- and within-subject analyses. These results are consistent with the idea that early, avoidance-motivated processes are associated with performance monitoring, and that such processes may also be related to orienting toward errors, the emergence of error awareness, or both.

MaqFACS (Macaque Facial Action Coding System) can be used to document facial movements in Barbary macaques (Macaca sylvanus)

Human and non-human primates exhibit facial movements or displays to communicate with one another. The evolution of form and function of those displays could be better understood through multispecies comparisons. Anatomically based coding systems (Facial Action Coding Systems: FACS) are developed to enable such comparisons because they are standardized and systematic and aid identification of homologous expressions underpinned by similar muscle contractions. To date, FACS has been developed for humans, and subsequently modified for chimpanzees, rhesus macaques, orangutans, hylobatids, dogs, and cats. Here, we wanted to test whether the MaqFACS system developed in rhesus macaques (Macaca mulatta) could be used to code facial movements in Barbary macaques (M. sylvanus), a species phylogenetically close to the rhesus macaques. The findings show that the facial movement capacity of Barbary macaques can be reliably coded using the MaqFACS. We found differences in use and form of some movements, most likely due to specializations in the communicative repertoire of each species, rather than morphological differences.

EquiFACS: The Equine Facial Action Coding System

Although previous studies of horses have investigated their facial expressions in specific contexts, e.g. pain, until now there has been no methodology available that documents all the possible facial movements of the horse and provides a way to record all potential facial configurations. This is essential for an objective description of horse facial expressions across a range of contexts that reflect different emotional states. Facial Action Coding Systems (FACS) provide a systematic methodology of identifying and coding facial expressions on the basis of underlying facial musculature and muscle movement. FACS are anatomically based and document all possible facial movements rather than a configuration of movements associated with a particular situation. Consequently, FACS can be applied as a tool for a wide range of research questions. We developed FACS for the domestic horse (Equus caballus) through anatomical investigation of the underlying musculature and subsequent analysis of naturally occurring behaviour captured on high quality video. Discrete facial movements were identified and described in terms of the underlying muscle contractions, in correspondence with previous FACS systems. The reliability of others to be able to learn this system (EquiFACS) and consistently code behavioural sequences was high—and this included people with no previous experience of horses. A wide range of facial movements were identified, including many that are also seen in primates and other domestic animals (dogs and cats). EquiFACS provides a method that can now be used to document the facial movements associated with different social contexts and thus to address questions relevant to understanding social cognition and comparative psychology, as well as informing current veterinary and animal welfare practices.

Human faces are slower than chimpanzee faces

BACKGROUND

While humans (like other primates) communicate with facial expressions, the evolution of speech added a new function to the facial muscles (facial expression muscles). The evolution of speech required the development of a coordinated action between visual (movement of the lips) and auditory signals in a rhythmic fashion to produce "visemes" (visual movements of the lips that correspond to specific sounds). Visemes depend upon facial muscles to regulate shape of the lips, which themselves act as speech articulators. This movement necessitates a more controlled, sustained muscle contraction than that produced during spontaneous facial expressions which occur rapidly and last only a short period of time. Recently, it was found that human tongue musculature contains a higher proportion of slow-twitch myosin fibers than in rhesus macaques, which is related to the slower, more controlled movements of the human tongue in the production of speech. Are there similar unique, evolutionary physiologic biases found in human facial musculature related to the evolution of speech?

METHODOLOGY/PRINICIPAL FINDINGS

Using myosin immunohistochemistry, we tested the hypothesis that human facial musculature has a higher percentage of slow-twitch myosin fibers relative to chimpanzees (Pan troglodytes) and rhesus macaques (Macaca mulatta). We sampled the orbicularis oris and zygomaticus major muscles from three cadavers of each species and compared proportions of fiber-types. Results confirmed our hypothesis: humans had the highest proportion of slow-twitch myosin fibers while chimpanzees had the highest proportion of fast-twitch fibers.

CONCLUSIONS/SIGNIFICANCE

These findings demonstrate that the human face is slower than that of rhesus macaques and our closest living relative, the chimpanzee. They also support the assertion that human facial musculature and speech co-evolved. Further, these results suggest a unique set of evolutionary selective pressures on human facial musculature to slow down while the function of this muscle group diverged from that of other primates.

The evolution of speech: vision, rhythm, cooperation

A full account of human speech evolution must consider its multisensory, rhythmic, and cooperative characteristics. Humans, apes, and monkeys recognize the correspondence between vocalizations and their associated facial postures, and gain behavioral benefits from them. Some monkey vocalizations even have a speech-like acoustic rhythmicity but lack the concomitant rhythmic facial motion that speech exhibits. We review data showing that rhythmic facial expressions such as lip-smacking may have been linked to vocal output to produce an ancestral form of rhythmic audiovisual speech. Finally, we argue that human vocal cooperation (turn-taking) may have arisen through a combination of volubility and prosociality, and provide comparative evidence from one species to support this hypothesis.

Of mice, monkeys, and men: physiological and morphological evidence for evolutionary divergence of function in mimetic musculature

Facial expression is a universal means of visual communication in humans and many other primates. Humans have the most complex facial display repertoire among primates; however, gross morphological studies have not found greater complexity in human mimetic musculature. This study examines the microanatomical aspects of mimetic musculature to test the hypotheses related to human mimetic musculature physiology, function, and evolutionary morphology. Samples from the orbicularis oris muscle (OOM) and the zygomaticus major (ZM) muscle in laboratory mice (N = 3), rhesus macaques (N = 3), and humans (N = 3) were collected. Fiber type proportions (slow-twitch and fast-twitch), fiber cross-sectional area, diameter, and length were calculated, and means were statistically compared among groups. Results showed that macaques had the greatest percentage of fast fibers in both muscles (followed by humans) and that humans had the greatest percentage of slow fibers in both muscles. Macaques and humans typically did not differ from one another in morphometrics except for fiber length where humans had longer fibers. Although sample sizes are low, results from this study may indicate that the rhesus macaque OOM and ZM muscle are specialized primarily to assist with maintenance of the rigid dominance hierarchy via rapid facial displays of submission and aggression, whereas human musculature may have evolved not only under pressure to work in facial expressions but also in development of speech.

Facial expressions and the evolution of the speech rhythm

In primates, different vocalizations are produced, at least in part, by making different facial expressions. Not surprisingly, humans, apes, and monkeys all recognize the correspondence between vocalizations and the facial postures associated with them. However, one major dissimilarity between monkey vocalizations and human speech is that, in the latter, the acoustic output and associated movements of the mouth are both rhythmic (in the 3- to 8-Hz range) and tightly correlated, whereas monkey vocalizations have a similar acoustic rhythmicity but lack the concommitant rhythmic facial motion. This raises the question of how we evolved from a presumptive ancestral acoustic-only vocal rhythm to the one that is audiovisual with improved perceptual sensitivity. According to one hypothesis, this bisensory speech rhythm evolved through the rhythmic facial expressions of ancestral primates. If this hypothesis has any validity, we expect that the extant nonhuman primates produce at least some facial expressions with a speech-like rhythm in the 3- to 8-Hz frequency range. Lip smacking, an affiliative signal observed in many genera of primates, satisfies this criterion. We review a series of studies using developmental, x-ray cineradiographic, EMG, and perceptual approaches with macaque monkeys producing lip smacks to further investigate this hypothesis. We then explore its putative neural basis and remark on important differences between lip smacking and speech production. Overall, the data support the hypothesis that lip smacking may have been an ancestral expression that was linked to vocal output to produce the original rhythmic audiovisual speech-like utterances in the human lineage.

A comparison of facial expression properties in five hylobatid species

Little is known about facial communication of lesser apes (family Hylobatidae) and how their facial expressions (and use of) relate to social organization. We investigated facial expressions (defined as combinations of facial movements) in social interactions of mated pairs in five different hylobatid species belonging to three different genera using a recently developed objective coding system, the Facial Action Coding System for hylobatid species (GibbonFACS). We described three important properties of their facial expressions and compared them between genera. First, we compared the rate of facial expressions, which was defined as the number of facial expressions per units of time. Second, we compared their repertoire size, defined as the number of different types of facial expressions used, independent of their frequency. Third, we compared the diversity of expression, defined as the repertoire weighted by the rate of use for each type of facial expression. We observed a higher rate and diversity of facial expression, but no larger repertoire, in Symphalangus (siamangs) compared to Hylobates and Nomascus species. In line with previous research, these results suggest siamangs differ from other hylobatids in certain aspects of their social behavior. To investigate whether differences in facial expressions are linked to hylobatid socio-ecology, we used a Phylogenetic General Least Square (PGLS) regression analysis to correlate those properties with two social factors: group-size and level of monogamy. No relationship between the properties of facial expressions and these socio-ecological factors was found. One explanation could be that facial expressions in hylobatid species are subject to phylogenetic inertia and do not differ sufficiently between species to reveal correlations with factors such as group size and monogamy level.

Development of the platysma muscle and the superficial musculoaponeurotic system (human specimens at 8-17 weeks of development)

There is controversy regarding the description of the different regions of the face of the superficial musculoaponeurotic system (SMAS) and its relationship with the superficial mimetic muscles. The purpose of this study is to analyze the development of the platysma muscle and the SMAS in human specimens at 8–17 weeks of development using an optical microscope. Furthermore, we propose to study the relationship of the anlage of the SMAS and the neighbouring superficial mimetic muscles. The facial musculature derives from the mesenchyme of the second arch and migrates towards the different regions of the face while forming premuscular laminae. During the 8th week of development, the cervical, infraorbital, mandibular, and temporal laminae are observed to be on the same plane. The platysma muscle derives from the cervical lamina and its mandibular extension enclosing the lower part of the parotid region and the cheek, while the SMAS derives from the upper region. During the period of development analyzed in this study, we have observed no continuity between the anlage of the SMAS and that of the superficial layer of the temporal fascia and the zygomaticus major muscle. Nor have we observed any structure similar to the SMAS in the labial region.

In Your Face: Risk of Punishment Enhances Cognitive Control and Error-Related Activity in the Corrugator Supercilii Muscle

Cognitive control is needed when mistakes have consequences, especially when such consequences are potentially harmful. However, little is known about how the aversive consequences of deficient control affect behavior. To address this issue, participants performed a two-choice response time task where error commissions were expected to be punished by electric shocks during certain blocks. By manipulating (1) the perceived punishment risk (no, low, high) associated with error commissions, and (2) response conflict (low, high), we showed that motivation to avoid punishment enhanced performance during high response conflict. As a novel index of the processes enabling successful cognitive control under threat, we explored electromyographic activity in the corrugator supercilii (cEMG) muscle of the upper face. The corrugator supercilii is partially controlled by the anterior midcingulate cortex (aMCC) which is sensitive to negative affect, pain and cognitive control. As hypothesized, the cEMG exhibited several key similarities with the core temporal and functional characteristics of the Error-Related Negativity (ERN) ERP component, the hallmark index of cognitive control elicited by performance errors, and which has been linked to the aMCC. The cEMG was amplified within 100 ms of error commissions (the same time-window as the ERN), particularly during the high punishment risk condition where errors would be most aversive. Furthermore, similar to the ERN, the magnitude of error cEMG predicted post-error response time slowing. Our results suggest that cEMG activity can serve as an index of avoidance motivated control, which is instrumental to adaptive cognitive control when consequences are potentially harmful.

Dissimilar processing of emotional facial expressions in human and monkey temporal cortex

Emotional facial expressions play an important role in social communication across primates. Despite major progress made in our understanding of categorical information processing such as for objects and faces, little is known, however, about how the primate brain evolved to process emotional cues. In this study, we used functional magnetic resonance imaging (fMRI) to compare the processing of emotional facial expressions between monkeys and humans. We used a 2×2×2 factorial design with species (human and monkey), expression (fear and chewing) and configuration (intact versus scrambled) as factors. At the whole brain level, neural responses to conspecific emotional expressions were anatomically confined to the superior temporal sulcus (STS) in humans. Within the human STS, we found functional subdivisions with a face-selective right posterior STS area that also responded to emotional expressions of other species and a more anterior area in the right middle STS that responded specifically to human emotions. Hence, we argue that the latter region does not show a mere emotion-dependent modulation of activity but is primarily driven by human emotional facial expressions. Conversely, in monkeys, emotional responses appeared in earlier visual cortex and outside face-selective regions in inferior temporal cortex that responded also to multiple visual categories. Within monkey IT, we also found areas that were more responsive to conspecific than to non-conspecific emotional expressions but these responses were not as specific as in human middle STS. Overall, our results indicate that human STS may have developed unique properties to deal with social cues such as emotional expressions.

Face to face with the social brain

Recent comparative evidence suggests that anthropoid primates are the only vertebrates to exhibit a quantitative relationship between relative brain size and social group size. In this paper, I attempt to explain this pattern with regard to facial expressivity and social bonding. I hypothesize that facial motor control increases as a secondary consequence of neocortical expansion owing to cortical innervation of the facial motor nucleus. This is supported by new analyses demonstrating correlated evolution between relative neocortex size and relative facial nucleus size. I also hypothesize that increased facial motor control correlates with enhanced emotional expressivity, which provides the opportunity for individuals to better gauge the trustworthiness of group members. This is supported by previous evidence from human psychology, as well as new analyses demonstrating a positive relationship between allogrooming and facial nucleus volume. I suggest new approaches to the study of primate facial expressivity in light of these hypotheses.

Facial muscle coordination in monkeys during rhythmic facial expressions and ingestive movements

Evolutionary hypotheses regarding the origins of communication signals generally suggest, particularly for the case of primate orofacial signals, that they derive by ritualization of noncommunicative behaviors, notably including ingestive behaviors such as chewing and nursing. These theories are appealing in part because of the prominent periodicities in both types of behavior. Despite their intuitive appeal, however, there are little or no data with which to evaluate these theories because the coordination of muscles innervated by the facial nucleus has not been carefully compared between communicative and ingestive movements. Such data are especially crucial for reconciling neurophysiological assumptions regarding facial motor control in communication and ingestion. We here address this gap by contrasting the coordination of facial muscles during different types of rhythmic orofacial behavior in macaque monkeys, finding that the perioral muscles innervated by the facial nucleus are rhythmically coordinated during lipsmacks and that this coordination appears distinct from that observed during ingestion.

The many facets of facial interactions in mammals

Facial interactions are prominent behaviors in primates. Primate facial signaling, which includes the expression of emotions, mimicking of facial movements, and gaze interactions, is visually dominated. Correspondingly, in primate brains an elaborate network of face processing areas exists within visual cortex. But other mammals also communicate through facial interactions using additional sensory modalities. In rodents, multisensory facial interactions are involved in aggressive behaviors and social transmission of food preferences. The eusocial naked mole-rat, whose face is dominated by prominent incisors, uses facial aggression to enforce reproductive suppression. In burrow-living mammals like the naked mole-rat in particular, and in rodents in general, somatosensory face representations in cortex are enlarged. Diversity of sensory domains mediating facial communication might belie underlying common mechanisms. As a case in point, neurogenetics has revealed strongly heritable traits in face processing and identified gene defects that disrupt facial interactions both in humans and rodents.

Monkeys and humans share a common computation for face/voice integration

Speech production involves the movement of the mouth and other regions of the face resulting in visual motion cues. These visual cues enhance intelligibility and detection of auditory speech. As such, face-to-face speech is fundamentally a multisensory phenomenon. If speech is fundamentally multisensory, it should be reflected in the evolution of vocal communication: similar behavioral effects should be observed in other primates. Old World monkeys share with humans vocal production biomechanics and communicate face-to-face with vocalizations. It is unknown, however, if they, too, combine faces and voices to enhance their perception of vocalizations. We show that they do: monkeys combine faces and voices in noisy environments to enhance their detection of vocalizations. Their behavior parallels that of humans performing an identical task. We explored what common computational mechanism(s) could explain the pattern of results we observed across species. Standard explanations or models such as the principle of inverse effectiveness and a "race" model failed to account for their behavior patterns. Conversely, a "superposition model", positing the linear summation of activity patterns in response to visual and auditory components of vocalizations, served as a straightforward but powerful explanatory mechanism for the observed behaviors in both species. As such, it represents a putative homologous mechanism for integrating faces and voices across primates.

Soft-tissue anatomy of the primates: phylogenetic analyses based on the muscles of the head, neck, pectoral region and upper limb, with notes on the evolution of these muscles

Apart from molecular data, nearly all the evidence used to study primate relationships comes from hard tissues. Here, we provide details of the first parsimony and Bayesian cladistic analyses of the order Primates based exclusively on muscle data. The most parsimonious tree obtained from the cladistic analysis of 166 characters taken from the head, neck, pectoral and upper limb musculature is fully congruent with the most recent evolutionary molecular tree of Primates. That is, this tree recovers not only the relationships among the major groups of primates, i.e. Strepsirrhini {Tarsiiformes [Platyrrhini (Cercopithecidae, Hominoidea)]}, but it also recovers the relationships within each of these inclusive groups. Of the 301 character state changes occurring in this tree, ca. 30% are non-homoplasic evolutionary transitions; within the 220 changes that are unambiguously optimized in the tree, ca. 15% are reversions. The trees obtained by using characters derived from the muscles of the head and neck are more similar to the most recent evolutionary molecular tree than are the trees obtained by using characters derived from the pectoral and upper limb muscles. It was recently argued that since the Pan/Homo split, chimpanzees accumulated more phenotypic adaptations than humans, but our results indicate that modern humans accumulated more muscle character state changes than chimpanzees, and that both these taxa accumulated more changes than gorillas. This overview of the evolution of the primate head, neck, pectoral and upper limb musculature suggests that the only muscle groups for which modern humans have more muscles than most other extant primates are the muscles of the face, larynx and forearm.

Brief communication: MaqFACS: A muscle-based facial movement coding system for the rhesus macaque

Over 125 years ago, Charles Darwin (1872) suggested that the only way to fully understand the form and function of human facial expression was to make comparisons with other species. Nevertheless, it has been only recently that facial expressions in humans and related primate species have been compared using systematic, anatomically based techniques. Through this approach, large-scale evolutionary and phylogenetic analyses of facial expressions, including their homology, can now be addressed. Here, the development of a muscular-based system for measuring facial movement in rhesus macaques (Macaca mulatta) is described based on the well-known FACS (Facial Action Coding System) and ChimpFACS. These systems describe facial movement according to the action of the underlying facial musculature, which is highly conserved across primates. The coding systems are standardized; thus, their use is comparable across laboratories and study populations. In the development of MaqFACS, several species differences in the facial movement repertoire of rhesus macaques were observed in comparison with chimpanzees and humans, particularly with regard to brow movements, puckering of the lips, and ear movements. These differences do not seem to be the result of constraints imposed by morphological differences in the facial structure of these three species. It is more likely that they reflect unique specializations in the communicative repertoire of each species.

Dynamic, rhythmic facial expressions and the superior temporal sulcus of macaque monkeys: implications for the evolution of audiovisual speech

Audiovisual speech has a stereotypical rhythm that is between 2 and 7 Hz, and deviations from this frequency range in either modality reduce intelligibility. Understanding how audiovisual speech evolved requires investigating the origins of this rhythmic structure. One hypothesis is that the rhythm of speech evolved through the modification of some pre-existing cyclical jaw movements in a primate ancestor. We tested this hypothesis by investigating the temporal structure of lipsmacks and teeth-grinds of macaque monkeys and the neural responses to these facial gestures in the superior temporal sulcus (STS), a region implicated in the processing of audiovisual communication signals in both humans and monkeys. We found that both lipsmacks and teeth-grinds have consistent but distinct peak frequencies and that both fall well within the 2-7 Hz range of mouth movements associated with audiovisual speech. Single neurons and local field potentials of the STS of monkeys readily responded to such facial rhythms, but also responded just as robustly to yawns, a nonrhythmic but dynamic facial expression. All expressions elicited enhanced power in the delta (0-3Hz), theta (3-8Hz), alpha (8-14Hz) and gamma (> 60 Hz) frequency ranges, and suppressed power in the beta (20-40Hz) range. Thus, STS is sensitive to, but not selective for, rhythmic facial gestures. Taken together, these data provide support for the idea that that audiovisual speech evolved (at least in part) from the rhythmic facial gestures of an ancestral primate and that the STS was sensitive to and thus 'prepared' for the advent of rhythmic audiovisual communication.

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.