A comparative study on the cortico-hypoglossal connections in primates, using biotin dextranamine

The Cortical Motor Areas and the Emergence of Motor Skills: A Neuroanatomical Perspective

What changes in neural architecture account for the emergence and expansion of dexterity in primates? Dexterity, or skill in performing motor tasks, depends on the ability to generate highly fractionated patterns of muscle activity. It also involves the spatiotemporal coordination of activity in proximal and distal muscles across multiple joints. Many motor skills require the generation of complex movement sequences that are only acquired and refined through extensive practice. Improvements in dexterity have enabled primates to manufacture and use tools and humans to engage in skilled motor behaviors such as typing, dance, musical performance, and sports. Our analysis leads to the following synthesis: The neural substrate that endows primates with their enhanced motor capabilities is due, in part, to (a) major organizational changes in the primary motor cortex and (b) the proliferation of output pathways from other areas of the cerebral cortex, especially from the motor areas on the medial wall of the hemisphere.

Sequence and hierarchy in vocal rhythms and phonology

I explore the neural and evolutionary origins of phonological hierarchy, building on Peter MacNeilage's frame/content model, which suggests that human speech evolved from primate nonvocal jaw oscillations, for example, lip smack displays, combined with phonation. Considerable recent data, reviewed here, support this proposition. I argue that the evolution of speech motor control required two independent components. The first, identified by MacNeilage, is the diversification of phonetic "content" within a simple sequential "frame," and would be within reach of nonhuman primates, by simply intermittently activating phonation during lip smack displays. Such voicing control requires laryngeal control, hypothesized to necessitate direct corticomotor connections to the nucleus ambiguus. The second component, proposed here, involves imposing additional hierarchical rhythmic structure upon the "flat" control sequences typifying mammalian vocal tract oscillations and is required for the flexible combinatorial capacity observed in modern phonology. I hypothesize that phonological hierarchy resulted from a marriage of a preexisting capacity for sequential structure seen in other primates, with novel hierarchical motor control circuitry (potentially evolved in tool use and/or musical contexts). In turn, this phonological hierarchy paved the way for phrasal syntactic hierarchy. I support these arguments using comparative and neural data from nonhuman primates and birdsong.

A Modular Approach to Vocal Learning: Disentangling the Diversity of a Complex Behavioral Trait

Vocal learning is a behavioral trait in which the social and acoustic environment shapes the vocal repertoire of individuals. Over the past century, the study of vocal learning has progressed at the intersection of ecology, physiology, neuroscience, molecular biology, genomics, and evolution. Yet, despite the complexity of this trait, vocal learning is frequently described as a binary trait, with species being classified as either vocal learners or vocal non-learners. As a result, studies have largely focused on a handful of species for which strong evidence for vocal learning exists. Recent studies, however, suggest a continuum in vocal learning capacity across taxa. Here, we further suggest that vocal learning is a multi-component behavioral phenotype comprised of distinct yet interconnected modules. Discretizing the vocal learning phenotype into its constituent modules would facilitate integration of findings across a wider diversity of species, taking advantage of the ways in which each excels in a particular module, or in a specific combination of features. Such comparative studies can improve understanding of the mechanisms and evolutionary origins of vocal learning. We propose an initial set of vocal learning modules supported by behavioral and neurobiological data and highlight the need for diversifying the field in order to disentangle the complexity of the vocal learning phenotype.

Neural Correlates of Vocal Repertoire in Primates

Understanding the nature of the relationship between vocal complexity and brain architecture across non-human primates may help elucidate some of the key elements underlying the evolution of human speech. Here, we report a positive correlation between vocal repertoire size and the relative size of cortical association areas (governing voluntary control over behavioural output) in non-human primates. We further demonstrate that a hominid grade shift in the relative volume of cortical association areas coincides with a similar grade shift in the hypoglossal nucleus (which is associated with the cranial nerve that innervates the muscles of the tongue). Our results support a qualitative continuity in the neural correlates of vocal repertoire, but a quantitative discontinuity in the extent to which the neural system supporting speech is innervated by cortical association areas in great apes and humans.

Sagittal Plane Kinematics of the Jaw and Hyolingual Apparatus During Swallowing in Macaca mulatta

Studies of mechanisms of feeding behavior are important in a society where aging- and disease-related feeding disorders are increasingly prevalent. It is important to evaluate the clinical relevance of animal models of the disease and the control. Our present study quantifies macaque hyolingual and jaw kinematics around swallowing cycles to determine the extent to which macaque swallowing resembles that of humans. One female and one male adult Macaca mulatta were trained to feed in a primate chair. Videofluoroscopy was used to record kinematics in a sagittal view during natural feeding on solid food, and the kinematics of the hyoid bone, thyroid cartilage, mandibular jaw, and anterior-, middle-, and posterior-tongue. Jaw gape cycles were defined by consecutive maximum gapes, and the kinematics of the swallow cycles were compared with those of the two consecutive non-swallow cycles preceding and succeeding the swallow cycles. Although there are size differences between macaques and humans, and macaques have shorter durations of jaw gape cycles and hyoid and thyroid upward movements, there are several important similarities between our macaque data and human data reported in the literature: (1) The durations of jaw gape cycles during swallow cycles are longer than those of non-swallow cycles as a result of an increased duration of the jaw-opening phase; (2) Hyoid and thyroid upward movement is linked with a posterior tongue movement and is faster during swallow than non-swallow cycles; (3) Tongue elevation propagates from anterior to posterior during swallow and non-swallow cycles. These findings suggest that macaques can be a useful experimental model for human swallowing studies.

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobule (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and audio-visual integration. I propose that the primary role of the ADS in monkeys/apes is the perception and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Perception of contact calls occurs by the ADS detecting a voice, localizing it, and verifying that the corresponding face is out of sight. The auditory cortex then projects to parieto-frontal visuospatial regions (visual dorsal stream) for searching the caller, and via a series of frontal lobe-brainstem connections, a contact call is produced in return. Because the human ADS processes also speech production and repetition, I further describe a course for the development of speech in humans. I propose that, due to duplication of a parietal region and its frontal projections, and strengthening of direct frontal-brainstem connections, the ADS converted auditory input directly to vocal regions in the frontal lobe, which endowed early Hominans with partial vocal control. This enabled offspring to modify their contact calls with intonations for signaling different distress levels to their mother. Vocal control could then enable question-answer conversations, by offspring emitting a low-level distress call for inquiring about the safety of objects, and mothers responding with high- or low-level distress calls. Gradually, the ADS and the direct frontal-brainstem connections became more robust and vocal control became more volitional. Eventually, individuals were capable of inventing new words and offspring were capable of inquiring about objects in their environment and learning their names via mimicry.

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobe (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and integration of calls with faces. I propose that the primary role of the ADS in non-human primates is the detection and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Detection of contact calls occurs by the ADS identifying a voice, localizing it, and verifying that the corresponding face is out of sight. Once a contact call is detected, the primate produces a contact call in return via descending connections from the frontal lobe to a network of limbic and brainstem regions. Because the ADS of present day humans also performs speech production, I further propose an evolutionary course for the transition from contact call exchange to an early form of speech. In accordance with this model, structural changes to the ADS endowed early members of the genus Homo with partial vocal control. This development was beneficial as it enabled offspring to modify their contact calls with intonations for signaling high or low levels of distress to their mother. Eventually, individuals were capable of participating in yes-no question-answer conversations. In these conversations the offspring emitted a low-level distress call for inquiring about the safety of objects (e.g., food), and his/her mother responded with a high- or low-level distress call to signal approval or disapproval of the interaction. Gradually, the ADS and its connections with brainstem motor regions became more robust and vocal control became more volitional. Speech emerged once vocal control was sufficient for inventing novel calls.

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobe (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and integration of calls with faces. I propose that the primary role of the ADS in non-human primates is the detection and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Detection of contact calls occurs by the ADS identifying a voice, localizing it, and verifying that the corresponding face is out of sight. Once a contact call is detected, the primate produces a contact call in return via descending connections from the frontal lobe to a network of limbic and brainstem regions. Because the ADS of present day humans also performs speech production, I further propose an evolutionary course for the transition from contact call exchange to an early form of speech. In accordance with this model, structural changes to the ADS endowed early members of the genus Homo with partial vocal control. This development was beneficial as it enabled offspring to modify their contact calls with intonations for signaling high or low levels of distress to their mother. Eventually, individuals were capable of participating in yes-no question-answer conversations. In these conversations the offspring emitted a low-level distress call for inquiring about the safety of objects (e.g., food), and his/her mother responded with a high- or low-level distress call to signal approval or disapproval of the interaction. Gradually, the ADS and its connections with brainstem motor regions became more robust and vocal control became more volitional. Speech emerged once vocal control was sufficient for inventing novel calls.

Cortical innervation of the hypoglossal nucleus in the non-human primate (Macaca mulatta)

The corticobulbar projection to the hypoglossal nucleus was studied from the frontal, parietal, cingulate, and insular cortices in the rhesus monkey by using high-resolution anterograde tracers and stereology. The hypoglossal nucleus received bilateral input from the face/head region of the primary (M1), ventrolateral pre- (LPMCv), supplementary (M2), rostral cingulate (M3), and caudal cingulate (M4) motor cortices. Additional bilateral corticohypoglossal projections were found from the dorsolateral premotor cortex (LPMCd), ventrolateral proisocortical motor area (ProM), ventrolateral primary somatosensory cortex (S1), rostral insula, and pregenual region of the anterior cingulate gyrus (areas 24/32). Dense terminal projections arose from the ventral region of M1, and moderate projections from LPMCv and rostral part of M2, with considerably fewer hypoglossal projections arising from the other cortical regions. These findings demonstrate that extensive regions of the non-human primate cerebral cortex innervate the hypoglossal nucleus. The widespread and bilateral nature of this corticobulbar connection suggests recovery of tongue movement after cortical injury that compromises a subset of these areas, may occur from spared corticohypoglossal projection areas located on the lateral, as well as medial surfaces of both hemispheres. Since functional imaging studies have shown that homologous cortical areas are activated in humans during tongue movement tasks, these corticobulbar projections may exist in the human brain.

Deviation of the fully protracted tongue: is it a reliable indicator for language cerebral dominance?

Since its introduction in 1960, The Wada test has been considered the gold standard for language lateralization prior to ablative brain surgery. Due to the invasive nature of The Wada test several non-invasive techniques have been alternatively adopted. Recently, it has been suggested that the tongue deviates toward the language dominant cerebral hemisphere on full protraction. This suggestion is based on the important role the tongue plays in articulation and on the close anatomical relationship between the cortical tongue motor area and the motor speech area. It was proposed that this phenomenon could serve as a reliable and simple method for language brain lateralization. However, this hypothesis is still open for verification. In an attempt to correlate tongue deviation and language cerebral dominance we present and discuss in this paper the results of a study conducted on 339 free adult Jordanian volunteers. Tongue deviation and handedness were determined and statistically correlated. Our results showed that 62% of test subjects did not show any tongue deviation on full protrusion. Additionally, 9% of test subjects showed left-sided tongue deviation on full protraction in spite of 90% right handedness with presumed left language dominant cerebral hemisphere. We conclude that, at least in Jordanians, tongue deviation cannot be considered as a reliable indicator for language lateralization.

Brain mechanisms of acoustic communication in humans and nonhuman primates: An evolutionary perspective

Any account of “what is special about the human brain” (Passingham 2008) must specify the neural basis of our unique ability to produce speech and delineate how these remarkable motor capabilities could have emerged in our hominin ancestors. Clinical data suggest that the basal ganglia provide a platform for the integration of primate-general mechanisms of acoustic communication with the faculty of articulate speech in humans. Furthermore, neurobiological and paleoanthropological data point at a two-stage model of the phylogenetic evolution of this crucial prerequisite of spoken language: (i) monosynaptic refinement of the projections of motor cortex to the brainstem nuclei that steer laryngeal muscles, presumably, as part of a “phylogenetic trend” associated with increasing brain size during hominin evolution; (ii) subsequent vocal-laryngeal elaboration of cortico-basal ganglia circuitries, driven by human-specificFOXP2mutations.;>This concept implies vocal continuity of spoken language evolution at the motor level, elucidating the deep entrenchment of articulate speech into a “nonverbal matrix” (Ingold 1994), which is not accounted for by gestural-origin theories. Moreover, it provides a solution to the question for the adaptive value of the “first word” (Bickerton 2009) since even the earliest and most simple verbal utterances must have increased the versatility of vocal displays afforded by the preceding elaboration of monosynaptic corticobulbar tracts, giving rise to enhanced social cooperation and prestige. At the ontogenetic level, the proposed model assumes age-dependent interactions between the basal ganglia and their cortical targets, similar to vocal learning in some songbirds. In this view, the emergence of articulate speech builds on the “renaissance” of an ancient organizational principle and, hence, may represent an example of “evolutionary tinkering” (Jacob 1977).

The babkin reflex in infants: clinical significance and neural mechanism

BACKGROUND

There have been very few studies concerning the Babkin reflex-opening of the mouth and flexion of the arms in response to stimulation of the palms. We attempted to clarify the clinical significance and neural mechanism of the reflex through systematic review.

METHODS

Searches were conducted on Medline, Embase, and Google Scholar from their inception through August 2012.

RESULTS

In normal term infants, the Babkin reflex can be elicited from the time of birth, becomes increasingly suppressed with age, and disappears in the great majority by the end of the fifth month of age. A marked response in the fourth or fifth month of age and persistence of the reflex beyond the fifth month of age are generally regarded as abnormal. On the other hand, because there are some normal infants showing no response during the neonatal period or early infancy, the absence of the response during these periods is not necessarily an abnormal finding.

CONCLUSIONS

Infants with these abnormal findings should be carefully observed for the appearance of neurological abnormalities including cerebral palsy and mental retardation. It is most likely that the Babkin reflex is mediated by the reticular formation of the brainstem, which receives inputs from the nonprimary motor cortices. On the basis of the hand-mouth reflex, more adaptive movement develops as control of the nonprimary motor cortices over the reflex mechanism in the reticular formation increases. Soon it evolves into the voluntary eye-hand-mouth coordination necessary for food intake as the control of the prefrontal cortex over the nonprimary motor cortices becomes predominant.

Mouse vocal communication system: are ultrasounds learned or innate?

Mouse ultrasonic vocalizations (USVs) are often used as behavioral readouts of internal states, to measure effects of social and pharmacological manipulations, and for behavioral phenotyping of mouse models for neuropsychiatric and neurodegenerative disorders. However, little is known about the neurobiological mechanisms of rodent USV production. Here we discuss the available data to assess whether male mouse song behavior and the supporting brain circuits resemble those of known vocal non-learning or vocal learning species. Recent neurobiology studies have demonstrated that the mouse USV brain system includes motor cortex and striatal regions, and that the vocal motor cortex sends a direct sparse projection to the brainstem vocal motor nucleus ambiguous, a projection previously thought be unique to humans among mammals. Recent behavioral studies have reported opposing conclusions on mouse vocal plasticity, including vocal ontogeny changes in USVs over early development that might not be explained by innate maturation processes, evidence for and against a role for auditory feedback in developing and maintaining normal mouse USVs, and evidence for and against limited vocal imitation of song pitch. To reconcile these findings, we suggest that the trait of vocal learning may not be dichotomous but encompass a broad spectrum of behavioral and neural traits we call the continuum hypothesis, and that mice possess some of the traits associated with a capacity for limited vocal learning.

Gestures, vocalizations, and memory in language origins

THIS ARTICLE DISCUSSES THE POSSIBLE HOMOLOGIES BETWEEN THE HUMAN LANGUAGE NETWORKS AND COMPARABLE AUDITORY PROJECTION SYSTEMS IN THE MACAQUE BRAIN, IN AN ATTEMPT TO RECONCILE TWO EXISTING VIEWS ON LANGUAGE EVOLUTION: one that emphasizes hand control and gestures, and the other that emphasizes auditory-vocal mechanisms. The capacity for language is based on relatively well defined neural substrates whose rudiments have been traced in the non-human primate brain. At its core, this circuit constitutes an auditory-vocal sensorimotor circuit with two main components, a "ventral pathway" connecting anterior auditory regions with anterior ventrolateral prefrontal areas, and a "dorsal pathway" connecting auditory areas with parietal areas and with posterior ventrolateral prefrontal areas via the arcuate fasciculus and the superior longitudinal fasciculus. In humans, the dorsal circuit is especially important for phonological processing and phonological working memory, capacities that are critical for language acquisition and for complex syntax processing. In the macaque, the homolog of the dorsal circuit overlaps with an inferior parietal-premotor network for hand and gesture selection that is under voluntary control, while vocalizations are largely fixed and involuntary. The recruitment of the dorsal component for vocalization behavior in the human lineage, together with a direct cortical control of the subcortical vocalizing system, are proposed to represent a fundamental innovation in human evolution, generating an inflection point that permitted the explosion of vocal language and human communication. In this context, vocal communication and gesturing have a common history in primate communication.

Convergent differential regulation of parvalbumin in the brains of vocal learners

Spoken language and learned song are complex communication behaviors found in only a few species, including humans and three groups of distantly related birds – songbirds, parrots, and hummingbirds. Despite their large phylogenetic distances, these vocal learners show convergent behaviors and associated brain pathways for vocal communication. However, it is not clear whether this behavioral and anatomical convergence is associated with molecular convergence. Here we used oligo microarrays to screen for genes differentially regulated in brain nuclei necessary for producing learned vocalizations relative to adjacent brain areas that control other behaviors in avian vocal learners versus vocal non-learners. A top candidate gene in our screen was a calcium-binding protein, parvalbumin (PV). In situ hybridization verification revealed that PV was expressed significantly higher throughout the song motor pathway, including brainstem vocal motor neurons relative to the surrounding brain regions of all distantly related avian vocal learners. This differential expression was specific to PV and vocal learners, as it was not found in avian vocal non-learners nor for control genes in learners and non-learners. Similar to the vocal learning birds, higher PV up-regulation was found in the brainstem tongue motor neurons used for speech production in humans relative to a non-human primate, macaques. These results suggest repeated convergent evolution of differential PV up-regulation in the brains of vocal learners separated by more than 65–300 million years from a common ancestor and that the specialized behaviors of learned song and speech may require extra calcium buffering and signaling.

Cerebellar contributions to speech production and speech perception: psycholinguistic and neurobiological perspectives

Articulate speech represents a unique trait of our species. Besides other structures, the cerebellum pertains to the brain network engaged in spoken language production. Data from different sources point at a dual role of this organ within the verbal domain: (i) the cerebellum appears to subserve the online sequencing of syllables into fast, smooth and rhythmically organized larger utterances, and (ii) furthermore, the cerebellum seems to participate in the temporal organization of internal speech, that is, a prearticulatory verbal code. Impaired prearticulatory verbal coding mechanisms could explain at least some of the perceptual and cognitive deficits observed in cerebellar disorders. Recent genetic studies indicate that distinct mutations of a specific regulatory gene (FOXP2) promoted the emergence of articulate speech during the course of hominid evolution. Conceivably, structural changes of the expressed FOXP2 protein supported the 'vocal elaboration' of phylogenetically older brain networks engaged in upper limb motor control, such as the cerebro-cerebellar loops.

Cortical memory mechanisms and language origins

We have previously proposed that cortical auditory-vocal networks of the monkey brain can be partly homologized with language networks that participate in the phonological loop. In this paper, we suggest that other linguistic phenomena like semantic and syntactic processing also rely on the activation of transient memory networks, which can be compared to active memory networks in the primate. Consequently, short-term cortical memory ensembles that participate in language processing can be phylogenetically tracked to more simple networks present in the primate brain, which became increasingly complex in hominid evolution. This perspective is discussed in the context of two current interpretations of language origins, the "mirror-system hypothesis" and generativist grammar.

Comparative anatomy of the facial motor nucleus in mammals, with an analysis of neuron numbers in primates

The facial motor nucleus (VII) contains motoneurons that innervate the facial muscles of expression. In this review, the comparative anatomy of this brainstem nucleus is examined. Several aspects of the anatomical organization of the VII appear to be common across mammals, such as the distribution of neuron types, general topography of muscle representation, and afferent connections from the midbrain and brainstem. Phylogenetic specializations are apparent in the proportion of neurons allocated to the representation of subsets of muscles and the degree of differentiation among subnuclei. These interspecific differences may be related to the elaboration of certain facial muscles in the context of socioecological adaptations such as whisking behavior, sound localization, vocalization, and facial expression. Furthermore, current evidence indicates that direct descending corticomotoneuron projections in the VII are present only in catarrhine primates, suggesting that this connectivity is an important substrate for the evolution of enhanced mobility and flexibility in facial expression. Data are also presented from a stereologic analysis of VII neuron numbers in 18 primate species and a scandentian. Using phylogenetic comparative statistics, it is shown that there is not a correlation between group size and VII neuron number (adjusted for medulla volume) among primates. Great apes and humans, however, display moderately more VII neurons that expected for their medulla size.

Monosynaptic pathway from rat vibrissa motor cortex to facial motor neurons revealed by lentivirus-based axonal tracing

The mammalian motor cortex typically innervates motor neurons indirectly via oligosynaptic pathways. However, evolution of skilled digit movements in humans, apes, and some monkey species is associated with the emergence of abundant monosynaptic cortical projections onto spinal motor neurons innervating distal limb muscles. Rats perform skilled movements with their whiskers, and we examined the possibility that the rat vibrissa motor cortex (VMC) projects monosynaptically onto facial motor neurons controlling the whisker movements. First, single injections of lentiviruses to VMC sites identified by intracortical microstimulations were used to label a distinct subpopulation of VMC axons or presynaptic terminals by expression of enhanced green fluorescent protein (GFP) or GFP-tagged synaptophysin, respectively. Four weeks after the injections, GFP and synaptophysin-GFP labeling of axons and putative presynaptic terminals was detected in the lateral portion of the facial nucleus (FN), in close proximity to motor neurons identified morphologically and by axonal back-labeling from the whisker follicles. The VMC projections were detected bilaterally, with threefold larger density of labeling in the contralateral FN. Next, multiple VMC injections were used to label a large portion of VMC axons, resulting in overall denser but still laterally restricted FN labeling. Ultrastructural analysis of the GFP-labeled VMC axons confirmed the existence of synaptic contacts onto dendrites and somata of FN motor neurons. These findings provide anatomical demonstration of monosynaptic VMC-to-FN pathway in the rat and show that lentivirus-based expression of GFP and GFP-tagged presynaptic proteins can be used as a high-resolution neuroanatomical tracing method.

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.