Cortical Orofacial Motor Representation in Old World Monkeys, Great Apes, and Humans

Abstract

Social life in anthropoid primates is mediated by interindividual communication, involving movements of the orofacial muscles for the production of vocalization and gestural expression. Although phylogenetic diversity has been reported in the auditory and visual communication systems of primates, little is known about the comparative neuroanatomy that subserves orofacial movement. The current study reports results from quantitative image analysis of the region corresponding to orofacial representation of primary motor cortex (Brodmann’s area 4) in several catarrhine primate species (Macaca fascicularis, Papio anubis, Pongo pygmaeus, Gorilla gorilla, Pan troglodytes, and Homo sapiens) using the Grey Level Index method. This cortical region has been implicated in the execution of skilled motor activities such as voluntary facial expression and human speech. Density profiles of the laminar distribution of Nissl-stained neuronal somata were acquired from high-resolution images to quantify cytoarchitectural patterns. Despite general similarity in these profiles across catarrhines, multivariate analysis showed that cytoarchitectural patterns of individuals were more similar within-species versus between-species. Compared to Old World monkeys, the orofacial representation of area 4 in great apes and humans was characterized by an increased relative thickness of layer III and overall lower cell volume densities, providing more neuropil space for interconnections. These phylogenetic differences in microstructure might provide an anatomical substrate for the evolution of greater volitional fine motor control of facial expressions in great apes and humans.

Ape gestures and language evolution

“…Signals involving orofacial movements (''facial gestures'') may be more open to action-understanding, a likely component of language evolution, than purely auditory signals (61). It is interesting, therefore, that bonobos show a degree of voluntary control over their facial musculature as reflected in games that consist of ''pulling'' unusual faces in situations that appear to lack emotional engagement (62,63).…”

Duchenne smiles and the perception of generosity and sociability in faces

“…Although the Duchenne smile can be difficult to fake (EKMAN and FRIESEN 1982), a data base of 105 posed smiles was once found to contain no less than 67% of Duchenne smiles (KANADE, COHN and TIAN 2000). This could be the result of an ability to control facial behaviour that is typical of great apes and humans (SHERWOOD et al 2004). The present paper shows that such ability could be adaptive, as it positively affected social judgements.…”

“…The degree of control that individuals gained over their face (SHERWOOD et al 2004;SHERWOOD et al 2005) and the effect of smiling on trait perception (LAU 1982;OTTA, ABROSIO and HOSHINO 1996;MEHU 2006) set forward the obvious problem that people could smile purposefully and claim specific traits whenever a relevant situation presents itself. This might have created a selection pressure for individuals who were particularly sensitive to smiling, as they became increasingly vulnerable to exploitation by socially skilled individuals.…”

The von Economo neurons in frontoinsular and anterior cingulate cortex in great apes and humans

“…Both individuals died suddenly of myocardial infarctions. The immunohistochemical method was adapted from Sherwood et al (2004). The primary antibody concentration was optimized using rabbit anti-DISC1 (human) (1:100 Zymed cat.…”