Gyrification in the cerebral cortex of primates

Mapping primary gyrogenesis during fetal development in primate brains: high-resolution in utero structural MRI of fetal brain development in pregnant baboons

The global and regional changes in the fetal cerebral cortex in primates were mapped during primary gyrification (PG; weeks 17-25 of 26 weeks total gestation). Studying pregnant baboons using high-resolution MRI in utero, measurements included cerebral volume, cortical surface area, gyrification index and length and depth of 10 primary cortical sulci. Seven normally developing fetuses were imaged in two animals longitudinally and sequentially. We compared these results to those on PG that from the ferret studies and analyzed them in the context of our recent studies of phylogenetics of cerebral gyrification. We observed that in both primates and non-primates, the cerebrum undergoes a very rapid transformation into the gyrencephalic state, subsequently accompanied by an accelerated growth in brain volume and cortical surface area. However, PG trends in baboons exhibited some critical differences from those observed in ferrets. For example, in baboons, the growth along the long (length) axis of cortical sulci was unrelated to the growth along the short (depth) axis and far outpaced it. Additionally, the correlation between the rate of growth along the short sulcal axis and heritability of sulcal depth was negative and approached significance (r = -0.60; p < 0.10), while the same trend for long axis was positive and not significant (p = 0.3; p = 0.40). These findings, in an animal that shares a highly orchestrated pattern of PG with humans, suggest that ontogenic processes that influence changes in sulcal length and depth are diverse and possibly driven by different factors in primates than in non-primates.

Development of structural MR brain imaging protocols to study genetics and maturation

Structural imaging research offers excellent translational benefits when non-human primate (NHP) models are employed. In this paper, we will discuss the development of anatomical MR imaging protocols for two important applications of structural imaging in NHPs: studies of genetic variability in brain morphology and longitudinal imaging of fetal brain maturation trends. In contrast with imaging studies of adult humans, structural imaging in the NHPs is challenging due to a comparatively small brain size (2- to 200-fold smaller volume, depending on the species). This difference in size is further accentuated in NHP studies of brain development in which fetal brain volumes are 10-50% of their adult size. The sizes of cortical gyri and sulci scale allometrically with brain size. Thus, achieving spatial sampling that is comparable to that of high-quality human studies (approximately 1.0 mm(3)) requires a brain-size-adjusted reduction in the sampling volumes of from 500-to-150 microm(3). Imaging at this spatial resolution while maintaining sufficient contrast and signal to noise ratio necessitates the development of specialized MRI protocols. Here we discuss our strategy to optimize the protocol parameters for two commonly available structural imaging sequences: MPRAGE and TrueFisp. In addition, computational tools developed for the analysis of human structural images were applied to the NHP studies. These included removal of non-brain tissues, correction for RF inhomogeneity, spatial normalization, building of optimized target brain and analysis of cerebral gyrification and individual cortical variability. Finally, recent findings in the genetics of cerebral gyrification and tracking of maturation trends in the fetal, newborn and adult brain are described.

Hominoid visual brain structure volumes and the position of the lunate sulcus

It has been argued that changes in the relative sizes of visual system structures predated an increase in brain size and provide evidence of brain reorganization in hominins. However, data about the volume and anatomical limits of visual brain structures in the extant taxa phylogenetically closest to humans-the apes-remain scarce, thus complicating tests of hypotheses about evolutionary changes. Here, we analyze new volumetric data for the primary visual cortex and the lateral geniculate nucleus to determine whether or not the human brain departs from allometrically-expected patterns of brain organization. Primary visual cortex volumes were compared to lunate sulcus position in apes to investigate whether or not inferences about brain reorganization made from fossil hominin endocasts are reliable in this context. In contrast to previous studies, in which all species were relatively poorly sampled, the current study attempted to evaluate the degree of intraspecific variability by including numerous hominoid individuals (particularly Pan troglodytes and Homo sapiens). In addition, we present and compare volumetric data from three new hominoid species-Pan paniscus, Pongo pygmaeus, and Symphalangus syndactylus. These new data demonstrate that hominoid visual brain structure volumes vary more than previously appreciated. In addition, humans have relatively reduced primary visual cortex and lateral geniculate nucleus volumes as compared to allometric predictions from other hominoids. These results suggest that inferences about the position of the lunate sulcus on fossil endocasts may provide information about brain organization.

Coming of age in Olduvai and the Zaire rain forest

Probably Homo habilis is two species not one; similarly for Pan troglodytes. Although amenable to training, in nature Pan paniscus may be a “specialized insular dwarf.” Language is uniquely human, but symbolic behavior and intelligence are widespread among animals with little respect for phylogenetic closeness to Homo sapiens.

Issues and nonissues in the origins of language

This response clarifies the nature of reappropriation and the definition of language. It explicates the relationship between neural systems and language and between homology and evolutionary gradualism. Through a review of ape capacities in the realms of language and tool use, it distinguishes human language acquisition from nonhuman learning. Finally, it suggests the appropriate sorts of evidence on which to base further evolutionary arguments relevant to the origins of language.

Is preadaptation for language a necessary assumption?

Preadaptation for language is an unnecessary assumption because intermediate stages of linguistic ability are possible and adaptive. Language could have evolved through gradual selection from structures exhibiting few features associated with modern structures. Without physical evidence pertaining to language ability in prehabilis hominids, it remains possible that selective pressures for language use preceded and necessitated modern neurolinguistic structures.

Neurolinguistic models and fossil reconstructions

Hominid-like morphology in habiline cranial endocasts does not necessarily imply the presence of language capacity. The cortical zone in question is not associated exclusively with language in humans, and its emergence in habilines might indicate the evolution of other cognitive functions special to humans that were preconditions for the later evolution of language.

Manual versus speech motor control and the evolution of language

Inferences made from endocasts of fossil skulls cannot provide information on the function of particular neocortical areas or the subcortical pathways to prefrontal cortex that form part of the neural substrate for speech, syntax, and certain aspects of cognition. The neural bases of syntax cannot be disassociated from “communication.” Manual motor control was probably a preadaptive factor in the evolution of humansyntactic ability, but neurophysiological data on living humans show that speech motor control and syntax are more closely linked. The evolution of fully modern speech occurred fairly recently; thoughHomo habilismay have had some degree of speech and syntactic ability, it was not fully modern. Stone tools are uncertain indices of language or cognition.

Language as a multimodal sensory enhancement system

Several claims made by Wilkins & Wakefield require qualification. First, the proposed delineation of the parietal-occipital-temporal junction (POT) is overly restrictive. Second, focusing exclusively on the evolutionary importance of manual manipulation oversimplifies interacting evolutionary preconditions for language. Finally, Wilkins and Wakefield's perspective adheres to a homocentric, formal, linguistic definition of language instead of viewing language as a multimodal sensory enhancement system unique to each species.

Evidence for POT expansion in earlyHomo: A pretty theory with ugly (or no) paleoneurological facts

If POT (parieto-occipital-temporal junction) reorganization came earlier in australopithecines than inHomo, it is likely that the selective pressures were different, and not necessarily directed toward language. The brain endocast evidence for the POT inA. afarensisis actually better than it is for earlyHomo.

Issues in neo- and paleoneurology of language

Wilkins and Wakefield's hypothesis that language is fundamentally a cognitive rather than cominunicational adaptation is reasonable, but there are flaws in their anatomical and fossil evidence. Their analysis of reorganization also needs clarification. Finally, the origin of language ability must have occurred with australopithecine rather than habiline adaptations on entry into the novel hominid adaptive zone.

Palaeoneurology of language: Grounds for scepticism

Wilkins & Wakefield's identification of anatomical features in the Koobi Fora endocast, which may be thought to carry some functional significance in relation to organization for language, raises fundamental problems of method: attention is drawn to some limitations of the evidence, of endocasts and of the neuroanatomical map used to interpret them.

Complex behaviors: Evolution and the brain

Three issues are addressed in this commentary. (1) Wilkins & Wakefield are commended for placing the complex behavior they discuss within an evolutionary matrix. (2) They err on a number of points in regard to their treatment of this complex behavior. These involve (a) their emphasis on the evolution of conceptual structure rather than language, (b) their equation of meaning with reference, (c) their minimalist view of learning theory, and (d) their separation of the evolution of speech from that of language. (3) They adopt a framework for neural preconditions that has been clearly discredited on both behavioral and neuroanatomical grounds. Finally, recent research involving modern neuroimaging techniques casts further doubt on the authors' postulated functions for the neural areas they discuss.

Brains evolution and neurolinguistic preconditions

This target article presents a plausible evolutionary scenario for the emergence of the neural preconditions for language in the hominid lineage. In pleistocene primate lineages there was a paired evolutionary expansion of frontal and parietal neocortex (through certain well-documented adaptive changes associated with manipulative behaviors) resulting, in ancestral hominids, in an incipient Broca's region and in a configurationally unique junction of the parietal, occipital, and temporal lobes of the brain (the POT). On our view, the development of the POT in our ancestors resulted in the neuroanatomical substrate consistent with the ability for representations in modality-neutral association cortex and, as a result of structure-imposing interaction with Broca's area, the hierarchically structured “conceptual structure.” Evidence from paleoneurology and comparative primate neuroanatomy is used to argue that Homo habilis (2.5–2 million years ago) was the first hominid to have the appropriate gross neuroanatomical configuration to support conceptual structure. We thus suggest that the neural preconditions for language are met in H. habilis. Finally, we advocate a theory of language acquisition that uses conceptual structure as input to the learning procedures, thus bridging the gap between it and language.

A case for auditory temporal processing as an evolutionary precursor to speech processing and language function

Wilkins & Wakefield suggest that changes in the hominid brain made it uniquely “preadaptive” for language, yet no precursor functions served as adaptive substrates to the emergence of language. We present contrary evidence that the ability to discriminate and process rapid and complex auditory information is a cross-species function subserving communication processes including, but not limited to, human speech perception. We suggest that auditory temporal processing served as an evolutionary precursor to speech processing and consequent language development in humans.

Finding the true place ofHomo habilisin language evolution

Despite some sound basic assumptions, Wilkins & Wakefield portray aHomo habilistoo linguistically sophisticated to fit in with the subsequent fossil record and thereby lose a reasoned explanation for human innovativeness. They err, too, in accepting a single-level model of conceptual structure and in deriving initial linguistic units from calls, a process far more dubious than the derivation of home-sign from naive gesture.

Neural preconditions for proto-language

Representation must be prior to communication in evolution. Wilkins & Wakefield's target article gives the impression that communicative pressures play a secondary role. We suggest that their evolutionary precursor is compatible with protolanguage rather than language itself. The difference between these two communicative systems should not be underestimated: only the former can be trivially reappropriated from a representational system.

Coevolution of neocortical size, group size and language in humans

Group size covaries with relative neocortical volume in nonhuman primates. This regression equation predicts a group size for modern humans very similar to that for hunter-gatherer and traditional horticulturalist societies. Similar group sizes are found in other contemporary and historical societies. Nonhuman primates maintain group cohesion through social grooming; among the Old World monkeys and apes, social grooming time is linearly related to group size. Maintaining stability of human-sized groups by grooming alone would make intolerable time demands. It is therefore suggested (1) that the evolution of large groups in the human lineage depended on developing a more efficient method for time-sharing the processes of social bonding and (2) that language uniquely fulfills this requirement. Data on the size of conversational and other small interacting groups of humans accord with the predicted relative efficiency of conversation compared to grooming as a bonding process. In human conversations about 60% of time is spent gossiping about relationships and personal experiences. Language may accordingly have evolved to allow individuals to learn about the behavioural characteristics of other group members more rapidly than was feasible by direct observation alone.

Reconstructing the ups and downs of primate brain evolution: implications for adaptive hypotheses and Homo floresiensis

Background

Brain size is a key adaptive trait. It is often assumed that increasing brain size was a general evolutionary trend in primates, yet recent fossil discoveries have documented brain size decreases in some lineages, raising the question of how general a trend there was for brains to increase in mass over evolutionary time. We present the first systematic phylogenetic analysis designed to answer this question.

Results

We performed ancestral state reconstructions of three traits (absolute brain mass, absolute body mass, relative brain mass) using 37 extant and 23 extinct primate species and three approaches to ancestral state reconstruction: parsimony, maximum likelihood and Bayesian Markov-chain Monte Carlo. Both absolute and relative brain mass generally increased over evolutionary time, but body mass did not. Nevertheless both absolute and relative brain mass decreased along several branches. Applying these results to the contentious case of Homo floresiensis, we find a number of scenarios under which the proposed evolution of Homo floresiensis' small brain appears to be consistent with patterns observed along other lineages, dependent on body mass and phylogenetic position.

Conclusions

Our results confirm that brain expansion began early in primate evolution and show that increases occurred in all major clades. Only in terms of an increase in absolute mass does the human lineage appear particularly striking, with both the rate of proportional change in mass and relative brain size having episodes of greater expansion elsewhere on the primate phylogeny. However, decreases in brain mass also occurred along branches in all major clades, and we conclude that, while selection has acted to enlarge primate brains, in some lineages this trend has been reversed. Further analyses of the phylogenetic position of Homo floresiensis and better body mass estimates are required to confirm the plausibility of the evolution of its small brain mass. We find that for our dataset the Bayesian analysis for ancestral state reconstruction is least affected by inclusion of fossil data suggesting that this approach might be preferable for future studies on other taxa with a poor fossil record.

The human parahippocampal region: I. Temporal pole cytoarchitectonic and MRI correlation

The temporal pole (TP) is the rostralmost portion of the human temporal lobe. Characteristically, it is only present in human and nonhuman primates. TP has been implicated in different cognitive functions such as emotion, attention, behavior, and memory, based on functional studies performed in healthy controls and patients with neurodegenerative diseases through its anatomical connections (amygdala, pulvinar, orbitofrontal cortex). TP was originally described as a single uniform area by Brodmann area 38, and von Economo (area TG of von Economo and Koskinas), and little information on its cytoarchitectonics is known in humans. We hypothesize that 1) TP is not a homogenous area and we aim first at fixating the precise extent and limits of temporopolar cortex (TPC) with adjacent fields and 2) its structure can be correlated with structural magnetic resonance images. We describe here the macroscopic characteristics and cytoarchitecture as two subfields, a medial and a lateral area, that constitute TPC also noticeable in 2D and 3D reconstructions. Our findings suggest that the human TP is a heterogeneous region formed exclusively by TPC for about 7 mm of the temporal tip, and that becomes progressively restricted to the medial and ventral sides of the TP. This cortical area presents topographical and structural features in common with nonhuman primates, which suggests an evolutionary development in human species.

Genetics of primary cerebral gyrification: Heritability of length, depth and area of primary sulci in an extended pedigree of Papio baboons

Genetic control over morphological variability of primary sulci and gyri is of great interest in the evolutionary, developmental and clinical neurosciences. Primary structures emerge early in development and their morphology is thought to be related to neuronal differentiation, development of functional connections and cortical lateralization. We measured the proportional contributions of genetics and environment to regional variability, testing two theories regarding regional modulation of genetic influences by ontogenic and phenotypic factors. Our measures were surface area, and average length and depth of eleven primary cortical sulci from high-resolution MR images in 180 pedigreed baboons. Average heritability values for sulcal area, depth and length (h(2)(Area)=.38+/-.22; h(2)(Depth)=.42+/-.23; h(2)(Length)=.34+/-.22) indicated that regional cortical anatomy is under genetic control. The regional pattern of genetic contributions was complex and, contrary to previously proposed theories, did not depend upon sulcal depth, or upon the sequence in which structures appear during development. Our results imply that heritability of sulcal phenotypes may be regionally modulated by arcuate U-fiber systems. However, further research is necessary to unravel the complexity of genetic contributions to cortical morphology.

Visualizing the entire cortical myelination pattern in marmosets with magnetic resonance imaging

Myeloarchitecture, the pattern of myelin density across the cerebral cortex, has long been visualized in histological sections to identify distinct anatomical areas of the cortex. In humans, two-dimensional (2D) magnetic resonance imaging (MRI) has been used to visualize myeloarchitecture in select areas of the cortex, such as the stripe of Gennari in the primary visual cortex and Heschl's gyrus in the primary auditory cortex. Here, we investigated the use of MRI contrast based on longitudinal relaxation time (T(1)) to visualize myeloarchitecture in vivo over the entire cortex of the common marmoset (Callithrix jacchus), a small non-human primate that is becoming increasingly important in neuroscience and neurobiology research. Using quantitative T(1) mapping, we found that T(1) at 7T in a cortical region with a high myelin content was 15% shorter than T(1) in a region with a low myelin content. To maximize this T(1) contrast for imaging cortical myelination patterns, we optimized a magnetization-prepared rapidly acquired gradient echo (MP-RAGE) sequence. In whole-brain, 3D T(1)-weighted images made in vivo with the sequence, we identified six major cortical areas with high myelination and confirmed the results with histological sections stained for myelin. We also identified several subtle features of myeloarchitecture, showing the sensitivity of our technique. The ability to image myeloarchitecture over the entire cortex may prove useful in studies of longitudinal changes of the topography of the cortex associated with development and neuronal plasticity, as well as for guiding and confirming the location of functional measurements.

The development of gyrification in childhood and adolescence

Gyrification is the process by which the brain undergoes changes in surface morphology to create sulcal and gyral regions. The period of greatest development of brain gyrification is during the third trimester of pregnancy, a period of time in which the brain undergoes considerable growth. Little is known about changes in gyrification during childhood and adolescence, although considering the changes in gray matter volume and thickness during this time period, it is conceivable that alterations in the brain surface morphology could also occur during this period of development. The formation of gyri and sulci in the brain allows for compact wiring that promotes and enhances efficient neural processing. If cerebral function and form are linked through the organization of neural connectivity, then alterations in neural connectivity, i.e., synaptic pruning, may also alter the gyral and sulcal patterns of the brain. This paper reviews developmental theories of gyrification, computational techniques for measuring gyrification, and the potential interaction between gyrification and neuronal connectivity. We also present recent findings involving alterations in gyrification during childhood and adolescence.