Este Armstrong

The human pattern of gyrification in the cerebral cortex.

SummaryThe degree of cortical folding found in adult human brains has been analyzed using a gyrification index (GI). This parameter permits the description of a mean value for the whole brain, but also a local specific analysis of different brain regions. Correlation analyses of the GI with age, body weight, body length, brain weight and volume of the prosencephalon and the cortex show no significant results. GI values do not differ significantly between male and female brains, right and left hemispheres or right and left sides of the superior temporal plane. The GI shows maximal values over the prefrontal and the parieto-temporo-occipital association cortex. A comparison between the rostro-caudal GI patterns of human brains and those of prosimians and Old World monkeys shows the largest difference over the prefrontal cortex. The mean GI increases from prosimians to human brains with the highest values for non-human primates being in the pongid group.

Prefrontal cortex in humans and apes: A comparative study of area 10

Area 10 is one of the cortical areas of the frontal lobe involved in higher cognitive functions such as the undertaking of initiatives and the planning of future actions. It is known to form the frontal pole of the macaque and human brain, but its presence and organization in the great and lesser apes remain unclear. It is here documented that area 10 also forms the frontal pole of chimpanzee, bonobo, orangutan, and gibbon brains. Imaging techniques and stereological tools are used to characterize this area across species and provide preliminary estimates of its absolute and relative size. Area 10 has similar cytoarchitectonic features in the hominoid brain, but aspects of its organization vary slightly across species, including the relative width of its cortical layers and the space available for connections. The cortex forming the frontal pole of the gorilla appears highly specialized, while area 10 in the gibbon occupies only the orbital sector of the frontal pole. Area 10 in the human brain is larger relative to the rest of the brain than it is in the apes, and its supragranular layers have more space available for connections with other higher-order association areas. This suggests that the neural substrates supporting cognitive functions associated with this part of the cortex enlarged and became specialized during hominid evolution.

Cytoarchitectonic identification and probabilistic mapping of two distinct areas within the anterior ventral bank of the human intraparietal sulcus

Anatomical studies in the macaque cortex and functional imaging studies in humans have demonstrated the existence of different cortical areas within the intraparietal sulcus (IPS). Such functional segregation, however, does not correlate with presently available architectonic maps of the human brain. This is particularly true for the classical Brodmann map, which is still widely used as an anatomical reference in functional imaging studies. The aim of this cytoarchitectonic mapping study was to use previously defined algorithms to determine whether consistent regions and borders can be found within the cortex of the anterior IPS in a population of 10 post‐mortem human brains. Two areas, the human intraparietal area 1 (hIP1) and the human intraparietal area 2 (hIP2), were delineated in serial histological sections of the anterior, lateral bank of the human IPS. The region hIP1 is located posterior and medial to hIP2, and the former is always within the depths of the IPS. The latter, on the other hand, sometimes reaches the free surface of the superior parietal lobule. The delineations were registered to standard reference space, and probabilistic maps were calculated, thereby quantifying the intersubject variability in location and extent of both areas. In the future, they can be a tool for analyzing structure–function relationships and a basis for determining degrees of homology in the IPS among anthropoid primates. We conclude that the human IPS has a more finely grained parcellation than shown in Brodmanns map. J. Comp. Neurol. 495:53–69, 2006.

Limbic frontal cortex in hominoids: A comparative study of area 13

The limbic frontal cortex forms part of the neural substrate responsible for emotional reactions to social stimuli. Area 13 is one of the cortical areas long known to be part of the posterior orbitofrontal cortex in several monkey species, such as the macaque. Its presence nevertheless in the human brain has been unclear, and the cortex of the frontal lobe of the great and lesser apes remains largely unknown. In this study area 13 was identified in human, chimpanzee, bonobo, gorilla, orangutan, and gibbon brains, and cortical maps were generated on the basis of its cytoarchitecture. Imaging techniques were used to characterize and quantify the microstructural organization of the area, and stereological tools were applied for estimates of the volume of area 13 in all species. Area 13 is conservative in its structure, and features such as size of cortical layers, density of neurons, and space available for connections are similar across hominoids with only subtle differences present. In contrast to the homogeneity found in its organization, variation is present in the relative size of this cortical area (as a percentage of total brain volume). The human and the bonobo include a complex orbitofrontal cortex and a relatively smaller area 13. On the contrary the orangutan stands out by having a shorter orbitofrontal region and a more expanded area 13. Differences in the organization and size of individual cortical areas involved in emotional reactions and social behavior can be related to behavioral specializations of each hominoid and to the evolution of emotions in hominids.

Cortical folding, the lunate sulcus and the evolution of the human brain

Abstract Controversy over the placement of the lunate sulcus on the Taung australopithecine endocast has been central in the debate as to whether cortical reorganization occurred independently of and preceded the expansion of the human brain. A new technique, the gyrification index, measures the degree of cortical folding. A comparison of the index among extant great ape and human brains shows that the values of the two taxa overlap only in the caudal cortex where the lunate sulcus is found. The similarity in degree of cortical folding in the caudal cortex contrasts to differences in all the other portions where the human cortex has a higher degree of folding than found in ape brains. The conservation in the degree of caudal cortical folding is unexpected if, during human evolution, the parietal association cortex had enlarged independently of brain size so much that the lunate sulcus moved posteriorly. The findings are more consistent with interpretations that the major differences in folding between human and ape parietal and occipital cortices arose concurrently with changes in brain size. Although these data do not localize the position of the lunate sulcus on the Taung endocast, the similarity in degrees of folding among hominoids is consistent with interpretations that, given its small cranial capacity, the lunate sulcus of the Taung specimen is in a pongid position.

Gender-Specific Left–Right Asymmetries in Human Visual Cortex

The structural correlates of gender differences in visuospatial processing are essentially unknown. Our quantitative analysis of the cytoarchitecture of the human primary visual cortex [V1/Brodmann area 17 (BA17)], neighboring area V2 (BA18), and the cytoarchitectonic correlate of the motion-sensitive complex (V5/MT+/hOc5) shows that the visual areas are sexually dimorphic and that the type of dimorphism differs among the areas. Gender differences exist in the interhemispheric asymmetry of hOc5 volumes and in the right-hemispheric volumetric ratio of hOc5 to BA17, an area that projects to V5/MT+/hOc5. Asymmetry was also observed in the surface area of hOc5 but not in its cortical thickness. The differences give males potentially more space in which to process additional information, a finding consistent with superior male processing in particular visuospatial tasks, such as mental rotation. Gender differences in hOc5 exist with similar volume fractions of cell bodies, implying that, overall, the visual neural circuitry is similar in males and females.

Brains, Bodies and Metabolism

The interrelationship of brain and body sizes has been the subject of investigations for over a hundred years. These studies have demonstrated that variation in brain weights is much smaller than that in body weights; consequently, scaling studies are ones of negative allometry. Furthermore, the variability in brain weight is greater when comparisons are between species rather than among individuals of the same species, and the degree of variability in brain size differs among orders. The largest shifts in brain sizes relative to changes in body weights are found when comparing different ontogenetic stages. Debate continues as to the importance of metabolism in determining the interrelationship of brain-body weights for interpreting differences in relative brain size. Although past advances in the study of brain-body size associations have come by increasing the size of the data bases and by improved statistical analyses, the recent utilization of transgenic animals may provide new insights into the mechanism of this association.

Relative Brain Size and Metabolism in Birds

Earlier studies have shown that the negatively allometric brain-body weight association in mature mammals changes to an isometric association when body weights are adjusted for their rates of oxygen consumption. Birds are endogenous homeotherms, and so their brain weights were analyzed according to their body weights and metabolism (estimated energy supply). As expected, the brain and body weights of the 83 species of neognathid birds have a negatively allometric association. The same species, however, have a brain weight-to-estimated energy supply which cannot be separated from isometry. While passerines have bigger brains for their body weights than altricial nonpasserines, the relative brain sizes of the two avian groups cannot be separated once the metabolic rate is used to adjust the body weights. Ratites or paleognathid birds may have a different brain-to-metabolism association. Consideration of bioenergetics helps clarify brain and body weight associations.

Enlarged limbic structures in the human brain: the anterior thalamus and medial mamillary body

Major paradigms about human evolution stress the increase of cognitive capabilities, while considering our emotional and limbic systems to be relatively unchanged. Morphometric analyses of two limbic structures, the anterior thalamic nuclei (AP) and the medial mamillary bodies (MB), in 27 primate species show, however, that: human nuclei are as large as MB or larger AP than differences in anthropoid brain sizes predict; and prosimians and anthropoid patterns differ.

The limbic system and culture

The human ability to live according to learned, shared rules of behavior requires cortical functions. Is the limbic system also necessary for culture or are its functions opposed to it, requiring cortical inhibition? The sizes of monkey and ape neocortical and major limbic structures scale with brain weight, but the neocortex expands more (has a steeper exponent) than limbic structures. As the human brain evolved it did not deviate from the scaling relationships found in nonhuman anthropoids. This evidence for conservation in scaling supports the idea that limbic functions are necessary for human symbolism and culture.