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Local relationships contributing to cytoarchitectonic allometry are depicted in this figure from Deacon (1990b). T h e top figure depicts the situation in a relatively small brain and the lower figure depicts these same relationships in a slightly larger brain. In small as compared to large brains projection neurons possess short, small diameter axons, with relatively less myelin, smaller cell bodies, lower neuron to glia ratio, higher neuron densities, lower ratios of local circuit neurons to projection neurons, smaller size differences between the smallest and largest neurons, etc.

Local relationships contributing to cytoarchitectonic allometry are depicted in this figure from Deacon (1990b). T h e top figure depicts the situation in a relatively small brain and the lower figure depicts these same relationships in a slightly larger brain. In small as compared to large brains projection neurons possess short, small diameter axons, with relatively less myelin, smaller cell bodies, lower neuron to glia ratio, higher neuron densities, lower ratios of local circuit neurons to projection neurons, smaller size differences between the smallest and largest neurons, etc.

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A critical review of past and current theories of mammalian brain evolution is presented in order to discuss conceptual problems that persist in the field. Problems with the concept of homology arise because of the interaction of cell lineages and axonal connectivity in the determination of structural features of the brain. Focusing on the continui...

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... the relative number of glia must also increase. T h e same constraints are not experienced by small local circuit neurons. Since local circuits remain rela- tively constant in volume (increasing no more than a few hundred percent across huge brain size differences) these neurons should have to change relatively little to compensate for size (see Fig. 6). Given these two extremes, it becomes obvious that the local cyto-and myelo- architecture of many brain structures will reflect the influence of size. In general, in large brains as compared to small brains there should be a number of regular trends: In large brains there should be (1) some much larger cell types, but also a much ...

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... This seemingly reduced level of specialization in humans is surprising in light of findings that larger absolute brain size is correlated with greater neural specialization across species (Striedter, 2005). As the number of neurons in the brain increases, the metabolic cost of maintaining the same proportion of interareal connectedness increases exponentially, leading to strong selective pressures for further local specialization (Deacon, 1990;Ringo, 1991). We speculate that this difference between human and non-human primates could be due to the change of the functional role of the tongue in humans. ...
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The English idiom “on the tip of my tongue” commonly acknowledges that something is known, but it cannot be immediately brought to mind. This phrase accurately describes sensorimotor functions of the tongue, which are fundamental for many tongue-related behaviors (e.g., speech), but often neglected by scientific research. Here, we review a wide range of studies conducted on non-primates, non-human and human primates with the aim of providing a comprehensive description of the cortical representation of the tongue’s somatosensory inputs and motor outputs across different phylogenetic domains. First, we summarize how the properties of passive non-noxious mechanical stimuli are encoded in the putative somatosensory tongue area, which has a conserved location in the ventral portion of the somatosensory cortex across mammals. Second, we review how complex self-generated actions involving the tongue are represented in more anterior regions of the putative somato-motor tongue area. Finally, we describe multisensory response properties of the primate and non-primate tongue area by also defining how the cytoarchitecture of this area is affected by experience and deafferentation.
... Alternatively, such a specialisation may simply arise as a consequence of the increased proportional size of neocortex. Larger brain regions send more axonal projections and compete more effectively for limited dendritic space [155,156]. For example, among mammals, proportionally larger neocortical size is correlated with deeper penetration of the spinal cord by corticospinal axons, which in turn mediates improved manual dexterity [42,43]. ...
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Humans are vocal modulators par excellence. This ability is supported in part by the dual representation of the laryngeal muscles in the motor cortex. Movement, however, is not the product of motor cortex alone but of a broader motor network. This network consists of brain regions that contain somatotopic maps that parallel the organization in motor cortex. We therefore present a novel hypothesis that the dual laryngeal representation is repeated throughout the broader motor network. In support of the hypothesis, we review existing literature that demonstrates the existence of network-wide somatotopy and present initial evidence for the hypothesis' plausibility. Understanding how this uniquely human phenotype in motor cortex interacts with broader brain networks is an important step toward understanding how humans evolved the ability to speak. We further suggest that this system may provide a means to study how individual components of the nervous system evolved within the context of neuronal networks. This article is part of the theme issue ‘Voice modulation: from origin and mechanism to social impact (Part I)’.
... The effect of allometric scaling on brain connectivity, a key factor in shaping brain function (Sporns et al. 2005), remains largely unknown. Comparative studies have noted that the white matter tends to take up more and more space in largersized brains compared with smaller brains (Deacon 1990;Rilling and Insel 1999a;Hofman 2001): The proportion of white matter to total brain volume ranges from an estimated 11% in mice to 27% in macaques to 40-41% in chimpanzees and humans (Zhang and Sejnowski 2000). While the proportion of white matter is higher in larger brains, the cortical surface has been noted to scale even faster (Hofman 1989;Mota et al. 2019). ...
... Despite the higher white matter proportion, our findings show that cortical surface area scales even faster and outpaces white matter volume and the CC, which is closely in line with earlier findings (Rilling and Insel 1999b). Higher proportions of white matter have been proposed to help maintain connectivity in the expanded cortex of larger brains, which requires disproportionately more white matter to remain connected (Deacon 1990;Ringo 1991;Hofman 2012). However, the observed negative allometric scaling exponents of cortical surface area with white matter volume (0.78) and CC cross-sectional area (0.88) indicate that there is less and less space available for white matter connectivity with increasing brain size. ...
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... These inconsistencies reveal the fundamental flaw in the EQ approach [Deacon, 1990;Striedter, 2005], it assumes that one process (be it proprioception or metabolic turnover) predominates in determining brain size to the extent that cognitive functions produce only minor deviations, so we can estimate their strength by taking ratios or residuals. Instead, we may expect a variety of processes to affect brain size, with some of them lineage specific and each potentially varying in strength across lineages. ...
... This produces a "progression index" for each species. However, due to various conceptual and statistical problems [Deacon, 1990] it has found little application. This index also predicts primate cognitive performance only marginally better than EQ and far worse than brain size per se [Gibson et al., 2001;Deaner et al., 2007], although it is unknown how well it does for other mammals or birds. ...
... The most popular measure is the neocortex ratio, i.e., the size of the neocortex relative to the rest of the brain [Dunbar, 1992]. However, all ratio measures have the fundamental drawback that they lack a clear neurobiological justification [Deacon, 1990;Deaner et al., 2000;Barton, 2006], and the neocortex ratio was favored simply because it yielded the best correlation with the putative selective pressure [Dunbar, 1992]. Like most other ratios, the neocortex ratio is clearly correlated with both overall brain and body size, as expected based on the fundamental brain allometries [Finlay and Darlington, 2001;Halley and Deacon, 2017]. ...
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... Alternatively, such a specialization may simply arise as a consequence of the increased proportional size of neocortex. Larger brain regions send more axonal projections and compete more effectively for limited dendritic space [155,156]. For example, among mammals, proportionally larger neocortical size is correlated with deeper penetration of the spinal cord by corticospinal axons, which in turn mediates improved manual dexterity [42,43]. ...
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Humans are vocal modulators par excellence. This ability is supported in part by the dual representation of the laryngeal muscles in the motor cortex. Movement, however, is not the product of motor cortex alone but of a broader motor network. This network consists of brain regions which contain somatotopic maps that parallel the organisation in motor cortex. We therefore present a novel hypothesis that the dual laryngeal representation is repeated throughout the broader motor network. In support of the hypothesis we review existing literature which demonstrates the existence of network-wide somatotopy, and present initial evidence for the hypothesis’ plausibility. Understanding how this uniquely human phenotype in motor cortex interacts with broader brain networks is an important step toward understanding how humans evolved the ability to speak. We further suggest that this system may provide a means to study how individual components of the nervous system evolved within the context of neuronal networks.
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... They are also identified in several species from rodents ( Grandjean et al., 2017 ) to primates ( Belcher et al., 2013 ). In evolutionary science, homology is defined as structural or functional similarities derived from common ancestry ( Deacon, 1990 ), and we considered that these networks are homologous in lemurs and humans. ...
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Measures of resting-state functional connectivity allow the description of neuronal networks in humans and provide a window on brain function in normal and pathological conditions. Characterizing neuronal networks in animals is complementary to studies in humans to understand how evolution has modelled network architecture. The mouse lemur (Microcebus murinus) is one of the smallest and more phylogenetically distant primates as compared to humans. Characterizing the functional organization of its brain is critical for scientists studying this primate as well as to add a link for comparative animal studies. Here, we created the first functional atlas of mouse lemur brain and describe for the first time its cerebral networks. They were classified as two primary cortical networks (somato-motor and visual), two high-level cortical networks (fronto-parietal and fronto-temporal) and two limbic networks (sensory-limbic and evaluative-limbic). Comparison of mouse lemur and human networks revealed similarities between mouse lemur high-level cortical networks and human networks as the dorsal attentional (DAN), executive control (ECN), and default-mode networks (DMN). These networks were however not homologous, possibly reflecting differential organization of high-level networks. Finally, cerebral hubs were evaluated. They were grouped along an antero-posterior axis in lemurs while they were split into parietal and frontal clusters in humans.
... That question is asked over and over. Ultimately, our driving interest in brain size stems from the large size of our own brain [Deacon, 1990]. There is a huge range of body size among mature cetaceans. ...
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Bowhead whales are one of the least encephalized mammals, possessing a small brain relative to their body size (e.g., a 3 kg brain in a 30,000 kg body). Features of the bowhead whale brain include a blunted temporal lobe and a gyrification index that is less than most cetaceans. Rather than having a cerebrum that is wider than long like odontocetes, the bowhead cerebrum is longer than it is wide. The hippocampus is very small and located within the lateral ventricle, which is ventral to the corpus callosum. The cytoarchitecture of the bowhead cerebral cortex is consistent with that of other cetaceans. The cortex is thin overall with a relatively thick, prominent layer I. As with other cetaceans, there is no granular layer IV. Notably, high numbers of von Economo neurons and fork neurons are found in all regions of the cortex. The highest numbers of these special neurons are observed at the apex of gyri.
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... Thus, early axon extension might favor competing for targets, and their overrepresentation in the mature isocortex (Catania 2001). On the other hand, prolonging the duration of axon extension and refinement might afford more time to outcompete targets, which would accordingly increase the territory of select pathways in the adult brain (Deacon 1990;Striedter 2005). These two seemingly contradictory hypotheses emphasize the importance of timing in generating variation in connections between species. ...
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