Enough species have now been subject to systematic quantitative analysis of the relationship betw... more Enough species have now been subject to systematic quantitative analysis of the relationship between the morphology and cellular composition of their brain that patterns begin to emerge and shed light on the evolutionary path that led to mammalian brain diversity. Based on an analysis of the shared and clade-specific characteristics of 41 modern mammalian species in 6 clades, and in light of the phylogenetic relationships among them, here we propose that ancestral mammal brains were composed and scaled in their cellular composition like modern afrotherian and glire brains: with an addition of neurons that is accompanied by a decrease in neuronal density and very little modification in glial cell density, implying a significant increase in average neuronal cell size in larger brains, and the allocation of approximately 2 neurons in the cerebral cortex and 8 neurons in the cerebellum for every neuron allocated to the rest of brain. We also propose that in some clades the scaling of di...
The pyramidal cell phenotype varies quite dramatically in structure among different cortical area... more The pyramidal cell phenotype varies quite dramatically in structure among different cortical areas in the primate brain. Comparative studies in visual cortex, in particular, but also in sensorimotor and prefrontal cortex, reveal systematic trends for pyramidal cell specialization in functionally related cortical areas. Moreover, there are systematic differences in the extent of these trends between different primate species. Recently we demonstrated differences in pyramidal cell structure in the cingulate cortex of the macaque monkey; however, in the absence of other comparative data it remains unknown as to whether the neuronal phenotype differs in cingulate cortex between species. Here we extend the basis for comparison by studying the structure of the basal dendritic trees of layer III pyramidal cells in the posterior and anterior cingulate gyrus of the vervet monkey (Brodmann's areas 23 and 24, respectively). Cells were injected with Lucifer Yellow in flat-mounted cortical s...
We describe representations of the visual field in areas 18, 19 and 21 of the ferret using standa... more We describe representations of the visual field in areas 18, 19 and 21 of the ferret using standard microelectrode mapping techniques. In all areas the azimuths are represented as islands of peripheral visual field surrounded by central visual field representation. The zero meridian was found at the 17/18 and 19/21 borders; at the 18/19 and anterior border of 21 the relative periphery of the visual field was found. In areas 18 and 19, elevations are represented in a smooth medio-lateral progression from lower to upper visual field. In several cases the elevations in area 21 evidenced a similar medio-lateral progression; however, in others the elevations exhibited a split representation of the horizontal meridian. Anatomically determined callosal connections coincided with the representation of azimuths near the zero meridian. Medio-lateral bands of callosal connectivity that straddle the 17/18 and 19/21 borders are connected by bridges of callosally projecting cells. Acallosal cortical islands corresponded to the peripheral visual field and were found straddling the 18/19 border and the anterior border of area 21. The results are discussed in relation to callosal connectivity and retinotopy in extrastriate visual cortex and to proposed homologies of carnivore and primate visual cortex.
The olfactory bulb is an evolutionarily old structure that antedates the appearance of a six-laye... more The olfactory bulb is an evolutionarily old structure that antedates the appearance of a six-layered mammalian cerebral cortex. As such, the neuronal scaling rules that apply to scaling the mass of the olfactory bulb as a function of its number of neurons might be shared across mammalian groups, as we have found to be the case for the ensemble of non-cortical, non-cerebellar brain structures. Alternatively, the neuronal scaling rules that apply to the olfactory bulb might be distinct in those mammals that rely heavily on olfaction. The group previously referred to as Insectivora includes small mammals, some of which are now placed in Afrotheria, a base group in mammalian radiation, and others in Eulipotyphla, a group derived later, at the base of Laurasiatheria. Here we show that the neuronal scaling rules that apply to building the olfactory bulb differ across eulipotyphlans and other mammals such that eulipotyphlans have more neurons concentrated in an olfactory bulb of similar si...
This study forms part of an ongoing investigation of pyramidal cell structure in the cingulate co... more This study forms part of an ongoing investigation of pyramidal cell structure in the cingulate cortex of primates. Recently we have demonstrated that layer III pyramidal cells in the anterior cingulate gyrus are considerably larger, more branched and more spinous than those in the posterior cingulate gyrus (areas 24 and 23, respectively) in the macaque and vervet monkeys. Moreover, the
Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan brains has... more Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in evolution in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of afrotherians, believed to be the first clade to radiate from the common eutherian ancestor. We find that afrotherians share non-neuronal scaling rules with rodents, primates and eulipotyphlans, as well as the coordinated scaling of numbers of neurons in the cerebral cortex and cerebellum. Afrotherians share with rodents and eulipotyphlans, but not with primates, the scaling of number of neurons in the cortex and in the cerebellum as a function of the number of neurons in the rest of the brain. Afrotheria also share with rodents and eulipotyphlans the neu...
The present study describes the distribution and cellular morphology of catecholaminergic neurons... more The present study describes the distribution and cellular morphology of catecholaminergic neurons in the diencephalon and midbrain of the bottlenose dolphin (Tursiops truncatus). Tyrosine hydroxylase immunohistochemistry was used to visualize these putatively dopaminergic neurons. The standard A1-A17, C1-C3, nomenclature is used for expediency; however, the neuroanatomical names of the various nuclei have also been given. Dolphins exhibit certain tyrosine hydroxylase
Quantitative analysis of the cellular composition of rodent, primate, insectivore, and afrotheria... more Quantitative analysis of the cellular composition of rodent, primate, insectivore, and afrotherian brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of artiodactyls, a group within the order Cetartiodactyla, believed to be a relatively recent radiation from the common Eutherian ancestor. We find that artiodactyls share non-neuronal scaling rules with all groups analyzed previously. Artiodactyls share with afrotherians and rodents, but not with primates, the neuronal scaling rules that apply to the cerebral cortex and cerebellum. The neuronal scaling rules that apply to the remaining brain areas are, however, distinct in artiodactyls. Importantly, we show that the folding index of the cerebral cortex scal...
What explains the superior cognitive abilities of the human brain compared to other, larger brain... more What explains the superior cognitive abilities of the human brain compared to other, larger brains? Here we investigate the possibility that the human brain has a larger number of neurons than even larger brains by determining the cellular composition of the brain of the African elephant. We find that the African elephant brain, which is about three times larger than the human brain, contains 257 billion (10(9)) neurons, three times more than the average human brain; however, 97.5% of the neurons in the elephant brain (251 billion) are found in the cerebellum. This makes the elephant an outlier in regard to the number of cerebellar neurons compared to other mammals, which might be related to sensorimotor specializations. In contrast, the elephant cerebral cortex, which has twice the mass of the human cerebral cortex, holds only 5.6 billion neurons, about one third of the number of neurons found in the human cerebral cortex. This finding supports the hypothesis that the larger absolute number of neurons in the human cerebral cortex (but not in the whole brain) is correlated with the superior cognitive abilities of humans compared to elephants and other large-brained mammals.
Observations on eight bottlenose dolphins located in the Southern Hemisphere during rest indicate... more Observations on eight bottlenose dolphins located in the Southern Hemisphere during rest indicated that they spent the majority of the time (85%) engaged in behaviors that can be considered clockwise. This is in contrast with many observations of sleeping/resting dolphins in the Northern Hemisphere that spend the majority of their time involved in counterclockwise activity. This observation leads to the possibility that the reason for preferential swimming biases in dolphins is the result of global forces rather than the result of the anatomy of the individual dolphins. Our observations also indicate that dolphins change overt behavior every 40 s, coincident with the respiration rate. The possibility is suggested that the salience of neural activity controlling respiration in the reticular system may effect/disrupt reticular attentional mechanisms, thus leading to the changes in overt behavior. It is hypothesized that this 40-s period may represent the possible attention span of the sleeping bottlenose dolphin.
Philosophical Transactions of the Royal Society B: Biological Sciences, 2008
Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytoche... more Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytochemically for calcium-binding proteins, gamma aminobutyric acid (GABA) or other antigens. Cyto- and myeloarchitecture revealed thalami that are fundamentally mammalian in organization, with the three principal divisions of the thalamus (epithalamus, dorsal thalamus and ventral thalamus) identifiable as in marsupials and eutherian mammals. The dorsal thalamus exhibits more nuclear parcellation than hitherto described, but lack of an internal medullary lamina, caused by splaying out of afferent fibre tracts that contribute to it in other mammals, makes identification of anterior, medial and intralaminar nuclear groups difficult. Differentiation of the ventral nuclei is evident with the ventral posterior nucleus of the platypus enormously expanded into the interior of the cerebral hemisphere, where it adopts a relationship to the striatum not seen in other mammals. Other nuclei such as the lateral dorsal become identifiable by expression of patterns of calcium-binding proteins identical to those found in other mammals. GABA cells are present in the ventral and dorsal thalamic nuclei, and in the ventral thalamus form a remarkable continuum with GABA cells of the two segments of the globus pallidus and pars reticulata of the substantia nigra.
Enough species have now been subject to systematic quantitative analysis of the relationship betw... more Enough species have now been subject to systematic quantitative analysis of the relationship between the morphology and cellular composition of their brain that patterns begin to emerge and shed light on the evolutionary path that led to mammalian brain diversity. Based on an analysis of the shared and clade-specific characteristics of 41 modern mammalian species in 6 clades, and in light of the phylogenetic relationships among them, here we propose that ancestral mammal brains were composed and scaled in their cellular composition like modern afrotherian and glire brains: with an addition of neurons that is accompanied by a decrease in neuronal density and very little modification in glial cell density, implying a significant increase in average neuronal cell size in larger brains, and the allocation of approximately 2 neurons in the cerebral cortex and 8 neurons in the cerebellum for every neuron allocated to the rest of brain. We also propose that in some clades the scaling of di...
The pyramidal cell phenotype varies quite dramatically in structure among different cortical area... more The pyramidal cell phenotype varies quite dramatically in structure among different cortical areas in the primate brain. Comparative studies in visual cortex, in particular, but also in sensorimotor and prefrontal cortex, reveal systematic trends for pyramidal cell specialization in functionally related cortical areas. Moreover, there are systematic differences in the extent of these trends between different primate species. Recently we demonstrated differences in pyramidal cell structure in the cingulate cortex of the macaque monkey; however, in the absence of other comparative data it remains unknown as to whether the neuronal phenotype differs in cingulate cortex between species. Here we extend the basis for comparison by studying the structure of the basal dendritic trees of layer III pyramidal cells in the posterior and anterior cingulate gyrus of the vervet monkey (Brodmann's areas 23 and 24, respectively). Cells were injected with Lucifer Yellow in flat-mounted cortical s...
We describe representations of the visual field in areas 18, 19 and 21 of the ferret using standa... more We describe representations of the visual field in areas 18, 19 and 21 of the ferret using standard microelectrode mapping techniques. In all areas the azimuths are represented as islands of peripheral visual field surrounded by central visual field representation. The zero meridian was found at the 17/18 and 19/21 borders; at the 18/19 and anterior border of 21 the relative periphery of the visual field was found. In areas 18 and 19, elevations are represented in a smooth medio-lateral progression from lower to upper visual field. In several cases the elevations in area 21 evidenced a similar medio-lateral progression; however, in others the elevations exhibited a split representation of the horizontal meridian. Anatomically determined callosal connections coincided with the representation of azimuths near the zero meridian. Medio-lateral bands of callosal connectivity that straddle the 17/18 and 19/21 borders are connected by bridges of callosally projecting cells. Acallosal cortical islands corresponded to the peripheral visual field and were found straddling the 18/19 border and the anterior border of area 21. The results are discussed in relation to callosal connectivity and retinotopy in extrastriate visual cortex and to proposed homologies of carnivore and primate visual cortex.
The olfactory bulb is an evolutionarily old structure that antedates the appearance of a six-laye... more The olfactory bulb is an evolutionarily old structure that antedates the appearance of a six-layered mammalian cerebral cortex. As such, the neuronal scaling rules that apply to scaling the mass of the olfactory bulb as a function of its number of neurons might be shared across mammalian groups, as we have found to be the case for the ensemble of non-cortical, non-cerebellar brain structures. Alternatively, the neuronal scaling rules that apply to the olfactory bulb might be distinct in those mammals that rely heavily on olfaction. The group previously referred to as Insectivora includes small mammals, some of which are now placed in Afrotheria, a base group in mammalian radiation, and others in Eulipotyphla, a group derived later, at the base of Laurasiatheria. Here we show that the neuronal scaling rules that apply to building the olfactory bulb differ across eulipotyphlans and other mammals such that eulipotyphlans have more neurons concentrated in an olfactory bulb of similar si...
This study forms part of an ongoing investigation of pyramidal cell structure in the cingulate co... more This study forms part of an ongoing investigation of pyramidal cell structure in the cingulate cortex of primates. Recently we have demonstrated that layer III pyramidal cells in the anterior cingulate gyrus are considerably larger, more branched and more spinous than those in the posterior cingulate gyrus (areas 24 and 23, respectively) in the macaque and vervet monkeys. Moreover, the
Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan brains has... more Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in evolution in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of afrotherians, believed to be the first clade to radiate from the common eutherian ancestor. We find that afrotherians share non-neuronal scaling rules with rodents, primates and eulipotyphlans, as well as the coordinated scaling of numbers of neurons in the cerebral cortex and cerebellum. Afrotherians share with rodents and eulipotyphlans, but not with primates, the scaling of number of neurons in the cortex and in the cerebellum as a function of the number of neurons in the rest of the brain. Afrotheria also share with rodents and eulipotyphlans the neu...
The present study describes the distribution and cellular morphology of catecholaminergic neurons... more The present study describes the distribution and cellular morphology of catecholaminergic neurons in the diencephalon and midbrain of the bottlenose dolphin (Tursiops truncatus). Tyrosine hydroxylase immunohistochemistry was used to visualize these putatively dopaminergic neurons. The standard A1-A17, C1-C3, nomenclature is used for expediency; however, the neuroanatomical names of the various nuclei have also been given. Dolphins exhibit certain tyrosine hydroxylase
Quantitative analysis of the cellular composition of rodent, primate, insectivore, and afrotheria... more Quantitative analysis of the cellular composition of rodent, primate, insectivore, and afrotherian brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of artiodactyls, a group within the order Cetartiodactyla, believed to be a relatively recent radiation from the common Eutherian ancestor. We find that artiodactyls share non-neuronal scaling rules with all groups analyzed previously. Artiodactyls share with afrotherians and rodents, but not with primates, the neuronal scaling rules that apply to the cerebral cortex and cerebellum. The neuronal scaling rules that apply to the remaining brain areas are, however, distinct in artiodactyls. Importantly, we show that the folding index of the cerebral cortex scal...
What explains the superior cognitive abilities of the human brain compared to other, larger brain... more What explains the superior cognitive abilities of the human brain compared to other, larger brains? Here we investigate the possibility that the human brain has a larger number of neurons than even larger brains by determining the cellular composition of the brain of the African elephant. We find that the African elephant brain, which is about three times larger than the human brain, contains 257 billion (10(9)) neurons, three times more than the average human brain; however, 97.5% of the neurons in the elephant brain (251 billion) are found in the cerebellum. This makes the elephant an outlier in regard to the number of cerebellar neurons compared to other mammals, which might be related to sensorimotor specializations. In contrast, the elephant cerebral cortex, which has twice the mass of the human cerebral cortex, holds only 5.6 billion neurons, about one third of the number of neurons found in the human cerebral cortex. This finding supports the hypothesis that the larger absolute number of neurons in the human cerebral cortex (but not in the whole brain) is correlated with the superior cognitive abilities of humans compared to elephants and other large-brained mammals.
Observations on eight bottlenose dolphins located in the Southern Hemisphere during rest indicate... more Observations on eight bottlenose dolphins located in the Southern Hemisphere during rest indicated that they spent the majority of the time (85%) engaged in behaviors that can be considered clockwise. This is in contrast with many observations of sleeping/resting dolphins in the Northern Hemisphere that spend the majority of their time involved in counterclockwise activity. This observation leads to the possibility that the reason for preferential swimming biases in dolphins is the result of global forces rather than the result of the anatomy of the individual dolphins. Our observations also indicate that dolphins change overt behavior every 40 s, coincident with the respiration rate. The possibility is suggested that the salience of neural activity controlling respiration in the reticular system may effect/disrupt reticular attentional mechanisms, thus leading to the changes in overt behavior. It is hypothesized that this 40-s period may represent the possible attention span of the sleeping bottlenose dolphin.
Philosophical Transactions of the Royal Society B: Biological Sciences, 2008
Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytoche... more Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytochemically for calcium-binding proteins, gamma aminobutyric acid (GABA) or other antigens. Cyto- and myeloarchitecture revealed thalami that are fundamentally mammalian in organization, with the three principal divisions of the thalamus (epithalamus, dorsal thalamus and ventral thalamus) identifiable as in marsupials and eutherian mammals. The dorsal thalamus exhibits more nuclear parcellation than hitherto described, but lack of an internal medullary lamina, caused by splaying out of afferent fibre tracts that contribute to it in other mammals, makes identification of anterior, medial and intralaminar nuclear groups difficult. Differentiation of the ventral nuclei is evident with the ventral posterior nucleus of the platypus enormously expanded into the interior of the cerebral hemisphere, where it adopts a relationship to the striatum not seen in other mammals. Other nuclei such as the lateral dorsal become identifiable by expression of patterns of calcium-binding proteins identical to those found in other mammals. GABA cells are present in the ventral and dorsal thalamic nuclei, and in the ventral thalamus form a remarkable continuum with GABA cells of the two segments of the globus pallidus and pars reticulata of the substantia nigra.
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