What can monotremes tell us about brain evolution?

Evolution of the nervous system by acquisition of retrovirus-derived genes in mammals

In the course of evolution, the most highly developed organ is likely the brain, which has become more complex over time and acquired diverse forms and functions in different species. In particular, mammals have developed complex and high-functioning brains, and it has been reported that several genes derived from retroviruses were involved in mammalian brain evolution, that is, generating the complexity of the nervous system. Especially, the sushi-ichi-related retrotransposon homolog (SIRH)/retrotransposon gag-like (RTL) genes have been suggested to play a role in the evolutionary processes shaping brain morphology and function in mammals. Genetic mutation and altered expression of genes are linked to neurological disorders, highlighting how the acquisition of virus-derived genes in mammals has both driven brain evolution and imposed a susceptibility to diseases. This review provides an overview of the functions, diversity, evolution and diseases associated with SIRH/RTL genes in the nervous system. The contribution of retroviruses to brain evolution is an important research topic in evolutionary biology and neuroscience, and further insights are expected to be gained through future studies.

Evolution of behavioural control from chordates to primates

This article outlines a hypothetical sequence of evolutionary innovations, along the lineage that produced humans, which extended behavioural control from simple feedback loops to sophisticated control of diverse species-typical actions. I begin with basic feedback mechanisms of ancient mobile animals and follow the major niche transitions from aquatic to terrestrial life, the retreat into nocturnality in early mammals, the transition to arboreal life and the return to diurnality. Along the way, I propose a sequence of elaboration and diversification of the behavioural repertoire and associated neuroanatomical substrates. This includes midbrain control of approach versus escape actions, telencephalic control of local versus long-range foraging, detection of affordances by the dorsal pallium, diversified control of nocturnal foraging in the mammalian neocortex and expansion of primate frontal, temporal and parietal cortex to support a wide variety of primate-specific behavioural strategies. The result is a proposed functional architecture consisting of parallel control systems, each dedicated to specifying the affordances for guiding particular species-typical actions, which compete against each other through a hierarchy of selection mechanisms.

This article is part of the theme issue ‘Systems neuroscience through the lens of evolutionary theory’.

Quantitative analysis of forebrain pallial morphology in monotremes and comparison with that in therians

We have made quantitative volumetric analyses of cerebral cortical (pallial) structures in the brains of three species of monotreme (Ornithorhynchus anatinus, Tachyglossus aculeatus, Zaglossus bruijni) and compared the findings with similar measurements in a range of therian mammals (6 marsupials and 50 placentals). We have found that although the iso- and periallocortical grey matter volume of the monotremes is about what would be expected for their brain size, the proportion of iso- and periallocortical white matter in monotremes is substantially lower than that in the forebrains of therians. This suggests that the forebrains of the three monotremes have fewer association, commissural and/or projection connections than those of similarly sized forebrains of therian mammals. We also found that the iso- and periallocortex of the platypus is relatively smooth-surfaced compared to similarly sized brains of therian mammals, with a distinct caudal shift in the positioning of cortical white matter in the forebrain, consistent with expansion of the posterior thalamic radiation. Central laminated olfactory structures (anterior olfactory nucleus and piriform cortex) are large in the tachyglossid monotremes (Tachyglossus aculeatus and Zaglossus bruijni) and large in xenarthran placental mammals, suggesting convergence of the forebrain structure of monotreme formivores with that of similarly specialized therians like the xenarthrans Myrmecophaga tridactyla and Dasypus novemcinctus.

The platypus: evolutionary history, biology, and an uncertain future

The platypus (Ornithorhynchus anatinus) is one of the world's most evolutionarily distinct mammals, one of five extant species of egg-laying mammals, and the only living species within the family Ornithorhynchidae. Modern platypuses are endemic to eastern mainland Australia, Tasmania, and adjacent King Island, with a small introduced population on Kangaroo Island, South Australia, and are widely distributed in permanent river systems from tropical to alpine environments. Accumulating knowledge and technological advancements have provided insights into many aspects of its evolutionary history and biology but have also raised concern about significant knowledge gaps surrounding distribution, population sizes, and trends. The platypus' distribution coincides with many of Australia's major threatening processes, including highly regulated and disrupted rivers, intensive habitat destruction, and fragmentation, and they were extensively hunted for their fur until the early 20th century. Emerging evidence of local population declines and extinctions identifies that ecological thresholds have been crossed in some populations and, if threats are not addressed, the species will continue to decline. In 2016, the IUCN Red Listing for the platypus was elevated to "Near Threatened," but the platypus remains unlisted on threatened species schedules of any Australian state, apart from South Australia, or nationally. In this synthesis, we review the evolutionary history, genetics, biology, and ecology of this extraordinary mammal and highlight prevailing threats. We also outline future research directions and challenges that need to be met to help conserve the species.

The Organization and Connections of Second Somatosensory Cortex in the Agouti

In order to understand how the mammalian sensory cortex has been structured during evolution, it is necessary to compare data from different species across distinct mammalian lineages. Here, we investigated the organization of the secondary somatosensory area (S2) in the agouti (Dasyprocta aguti), a medium-sized Amazonian rodent, using microelectrode mapping techniques and neurotracer injections. The topographic map obtained from multiunit electrophysiological recordings were correlated with both cytochrome oxidase (CO) histochemistry and with patterns of corticocortical connections in tangential sections. The electrophysiological mapping of the lateral strip of parietal cortex adjacent to the primary somatosensory area (S1) revealed that S2 displays a mirror-reversed topographical representation of S1, but with a smaller cortical magnification factor. The caudal border of S2 is surrounded by sensory fields which also respond to auditory stimulation. BDA injections into the forelimb representation of S2 revealed a dense homotopic ipsilateral projection to S1, supplemented by a less dense projection to the caudolateral cortex located near the rhinal sulcus (parietal rhinal area) and to a frontal region probably associated with the motor cortex. Our findings were similar to those described in other mammalian species, reinforcing the existence of a common plan of organization for S2 in the mammalian parietal cortex.

Territorial Behavior and Social Stability in the Mouse Require Correct Expression of Imprinted Cdkn1c

Genomic imprinting, the epigenetic process by which transcription occurs from a single parental allele, is believed to influence social behaviors in mammals. An important social behavior is group living, which is enriched in Eutherian mammals relative to monotremes and marsupials. Group living facilitates resource acquisition, defense of territory and co-care of young, but requires a stable social group with complex inter-individual relationships. Co-occurring with increased group living in Eutherians is an increase in the number of imprinted loci, including that spanning the maternally expressed Cdkn1c. Using a 'loss-of-imprinting' model of Cdkn1c (Cdkn1cBACx1), we demonstrated that twofold over expression of Cdkn1c results in abnormal social behaviors. Although, our previous work indicated that male Cdkn1cBACx1 mice were more dominant as measured by tube test encounters with unfamiliar wild-type (WT) males. Building upon this work, using more ecologically relevant assessments of social dominance, indicated that within their normal social group, Cdkn1cBACx1 mice did not occupy higher ranking positions. Nevertheless, we find that presence of Cdkn1cBACx1 animals within a group leads to instability of the normal social hierarchy, as indicated by greater variability in social rank within the group over time and an increase in territorial behavior in WT cage-mates. Consequently, these abnormal behaviors led to an increased incidence of fighting and wounding within the group. Taken together these data indicate that normal expression of Cdkn1c is required for maintaining stability of the social group and suggests that the acquisition of monoallelic expression of Cdkn1c may have enhanced social behavior in Eutherian mammals to facilitate group living.

Orientation selectivity in the visual cortex of the nine-banded armadillo

Orientation selectivity in primary visual cortex (V1) has been proposed to reflect a canonical computation performed by the neocortical circuitry. Although orientation selectivity has been reported in all mammals examined to date, the degree of selectivity and the functional organization of selectivity vary across mammalian clades. The differences in degree of orientation selectivity are large, from reports in marsupials that only a small subset of neurons are selective to studies in carnivores, in which it is rare to find a neuron lacking selectivity. Furthermore, the functional organization in cortex varies in that the primate and carnivore V1 is characterized by an organization in which nearby neurons share orientation preference while other mammals such as rodents and lagomorphs either lack or have only extremely weak clustering. To gain insight into the evolutionary emergence of orientation selectivity, we examined the nine-banded armadillo, a species within the early placental clade Xenarthra. Here we use a combination of neuroimaging, histological, and electrophysiological methods to identify the retinofugal pathways, locate V1, and for the first time examine the functional properties of V1 neurons in the armadillo (Dasypus novemcinctus) V1. Individual neurons were strongly sensitive to the orientation and often the direction of drifting gratings. We uncovered a wide range of orientation preferences but found a bias for horizontal gratings. The presence of strong orientation selectivity in armadillos suggests that the circuitry responsible for this computation is common to all placental mammals.NEW & NOTEWORTHY The current study shows that armadillo primary visual cortex (V1) neurons share the signature properties of V1 neurons of primates, carnivorans, and rodents. Furthermore, these neurons exhibit a degree of selectivity for stimulus orientation and motion direction similar to that found in primate V1. Our findings in armadillo visual cortex suggest that the functional properties of V1 neurons emerged early in the mammalian lineage, near the time of the divergence of marsupials.

Internally coupled ears in living mammals

It is generally held that the right and left middle ears of mammals are acoustically isolated from each other, such that mammals must rely on neural computation to derive sound localisation cues. There are, however, some unusual species in which the middle ear cavities intercommunicate, in which case each ear might be able to act as a pressure-difference receiver. This could improve sound localisation at lower frequencies. The platypus Ornithorhynchus is apparently unique among mammals in that its tympanic cavities are widely open to the pharynx, a morphology resembling that of some non-mammalian tetrapods. The right and left middle ear cavities of certain talpid and golden moles are connected through air passages within the basicranium; one experimental study on Talpa has shown that the middle ears are indeed acoustically coupled by these means. Having a basisphenoid component to the middle ear cavity walls could be an important prerequisite for the development of this form of interaural communication. Little is known about the hearing abilities of platypus, talpid and golden moles, but their audition may well be limited to relatively low frequencies. If so, these mammals could, in principle, benefit from the sound localisation cues available to them through internally coupled ears. Whether or not they actually do remains to be established experimentally.

A connection to the past: Monodelphis domestica provides insight into the organization and connectivity of the brains of early mammals

The current experiment is one of a series of comparative studies in our laboratory designed to determine the network of somatosensory areas that are present in the neocortex of the mammalian common ancestor. Such knowledge is critical for appreciating the basic functional circuitry that all mammals possess and how this circuitry was modified to generate species-specific, sensory-mediated behavior. Our animal model, the gray short-tailed opossum (Monodelphis domestica), is a marsupial that is proposed to represent this ancestral state more closely than most other marsupials and, to some extent, even monotremes. We injected neuroanatomical tracers into the primary somatosensory area (S1), rostral and caudal somatosensory fields (SR and SC, respectively), and multimodal cortex (MM) and determined their connections with other architectonically defined cortical fields. Our results show that S1 has dense intrinsic connections, dense projections from the frontal myelinated area (FM), and moderate projections from S2 and SC. SR has strong projections from several areas, including S1, SR, FM, and piriform cortex. SC has dense projections from S1, moderate to strong projections from other somatosensory areas, FM, along with connectivity from the primary (V1) and second visual areas. Finally, MM had dense intrinsic connections, dense projections from SC and V1, and moderate projections from S1. These data support the proposition that ancestral mammals likely had at least four specifically interconnected somatosensory areas, along with at least one multimodal area. We discuss the possibility that these additional somatosensory areas (SC and SR) are homologous to somatosensory areas in eutherian mammals.

Reptiles: a new model for brain evo-devo research

Vertebrate brains exhibit vast amounts of anatomical diversity. In particular, the elaborate and complex nervous system of amniotes is correlated with the size of their behavioral repertoire. However, the evolutionary mechanisms underlying species-specific brain morphogenesis remain elusive. In this review we introduce reptiles as a new model organism for understanding brain evolution. These animal groups inherited ancestral traits of brain architectures. We will describe several unique aspects of the reptilian nervous system with a special focus on the telencephalon, and discuss the genetic mechanisms underlying reptile-specific brain morphology. The establishment of experimental evo-devo approaches to studying reptiles will help to shed light on the origin of the amniote brains.

Pallio-pallial tangential migrations and growth signaling: new scenario for cortical evolution?

Observations accruing in recent years imply that the areal patterning and size dimensioning of the mammalian neocortex are influenced by diverse sets of tangentially migrating glutamatergic neurons that invade the cortical plate and, in so doing, modify the properties of the neopallial proliferative compartments. This developmental scenario sheds new light upon the old issue of how the mammalian neocortex evolved its more complex structure from nonmammalian antecedent forms. In reviewing these novelties, I first point out the topological position of the neopallial island as a central component of the pallium in all gnathostomes, surrounded by a ring of prospective allocortical pallial regions and a more distant set of peripheral neighboring forebrain areas. Early patterning arises from the periphery via passive planar signaling. This process probably establishes the pallium field and its basic island plus allocortical ring organization, as well as a rough prepatterning of some regional subareas. Afterwards, patterning and modulated growth are also actively influenced by the convergence of separate streams of tangentially migrating subpial cells (partly peripheral and partly allocortical in origin) which collectively form the Cajal-Retzius neuronal population in layer I. Effects of these cells include the inside-out stratification of the cortical plate and they may also contribute to the evolutionary emergence of the 6-layered neocortical structure. The most recent addition to our knowledge of pallio-pallial migrations is the existence of a subsequent deep tangential migration of ventropallial cells into the neopallial primordium, whose signaling influence upon local progenitors magnifies the cortex population by 20%. These glutamatergic cells dispersedly invade the entire cortex but largely die postnatally. The crucial implications of these data for comparative thinking on mammalian neocortex evolution and interpretation of potential homologs in sauropsids are explored. Finally, a new conjecture regarding a possible role of the hitherto disregarded lateral pallium is advanced.

Thalamic nuclei in the opossum Monodelphis domestica

We investigated nuclear divisions of the thalamus in the gray short-tailed opossum (Monodelphis domestica) to gain detailed information for further developmental and comparative studies. Nissl and myelin staining, histochemistry for acetylcholinesterase and immunohistochemistry for calretinin and parvalbumin were performed on parallel series of sections. Many features of the Monodelphis opossum thalamus resemble those in Didelphis and small eutherians showing no particular sensory specializations, particularly in small murid rodents. However, several features of thalamic organization in Monodelphis were distinct from those in rodents. In the opossum the anterior and midline nuclear groups are more clearly separated from adjacent structures than in eutherians. The dorsal lateral geniculate nucleus (LGNd) starts more rostrally and occupies a large part of the lateral wall of the thalamus. As in other marsupials, two cytoarchitectonically different parts, alpha and beta are discernible in the LGNd of the opossum. Each of them may be subdivided into two additional bands in acetylcholinesterase staining, while in murid rodents the LGNd consists of a homogeneous mass of cells. Therefore, differentiation of the LGNd of the Monodelphis opossum is more advanced than in murid rodents. The medial geniculate body consists of three nuclei (medial, dorsal and ventral) that are cytoarchitectonically distinct and stain differentially for parvalbumin. The relatively large size of the MG and LGNd points to specialization of the visual and auditory systems in the Monodelphis opossum. In contrast to rodents, the lateral dorsal and lateral posterior nuclei in the opossum are poorly differentiated cytoarchitectonically.

The evolution of the complex sensory and motor systems of the human brain

Inferences about how the complex sensory and motor systems of the human brain evolved are based on the results of comparative studies of brain organization across a range of mammalian species, and evidence from the endocasts of fossil skulls of key extinct species. The endocasts of the skulls of early mammals indicate that they had small brains with little neocortex. Evidence from comparative studies of cortical organization from small-brained mammals of the six major branches of mammalian evolution supports the conclusion that the small neocortex of early mammals was divided into roughly 20-25 cortical areas, including primary and secondary sensory fields. In early primates, vision was the dominant sense, and cortical areas associated with vision in temporal and occipital cortex underwent a significant expansion. Comparative studies indicate that early primates had 10 or more visual areas, and somatosensory areas with expanded representations of the forepaw. Posterior parietal cortex was also expanded, with a caudal half dominated by visual inputs, and a rostral half dominated by somatosensory inputs with outputs to an array of seven or more motor and visuomotor areas of the frontal lobe. Somatosensory areas and posterior parietal cortex became further differentiated in early anthropoid primates. As larger brains evolved in early apes and in our hominin ancestors, the number of cortical areas increased to reach an estimated 200 or so in present day humans, and hemispheric specializations emerged. The large human brain grew primarily by increasing neuron number rather than increasing average neuron size.

The functional and anatomical organization of marsupial neocortex: evidence for parallel evolution across mammals

Marsupials are a diverse group of mammals that occupy a large range of habitats and have evolved a wide array of unique adaptations. Although they are as diverse as placental mammals, our understanding of marsupial brain organization is more limited. Like placental mammals, marsupials have striking similarities in neocortical organization, such as a constellation of cortical fields including S1, S2, V1, V2, and A1, that are functionally, architectonically, and connectionally distinct. In this review, we describe the general lifestyle and morphological characteristics of all marsupials and the organization of somatosensory, motor, visual, and auditory cortex. For each sensory system, we compare the functional organization and the corticocortical and thalamocortical connections of the neocortex across species. Differences between placental and marsupial species are discussed and the theories on neocortical evolution that have been derived from studying marsupials, particularly the idea of a sensorimotor amalgam, are evaluated. Overall, marsupials inhabit a variety of niches and assume many different lifestyles. For example, marsupials occupy terrestrial, arboreal, burrowing, and aquatic environments; some animals are highly social while others are solitary; different species are carnivorous, herbivorous, or omnivorous. For each of these adaptations, marsupials have evolved an array of morphological, behavioral, and cortical specializations that are strikingly similar to those observed in placental mammals occupying similar habitats, which indicate that there are constraints imposed on evolving nervous systems that result in recurrent solutions to similar environmental challenges.

Peripheral variability and central constancy in mammalian visual system evolution

Neural systems are necessarily the adaptive products of natural selection, but a neural system, dedicated to any particular function in a complex brain, may be composed of components that covary with functionally unrelated systems, owing to constraints beyond immediate functional requirements. Some studies support a modular or mosaic organization of the brain, whereas others emphasize coordination and covariation. To contrast these views, we have analysed the retina, striate cortex (V1) and extrastriate cortex (V2, V3, MT, etc.) in 30 mammals, examining the area of the neocortex and individual neocortical areas and the relative numbers of rods and cones. Controlling for brain size and species relatedness, the sizes of visual cortical areas (striate, extrastriate) within the brains of nocturnal and diurnal mammals are not statistically different from one another. The relative sizes of all cortical areas, visual, somatosensory and auditory, are best predicted by the total size of the neocortex. In the sensory periphery, the retina is clearly specialized for niche. New data on rod and cone numbers in various New World primates confirm that rod and cone complements of the retina vary substantially between nocturnal and diurnal species. Although peripheral specializations or receptor surfaces may be highly susceptible to niche-specific selection pressures, the areal divisions of the cerebral cortex are considerably more conservative.

CLARIFYING HOMOLOGIES IN THE MAMMALIAN CEREBRAL CORTEX: THE CASE OF THE THIRD VISUAL AREA (V3)

1. Experiments in mammalian models are the main source of information on the neural architecture underlying human visual perception, establishing scientific boundaries for the interpretation of experiments using non-invasive techniques in humans and for the realistic modelling of visual processes. Thus, it is important to define the homology between visual areas in different species. 2. To date, relatively few visual areas can be defined with certainty across mammalian Orders. Here, we review the evidence pointing to the fact that the third visual area (V3; or area 19) is a crucial node of a system involved in shape recognition that exists in most, if not all, eutherian mammals. 3. The size and shape of area V3 are variable, even between species that belong to the same Order. Although some features of the visuotopic organization of V3 are constant (including the relative location of the representations of the upper and lower quadrant and correspondence between the anterior border and the representation of the vertical meridian of the visual field), others are variable between species and even individuals. A complex pattern of representation, involving topological discontinuities, can exist. 4. In addition to its location in relation to the first (V1) and second (V2) visual areas, the identification of V3 homologues can be aided by certain other features, including low myelination, weak cytochrome oxidase reactivity, response properties that are indicative in the processing of stimulus shape, relationship to clusters of neurons forming interhemispheric connections and projections from the koniocellular (W-cell-like) components of the lateral geniculate nucleus. 5. Recent research in primates has clarified the organization of the V3 homologue in members of this Order. Regions of cortex that were formerly thought to belong to V3 (including a densely myelinated region near the dorsal midline) are better considered as part of a separate dorsomedial area, involved in motion analysis and visuomotor integration. The redefined V3, which includes the 'ventral posterior area' and parts of the dorsolateral complex proposed by earlier studies, is very similar to V3 (area 19) of other species in terms of structure and function.

The anatomy of the cerebral cortex of the echidna (Tachyglossus aculeatus)

The cerebral cortex of the echidna is notable for its extensive folding and the positioning of major functional areas towards its caudal extremity. The gyrification of the echidna cortex is comparable in magnitude to prosimians and cortical thickness and neuronal density are similar to that seen in rodents and carnivores. On the other hand, many pyramidal neurons in the cerebral cortex of the echidna are atypical with inverted somata and short or branching apical dendrites. All other broad classes of neurons noted in therian cortex are also present in the echidna, suggesting that the major classes of cortical neurons evolved prior to the divergence of proto- and eutherian lineages. Dendritic spine density on dendrites of echidna pyramidal neurons in somatosensory cortex and apical dendrites of motor cortex pyramidal neurons is lower than that found in eutheria. On the other hand, synaptic morphology, density and distribution in somatosensory cortex are similar to that in eutheria. In summary, although the echidna cerebral cortex displays some structural features, which may limit its functional capacities (e.g. lower spine density on pyramidal neurons), in most structural parameters (e.g. gyrification, cortical area and thickness, neuronal density and types, synaptic morphology and density), it is comparable to eutheria.

Orientation and spatiotemporal tuning of cells in the primary visual cortex of an Australian marsupial, the wallaby Macropus eugenii

The metatherians (marsupials) have been separated from eutherians (placentals) for approximately 135 million years. It might, therefore, be expected that significant independent evolution of the visual system has occurred. The present paper describes for the first time the orientation, direction and spatiotemporal tuning of neurons in the primary visual cortex of an Australian marsupial, the wallaby Macropus eugenii. The stimuli consisted of spatial sinusoidal gratings presented within apertures covering the classical receptive fields of the cells. The neurons can be classified as those with clear ON and OFF zones and those with less well-defined receptive field structures. Seventy-percent of the total cells encountered were strongly orientation selective (tuning functions at half height were less than 45 degrees ). The preferred orientations were evenly distributed throughout 360 degrees for cells with uniform receptive fields but biased towards the vertical and horizontal for cells with clear ON-OFF zones. Many neurons gave directional responses but only a small percentage of them (4%) showed motion opponent properties (i.e. they were excited by motion in one direction and actively inhibited by motion in the opposite direction). The median peak temporal tuning for cells with clear ON-OFF zones and those without were 3 Hz and 6 Hz, respectively. The most common peak spatial frequency tuning for the two groups were 2 cycles per degree and 0.5 cycles per degree, respectively. Spatiotemporal tuning was not always the same for preferred and antipreferred direction motion. In general, the physiology of the wallaby cortex was similar to well studied eutherian mammals suggesting either convergent evolution or a highly conserved architecture that stems from a common therian ancestor.

Massive cross-modal cortical plasticity and the emergence of a new cortical area in developmentally blind mammals

In the current investigation, the neurophysiological organization of the neocortex was examined in adult animals that were bilaterally enucleated very early in life, before the retino-geniculo-cortical pathway was established. Our results indicate that some aspects of development of cortical fields are not mediated by specific sensory inputs. However, the current study also demonstrates that peripheral innervation plays a large role in the organization of the neocortex, as cortical territories normally involved in visual processing are completely captured by the auditory and somatosensory system. Thus, a large degree of phenotypic variability in cortical organization can be accomplished solely by removing or modifying sensory inputs.

Hippocampal fields in the hedgehog tenrec. Their architecture and major intrinsic connections

The Madagascan lesser hedgehog tenrec was investigated to get insight into the areal evolution of the hippocampal formation in mammals with poorly differentiated brains. The hippocampal subdivisions were analyzed using cyto- and chemoarchitectural criteria; long associational and commissural connections were demonstrated with tracer techniques. The hedgehog tenrec shows a well differentiated dentate gyrus, CA3 and CA1. Their major intrinsic connections lie within the band of variations known from other species. The dentate hilar region shows calretinin-positive mossy cells with extensive projections to the molecular layer. The calbindin- and enkephalin-positive granule mossy fibers form a distinct endbulb and do not invade the CA1 as reported in the erinaceous hedgehog. Isolated granule cells with basal dendrites were also noted. A CA2 region is hard to identify architecturally; its presence is suggested due to its contralateral connections. Subicular and perisubicular regions are clearly present along the dorsal aspects of the hemisphere, but we failed to identify them unequivocally along the caudal and ventral tip of the hippocampus. A temporal portion of the subiculum, if present, differs in its chemoarchitecture from its dorsal counterpart. The perisubicular region, located medially adjacent to the dorsal subiculum may be equivalent to the rat's presubiculum; evidence for the presence of a parasubiculum was rather weak.

Multiple somatosensory areas in the anterior parietal cortex of the California ground squirrel (Spermophilus beecheyii)

Multiunit electrophysiological recording techniques were used to explore the somatosensory cortex of the California ground squirrel (Spermophilus beecheyii). Cortex rostral and caudal to the primary somatosensory area (SI) contained neurons that responded to stimulation of deep receptors and to muscle and joint manipulation. The region of cortex rostral to SI was termed the rostral field (R) because of possible homologies with a similar field described in other mammals. Cortex caudal to SI had neurons that responded to stimulation of deep receptors and has been termed the parietal medial area (PM), as in previous investigations in squirrels. Like SI, both R and PM contained a complete or almost complete representation of the body surface, although the receptive field size for clusters of neurons in these regions was somewhat larger than those for clusters of neurons in SI. Electrophysiological recording results were correlated with histologically processed tissue that had been sectioned tangentially. Although SI was clearly identified as a myelin-dense region, both R and PM stained much less densely for myelin. Our results indicate that as in a number of other mammals including monotremes, marsupials, carnivores, and primates, the anterior parietal cortex of the California ground squirrel contains multiple representations of the sensory epithelium. This work, as well as a growing body of studies of somatosensory cortex organization in a variety of mammals, indicates that anterior parietal fields other than SI existed early in mammalian evolution, and were present in the common ancestor of all mammals.

Formation of cortical fields on a reduced cortical sheet

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (; ). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

Formation of cortical fields on a reduced cortical sheet

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (; ). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

The evolution of visual cortex: where is V2?

A comparative analysis of the area of the cortex that is adjacent to the primary visual area (V1), indicates that the lateral extrastriate cortex of primitive mammals was likely to contain only a single visuotopically organized field, the second visual area (V2). Few, if any, other visual areas existed. The opposing hypothesis, that primitive mammals had a 'string' of small visual areas in the cortex lateral to V1 (as in some rodents), is not supported by studies of the organization of extrastriate cortex in other mammals, nor by the variability in this organization among extant rodents. A critical re-analysis of published evidence on the presence of multiple areas adjacent to V1 in some rodents has led to alternative interpretations of the organization of the areas in these regions.

Organization of visual cortex in the northern quoll, Dasyurus hallucatus: evidence for a homologue of the second visual area in marsupials

Two visual areas, V1 and V2 (first and second visual areas), appear to be present in the posterior neocortex of all eutherian mammals investigated so far. However, previous studies have not established whether an area homologous to V2 also exists in metatherian mammals (marsupials). Using electrophysiological techniques, we mapped the visual receptive fields of neurons in the striate and peristriate cortices of the northern quoll, an Australian marsupial. We found that neurons in a 2-mm-wide strip of cortex rostrolateral to V1 form a single, relatively simple representation of the complete contralateral hemifield. This area resembles V2 of eutherians in several respects: (i) neurons in the medial half of the peristriate area represent the lower visual quadrant, whereas those in the lateral half represent the upper visual quadrant; (ii) the vertical meridian of the visual field is represented adjacent to V1, while the visual field periphery is represented along the lateral and rostrolateral borders of the peristriate area; (iii) there is a marked anisotropy in the representation, with a larger magnification factor parallel to the V1 border than perpendicular to this border; and (iv) receptive fields of multiunit clusters in the peristriate cortex are much larger than those of cells in V1 at comparable eccentricities. The cortex immediately rostral and lateral to V2 did not respond to visual stimulation under our recording conditions. These results suggest that V1 and V2 together form a 'core' of homologous visual areas, likely to exist in all therian mammals.

Organization of somatosensory cortex in three species of marsupials, Dasyurus hallucatus, Dactylopsila trivirgata, and Monodelphis domestica: neural correlates of morphological specializations

The organization of somatosensory neocortex was investigated in three species of marsupials, the northern quoll (Dasyurus hallucatus), the striped possum (Dactylopsila trivirgata), and the short-tailed opossum (Monodelphis domestica). In these species, multiunit microelectrode mapping techniques were used to determine the detailed organization of the primary somatosensory area (SI). In the striped possum and quoll, the topography of somatosensory regions rostral (R), and caudal (C) to SI were described as well. Lateral to SI, two fields were identified in the striped possum, the second somatosensory area (SII) and the parietal ventral area (PV); in the quoll, there appeared to be only one additional lateral field which we term SII/PV. Visual and auditory cortices adjacent to somatosensory cortex were also explored, but the details of organization of these regions were not ascertained. In these animals, electrophysiological recording results were related to cortical myeloarchitecture and/or cytochrome oxidase staining. In one additional species, the fat-tailed dunnart (Sminthopsis crassicaudata), an architectonic analysis alone was carried out, and compared with the cortical architecture and electrophysiological recording results in the other three species. We discuss our results on the internal organization of SI in relation to the morphological specializations that each animal possesses. In addition, we discuss the differences in the organization of SI, and how evolutionary processes and developmental and adult neocortical plasticity may contribute to the observed variations in SI.