Somatosensory cortical representation in the Australian ghost bat, Macroderma gigas

The Combinatorial Creature: Cortical Phenotypes within and across Lifetimes

The neocortex is one of the most distinctive structures of the mammalian brain, yet also one of the most varied in terms of both size and organization. Multiple processes have contributed to this variability, including evolutionary mechanisms (i.e., alterations in gene sequence) that alter the size, organization, and connections of neocortex, and activity dependent mechanisms that can also modify these same features. Thus, changes to the neocortex can occur over different time-scales, including within a single generation. This combination of genetic and activity dependent mechanisms that create a given cortical phenotype allows the mammalian neocortex to rapidly and flexibly adjust to different body and environmental contexts, and in humans permits culture to impact brain construction.

Cortical plasticity within and across lifetimes: how can development inform us about phenotypic transformations?

The neocortex is the part of the mammalian brain that is involved in perception, cognition, and volitional motor control. It is a highly dynamic structure that is dramatically altered within the lifetime of an animal and in different lineages throughout the course of evolution. These alterations account for the remarkable variations in behavior that species exhibit. Of particular interest is how these cortical phenotypes change within the lifetime of the individual and eventually evolve in species over time. Because we cannot study the evolution of the neocortex directly we use comparative analysis to appreciate the types of changes that have been made to the neocortex and the similarities that exist across taxa. Developmental studies inform us about how these phenotypic transitions may arise by alterations in developmental cascades or changes in the physical environment in which the brain develops. Both genes and the sensory environment contribute to aspects of the phenotype and similar features, such as the size of a cortical field, can be altered in a variety of ways. Although both genes and the laws of physics place constraints on the evolution of the neocortex, mammals have evolved a number of mechanisms that allow them to loosen these constraints and often alter the course of their own evolution.

Cortical evolution in mammals: the bane and beauty of phenotypic variability

Evolution by natural selection, the unifying theory of all biological sciences, provides a basis for understanding how phenotypic variability is generated at all levels of organization from genes to behavior. However, it is important to distinguish what is the target of selection vs. what is transmitted across generations. Physical traits, behaviors, and the extended phenotype are all selected features of an individual, but genes that covary with different aspects of the targets of selection are inherited. Here we review the variability in cortical organization, morphology, and behavior that have been observed across species and describe similar types of variability within species. We examine sources of variability and the constraints that limit the types of changes that evolution has and can produce. Finally, we underscore the importance of how genes and genetic regulatory networks are deployed and interact within an individual, and their relationship to external, physical forces within the environment that shape the ultimate phenotype.

Organization of the primary somatosensory cortex and wing representation in the Big Brown Bat, Eptesicus fuscus

Bats are the only mammals capable of true powered flight. The bat wing exhibits specializations, allowing these animals to perform complicated flight maneuvers like landing upside-down, and hovering. The wing membrane contains various tactile receptors, including hair-associated Merkel receptors that might be involved in stabilizing bat flight. Here, we studied the neuronal representation of the wing membrane in the primary somatosensory cortex (S1) of the anesthetized Big Brown Bat, Eptesicus fuscus, using tactile stimulation with calibrated monofilaments (von Frey hairs) while recording from multi-neuron clusters. We also measured cortical response thresholds to tactile stimulation of the wings.The body surface is mapped topographically across the surface of S1, with the head, foot, and wing being overrepresented. The orientation of the wing representation is rotated compared to the hand representaion of terrestrial mammals, confirming results from other bat species. Although different wing membrane parts derive embryologically from different body parts, including the flank (plagiopatagium), the tactile sensitivity of the entire flight membrane (0.2-1.2 mN) is remarkably close or even higher (dactylopatagium) than the average tactile sensitivity of the human fingertip.

Distribution and morphology of cholinergic, catecholaminergic and serotonergic neurons in the brain of Schreiber's long-fingered bat, Miniopterus schreibersii

The current study describes the nuclear parcellation and neuronal morphology of the cholinergic, catecholaminergic and serotonergic systems within the brain of a representative species of microbat. While these systems have been investigated in detail in the laboratory rat, and examined in several other mammalian species, no chiropterans, to the author's knowledge, have been examined. Using immunohistochemical stains for choline-acetyltransferase, tyrosine hydroxylase and serotonin, we were able to observe and document these systems in relation to the cytoarchitecture. The majority of cholinergic nuclei typically found in mammals were evident in the microbat, however we could not find evidence for choline-acetyltransferase immunopositive neurons in the Edinger-Westphal nucleus, parabigeminal nucleus, and the medullary tegmental field, as seen in several other mammalian species. A typically mammalian appearance of the catecholaminergic nuclei was observed, however, the anterior hypothalamic groups (A15 dorsal and ventral), the dorsal and dorsal caudal subdivisions of the ventral tegmental area (A10d and A10dc), and the ventral (pars reticulata) substantia nigra (A9v) were not present. The serotonergic nuclei were similar to that reported in all eutherian mammalian species studied to date. The overall complement of nuclei of these systems in the microbat, while different to the species examined in other orders of mammals, resembles most closely the complement seen in earlier studies of insectivore species, and is clearly distinguished from that seen in rodents, carnivores and primates. This data is discussed in terms of the phylogenetic relationships of the chiropterans.

Representation of face and intra-oral structures in area 3b of macaque monkey somatosensory cortex

Details of the representation of body regions innervated by the trigeminal nerve were elucidated in monkey cerebral cortex. Microelectrode recording was used to generate somatosensory maps in the posterior bank of the central sulcus and on the exposed cortical surface lateral to the lateral tip of the central sulcus in Macaca nemestrina. The area innervated by the contralateral trigeminal nerve is represented in an 8-mm mediolateral extent of area 3b lateral to the representation of the hand. Lateral to this, still within area 3b, there is an expanded representation of ipsilateral intra-oral structures measuring 6 mm in mediolateral extent. Both representations fill area 3b anteroposteriorly. The ipsilateral representation forms approximately 40% of the trigeminal representation, consistent with the amount of the ventroposterior medial (VPM) thalamic nucleus devoted to representation of ipsilateral intra-oral structures. Comparison of the present results with maps of the face representation in other species of monkey shows a consistent somatotopy of the face between species; size variations are mainly related to the enlarged ipsi- and contralateral representations of the cheek pouches in macaques. The general somatotopy of the trigeminal representation in monkeys is consistent with that in other mammalian species.

The organization of neocortex in mammals: are species differences really so different?

By examining a variety of mammals, it is possible to determine common features of cortical organization, and from these infer homologies across species. Such analysis also enables differences in the organization of the neocortex to be identified. Species differ in the amount of cortex that is devoted to a particular sensory system, in the size and configuration of a cortical field, in the number of cortical fields, and in the pattern of connections of homologous fields. It is suggested that the plan of organization that is retained is the result of homologous developmental events, and that modifications to this plan are generated by a limited set of mechanisms. These types of changes to the common network might account for the sensory and behavioural diversity that is observed in extant mammals.

Representation of the body surface in the gracile, cuneate, and spinal trigeminal nuclei of the little red flying fox (Pteropus scapulatus)

The body surface representation in the gracile, cuneate, and spinal trigeminal nuclei of the little red flying fox (Pteropus scapulatus) was examined. As in other species, it was found that any single cross-section through all three nuclei contains a representation of most, or all, of the body surface. In the little red flying fox, however, this representation is arranged as a series of dorsolateral to ventromedially oriented bands, within which there are no apparent topographies. These bands are arranged in such a way that the spatial relationships between body regions in the representation do not reflect those at the periphery.

Retinotopic organization of the primary visual cortex of flying foxes (Pteropus poliocephalus and Pteropus scapulatus)

The representation of the visual field in the occipital cortex was studied by multiunit recordings in seven flying foxes (Pteropus spp.), anesthetized with thiopentone/N2O and immobilized with pancuronium bromide. On the basis of its visuotopic organization and architecture, the primary visual area (V1) was distinguished from neighboring areas. Area V1 occupies the dorsal surface of the occipital pole, as well as most of the tentorial surface of the cortex, the posterior third of the mesial surface of the brain, and the upper bank of the posterior portion of the splenial sulcus. In each hemisphere, it contains a precise, visuotopically organized representation of the entire extent of the contralateral visual hemifield. The representation of the vertical meridian, together with 8-15 degrees of ipsilateral hemifield, forms the anterior border of V1 with other visually responsive areas. The representation of the horizontal meridian runs anterolateral to posteromedial, dividing V1 so that the lower visual quadrant is represented medially, and the upper quadrant laterally. The total surface area of V1 is about 140 mm2 for P. poliocephalus, and 110 mm2 for P. scapulatus. The representation of the central visual field is greatly magnified relative to that of the periphery. The cortical magnification factor decreases with increasing eccentricity, following a negative power function. Conversely, receptive field sizes increase markedly with increasing eccentricity, and therefore the point-image size is approximately constant throughout V1. The emphasis in the representation of the area centralis in V1 is much larger than that expected on the basis of ganglion cell counts in flat-mounted retinas. Thus, a larger degree of convergence occurs at the peripheral representations in the retino-geniculo-cortical pathway, in comparison with the central representations. The marked emphasis in the representation of central vision, the wide extent of the binocular field of vision, and the relatively large surface area of V1 reflect the importance of vision in megachiropterans.

Five topographically organized fields in the somatosensory cortex of the flying fox: microelectrode maps, myeloarchitecture, and cortical modules

Five somatosensory fields were defined in the grey-headed flying fox by using microelectrode mapping procedures. These fields are: the primary somatosensory area, SI or area 3b; a field caudal to area 3b, area 1/2; the second somatosensory area, SII; the parietal ventral area, PV; and the ventral somatosensory area, VS. A large number of closely spaced electrode penetrations recording multiunit activity revealed that each of these fields had a complete somatotopic representation. Microelectrode maps of somatosensory fields were related to architecture in cortex that had been flattened, cut parallel to the cortical surface, and stained for myelin. Receptive field size and some neural properties of individual fields were directly compared. Area 3b was the largest field identified and its topography was similar to that described in many other mammals. Neurons in 3b were highly responsive to cutaneous stimulation of peripheral body parts and had relatively small receptive fields. The myeloarchitecture revealed patches of dense myelination surrounded by thin zones of lightly myelinated cortex. Microelectrode recordings showed that myelin-dense and sparse zones in 3b were related to neurons that responded consistently or habituated to repetitive stimulation respectively. In cortex caudal to 3b, and protruding into 3b, a complete representation of the body surface adjacent to much of the caudal boundary of 3b was defined. Neurons in this area habituated rapidly to repetitive stimulation. We termed this caudal field area 1/2 because it had properties of both area 1 and area 2 of primates. In cortex caudolateral to 3b and lateral to area 1/2 (cortex traditionally defined as SII) we describe three separate representations of the body surface coextensive with distinct myeloarchitectonic appearances. The second somatosensory area, SII, shared a congruent border with 3b at the representation of the nose. In SII, the overall orientation of the body representation was erect. The lips were represented rostrolaterally, the digits were represented laterally, and the toes were caudolateral to the digits. The trunk was represented caudally and the head was represented medially. A second complete representation, PV, had an inverted body representation with respect to SII and bordered SII at the representation of the distal limbs. The proximal body parts were represented rostrolaterally in PV. Finally, caudal to both SII and PV, an additional representation, VS, shared a congruent border with the distal hindlimb representation of both SII and PV. VS had a crude topography, and receptive fields of neurons in VS were relatively large. Many neurons in VS responded to both somatosensory and auditory stimulation.