Dendritic structure varies as a function of eccentricity in V1: a quantitative study of NADPH diaphorase neurons in the diurnal South American rodent agouti, Dasyprocta prymnolopha.

The cerebral cortex is often described as a composite of repeated units or columns, integrating the same basic circuit. The 'ice-cube' model of cortical organization, and 'canonical' circuit, born from insights into functional architecture, still require systematic comparative data. Here we probed the anatomy of an individual neuronal type within V1 to determine whether or not its dendritic trees are consistent with the 'ice-cube' model and theories of canonical circuits. In a previous report we studied the morphometric variability of NADPH-diaphorase (NADPH-d) neurons in the rat auditory, visual and somatosensory primary cortical areas. Our results suggested that the nitrergic cortical circuitry of primary sensory areas are differentially specialized, probably reflecting peculiarities of both habit and behavior of the species. In the present report we specifically quantified the dendritic trees of NADPH-d type I neurons as a function of eccentricity within V1. Individual neurons were reconstructed in 3D, and the size, branching and space-filling of their dendritic trees were correlated with their location within the visuotopic map. We found that NADPH-d neurons became progressively smaller and less branched with progression from the central visual representation to the intermediate and peripheral visual representation. This finding suggests that aspects of cortical circuitry may vary across the cortical mantle to a greater extent that envisaged as natural variation among columns in the 'ice-cube' model. The systematic variation in neuronal structure as a function of eccentricity warrants further investigation to probe the general applicability of columnar models of cortical organization and canonical circuits.

Density and morphology of dendritic spines in mouse neocortex.

Dendritic spines of pyramidal cells are the main postsynaptic targets of cortical excitatory synapses and as such, they are fundamental both in neuronal plasticity and for the integration of excitatory inputs to pyramidal neurons. There is significant variation in the number and density of dendritic spines among pyramidal cells located in different cortical areas and species, especially in primates. This variation is believed to contribute to functional differences reported among cortical areas. In this study, we analyzed the density of dendritic spines in the motor, somatosensory and visuo-temporal regions of the mouse cerebral cortex. Over 17,000 individual spines on the basal dendrites of layer III pyramidal neurons were drawn and their morphologies compared among these cortical regions. In contrast to previous observations in primates, there was no significant difference in the density of spines along the dendrites of neurons in the mouse. However, systematic differences in spine dimensions (spine head size and spine neck length) were detected, whereby the largest spines were found in the motor region, followed by those in the somatosensory region and those in visuo-temporal region.

Alterations in the phenotype of neocortical pyramidal cells in the Dyrk1A+/- mouse.

The gene encoding the dual-specificity tyrosine-regulated kinase DYRK1A maps to the chromosomal segment HSA21q22.2, which lies within the Down syndrome critical region. The reduction in brain size and behavioral defects observed in mice lacking one copy of the murine homologue Dyrk1A (Dyrk1A+/-) support the idea that this kinase may be involved in monosomy 21 associated mental retardation. However, the structural basis of these behavioral defects remains unclear. In the present work, we have analyzed the microstructure of cortical circuitry in the Dyrk1A+/- mouse and control littermates by intracellular injection of Lucifer Yellow in fixed cortical tissue. We found that labeled pyramidal cells were considerably smaller, less branched and less spinous in the cortex of Dyrk1A+/- mice than in control littermates. These results suggest that Dyrk1A influences the size and complexity of pyramidal cells, and thus their capability to integrate information.

Specialization in pyramidal cell structure in the sensory-motor cortex of the vervet monkey (Cercopethicus pygerythrus).

Recent studies have revealed systematic differences in the pyramidal cell structure between functionally related cortical areas of primates. Trends for a parallel in pyramidal cell structure and functional complexity have been reported in visual, somatosensory, motor, cingulate and prefrontal cortex in the macaque monkey cortex. These specializations in structure have been interpreted as being fundamental in determining cellular and systems function, endowing circuits in these different cortical areas with different computational power. In the present study we extend our initial finding of systematic specialization of pyramidal cell structure in sensory-motor cortex in the macaque monkey [Cereb Cortex 12 (2002) 1071] to the vervet monkey. More specifically, we investigated pyramidal cell structure in somatosensory and motor areas 1/2, 5, 7, 4 and 6. Neurones in fixed, flat-mounted, cortical slices were injected intracellularly with Lucifer Yellow and processed for a light-stable 3,3'-diaminobenzidine reaction product. The size of, number of branches in, and spine density of the basal dendritic arbors varied systematically such that there was a trend for increasing complexity in arbor structure with progression through 1/2, 5 and 7. In addition, cells in area 6 were larger, more branched, and more spinous than those in area 4.

Synaptology of the proximal segment of pyramidal cell basal dendrites.

Pyramidal neurons are covered with dendritic spines, the main postsynaptic targets of excitatory (asymmetrical) synapses. However, the proximal portion of both the apical and basal dendrites is devoid of spines, suggesting a lack of excitatory inputs to this region. In the present study we used electron microscopy to analyse the proximal region of the basal dendrites of supra- and infragranular pyramidal cells to determine if this is the case. The proximal region of 80 basal dendrites sampled from the rat hindlimb representation in the primary somatosensory cortex was studied by electron microscopy. A total of 317 synapses were found within this region of the dendrites, all of which were of the symmetrical type. These results suggest that glutamate receptors, although present in the cytoplasm, are not involved in synaptic junctions in the proximal portion of the dendrites. These data further support the idea that inhibitory terminals exclusively innervate the proximal region of basal dendrites.

Alterations of neocortical pyramidal cell phenotype in the Ts65Dn mouse model of Down syndrome: effects of environmental enrichment.

Mental retardation in individuals with Down syndrome (DS) is thought to result from anomalous development and function of the brain; however, the underlying neuropathological processes have yet to be determined. Early implementation of special care programs result in limited, and temporary, cognitive improvements in DS individuals. In the present study, we investigated the possible neural correlates of these limited improvements. More specifically, we studied cortical pyramidal cells in the frontal cortex of Ts65Dn mice, a partial trisomy of murine chromosome 16 (MMU16) model characterized by cognitive deficits, hyperactivity, behavioral disruption and reduced attention levels similar to those observed in DS, and their control littermates. Animals were raised either in a standard or in an enriched environment. Environmental enrichment had a marked effect on pyramidal cell structure in control animals. Pyramidal cells in environmentally enriched control animals were significantly more branched and more spinous than non-enriched controls. However, environmental enrichment had little effect on pyramidal cell structure in Ts65Dn mice. As each dendritic spine receives at least one excitatory input, differences in the number of spines found in the dendritic arbors of pyramidal cells in the two groups reflect differences in the number of excitatory inputs they receive and, consequently, complexity in cortical circuitry. The present results suggest that behavioral deficits demonstrated in the Ts65Dn model could be attributed to abnormal circuit development.

Pyramidal cell heterogeneity in the visual cortex of the nocturnal New World owl monkey (Aotus trivirgatus).

Recent studies have revealed marked variation in pyramidal cell structure in the visual cortex of macaque and marmoset monkeys. In particular, there is a systematic increase in the size of, and number of spines in, the arbours of pyramidal cells with progression through occipitotemporal (OT) visual areas. In the present study we extend the basis for comparison by investigating pyramidal cell structure in OT visual areas of the nocturnal owl monkey. As in the diurnal macaque and marmoset monkeys, pyramidal cells became progressively larger and more spinous with anterior progression through OT visual areas. These data suggest that: 1. the trend for more complex pyramidal cells with anterior progression through OT visual areas is a fundamental organizational principle in primate cortex; 2. areal specialization of the pyramidal cell phenotype provides an anatomical substrate for the reconstruction of the visual scene in OT areas; 3. evolutionary specialization of different aspects of visual processing may determine the extent of interareal variation in the pyramidal cell phenotype in different species; and 4. pyramidal cell structure is not necessarily related to brain size.

The human temporal cortex: characterization of neurons expressing nitric oxide synthase, neuropeptides and calcium-binding proteins, and their glutamate receptor subunit profiles.

Immunocytochemical techniques were used to examine the distribution of neurons immunoreactive (-ir) for nitric oxide synthase (nNOS), somatostatin (SOM), neuropeptide Y (NPY), parvalbumin (PV), calbindin (CB) and calretinin (CR), in the inferotemporal gyrus (Brodmann's area 21) of the human neocortex. Neurons that colocalized either nNOS or SOM with PV, CB or CR were also identified by double-labeling techniques. Furthermore, glutamate receptor subunit profiles (GluR1, GluR2/3, GluR2/4, GluR5/6/7 and NMDAR1) were also determined for these cells. The number and distribution of cells containing nNOS, SOM, NPY, PV, CB or CR differed for each antigen. In addition, distinct subpopulations of neurons displayed different degrees of colocalization of these antigens depending on which antigens were compared. Moreover, cells that contained nNOS, SOM, NPY, PV, CB or CR expressed different receptor subunit profiles. These results show that specific subpopulations of neurochemically identified nonpyramidal cells may be activated via different receptor subtypes. As these different subpopulations of cells project to specific regions of pyramidal cells, facilitation of subsets of these cells via different receptor subunits may activate different inhibitory circuits. Thus, various distinct, but overlapping, inhibitory circuits may act in concert in the modulation of normal cortical function, plasticity and disease.

The pyramidal cell in cognition: a comparative study in human and monkey.

Here we present evidence that the pyramidal cell phenotype varies markedly in the cortex of different anthropoid species. Regional and species differences in the size of, number of bifurcations in, and spine density of the basal dendritic arbors cannot be explained by brain size. Instead, pyramidal cell morphology appears to accord with the specialized cortical function these cells perform. Cells in the prefrontal cortex of humans are more branched and more spinous than those in the temporal and occipital lobes. Moreover, cells in the prefrontal cortex of humans are more branched and more spinous than those in the prefrontal cortex of macaque and marmoset monkeys. These results suggest that highly spinous, compartmentalized, pyramidal cells (and the circuits they form) are required to perform complex cortical functions such as comprehension, perception, and planning.

Interlaminar differences in the pyramidal cell phenotype in cortical areas 7 m and STP (the superior temporal polysensory area) of the macaque monkey.

Pyramidal neurones were injected with Lucifer Yellow in slices cut tangential to the surface of area 7 m and the superior temporal polysensory area (STP) of the macaque monkey. Comparison of the basal dendritic arbors of supra- and infragranular pyramidal neurones (n = 139) that were injected in the same putative modules in the different cortical areas revealed variation in their structure. Moreover, there were relative differences in dendritic morphology of supra- and infragranular pyramidal neurones in the two cortical areas. Sholl analyses revealed that layer III pyramidal neurones in area STP had considerably higher peak complexity (maximum number of dendritic intersections per Sholl circle) than those in layer V, whereas peak complexities were similar for supra- and infragranular pyramidal neurones in area 7 m. In both cortical areas, the basal dendritic trees of layer III pyramidal neurones were characterized by a higher spine density than those in layer V. Calculations of the total number of dendritic spines in the "average" basal dendritic arbor revealed that layer V pyramidal neurones in area 7 m had twice as many spines as cells in layer III (4535 and 2294, respectively). A similar calculation for neurones in area STP revealed that layer III pyramidal neurones had approximately the same number of spines as cells in layer V (3585 and 3850 spines, respectively). Relative differences in the branching patterns of, and the number of spines in, the basal dendritic arbors of supra- and infragranular pyramidal neurones in the different cortical areas may allow for integration of different numbers of inputs, and different degrees of dendritic processing. These results support the thesis that intra-areal circuitry differs in different cortical areas.

Pyramidal cells of the frontal lobe: all the more spinous to think with.

The basal dendritic arbors of pyramidal cells in prefrontal areas 10, 11, and 12 of the macaque monkey were revealed by intracellular injection in fixed, flat-mounted, cortical slices. The size, number of branches, and spine density of the basal dendrites were quantified and compared with those of pyramidal cells in the occipital, parietal, and temporal lobes. These analyses revealed that cells in the frontal lobe were significantly more spinous than those in the other lobes, having as many as 16 times more spines than cells in the primary visual area (V1), four times more those in area 7a, and 45% more than those in area TE. As each dendritic spine receives at least one excitatory input, the large number of spines reported for layer III cells in prefrontal cortex suggests that they are capable of integrating a greater number of excitatory inputs than layer III pyramidal cells so far studied in the occipital, parietal, and temporal lobes. The ability to integrate a large number of excitatory inputs may be important for the sustained tonic activity characteristic of neurons in prefrontal cortex and their role in memory and cognition.

Visual responses of neurons in the middle temporal area of new world monkeys after lesions of striate cortex.

In primates, lesions of striate cortex (V1) result in scotomas in which only rudimentary visual abilities remain. These aspects of vision that survive V1 lesions have been attributed to direct thalamic pathways to extrastriate areas, including the middle temporal area (MT). However, studies in New World monkeys and humans have questioned this interpretation, suggesting that remnants of V1 are responsible for both the activation of MT and residual vision. We studied the visual responses of neurons in area MT in New World marmoset monkeys in the weeks after lesions of V1. The extent of the scotoma in each case was estimated by mapping the receptive fields of cells located near the lesion border and by histological reconstruction. Two response types were observed among the cells located in the part of MT that corresponds, in visuotopic coordinates, to the lesioned part of V1. Many neurons (62%) had receptive fields that were displaced relative to their expected location, so that they represented the visual field immediately surrounding the scotoma. This may be a consequence of a process analogous to the reorganization of the V1 map after retinal lesions. However, another 20% of the cells had receptive fields centered inside the scotoma. Most of these neurons were strongly direction-selective, similar to normal MT cells. These results show that MT cells differ in their responses to lesioning of V1 and that only a subpopulation of MT neurons can be reasonably linked to residual vision and blindsight.

Pyramidal cells, patches, and cortical columns: a comparative study of infragranular neurons in TEO, TE, and the superior temporal polysensory area of the macaque monkey.

The basal dendritic arbors of layer III pyramidal neurons are known to vary systematically among primate visual areas. Generally, those in areas associated with "higher" level cortical processing have larger and more spinous dendritic arbors, which may be an important factor for determining function within these areas. Moreover, the tangential area of their arbors are proportional to those of the periodic supragranular patches of intrinsic connections in many different areas. The morphological parameters of both dendritic and axon arbors may be important for the sampling strategies of cells in different cortical areas. However, in visual cortex, intrinsic patches are a feature of supragranular cortex, and are weaker or nonexistent in infragranular cortex. Thus, the systematic variation in the dendritic arbors of pyramidal cells in supragranular cortex may reflect intrinsic axon projections, rather than differences in columnar organization. The present study was aimed at establishing whether cells in the infragranular layers also vary in terms of dendritic morphology among different cortical areas, and whether these variations mirror the ones demonstrated in supragranular cortex. Layer V pyramidal neurons were injected with Lucifer yellow in flat-mounted cortical slices taken from cytoarchitectonic areas TEO and TE and the superior polysensory area (STP) of the macaque monkey. The results demonstrate that cells in STP were larger, had more bifurcations, and were more spinous than those in TE, which in turn were larger, had more bifurcations and were more spinous than those in TEO. These results parallel morphological variation seen in layer III pyramidal neurons, suggesting that increasing complexity of basal dendritic arbors of cells, with progression through higher areas of the temporal lobe, is a general organizational principle. It is proposed that the differences in microcircuitry may contribute to the determination of the functional signatures of neurons in different cortical areas. Furthermore, these results provide evidence that intrinsic circuitry differs across cortical areas, which may be important for theories of columnar processing.

Cellular heterogeneity in cerebral cortex: a study of the morphology of pyramidal neurones in visual areas of the marmoset monkey.

The morphological characteristics of the basal dendritic fields of layer III pyramidal neurones were determined in visual areas in the occipital, parietal, and temporal lobes of adult marmoset monkeys by means of intracellular iontophoretic injection of Lucifer yellow. Neurones in the primary visual area (V1) had the least extensive and least complex (as determined by Sholl analysis) dendritic trees, followed by those in the second visual area (V2). There was a progressive increase in size and complexity of dendritic trees with rostral progression from V1 and V2, through the "ventral stream," including the dorsolateral area (DL) and the caudal and rostral subdivisions of inferotemporal cortex (ITc and ITr, respectively). Neurones in areas of the dorsal stream, including the dorsomedial (DM), dorsoanterior (DA), middle temporal (MT), and posterior parietal (PP) areas, were similar in size and complexity but were larger and more complex than those in V1 and V2. Neurones in V1 had the lowest spine density, whereas neurones in V2, DM, DA, and PP had similar spine densities. Neurones in MT and inferotemporal cortex had relatively high spine densities, with those in ITr having the highest spine density of all neurones studied. Calculations based on the size, number of branches, and spine densities revealed that layer III pyramidal neurones in ITr have 7.4 times more spines on their basal dendritic fields than those in V1. The differences in the extent of, and the number of spines in, the basal dendritic fields of layer III pyramidal neurones in the different visual areas suggest differences in the ability of neurones to integrate excitatory and inhibitory inputs. The differences in neuronal morphology between visual areas, and the consistency in these differences across New World and Old World monkey species, suggest that they reflect fundamental organisational principles in primate visual cortical structure.

Variation in the spatial relationship between parvalbumin immunoreactive interneurones and pyramidal neurones in rat somatosensory cortex.

The morphology of pyramidal neurones was revealed by intracellular injection of Lucifer Yellow (LY) in fixed tangential cortical slices taken from rat primary somatosensory cortex. Slices were processed with a combination of antibodies to allow visualization of both the LY-injected neurones and parvalbumin immunoreactive (PV-ir) cell bodies, by confocal microscopy. Basal dendritic fields of pyramidal neurones in layer V were larger and more complex than those of layer III. Furthermore, the number of PV-ir cell bodies contained within the basal dendritic territories of pyramidal neurones in layer V was significantly greater than in layer III (mean +/- s.e.m., 36.3 +/- 3.0 and 20.9 +/- 1.6, respectively). These findings have functional implications both in terms of physiological characteristics, and inhibitory modulation of receptive field properties, of cortical neurones.

Supragranular pyramidal neurones in the medial posterior parietal cortex of the macaque monkey: morphological heterogeneity in subdivisions of area 7.

Pyramidal neurones were injected with Lucifer Yellow in cortical slices taken from layer III of the medial subdivision of cytoarchitectonic area 7 (7m) of the macaque monkey. Cross-sectional area, branching complexity and spine density of the basal dendritic fields were determined and compared with those of neurones in other areas of the dorsal processing stream. Layer III pyramidal neurones in area 7m have an average basal dendritic field area of 109.57 +/- 13.03 x 10(3) microm2, which is significantly greater than that obtained for neurones in the lateral intraparietal area (LIP) and area 7a. Moreover, Sholl analyses revealed that neurones in area 7m are significantly more complex in their branching patterns than those in LIP and area 7a. These results reinforce the view that, behind the apparent architectural uniformity of Brodmann's area 7, there is a significant diversity of neuronal structure and function.

Distribution and patterns of connectivity of interneurons containing calbindin, calretinin, and parvalbumin in visual areas of the occipital and temporal lobes of the macaque monkey.

Immunocytochemical techniques were used to examine the distribution of double-bouquet cells and chandelier cells that were immunoreactive (-ir) for the calcium-binding proteins calbindin (CB), calretinin (CR), and parvalbumin (PV) in the primary visual area (V1), the second visual area (V2), and cytoarchitectonic area TE in the macaque monkey. Furthermore, the connections between CB-, CR-, and PV-ir neurons in these visual areas were investigated at the light microscope level by using a dual-immunocytochemical staining procedure. The most significant findings were three-fold. First, the number and distribution of CB-ir and CR-ir double-bouquet cells and PV-ir chandelier cells differed considerably between different visual areas. In particular, the different distribution of double-bouquet cells was illustrated dramatically at the V1/V2 border, where CB-ir double-bouquet axons were very few or lacking in V1 but were very numerous in V2. Furthermore, PV-ir chandelier cell terminals were relatively sparse in V1, more frequent in V2, and most frequent in area TE. Second, the percentage of CB-, CR-, and PV-ir neurons receiving multiple contacts on their somata and proximal dendrites from other calcium-binding protein neurons varied between 22% and 85%. The highest percentage of contacts found between immunolabelled cells and multiterminals were for the combinations CR/CB (76-85%; percent of cells immunoreactive for CB that were innervated by multiterminals immunoreactive for CR), followed by the combination PV/CR (42-48%), and then by the other combinations that had similar percentages (22-32% for CR/PV; 26-37% for CB/CR; 29-42% for CR/PV). Third, differences in the relative proportions of CB, CR, and PV terminals in contact with CB-, CR-, and PV-ir neurons were consistent between the different cortical areas studied. Thus, certain characteristics of intraareal circuits differ, whereas others remain similar, in different areas of the occipitotemporal visual pathway. The differences may represent regional specializations related to the different processing of visual stimuli, whereas the similarities may be attributed to general functional requisites for interneuronal circuitry.

Cortical integration in the visual system of the macaque monkey: large-scale morphological differences in the pyramidal neurons in the occipital, parietal and temporal lobes.

Layer III pyramidal neurons were injected with Lucifer yellow in tangential cortical slices taken from the inferior temporal cortex (area TE) and the superior temporal polysensory (STP) area of the macaque monkey. Basal dendritic field areas of layer III pyramidal neurons in area STP are significantly larger, and their dendritic arborizations more complex, than those of cells in area TE. Moreover, the dendritic fields of layer III pyramidal neurons in both STP and TE are many times larger and more complex than those in areas forming 'lower' stages in cortical visual processing, such as the first (V1), second (V2), fourth (V4) and middle temporal (MT) visual areas. By combining data on spine density with those of Sholl analyses, we were able to estimate the average number of spines in the basal dendritic field of layer III pyramidal neurons in each area. These calculations revealed a 13-fold difference in the number of spines in the basal dendritic field between areas STP and V1 in animals of similar age. The large differences in complexity of the same kind of neuron in different visual areas go against arguments for isopotentiality of different cortical regions and provide a basis that allows pyramidal neurons in temporal areas TE and STP to integrate more inputs than neurons in more caudal visual areas.

Morphology of pyramidal neurones in cytochrome oxidase modules of the S-I bill representation of the platypus.

The primary somatosensory cortex of the platypus (Ornithorhynchus anatinus) is characterized by a distinct array of functionally specific cytochrome oxidase (CO) modules, forming alternate CO-rich and CO-poor bands. In the current study, we undertook to establish whether the cellular morphology of layer V pyramidal neurones reflects this modular organization. To this end, we injected neurones with Lucifer Yellow in 250 microm thick, flat-mounted cortical slices and processed the tissue to reveal a light-stable reaction product. By aligning blood vessels in serial sections in which we injected individual neurones with sections processed for CO, we were able to establish the exact location of injected cells with respect to the pattern of CO bands. Pyramidal neurones in the CO-poor bands (which are responsive to both mechano- and electroreceptive stimuli) had basal dendritic fields that were larger than those in the CO-rich bands. The large basal dendritic fields of layer V pyramidal neurones in the CO-poor bands may allow for integration of a greater number of more diverse inputs, thus allowing for averaging of stimuli to improve the signal-to-noise ratio or enhance spatial discrimination of peripheral stimuli. In some instances, neurones located within approximately 100 microm of the boundaries of the CO bands had dendritic fields that appeared to conform to the CO bands, the dendrites preferentially arborizing within a single band and avoiding the neighbouring band. However, the bias was not absolute, as we observed many examples of cells with dendrites that crossed the boundary between bands. Furthermore, many cells had dendrites that showed distinct dendritic bias that bore no obvious relationship to the CO boundaries.

The organization and connections of somatosensory cortex in the brush-tailed possum (Trichosurus vulpecula): evidence for multiple, topographically organized and interconnected representations in an Australian marsupial.

Microelectrode mapping techniques were used to determine the organization of somatosensory cortex in the Australian brush-tailed possum (Trichosurus vulpecula). The results of electrophysiological mapping were combined with data on the cyto- and myeloarchitecture, and patterns of corticocortical connections, using sections cut tangential to the pial surface. We found evidence for three topographically organized representations of the body surface that were coextensive with architectonic subdivisions. A large, discontinuous cutaneous representation in anterior parietal cortex was termed the primary somatosensory area (SI). Lateral to SI we found evidence for two further areas, the second somatosensory area (SII) and the parietal ventral area (PV). While neurones in all of these areas were responsive to cutaneous stimulation, those of SI were non-habituating, whereas those in SII and PV often habituated to the stimuli. Moreover, neuronal receptive fields in SII and PV were, in general, larger than those in SI. Neurones in cortex adjacent to the rostral and caudal boundaries of SI, including cortex that interdigitated between the discontinuous SI head and body representations, required stimulation of deep receptors in the periphery to elicit responses. Within the region of cortex containing neurones responsive to stimulation of deep receptors, body parts were represented in a mediolateral progression. Injections of anatomical tracers placed in electrophysiologically identified locations in SI revealed ipsilateral connections with other parts of SI, as well as cortex rostral to, caudal to, and interdigitating between, SI. Injections in SI also resulted in labelling in PV, SII, motor cortex, posterior parietal cortex and perirhinal cortex. The patterns of contralateral projections reflected those of ipsilateral projections, although they were relatively less dense. The present findings support recent observations in other marsupials in which multiple representations of the body surface were described, and suggest that multiple interconnected sensory representations may be a common feature of cortical organization and function in marsupials.