Interoperable atlases of the human brain.

The last two decades have seen an unprecedented development of human brain mapping approaches at various spatial and temporal scales. Together, these have provided a large fundus of information on many different aspects of the human brain including micro- and macrostructural segregation, regional specialization of function, connectivity, and temporal dynamics. Atlases are central in order to integrate such diverse information in a topographically meaningful way. It is noteworthy, that the brain mapping field has been developed along several major lines such as structure vs. function, postmortem vs. in vivo, individual features of the brain vs. population-based aspects, or slow vs. fast dynamics. In order to understand human brain organization, however, it seems inevitable that these different lines are integrated and combined into a multimodal human brain model. To this aim, we held a workshop to determine the constraints of a multi-modal human brain model that are needed to enable (i) an integration of different spatial and temporal scales and data modalities into a common reference system, and (ii) efficient data exchange and analysis. As detailed in this report, to arrive at fully interoperable atlases of the human brain will still require much work at the frontiers of data acquisition, analysis, and representation. Among them, the latter may provide the most challenging task, in particular when it comes to representing features of vastly different scales of space, time and abstraction. The potential benefits of such endeavor, however, clearly outweigh the problems, as only such kind of multi-modal human brain atlas may provide a starting point from which the complex relationships between structure, function, and connectivity may be explored.

Towards a European health research and innovation cloud (HRIC).

The European Union (EU) initiative on the Digital Transformation of Health and Care (Digicare) aims to provide the conditions necessary for building a secure, flexible, and decentralized digital health infrastructure. Creating a European Health Research and Innovation Cloud (HRIC) within this environment should enable data sharing and analysis for health research across the EU, in compliance with data protection legislation while preserving the full trust of the participants. Such a HRIC should learn from and build on existing data infrastructures, integrate best practices, and focus on the concrete needs of the community in terms of technologies, governance, management, regulation, and ethics requirements. Here, we describe the vision and expected benefits of digital data sharing in health research activities and present a roadmap that fosters the opportunities while answering the challenges of implementing a HRIC. For this, we put forward five specific recommendations and action points to ensure that a European HRIC: i) is built on established standards and guidelines, providing cloud technologies through an open and decentralized infrastructure; ii) is developed and certified to the highest standards of interoperability and data security that can be trusted by all stakeholders; iii) is supported by a robust ethical and legal framework that is compliant with the EU General Data Protection Regulation (GDPR); iv) establishes a proper environment for the training of new generations of data and medical scientists; and v) stimulates research and innovation in transnational collaborations through public and private initiatives and partnerships funded by the EU through Horizon 2020 and Horizon Europe.

Three-dimensional topography of corticopontine projections from rat barrel cortex: correlations with corticostriatal organization.

Subcortical re-entrant projection systems connecting cerebral cortical areas with the basal ganglia and cerebellum are topographically specific and therefore considered to be parallel circuits or "closed loops." The precision of projections within these circuits, however, has not been characterized sufficiently to indicate whether cortical signals are integrated within or among presumed compartments. To address this issue, we studied the first link of the rat cortico-ponto-cerebellar pathway with anterograde axonal tracing from physiologically defined, individual whisker "barrels" of the primary somatosensory cortex (SI). The labeled axons in the pontine nuclei formed several, sharply delineated clusters. Dual tracer injections into different SI whisker barrels gave rise to partly overlapping, paired clusters, indicating somatotopic specificity. Three-dimensional reconstructions revealed that the clusters were components of concentrically organized lamellar subspaces. Whisker barrels in the same row projected to different pontine lamellae (side by side), the somatotopic representation of which followed an inside-out sequence. By contrast, whisker barrels from separate rows projected to clusters located within the same lamellar subspace (end to end). In the neostriatum, this lamellar topography was the opposite, with barrels in the same row contacting different parts of the same lamellar subspace (end to end). The degree of overlap among pontine clusters varied as a function of the proximity of the cortical injections. Furthermore, corticopontine overlap was higher among projections from barrels in the same row than among projections from different whisker barrel rows. This anisotropy was the same in the corticostriatal projection. These findings have important implications for understanding convergence and local integration in somatosensory-related subcortical circuits.

Three-dimensional chemoarchitecture of the basal forebrain: spatially specific association of cholinergic and calcium binding protein-containing neurons.

The basal forebrain refers to heterogeneous structures located close to the medial and ventral surfaces of the cerebral hemispheres. It contains diverse populations of neurons, including the cholinergic cortically projecting cells that show severe loss in Alzheimer's and related neurodegenerative diseases. The basal forebrain does not display any cytoarchitectural or other structural features that make it easy to demarcate functional boundaries, a problem that allowed different investigators to propose different organizational schemes. The present paper uses novel three-dimensional reconstructions and numerical analyses for studying the spatial organization of four major basal forebrain cell populations, the cholinergic, parvalbumin, calbindin and calretinin containing neurons in the rat. Our analyses suggest that the distribution of these four cell populations is not random but displays a general pattern of association. Within the cholinergic space (i.e. the volume occupied by the cortically projecting cholinergic cell bodies) the three other cell types form twisted bands along the longitudinal axis of a central dense core of cholinergic cells traversing the traditionally defined basal forebrain regions, (i.e. the medial septum, diagonal bands, the substantia innominata, pallidal regions and the bed nucleus of the stria terminalis). At a smaller scale, the different cell types within the cholinergic space occupy overlapping high-density cell clusters that are either chemically uniform or mixed. However, the cell composition of these high-density clusters is regionally specific. The proposed scheme of basal forebrain organization, using cell density or density relations as criteria, offers a new perspective on structure-function relationship, unconstrained by traditional region boundaries.

Database and tools for analysis of topographic organization and map transformations in major projection systems of the brain.

Integration of dispersed and complicated information collected from the brain is needed to build new knowledge. But integration may be hampered by rigid presentation formats, diversity of data formats among laboratories, and lack of access to lower level data. We have addressed some of the fundamental issues related to this challenge at the level of anatomical data, by producing a coordinate based digital atlas and database application for a major projection system in the rat brain: the cerebro-ponto-cerebellar system. This application, Functional Anatomy of the Cerebro-Cerebellar System in rat (FACCS), is available via the Rodent Brain WorkBench (http://www.rbwb.org). The data included are x,y,z-coordinate lists describing exact distributions of tissue elements (axonal terminal fields of axons, or cell bodies) that are labeled with axonal tracing techniques. All data are translated to a common local coordinate system to facilitate across animal comparison. A search capability allows queries based on, e.g. location of tracer injection sites, tracer category, size of the injection sites, and contributing author. A graphic search tool allows the user to move a volume cursor inside a coordinate system to detect particular injection sites having connections to a specific tissue volume at chosen density levels. Tools for visualization and analysis of selected data are included, as well as an option to download individual data sets for further analysis. With this application, data and metadata from different experiments are mapped into the same information structure and made available for re-use and re-analysis in novel combinations. The application is prepared for future handling of data from other projection systems as well as other data categories.

Statistical analysis of corticopontine neuron distribution in visual areas 17, 18, and 19 of the cat.

The spatial organization of visual corticopontine neurons was studied both at a "large scale" (in relation to cortical visual field maps) and at a "small scale" (in relation to cortical modular organization). Large injections of horse-radish peroxidase-wheat germ agglutinin were made in the pontine nuclei. In complete series of sections from parts of areas 17, 18, and 19, the position of each retrogradely labeled neuron was recorded with an x-y plotter connected to the microscope stage. Each cell was thus given a set of x, y, and z coordinates. After alignment of the sections, three-dimensional computer reconstructions of the distribution of the labeled cells were made. With program RPOP (developed by Blackstad and Bjaalie, '88), the reconstructions were studied with different rotations, scaling, etc. In addition, section-independent parts of reconstructions were isolated ("windows") and further analyzed. Curved parts were automatically unfolded for inspection of distribution patterns and determination of cell densities. The spatial distribution of the labeled cells was analyzed within small windows, where density gradients are negligible. We confirm and extend previous demonstrations of a large-scale aggregation of visual corticopontine cells due to density gradients by showing that densities of corticopontine neurons increase linearly as a function of distance from paracentral to lower visual field representations in area 17 (and partly in areas 18 and 19). We demonstrate that density gradients are steeper in area 17 than in area 18. For example, clear-cut differences between the areas in mediolateral density gradients are found. These findings are discussed in relation to the different visual field maps of the areas and the existence of a similar visual field representation in corticopontine projections from different visual areas. The type of small-scale distribution (randomness or non-randomness, aggregation into clusters, bands, etc.) was studied with statistical methods. Such analysis shows that the labeled cells within small zones are non-randomly distributed in all three areas. In most cases, the analysis indicates an aggregated spatial distribution. A possible relationship to the cortical map of direction selectivity is discussed. To our knowledge, this study is the first to combine the use of three-dimensional computer reconstructions of a population of labeled neurons, with subsequent statistical analysis of spatial point (cell distribution) patterns.

Distribution in area 17 of neurons projecting to the pontine nuclei: a quantitative study in the cat with retrograde transport of HRP-WGA.

The number and distribution of corticopontine neurons within area 17 of the cat were studied quantitatively with the use of retrograde transport of horseradish peroxidase-wheat germ agglutinin. Eight cats received stereotactic injections in the pontine nuclei; in three of these complete staining of the parts of the pontine nuclei receiving fibers from the visual cortex was achieved. Labeled cells were counted in frontal sections through the hemisphere, spaced at 0.5 mm. The borders of area 17 were determined cyto- and myeloarchitectonically and a flat map was produced for each animal. A map of the representation of the visual field in 10 degrees X 10 degrees blocks in the first visual area (Tusa et al., '78, '81) was transferred to our maps of area 17. The density and number of labeled corticopontine cells could then be determined within blocks of the cortex representing 10 degrees X 10 degrees of the visual field. The cell density (number of labeled cells per mm2 cortex) was found in general to be highest in parts of the cortex representing peripheral parts of the visual field. The cell density is low in cortex representing the central visual field, but the lowest density was found in the representation of a para-central region in the upper visual field. Furthermore, cortical regions representing the lower part of the visual field have a higher cell density than those representing the upper part; in four cases, 68-86% of all labeled cells were found in parts of area 17 representing the visual field below the horizontal meridian. Since there is an enlarged cortical representation of central vision, the much lower cell densities in "central" parts of area 17 than in "peripheral" parts may mean that all parts of the visual field are represented with equal numbers of corticopontine neurons ("linear" representation). This is not the case, however, since the number of labeled cells per 10 degrees X 10 degrees is considerably higher in the cortex representing the central 10 degrees and medial parts of the lower visual field than in the rest of area 17. Assuming that the corticopontine cells in the visual cortex transmit spatially relevant information, we conclude that there is an overrepresentation of central vision and the medial parts of the lower visual field in the corticopontine projection from area 17.

Topographical organization in the early postnatal corticopontine projection: a carbocyanine dye and 3-D computer reconstruction study in the rat.

We have explored basic rules guiding the early development of topographically organized projections, employing the rat corticopontine projection as a model system. Using anterograde in vivo tracing with 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), we studied the distribution of labelled fibers in the pontine nuclei in relation to cortical site of origin during the first postnatal week. Labelled corticopontine fibers enter the pontine nuclei in distinct, sharply defined zones. The putative terminal fibers typically occupy lamella-like subspaces. Related to changes in cortical site of origin, we describe mediolateral, internal to external, and caudorostral distribution gradients in the pontine nuclei. Fibers originating in the anterolateral cortex occupy an internal central core, while implantations at increasing distance from the anterolateral cortex produce 1) more externally located lamellae, and 2) a caudal to rostral shift in fiber location. Previous investigations have shown that pontocerebellar neurons migrate into the ventral pons in a temporal sequence (Altman and Bayer [1987] J. Comp. Neurol. 257:529). The earliest arriving neurons occupy the central core and later arriving neurons settle in more externally and rostrally located subspaces. We hypothesize that the earliest arriving corticopontine fibers grow into the then only available zone of pontocerebellar neurons (central core), attracted by a diffusible chemotropic cue. Later arriving fibers grow into correspondingly later and more externally and rostrally located contingents of pontocerebellar neurons. Thus, we propose that the topographical organization in the early postnatal corticopontine projection is determined by simple temporal and spatial gradients operative within source (cerebral cortex) and target region (pontine nuclei).

Monkey somatosensory cerebrocerebellar pathways: uneven densities of corticopontine neurons in different body representations of areas 3b, 1, and 2.

We have studied the anatomic organization of corticopontine neurons in the monkey cytoarchitectonic areas 3a, 3b, 1, and 2. The purpose was to provide information about the composition of somatosensory cortical influence on cerebellar operations. Large tracer injections were made in the pontine nuclei. Retrogradely labeled neurons were confined to cortical layer 5, with the largest cell bodies located in area 3a and the smallest in area 3b. The distribution of labeled cells was quantitatively recorded and displayed in three-dimensional reconstructions and in flat maps. We have: (1) compared the average densities of labeled cells among the cytoarchitectonic areas, and (2) outlined the distribution of labeled cells within the cortical map of the body surface representation. The average density of labeled cells was considerably higher in areas 3a, 1, and 2, compared to area 3b. This finding suggests that areas 3a, 1, and 2 are more engaged in cerebellar operations than area 3b. We found marked density gradients of labeled cells within areas 3b, 1, and 2, but not within area 3a. When the density maps from areas 3b, 1, and 2 were superimposed on previously published somatotopic maps, we found higher average densities of corticopontine neurons in regions representing the trunk and proximal limbs, than in regions representing the distal forelimb. Thus, the distal forelimb representation, which is known to be strongly emphasized in terms of cortical volume, appears not to be correspondingly emphasized in the corticopontine projection.

Laminar organization of frequency-defined local axons within and between the inferior colliculi of the guinea pig.

We present a comprehensive description of the local (intrinsic and commissural) connections in the central nucleus of the inferior colliculi (CNICs) in guinea pig. Focal injections of the anterograde tracer biocytin were made into physiologically identified loci of the CNIC and the spatial organisation of the labeled fibres was revealed with computer-assisted three-dimensional (3-D) reconstruction. The intrinsic fibres form a series of V-shaped laminar plexuses composed of fibres bearing both terminal and en passant boutons. Each laminar plexus has a central wing located in the CNIC that extends into the dorsal cortex and an external wing located in the external cortex. The edge where the two wings intersect delimits the lateral border of the central nucleus with the external cortex. The density of labeled terminals was consistently lower in the cortices than in the CNIC. The laminar plexus connects points of similar frequency within the CNIC. Seen in 3-D, the location, orientation, shape, and area of the laminar plexus vary as a function of best frequency. The commissural fibres ending in the contralateral IC to the injection also form a laminar plexus which is symmetrical to the ipsilateral plexus. Electrolytic lesions placed in the contralateral IC at sites with best frequencies corresponding to those of the injection coincided with the terminals of the commissural fibres in most instances. Possible patterns for the organisation of these connections (point-to-point and diverging) are discussed. Three systems of peripheral axons to the laminar plexus are described: parallel, oblique, and perpendicular to the central wing. The novel parallel system has terminals in both ICs that run parallel to the central wing. It might constitute the anatomical basis for across-frequency interactions. The oblique and perpendicular systems are fibres of passage projecting to the commissure and brachium of the IC, respectively.

Topographic organization of the dorsal nucleus of the lateral lemniscus in the cat.

The dorsal nucleus of the lateral lemniscus (DNLL) is an auditory structure of the brainstem. It plays an important role in binaural processing and sound localization and it provides the inferior colliculus with an inhibitory projection. The DNLL is a highly conserved auditory structure across mammals, but differences among species in its detailed organization have been reported. The main goal of this study was to analyze the topographic organization of the cat DNLL. Single, small iontophoretic injections of biotinylated dextran amine were made at different loci in the central nucleus of the inferior colliculus (CNIC). The distribution of the labeled structures in the ipsi- and contralateral DNLL was computer reconstructed in three dimensions. In individual sections, a band of labeling is seen in the DNLL on both sides. These two labeled bands occupy symmetric locations and are made of retrogradely labeled neurons with flattened dendritic arbors oriented parallel to each other. Moreover, the ipsilateral labeled band contains labeled terminal fibers parallel to the labeled dendrites. With three-dimensional reconstructions, it becomes evident that the labeled band seen in each individual DNLL section represents a slice through a rostrocaudally oriented lamina. The shape, size, orientation, and location of this lamina change as the injection site is shifted along the tonotopic axis of the CNIC. An injection in the low-frequency region of the CNIC, produces a lamina that resembles a flattened tube located in the dorsolateral corner of the DNLL. An injection in the high-frequency region of the CNIC, by contrast, results in a lamina that is an elongated sheet located at the ventromedial surface of the DNLL. The laminae of the DNLL might constitute the structural basis for its tonotopical organization. Previous studies (Merchan MA, et al. 1994. J Comp Neurol 342:259-278) in conjunction with our current results suggest that the laminar organization in the DNLL might be common among mammals.

Rat somatosensory cerebropontocerebellar pathways: spatial relationships of the somatotopic map of the primary somatosensory cortex are preserved in a three-dimensional clustered pontine map.

In the primary somatosensory cortex (SI), the body surface is mapped in a relatively continuous fashion, with adjacent body regions represented in adjacent cortical domains. In contrast, somatosensory maps found in regions of the cerebellar hemispheres, which are influenced by the SI through a monosynaptic link in the pontine nuclei, are discontinuous ("fractured") in organization. To elucidate this map transformation, the authors studied the organization of the first link in the SI-cerebellar pathway, the SI-pontine projection. After injecting anterograde axonal tracers into electrophysiologically defined parts of the SI, three-dimensional reconstruction and computer-graphic visualization techniques were used to analyze the spatial distribution of labeled fibers. Several target regions in the pontine nuclei were identified for each major body representation. The labeled axons formed sharply delineated clusters that were distributed in an inside-out, shell-like fashion. Upper lip and other perioral representations were located in a central core, whereas extremity and trunk representations were found more externally. The multiple clusters suggest that the pontine nuclei contain several representations of the SI map. Within each representation, the spatial relationships of the SI map are largely preserved. This corticopontine projection pattern is compatible with recently proposed principles for the establishment of subcortical topographic patterns during development. The largely preserved spatial relationships in the pontine somatotopic map also suggest that the transformation from an organized topography in SI to a fractured map in the cerebellum takes place primarily in the mossy fiber pontocerebellar projection.

The corticopontine projection from area 20 and surrounding areas in the cat: terminal fields and distribution of cells of origin as compared to other visual cortical areas.

Visual area 20 in the cat projects to the pontine nuclei and thereby gives input to the cerebellum. The termination of the fibres and the distribution of the cells of origin were studied with anterograde and retrograde transport of horseradish peroxidase-wheat germ agglutinin. Fibres from area 20 terminate ipsilaterally in the medial half of the rostral three-quarters of the pontine nuclei. The labelled fibres occur in multiple well-restricted patches usually some distance away from the peduncle. The retrogradely labelled cells in areas 20a, 20b, and the adjacent posterior suprasylvian visual area were quantitatively mapped. Areal borders were placed according to the maps of Tusa and Palmer (J. comp. Neurol. 193, 147-164) and Updyke (J. comp. Neurol. 246, 265-280). In terms of cortical densities (cells per mm2 of cortex) area 20a is among the visual areas with the highest densities of corticopontine neurons (data from other visual areas from the author's previous works). Densities are 30-60% lower in area 20b than in 20a. The posterior suprasylvian visual area was found to be among the visual areas with the lowest densities of corticopontine neurons. Due to the small size of the areas investigated, the total number of corticopontine cells within them is small compared to many other areas. Within areas 20a and 20b, cortical densities are higher in the representation of visual space below than above the horizontal meridian. Furthermore, cortical densities are generally somewhat higher in the region devoted to central vision compared to regions devoted to the visual periphery. Since the part of cortex in areas 20a and 20b devoted to central vision is weakly over-represented also in terms of cortical volume, it follows that the "visual field density" (cells per degree of visual field) of corticopontine cells is highest in the central visual field representation. The finding of an over-representation of central vision compared to peripheral vision in the corticopontine projection from areas 20a and 20b is particularly interesting in conjunction with previous findings in other visual areas. In the cortical representations of central vision, areas with high magnification factors (i.e. areas that greatly emphasize central vision in terms of cortical volume, such as areas 17, 18, and 19) have relatively low cortical densities of pontine projecting cells, whereas areas with low magnification factors (such as areas 20a, 20b, and some of the lateral suprasylvian visual areas) have relatively high cortical densities. The "visual field density" of corticopontine neurons appears therefore to be fairly constant from area to area as concerns central vision.

Distribution of corticopontine neurons in visual areas of the middle suprasylvian sulcus: quantitative studies in the cat.

This study deals with the distribution of corticopontine neurons in four of the lateral suprasylvian visual areas, the anteromedial, anterolateral, posteromedial and posterolateral areas. Following large injections of horseradish peroxidase-wheat germ agglutinin in the pontine nuclei, labelled cells were quantitatively mapped. The borders of the four areas were determined cyto- and myeloarchitectonically or from standard retinotopic maps presented in frontal sections (Tusa et al., 1981). Flat maps were constructed of each area showing the distribution of retrogradely labelled neurons. Maps of the retinotopic organization in the lateral suprasylvian visual areas (Palmer et al., 1978; Tusa et al., 1981) were transferred to the flat maps. Thus, the density and number of labelled cells within each area and within smaller zones representing different visual field blocks could be determined. The average cell density (average number of labelled cells per mm2 of flattened cortex) is highest in the anteromedial area. The average density in this area is higher than in areas 17, 18 and 19. The total number of labelled cells is highest in the posteromedial area. The lateral syprasylvian visual areas together have a stronger pontine projection in terms of actual number of cells than each of areas 17, 18 and 19. [Data on areas 17, 18 and 19 from Bjaalie and Brodal (1983) and Bjaalie (1985)]. The four areas investigated have different internal distribution of corticopontine cells. The posteromedial and posterolateral areas have lower densities of labelled cells in the representations of the retinal central area than in the representations of the visual periphery. However, due to the enlarged cortical representations of central vision, the actual number of corticopontine cells per visual field block is higher in the cortex representing the central area; i.e. the central visual field region is moderately over-represented in the corticopontine projection from these areas. In the anteromedial and anterolateral areas, unlike in all other areas so far investigated, the cortical representations of the central visual field contain higher densities of corticopontine cells than the representations of the lower visual periphery. However, since the central area is only very weakly over-represented compared to the lower visual periphery in terms of cortical volume in these areas, the over-representation in the corticopontine projection of central vision compared to the lower visual periphery is fairly moderate.(ABSTRACT TRUNCATED AT 400 WORDS)

Visual pathways to the cerebellum: segregation in the pontine nuclei of terminal fields from different visual cortical areas in the cat.

The cerebellum receives input from visual cortical areas via a relay in the pontine nuclei. We have compared the location in the pontine nuclei of terminal fields of fibres from visual areas 18 and 20, and the posteromedial lateral suprasylvian visual area. Due to individual variations in the precise location of terminal fields, comparisons were performed in individual animals. Horseradish peroxidase-wheat germ agglutinin conjugate was used as an anterograde tracer in combination with the Fink and Heimer method for visualization of anterograde degeneration. Most of the terminal fields of area 20 are widely separated from those of area 18. Fibres from the posteromedial lateral suprasylvian visual area and area 20 terminate close to each other but overlap of terminal fields is limited. Area 18 and the posteromedial area have in some places completely overlapping terminal fields; in other places, however, there is only partial overlap or complete separation. Generally, segregation of terminal fields from different areas is most pronounced in the caudal part of the recipient zone of the pontine nuclei. The terminal fields of fibres from the three cortical areas studied appear as numerous patches arranged in a complicated mosaic that tend to form concentric lamellae around the ventromedial aspect of the peduncle. Within these lamellae, area 18 projects mainly to the innermost one, area 20 to the outermost, and the posteromedial area to an intermediate lamella. Whether terminal fibres from different areas are segregated (non-overlapping) or overlapping in the pontine nuclei is of relevance for the functional organization of the cerebrocerebellar pathway. Segregation of terminal fields from different areas would mean that the areas in question influence different sets of pontocerebellar neurons and thereby relay information to the cerebellum in separate channels. Overlap of terminal fields from different areas could mean that convergence on the same pontocerebellar neurons occurs (although convergence cannot be proved with the techniques employed in this study). This study indicates that information from visual areas is relayed at least in part in separate channels from the cortex to the cerebellum.

Spatial correspondence between tactile projection patterns and the distribution of the antigenic Purkinje cell markers anti-zebrin I and anti-zebrin II in the cerebellar folium crus IIA of the rat.

We have compared the band-like distribution of the Purkinje cell-specific polypeptides zebrin I and zebrin II with the spatial organization of tactile projections to crus IIa in the cerebellar hemisphere of the rat. Maps of tactile responses in the granular layer of the cerebellar hemispheres are fractured into discontinuous regions, termed "patches". High-density micromapping was used to identify specific patches and their boundaries within this fractured somatotopic map. In one series of experiments, medial and lateral boundaries of the large central ipsilateral upper lip-related patch were identified and labeled with either Fast Blue or India Ink. Following immunocytochemical processing, the band-like distribution of immunostained Purkinje cells (zebrin-positive bands) and the identified patch boundaries were digitized and reconstructed in three dimensions. Comparisons between these two features demonstrate a spatial correspondence between zebrin transitions and the boundaries of the electrophysiologically defined upper lip-related patch. In another series of experiments, we outlined the boundaries or centers of several smaller patches consistently located in the medial portion of the folium. Again, we found a correspondence between the distribution of granule cell layer tactile patches and the zebrin staining pattern. The correspondence between tactile projection patterns and molecular features demonstrated in the present study implies that there is a distinct and largely fixed spatial pattern of organization in the cerebellar hemispheres. We discuss possible causal connections and developmental determinates, as well as the physiological significance of the correspondence between the two features.

Salient anatomic features of the cortico-ponto-cerebellar pathway.

Recent studies of the primate corticopontine projection show that the neocerebellum--in addition to connections from motor and sensory areas--receives connections from various association areas of the cerebral cortex, some of which are thought to be primarily engaged in cognitive tasks. The quantities of such connections in relation to those from more clearly motor-related parts of the cortex need to be more precisely determined, however. Furthermore, the anatomic data on origin of corticopontine fibers needs to be supplemented with physiological experiments to clarify their functional properties at the single-cell level. For example, nothing is known of the functional role of the large input from the cingulate gyrus, nor is the input from the posterior parietal cortex physiologically characterized. Finally, the scarcity of corticopontine connections from the prefrontal cortex in the monkey (and probably also in man) may not seem readily compatible with a prominent role of the neocerebellum in certain cognitive tasks. We discuss data--in particular from three-dimensional reconstructions--indicating that both corticopontine projects and pontocerebellar neurons are arranged in a lamellar pattern. Corticopontine and pontocerebellar lamellae have similar shapes and orientations but appear to differ in other respects. Corticopontine terminal fields are sharply delimited, apparently without gradual overlap between projections from different sites in the cortex, whereas pontocerebellar lamellae are more fuzzy and exhibit gradual overlap of neuronal populations projecting to different targets. In spite of the sharpness of the corticopontine projection, there may be many opportunities for convergence of inputs from different parts of the cortex. Thus, the wide divergence of corticopontine projections produces many sites of overlap, and extensive interfaces between different terminal fields enabling convergence of inputs onto each neuron. We suggest that the lamellar arrangement of corticopontine terminal fields and of pontocerebellar neurons serve to create diversity of pontocerebellar neuronal properties. Thus, each small part of the cerebellar cortex would receive a specific combination of messages from many different sites in the cerebral cortex. The spatial arrangement of cerebrocerebellar connections have to be understood both in terms of fairly simple large-scale, gradual topographic relationships and an apparently highly complex pattern of divergence and convergence. Developmental studies of corticopontine and of pontocerebellar projections together with three-dimensional reconstructions in adults suggest that the highly complex adult connectional pattern may be created by simple rules operating during development.

Corticopontine terminal fibres form small scale clusters and large scale lamellae in the cat.

We investigated whether terminal fibres in the pontine nuclei are arranged in a lamellar pattern like that demonstrated earlier for pontocerebellar neurones. Following tracer injections in visual and parietal cortices and subsequent computer-based 3-D analysis, we found that labelled corticopontine terminal fibres form numerous sharply delimited aggregates of variable shape. Several of the aggregates are cylindroids (diameter 200-300 microns, length 1-3 mm). The aggregates are confined to a lamellar subspace, the position of which depends on the anteroposterior location of the cortical injections. These findings suggest that the cerebroponto-cerebellar system may be organized according to fairly simple, topographical rules. We discuss the implications of our results in relation to the development of corticopontine topographical organization.

Semi-automatic data acquisition for quantitative neuroanatomy. MicroTrace--computer programme for recording of the spatial distribution of neuronal populations.

We present a computer programme, MicroTrace, designed for user-guided digitisation of objects in biological sections. The programme is optimised for recording the spatial distribution of neuronal structures, such as large populations of tracer-labelled cell bodies or axonal plexuses, regional borders, and surfaces. System requirements are a PC running Microsoft Windows, a microscope equipped with stepping motors, and a drawing tube. A computer generated drawing area, surrounded by menus and icons, is projected into the microscope field of view via the drawing tube. Different 'object' icons are assigned to individual object categories (cell types, surfaces, etc.). Digitisation is performed by pointing the cursor at objects in the section. Computer graphical symbols are superimposed on the digitised objects. All object categories are digitised, before moving the stage to other fields of view by manipulating the joystick or scroll bars. Movement of the microscope stage is accompanied by a translation of the graphical image, so that continuous feedback on the progress of the digitisation is provided. MicroTrace can readily be adapted to the specific needs of the user. We show its use in different experimental neuroanatomical techniques. Two-dimensional images and three-dimensional reconstructions of neuronal distribution and surfaces are demonstrated.

Organization of the pontine nuclei.

The pontine nuclei provide the cerebellar hemispheres with the majority of their mossy fiber afferents, and receive their main input from the cerebral cortex. Even though the vast majority of pontine neurons send their axons to the cerebellar cortex, and are contacted monosynaptically by (glutamatergic) corticopontine fibers, the information-processing taking place is not well understood. In addition to typical projection neurons, the pontine nuclei contain putative GABA-ergic interneurons and complex synaptic arrangements. The corticopontine projection is characterized by a precise but highly divergent terminal pattern. Large and functionally diverse parts of the cerebral cortex contribute; in the monkey the most notable exception is the almost total lack of projections from large parts of the prefrontal and temporal cortices. Within corticopontine projections from visual and somatosensory areas there is a de-emphasis of central vision and distal parts of the extremities as compared with other connections of these sensory areas. Subcorticopontine projections provide only a few percent of the total input to the pontine nuclei. Certain cell groups, such as the reticular formation, project in a diffuse manner whereas other nuclei, such as the mammillary nucleus, project to restricted pontine regions only, partially converging with functionally related corticopontine connections. The pontocerebellar projection is characterized by a highly convergent pattern, even though there is also marked divergence. Neurons projecting to a single cerebellar folium appear to be confined to a lamella-shaped volume in the pontine nuclei. The organization of the pontine nuclei suggests that they ensure that information from various, functionally diverse, parts of the cerebral cortex and subcortical nuclei are brought together and integrated in the cerebellar cortex.