Biomedical subjects

T A Coogan

Publications and source records attributed to T A Coogan.

5 recordsLinked to original sources

PubMed1996

Development of connections within and between areas V1 and V2 of macaque monkeys.

We have investigated the development of intrinsic and interareal connections in areas V1 and V2 of the macaque monkey using postmortem transport of the lipophilic fluorescent tracer diI, applied to brains fixed at different pre- and postnatal ages. Intrinsic connections in the deep layers of V1 are evident on embryonic day 108 (E108), but are not robust in the superficial layers until around E118, when migration is largely complete. Both intrinsic horizontal projections and extrinsic projections to V2 initially have a continuous distribution. Patchy projections are first evident in V1 around E145, the same age at which cytochrome oxidase blobs appear, presumably signaling the differentiation of the blob-dominated and interblob-dominated streams in the primary visual cortex. The magnocellular-dominated stream becomes distinct at earlier stages (by E122), as judged by connectional and histochemical criteria. In area V2, intrinsic connections initially (at E108) involve only deep layer cells and do not have a clustered organization. By E130, superficial layer cells are involved and the V2 intrinsic connections have a patchy distribution; by E145, an adult-like pattern is present. The projection from V2 to V1 passes through an early stage (up to E133) of originating principally from deep layer cells, and thereafter originating from superficial as well as deep layers. We found evidence for changes in dendritic morphology during development. Most notably, at E118, many neurons in layer 6 which are involved in intrinsic or interareal connections have dendrites that extend well into the superficial layers, even into layer 1, a characteristic not reported in the adult.

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PubMed1996

Computerized mappings of the cerebral cortex: a multiresolution flattening method and a surface-based coordinate system.

We present a new method for generating two-dimensional maps of the cerebral cortex. Our computerized, two-stage flattening method takes as its input any well-defined representation of a surface within the three-dimensional cortex. The first stage rapidly converts this surface to a topologically correct two-dimensional map, without regard for the amount of distortion introduced. The second stage reduces distortions using a multiresolution strategy that makes gross shape changes on a coarsely sampled map and further shape refinements on progressively finer resolution maps. We demonstrate the utility of this approach by creating flat maps of the entire cerebral cortex in the macaque monkey and by displaying various types of experimental data on such maps. We also introduce a surface-based coordinate system that has advantages over conventional stereotaxic coordinates and is relevant to studies of cortical organization in humans as well as non-human primates. Together, these methods provide an improved basis for quantitative studies of individual variability in cortical organization.

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PubMed1993

Hierarchical organization of areas in rat visual cortex.

To test the hypothesis that areas within rat visual cortex are organized in a multilevel hierarchy, we have employed Phaseolus vulgaris leucoagglutinin as an anterograde axonal tracer to visualize the laminar patterns of connections between different cortical areas. For identification of cortical areas, we used a combination of markers that included callosal connections, the patterns of inputs and outputs to ipsilateral cortical and subcortical targets, and geographical location. Projections from area 17 to every identified extrastriate target area extend throughout all layers of cortex and include layer 4. Area LM (lateromedial), contained within the cytoarchitectonic subdivision 18a, projects to area 17, area AL (anterolateral), area RL (rostrolateral), multiple sites within the posterior complex (PX), the anterior complex (AX), the far lateral complex (FLX), the medial complex (MX), perirhinal, entorhinal, retrosplenial, and presubicular cortex. Each of the projections to extrastriate areas resembles those originating from area 17. Only the projection to area 17 differs, and terminates largely in layers outside of lamina 4. Such projections are designated as feedback (Coogan and Burkhalter, 1990). The projections of a second area, AL, of the cytoarchitectonic subdivision 18a are similar to those of LM: all terminate in layers 1-6, except the inputs to area 17, LM, and a site in FLX, which spare layer 4. The feedback projection to LM provides further support that LM and AL constitute distinct cortical areas. Projections from additional distinct sites within area 18a that are located immediately lateral to LM and AL and are designated FLX make feedback projections to area 17 and projections involving all layers to LM and AL. Thus, unlike the asymmetrical laminar organization of reciprocal connections between area 17 and LM, 17 and AL, and LM and AL, the connections between LM and at least one site in FLX are symmetrical. Projections that include layer 4 can, therefore, be components of connections between different hierarchical levels as well as components of connections on the same hierarchical level. The MX sites contained within the cytoarchitectonic subdivision 18b send feedback projections to striate cortex, LM, AL, and PX within 18a. Thus, the connections between these areas are reciprocal and the laminar organization is asymmetrical. The projections to FLX include layer 4, and the projections to frontal, cingulate, and retrosplenial cortex resemble forward projections. Although the areal organization of extrastriate cortex is not yet fully resolved, using the patterns of intracortical connections we are able to construct a provisional hierarchy of cortical areas.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1990

Conserved patterns of cortico-cortical connections define areal hierarchy in rat visual cortex.

The prevalence of reciprocal connections in the cerebral cortex indicates that they play a fundamental role in the processing of sensory information. We have investigated the laminar termination patterns of such paired connections between different visual cortical areas of the rat, and have found two basic projection types: one which includes layer 4 and a second which includes layer 1 and avoids layer 4. The projections from primary visual cortex (area 17) to extrastriate visual cortical targets in the cytoarchitectonical areas 18a and 18b, and from 18a to a site in 18b, are of the first type. In contrast, the return projections from 18a and 18b to area 17 and from 18b to 18a, are of the second type. Thus each pair of connections has one element of each type, giving every circuit a nearly identical asymmetric structure. These laminar patterns resemble those of forward and feedback connections in primate cortex, indicating that corticocortical connectivity patterns are highly conserved through evolution, and that, as in monkeys, these connections define a hierarchical organization of areas in rat visual cortex.

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PubMed1988

Sequential development of connections between striate and extrastriate visual cortical areas in the rat.

In these experiments we have asked whether the projection from the rat's primary visual cortex, area 17, to the extrastriate visual cortical area 18a is formed in a sequence and whether that sequence resembles the pattern of inside-out cortical neurogenesis. For this purpose fluorescent retrograde tracers were injected into area 18a at different postnatal ages (P1, P5, adult). Animals survived until 3-4 weeks of age, after migration is complete and neurons have arrived at their final laminar location. In the ipsilateral cortex, P1 injections retrogradely labeled cells in layers 5 and 6 of area 17. Labeling after P5 injections extended into more superficial layers and included the bottom of layer 2/3 and layers 4-6. After P5, more labeled cells were found at the top of layer 2/3, producing the adult laminar pattern, where the projection originates predominantly from layer 2/3. A similar sequence of laminar labeling was observed in the transcallosal connection of area 18a. This sequence of labeling, deep layers before superficial, resembles the pattern in which cortical neurons are born and indicates that axons arrive at their cortical targets in the order the cells were generated.

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