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A Burkhalter

Publications and source records attributed to A Burkhalter.

At least 19 recordsLinked to original sources
PubMed1999

Differential subcellular localization of forward and feedback interareal inputs to parvalbumin expressing GABAergic neurons in rat visual cortex.

In rat visual cortex, forward and feedback interareal pathways innervate both pyramidal and gamma-aminobutyric acid (GABA)ergic (Johnson and Burkhalter [1996] J. Comp. Neurol. 368:383-398). GABAergic neurons consist of different cell types of which the largest group expresses parvalbumin (PV; Gonchar and Burkhalter [1997] Cereb. Cortex 4:347-358). Here, we report that PV neurons in layers 2/3 are synaptic targets of forward and feedback projections between area 17 and the lateromedial area (LM) of rat visual cortex. In both forward and feedback pathways, approximately 90% of axon terminals in layer 2/3 labeled by tracing with biotinylated dextran amine formed synapses with PV-negative profiles. In both pathways, most of these profiles resembled dendritic spines. Although there were no differences in the innervation of PV-negative targets, the two pathways differed in the innervation of PV-positive neurons. In each pathway, approximately 10% of terminals formed synapses with PV-positive profiles. However, in the forward pathway, the size of the contacted PV-positive profiles was larger than in the feedback pathway. Moreover, in the forward pathway, axon terminals on PV-positive profiles were larger, contained more mitochondria and docked synaptic vesicles than feedback synapses on PV neurons. Our results show that PV neurons provide a major target for area 17 <-> LM forward and feedback pathways terminating in upper layers. In each pathway, the proportion of axons contacting PV neurons is similar. However, both pathways differ in the subcellular localization and morphology of synapses on PV neurons. These asymmetries may contribute to the inequality in the strength of disynaptic inhibition evoked by forward and feedback inputs (Shao and Burkhalter [1996] J. Neurosci. 16:7353-7365).

Afferent Pathways↗
PubMed1999

Role of GABAB receptor-mediated inhibition in reciprocal interareal pathways of rat visual cortex.

In neocortex, synaptic inhibition is mediated by gamma-aminobutyric acid-A (GABAA) and GABAB receptors. By using intracellular and patch-clamp recordings in slices of rat visual cortex we studied the balance of excitation and inhibition in different intracortical pathways. The study was focused on the strength of fast GABAA- and slow GABAB-mediated inhibition in interareal forward and feedback connections between area 17 and the secondary, latero-medial visual area (LM). Our results demonstrate that in most layer 2/3 neurons forward inputs elicited excitatory postsynaptic potentials (EPSPs) that were followed by fast GABAA- and slow GABAB-mediated hyperpolarizing inhibitory postsynaptic potentials (IPSPs). These responses resembled those elicited by horizontal connections within area 17 and those evoked by stimulation of the layer 6/white matter border. In contrast, in the feedback pathway hyperpolarizing fast and slow IPSPs were rare. However weak fast and slow IPSPs were unmasked by bath application of GABAB receptor antagonists. Because in the feedback pathway disynaptic fast and slow IPSPs were rare, polysynaptic EPSPs were more frequent than in forward, horizontal, and interlaminar circuits and were activated over a broader stimulus range. In addition, in the feedback pathway large-amplitude polysynaptic EPSPs were longer lasting and showed a late component whose onset coincided with that of slow IPSPs. In the forward pathway these late EPSPs were only seen with stimulus intensities that were below the activation threshold of slow IPSPs. Unlike strong forward inputs, feedback stimuli of a wide range of intensities increased the rate of ongoing neuronal firing. Thus, when forward and feedback inputs are simultaneously active, feedback inputs may provide late polysynaptic excitation that can offset slow IPSPs evoked by forward inputs and in turn may promote recurrent excitation through local intracolumnar circuits. This may provide a mechanism by which feedback inputs from higher cortical areas can amplify afferent signals in lower areas.

Animals↗
PubMed1997

Nasotemporal asymmetries in V1: ocular dominance columns of infant, adult, and strabismic macaque monkeys.

To quantify asymmetries of input from the two eyes into each cerebral hemisphere, we measured ocular dominance column (ODC) widths and areas in the striate visual cortex (area V1) of macaque monkeys. Ocular dominance stripes in layer 4C were labeled by using transneuronal transport of intraocularly injected wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or cytochrome oxidase (CO) histochemistry, after deafferentation of one eye or even by leaving afferent input intact. In infant monkey aged 4 and 8 weeks, ocular dominance stripes labeled by WGA-HRP appeared adultlike with smooth, sharply defined borders. In normal infant and normal adult macaque, ocular dominance stripes driven by the nasal retina (i.e., contralateral eye) were consistently wider than stripes driven by the temporal retina (i.e., ipsilateral eye). Asymmetries in the percentage of area V1 driven by nasal vs. temporal ODCs showed a similar "nasal bias": in infant macaque, approximately 58% of ODCs in V1 were driven by nasal retina, and in adult macaque approximately 57%. The asymmetries tended to be slightly smaller in opercular V1 and greater in calcarine V1. "Spontaneous" ocular dominance stripes were revealed by CO staining of V1 in a naturally strabismic monkey and in a monkey made strabismic by early postnatal alternating monocular occlusion. In these animals, ocular dominance stripes and CO blobs corresponding to the nasal retina stained more intensely for CO in both the right and left V1. ODC spacing and the nasotemporal asymmetry in ODC width and area were similar in strabismic and normal monkeys. Our results in normal monkeys extend the observations of previous investigators and verify that nasotemporal inputs to opercular and calcarine V1 are unequal, with a consistent bias favoring inputs from the nasal retina. The CO results in strabismic macaque suggest that the nasal ODC bias promotes interocular suppression when activity in neighboring ODCs is decorrelated by abnormal binocular experience in infancy.

Aging↗
PubMed1997

A polysynaptic feedback circuit in rat visual cortex.

Feedback connections from extrastriate cortex to primary visual cortex (V1) in the primate may provide "top-down" information that plays a role in visual attention and object recognition. Our work in a rodent model of corticocortical circuitry demonstrates that feedback pathways synapse preferentially with pyramidal cells in V1 () and favor excitation over inhibition in cortical microcircuits (). To investigate the polysynaptic circuits activated by feedback inputs, we studied chains of neurons postsynaptic to feedback connections using a combination of axonal tract tracing and anterograde degeneration. This approach enabled independent labeling of local collaterals of forward-projecting neurons in V1 and feedback connections from extrastriate lateromedial (LM) visual area to V1. Postsynaptic targets were identified in the electron microscope after retrograde transport of biotinylated dextran amine (BDA) to identify dendrites of forward-projecting neurons (i.e., from V1 to LM) and postembedding immunogold labeling to identify GABAergic interneurons. The results show that feedback connections provide strong monosynaptic input to forward-projecting neurons in V1. These neurons in turn make local connections that preferentially form synapses with other pyramidal cells ( approximately 97%), many of which were identified as forward-projecting neurons. This indicates that feedback pathways provide input directly to neurons which make the reciprocal forward connection, and that feedback-recipient forward-projecting neurons are strongly interconnected. The function of these excitatory networks within V1 may be to amplify feedback activity and provide a circuit for modulation of striate cortical activity by top-down influences.

Animals↗
PubMed1997

Cortical connections of areas V3 and VP of macaque monkey extrastriate visual cortex.

The cortical connections of visual area 3 (V3) and the ventral posterior area (VP) in the macaque monkey were studied by using combinations of retrograde and anterograde tracers. Tracer injections were made into V3 or VP following electrophysiological recording in and near the target area. The pattern of ipsilateral cortical connections was analyzed in relation to the pattern of interhemispheric connections identified after transection of the corpus callosum. Both V3 and VP have major connections with areas V2, V3A, posterior intraparietal area (PIP), V4, middle temporal area (MT), medial superior temporal area (dorsal) (MSTd), and ventral intraparietal area (VIP). Their connections differ in several respects. Specifically, V3 has connections with areas V1 and V4 transitional area (V4t) that are absent for VP; VP has connections with areas ventral occipitotemporal area (VOT), dorsal prelunate area (DP), and visually responsive portion of temporal visual area F (VTF) that are absent or occur only rarely for V3. The laminar pattern of labelled terminals and retrogradely labeled cell bodies allowed assessment of the hierarchical relationships between areas V3 and VP and their various targets. Areas V1 and V2 are at a lower level than V3 and VP; all of the remaining areas are at a higher level. V3 receives major inputs from layer 4B of V1, suggesting an association with the magnocellular-dominated processing stream and a role in routing magnocellular-dominated information along pathways leading to both parietal and temporal lobes. The convergence and divergence of pathways involving V3 and VP underscores the distributed nature of hierarchical processing in the visual system.

Animals↗
PubMed1997

Three distinct families of GABAergic neurons in rat visual cortex.

In the cortex inhibition is mediated predominantly by GABAergic interneurons. Although all of these neurons use the same neurotransmitter, studies in the rat frontal cortex have shown that they are molecularly and physiologically diverse. It is not known whether similar subgroups of GABAergic neurons exist in primary visual cortex and how these different inhibitory neurons are inserted into specific cortical circuits. We have used immunostaining with antibodies against gamma aminobutyric acid (GABA), parvalbumin (PV), calretinin (CR), somatostatin (SOM), calbindin (CB) and nitric oxide synthase (NOS) to probe for colocalization of known markers of GABAergic interneurons. The results show that the majority of PV (100%), SOM (89.8%) and CR (93.9%) staining neurons are GABA positive. PV immunoreactive neurons constitute a distinct group that show no overlap with CR, SOM and NOS expressing cells and only a minor overlap (5.3%) with CB. PV immunoreactive cells account for 50.8% of GABAergic neurons. A second group of SOM expressing neurons accounts for 16.9% of GABAergic cells. None of these cells colocalize PV or CR, but 1.7% of SOM neurons stain for NOS and 86.3% show CB immunoreactivity. The third distinct group of CR expressing cells accounts for 17.0% of GABAergic neurons. All of these are PV, CB, SOM and NOS negative. CB expressing neurons represent a heterogeneous group that includes GABAergic and non-GABAergic cells. Our findings indicate that GABAergic neurons in rat area 17 are organized in at least three separate families that can be identified by the expression of PV, CR and SOM. These cells account for 84.9% of GABAergic neurons. These results extend previous observations in rat frontal agranular cortex and suggest that in visual cortex the inhibitory network is composed of similar cell types.

Animals↗
PubMed1997

Malignant melanoma in situ colonizing basal cell carcinoma. A simulator of invasive melanoma.

Coexisting (collision) cutaneous neoplasms of various types and combinations are well documented but relatively uncommon. This report describes the unusual occurrence of a malignant melanoma in situ (MMIS) colonizing a basal cell carcinoma (BCC). A 69-year-old man was considered clinically to have a melanocytic neoplasm or a pigmented BCC of the right ear. Biopsy showed an MMIS, lentigo maligna type, juxtaposed to a typical BCC. The MMIS extended peripherally and into the BCC. Interspersed among the basaloid epithelial cell aggregates that extended 1.70 mm into the dermis were atypical melanocytes. Immunoperoxidase stains with high molecular weight cytokeratin (903) stained the basaloid keratinocytes but not the melanocytes. Conversely, HMB-45 and Mel-5 intensely stained the melanocytes throughout the BCC and in the epidermis but did not demonstrate any separate aggregates of invasive melanoma. These findings suggest that the position of the MMIS was a consequence of BCC colonization. Because the prognosis of malignant melanoma correlates most closely with the thickness of the lesion, this case poses a unique problem in predicting the biology of the lesion, which we believe should not be considered invasive melanoma.

Aged↗
PubMed1996

Different balance of excitation and inhibition in forward and feedback circuits of rat visual cortex.

Different cortical areas are linked reciprocally via forward and feedback connections. Forward connections are involved in the representation of retinal images, whereas feedback pathways may play a role in the selection and interpretation of visual information. To examine the synaptic mechanisms of forward and feedback connections between primary and secondary visual cortical areas directly, we have performed intracellular recordings in slices of rat visual cortex. Irrespective of stimulus intensity and membrane potential, 78% (45/58) of the cells in striate cortex activated by feedback input showed monosynaptic responses that were depolarizing only, and inhibitory inputs were evident merely as a slight acceleration in the decay of EPSPs. In contrast, in 89% (17/19) of the cells, stimulation of forward input evoked monosynaptic excitatory postsynaptic potentials (EPSPs), followed by disynaptic, hyperpolarizing inhibitory postsynaptic potentials (IPSPs). EPSPs followed by IPSPs also were recorded after stimulation of local connections within primary visual cortex (92%, 12/13) and after activation of thalamocortical input (91%, 10/11). These results suggest that the synaptic organization of feedback connections are distinct from forward, local, and thalamocortical circuits. The findings further indicate that intracortical back projections exert modulatory influences via synaptic mechanisms in which weak inhibitory input is strongly dominated by excitation.

Animals↗
PubMed1996

Regional and laminar differences in synaptic localization of NMDA receptor subunit NR1 splice variants in rat visual cortex and hippocampus.

Changes in N-methyl-D-aspartate (NMDA) receptor expression may represent a molecular substrate for differences in synaptic plasticity between early postnatal and adult brains (Fox and Zahs [1994] Curr. Opinion Neurobiol. 4:112-119). We have, therefore, examined the regional and laminar distribution of NR1, the essential subunit of the NMDA receptor, in two regions in which synaptic plasticity has been most thoroughly studied: primary visual cortex and hippocampus. To study NR1 expression at the light and electron microscopic levels we have used a new antiserum (NR1-C1; Sheng et al. [1994] Nature 368:144-147) directed against a differentially spliced C-terminal exon ("C1"). The most striking result was that the pattern of NR1-C1 labeling in the adult was more restricted than that of previously published NR1-specific antibodies. Specifically, NR1-C1 did not label cells in the CA3, dentate gyrus or subicular regions of the hippocampus or in layer 4 of the visual cortex. Quantitative ultrastructural analysis revealed that these differences were paralleled by differential expression of NR1-C1 at synapses. In sharp contrast to the pattern in the adult, NR1-C1 immunoreactivity was distributed more widely in the developing brain. At postnatal day 11, NR1-C1 splice variants were expressed in all layers of the visual cortex and in all regions of the hippocampus. The transient expression of NR1-C1 splice variants in layer 4 of visual cortex suggests that NR1-C1 may play a role in determining the critical period for binocular plasticity. Continued expression of NR1-C1 in upper and lower layers of the adult cortex and in CA1 of the hippocampus may provide a substrate for plasticity in corticocortical connections and Schaffer collateral synapses beyond the critical period. In addition to abundant postsynaptic staining, NR1-C1 immunoreactivity was found in a large number of axon terminals in the dorsal subiculum, but in very few terminals in visual cortex. This strongly suggests that presynaptic NMDA receptors play a major role in neuronal processing of hippocampal output through the subiculum, but play a relatively minor role in visual processing.

Alternative Splicing↗
PubMed1996

Microcircuitry of forward and feedback connections within rat visual cortex.

Visual cortex in mammals is composed of many distinct areas that are linked by reciprocal connections to form a multilevel hierarchy. Ascending information is sent via forward connections from lower to higher areas and is thought to contribute to the emergence of increasingly complex receptive field properties at higher levels. Descending signals are transmitted via feedback connections from higher to lower areas and are believed to provide information about the context in which a stimulus appears, to contribute to modulation of visual responses by attention, and to play a role in memory processes. To determine whether forward and feedback pathways in rat visual cortex constitute distinct intracortical circuits, we have studied the distribution of reciprocal corticocortical inputs to pyramidal cells and gamma-aminobutyric acid (GABA)ergic interneurons. For this purpose, we chose forward and feedback connections between primary visual cortex and the secondary extrastriate lateromedial (LM) area as a model system. Pathways were traced with the axonal marker phaseolus vulgaris-leucoagglutinin. Labeled terminals were identified in the electron microscope, and GABA immunocytochemistry was used to identify the postsynaptic dendritic shafts of GABAergic interneurons. In both pathways, inputs to pyramidal cells were directed preferentially to dendritic spines and not to shafts. In the forward pathway, 90% of labeled inputs were distributed to pyramidal cells and 10% to interneurons. This proportion was similar to that of nearby unlabeled connections in the neuropil, indicating that forward connections are not selective for pyramidal cells or interneurons. In sharp contrast, feedback connections were significantly different from the unlabeled connections and supplied almost exclusively pyramidal cells (98%). Feedback inputs to GABAergic neurons were five times weaker (2%) relative to the forward direction. These structural differences suggest that disynaptic GABAergic inhibition is much stronger in forward than in feedback pathways. Recent physiological experiments have confirmed this prediction (Shao et al. [1995] Soc. Neurosci. Abstr., 21:1274) and we, therefore, conclude that relatively small anatomical differences in the microcircuitry can have important functional consequences. It remains an open question whether generally reciprocal interareal circuits at all levels of the cortical hierarchy are organized in similar fashion.

Animals↗
PubMed1996

[Functional and structural abnormalities in the visual cortex in early childhood strabismus].

BACKGROUND: To investigate the structural basis for functional deficits in infantile strabismus we used a neuroanatomic tracer and a histochemical label to examine the primary visual cortex (area V1) of adult esotropic macaque monkeys. ANIMALS AND METHODS: The animals had developed natural esotropia in the first months of life, alternated fixation, and exhibited the ocular motor signs that typify strabismus with onset in infancy. After behavioral and VEP testing, ocular dominance columns (ODCs) in V1 were injected with a neuronal traser and labeled for cytochrome oxidase activity. RESULTS: The strabismic monkeys showed striking deficits in binocular luminance visually-evoked potentials (VEPs) and characteristic directional asymmetries in motion VEPs. Binocular horizontal connections between ODCs were reduced an average of 50-60% in strabismic as compared to normal monkeys. ODCs also showed unequal metabolic activity with contralaterally (i.e. nasal retina) driven ODCs showing greater activity in each V1. DISCUSSION AND CONCLUSION: The maldevelopment of connections in upper V1 layers correlates with the abnormalities in binocular and motion VEPs. The unequal metabolic activity suggests interocular suppression. These results provide the first neuroanatomic evidence for cerebral cortex maldevelopments in natural infantile esotropia.

Animals↗
PubMed1996

Bronchoalveolar lavage cytology in pulmonary alveolar proteinosis.

Pulmonary alveolar proteinosis (PAP) is an uncommon disease in which alveoli are progressively filled with surfactant-related material. Although a definitive diagnosis is usually made by an open lung biopsy, bronchoalveolar lavage (BAL) cytology may play a decisive role in the clinical work-up of these patients, and, in some cases, may spare a patient a more invasive diagnostic procedure. The authors present three patients in whom BAL cytology specimens contained the characteristic (although not specific) globules of amorphous proteinaceous PAS-positive material accompanied by only rare background macrophages and inflammatory cells. The patients include a 40-year-old man with an 8-year history of fever of unknown origin, a 30-year-old man with a chronic nonproductive cough, and a 6-year-old boy diagnosed at 5 months of age with osteopetrosis and hypogammaglobulinemia who subsequently developed a disseminated Mycobacterium avium-intracellulare infection. All specimens stained with Gomori methenamine silver (3) and Ziehl-Neelsen (2) were negative for microbial organisms, Ultrastructural examination of two specimens revealed the characteristic lamellar structures of surfactant, increasing diagnostic specificity. Lung biopsies and/or autopsy subsequently confirmed the diagnosis in all three cases. The characteristic cytologic and ultrastructural features of PAP in BAL specimens are presented along with the morphologic differential features of other entities which potentially could be confused with PAP.

Adult↗
PubMed1995

Patterns of synaptic activity in forward and feedback pathways within rat visual cortex.

1. The laminar and temporal distribution of synaptic activity supplied by forward and feedback connections between different areas of rat visual cortex was determined with the use of current source density (CSD) analysis in in vitro slices. In forward connections, synaptic potentials were evoked by electrically stimulating area 17 and recording in the extrastriate area LM (lateromedial), that ranks at the second hierarchical level, one step above primary visual cortex. For activating feedback connections, the location of stimulating and recording electrodes was reversed. 2. The synaptic interactions in reciprocal intracortical circuits are excitatory, and they are mediated through glutamate receptors that are blocked by kynurenic acid. 3. Forward connections from area 17 to area LM provide input to all layers including a strong input to layer 4. In contrast, feedback input to layer 4 is weak and is mainly directed to superficial and deep layers. This laminar distribution closely resembles that seen anatomically. 4. Both forward and feedback connections evoke distinct temporal patterns of synaptic activation in different layers. Although onset and peak latencies are slightly shorter in the forward than in the feedback pathway, the difference is not statistically significant. 5. The spatiotemporal distribution of synaptic activation by forward connections resembles the pattern evoked by geniculocortical inputs. Feedback connections show greater similarities to long-range connections within area 17, although they are not identical. Our results support the notion derived from anatomic and in vivo physiological studies that forward and feedback pathways belong to functionally distinct cortical circuits.

Animals↗
PubMed1994

Evidence for excitatory amino acid neurotransmitters in forward and feedback corticocortical pathways within rat visual cortex.

It is a commonly accepted notion that cells which make projections between the multiple cortical areas found in the mammalian visual system are excitatory, but there is little direct evidence that this is the case. Here we demonstrate using retrograde tracing with D-[3H]aspartate that connections in the rat which project from lower to higher visual areas (i.e. forward) and those which project from higher to lower areas (i.e. feedback) may use excitatory amino acid neurotransmitters. Following injection into the primary visual cortex, clusters of retrogradely labelled cells were found in several extrastriate areas within the cytoarchitectonic subdivisions 18a ('areas' LM, AL, PX, FLX, RL, AX) and 18b ('area' MX), and in the retrosplenial cortex. In all of these areas D-[3H]aspartate-labelled cells were surrounded by diffuse label which may represent anterograde labelling of axon terminals. This suggests that both legs of reciprocal intracortical circuits have similar chemospecificity. To directly demonstrate excitatory amino acid localization in forward projections, D-[3H]aspartate was injected into extrastriate area LM. As expected, the results revealed retrogradely labelled neurons within area 17. Outside area 17, LM injections labelled neurons in AL, PX, FLX, RL, AX and MX. Taken in the context of the hierarchy of areas in rat cerebral cortex (Coogan and Burkhalter, J. Neurosci., 13, 3749-3772, 1993), these results show that D-[3H]aspartate labels: (1) forward connections from area 17 to LM, AL, PX, RL, AX and MX, (2) feedback connections from LM, AL, FLX, PX, RL, AX and MX to area 17, (3) feedback connections from AL, PX, RL, AX and MX to LM, and (4) lateral connections between FLX and LM. These findings strongly indicate that both forward and feedback connections as well as lateral connections at several different levels of the cortical hierarchy use excitatory amino acid neurotransmitters.

Animals↗
PubMed1993

Visualization of dendritic morphology of cortical projection neurons by retrograde axonal tracing.

Currently there is no reliable retrograde tracing technique for visualization of dendritic morphologies of projection neurons. Here we describe a simple and efficient method that can be used to label neurons in Golgi-like fashion. The approach relies on activity-dependent uptake of tracer. For this purpose we inject the glutamate receptor agonist N-methyl-D,L-aspartic acid (NMDA) at the tracer injection site to massively stimulate neurons and to thereby promote uptake of biocytin or biotinylated dextran amine (BDA) by axon terminals. The results show that co-injections of NMDA/biocytin and NMDA/BDA into the extrastriate lateromedial area (LM) of rat visual cortex labels large numbers of neurons in area 17 in Golgi-like fashion. Similarly injections of the lateral geniculate nucleus (LGN) lead to Golgi-like labeling of corticogeniculate neurons in area 17. The distribution of labeled neurons is highly topographic. In addition the method allows excellent preservation of ultrastructure, indicating that this approach is useful for determining the organization of neuronal circuits within the central nervous system.

Animals↗
PubMed1993

Organization of long-range inhibitory connections with rat visual cortex.

We have studied the laminar organization of local long-range inhibitory connections within rat primary visual cortex (area 17) by combining retrograde tracing of nerve cell bodies with glutamic acid decarboxylase immunocytochemistry. While most inhibitory connections are confined to within 0.4 mm of the injection site, a subset of neurons at the layer 5/6 border provide long-range (> 1 mm) inhibitory connections within area 17. However, other cell layers that contain similar local long-range horizontal connections, that is, lower layer 2/3, upper layer 5, and lower layer 6 (Burkhalter and Charles, 1990), show a much more restricted distribution of inhibitory connections. This suggests that cells at the layer 5/6 border play a role in the direct inhibition of neurons at a distant point of the topographic map. Similar double labeling studies reveal long-range inhibitory connections between visual areas. Following injections of fluorescent tracers into area 17, in horizontal sections inhibitory connections can be identified that are up to 8 mm long, linking the extrastriate subdivisions 18a and 18b with striate cortex. Conversely, injections of fluorescent tracers into the cytoarchitectonic subdivision 18a reveal local long-range inhibitory connections within 18a, long-range inhibitory connections between 18a and the cytoarchitectonic subdivision 18b, and inhibitory forward connections from area 17 to 18a. These results suggest that the communication between different cortical areas can be influenced by direct inhibitory connections.

Animals↗
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)

Animals↗