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Multiple ischemic lesions of the optic pathways may cause a number of complex visual field defects; their analysis is of substantial diagnostic significance, especially if the computer tomogram is negative. Perimetry is often complicated by an accompanying organic psychosyndrome.
Visual inputs to field 29 of the limbic retrosplenial cortex have been studied by means of paired stimuli, i.e. photic and electrical stimulation of the anterodorsal nucleus (ADN). During stimulation of this nucleus, the evoked potentials in the field 29 decreased in the initial phase of visual stimulation, indicating the involvement of the ADN into the visual system. This finding confirms earlier morphological data [2]. Besides this, in the field 29 neurones were found which responded to light but not to ADN stimulation, suggesting the possibility of termination of different visual inputs on the neurones of the limbic cortex.
Pseudorabies virus (PRV) has been shown to be an effective transneuronal tracer within both the peripheral and the central nervous system. The only investigations of this virus in the visual system have examined anterograde transport of PRV from injection sites in the retina. In the present study, we injected attenuated forms of PRV into the primary visual cortex of both rats and cats to determine whether transneuronal retrograde infection would occur back to the retina. In rats, we made small injections into visual cortex of a strain of PRV (Bartha Blu) that contained a beta-galactosidase promoter insert. In cats, we injected PRV-M201 into area V1 of visual cortex. After a 2- to 4-day incubation period, we examined tissue from these animals for the presence of the beta-galactosidase marker (rats) or the virus itself (cats). Cortical PRV injections resulted in transneuronal retrograde infection of the lateral geniculate nucleus (LGN), thalamic reticular nucleus (TRN), and retina. PRV was retinotopically distributed in the pathway. In addition, double-labeling experiments in cats using an antibody against gamma-aminobutyric acid (GABA) were conducted to reveal PRV-labeled interneurons within the LGN and TRN. All TRN neurons were GABA+, as was a subset of LGN neurons. Only the subset of TRN neurons adjacent to the PRV-labeled sector of LGN was labeled with PRV. In addition, a subset of GABA+ interneurons in LGN was also labeled with PRV. We processed some tissue for electron microscopy to examine the morphology of the virus at various replication stages. No mature virions were detected in terminals from efferent pathways, although forms consistent with retrograde infection were encountered. We conclude that the PRV strains we have used produce a local infection that progresses primarily in the retrograde direction in the central visual pathways. The infection is transneuronal and viral replication maintains the intensity of the label throughout the chain of connected neurons, providing a means of examining detailed circuitry within the visual pathway.
Brain metabolic mapping techniques, such as positron emission tomography (PET), can provide information about the functional interactions within entire neural systems. With the large quantity of data that can accumulate from a mapping study, a network analysis, which makes sense of the complex interactions among neural elements, is necessary. A network analysis was performed on data obtained from a PET study that examined both the changes in regional cerebral blood flow (rCBF) and interregional correlations among human cortical areas during performance of an object vision (face matching) and spatial vision (dot-location matching) task. Brain areas for the network were selected based on regions showing significant rCBF or interregional correlations between tasks. Anterior temporal and frontal lobe regions were added to the network using a principal components analysis. Interactions among selected regions were quantified with structural equation modeling. In the structural equation models, connections between brain areas were based on known neuroanatomy and the interregional correlations were used to calculate path coefficients representing the magnitude of the influence of each directional path. The combination of the anatomical network and interregional correlations created a functional network for each task. The functional network for the right hemisphere showed that in the object vision task, dominant path influences were among occipitotemporal areas, while in the spatial vision task, occipitoparietal interactions were stronger. The network for the spatial vision task also had a strong feedback path from area 46 to occipital cortex, an effect that was absent in the object vision task. There were strong interactions between dorsal and ventral pathways in both networks. Functional networks for the left hemisphere did not differ between tasks. Networks for the interhemispheric interactions showed that the dominant pathway in the right hemisphere also had stronger effects on homologous left hemisphere areas and are consistent with a hypothesis that intrahemispheric interactions were greater in the right hemisphere in both tasks, and that these influences were transmitted callosally to the left hemisphere.
Using path analysis to determine the systems-level neural networks mediating specific tasks from regional cerebral blood flow (rCBF) data obtained by positron emission tomography (PET), we recently found in young subjects strong functional linkages during a face matching task along a right hemisphere ventral network including occipital, temporal, and frontal regions. In this study, PET data obtained during a face matching task from mildly affected patients with dementia of the Alzheimer type (DAT) and healthy matched controls showed that (1) the neural model obtained in young subjects provides a good fit to data from old subjects; (2) although the DAT patients could perform this task with the same accuracy as controls, they did not use the same functional network.
The axoplasmic migration of ribosomes has been detected in the visual system of the chick. Monocular injection of radioactive uridine or an amino acid mixture was followed by sedimentation analysis in sucrose or cesium sulfate density gradients, of ribosomes prepared from the retinae of injected eyes and the left and right optic lobes. By this means both RNA and protein components of ribosomes were found to migrate from the retina to the innervated contralateral optic lobe. Following denervation of the distal nerve segment by eye removal, the stability of the transported RNA was reduced, suggesting its presynaptic location. The transport of RNA was not significantly imparied by intraocular injection of inhibitors of informational RNA or mitochondrial RNA synthesis prior to injection of radioactive uridine but was depressed by a low dose of actinomycin D.
How does the cortical circuitry analyze the visual scene? Here we explore the earliest levels of striate cortical processing: the first stage, where orientation sensitivity emerges, and the second stage, where stimulus selectivity is further refined. The approach is whole cell recording from cat in vivo. Neurons in the lateral geniculate nucleus of the thalamus have circular receptive fields whose subregions, center and surround are concentrically arranged and have the reverse sign, on or off. These neurons supply cortical simple cells, whose receptive fields have on and off subregions that are elongated and lie side by side. Feedforward models hold that orientation sensitivity depends on this thalamocortical change in receptive field structure and an arrangement within subregions such that stimuli of the reverse contrast evoke synaptic responses of the opposite polarity-push-pull. Our work provides support for feedforward models and emphasizes that push-pull is key in the geniculostriate pathway, preserved from retina by thalamic relay cells and reiterated, point by point, by cortical simple cells. Also, we help define the cortical push-pull circuit by identifying inhibitory simple cells. Lastly, separate experiments that compare the first and second levels of cortical processing suggest that differences in the synaptic physiology of connections at the two (thalamocortical versus intracortical) stages underlie differential selectivity for properties such as motion.
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Functional neuroimaging has provided a new view of activity in human visual cortex. There have been a series of interesting developments in understanding the relationship between the functional signals, particularly functional MRI, and basic measurements of action potentials and local field potentials. The new human neuro-imaging measurements have clarified some of the similarities and differences between the general organization of visual areas in human and macaque visual cortex, and there have been some interesting new results concerning cortical visual plasticity and dysfunction. The new fMRI focus on measurements of the human brain will drive new relationships between neurology and visual neuroscience that should help us learn much more about the neural basis of perception.
The anatomical characteristics of the avian visual system are well known. However, there are wide gaps in our knowledge with respect to the physiological characteristics of their visual system. For example, we lack both an operational identification of the different ganglion cell types present in the retinae of birds, and a description of their presumptive differential central projections. The results presented here address this latter point by classifying the conduction velocity groups of fibers present in the optic tract of the pigeon. We report the existence of at least 5 groups of axons in the optic tract of the pigeon, with conduction velocities of 22-18 m/s, 12-10 m/s, 8 m/s, 6 m/s and less than 2.5 m/s. All five groups project to the tectum but only the four fastest groups project to the dorsal thalamic complex. The homologies with the populations of retinal axons found in cats are discussed.
Recent studies of the primate visual system have focused on the proposal that the perception of form and motion are processed by two parallel pathways that originate from separate populations of cells in the retina. Earlier proposals for parallel processing of visual signals identified a third pathway that could be traced from the retina to the visual cortex. This third pathway was assumed to be unimportant. A growing body of evidence suggests that this pathway to cortex is distinct anatomically, physiologically and neurochemically, and is well represented in primates. These findings raise new and interesting questions not only about the role of this pathway, but also about the intracortical integration of afferent parallel signals.
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.
We employed a monoclonal antibody raised against Drosophila brain homogenate for a comparative immunocytochemical analysis of visual and olfactory pathways in brains of two insect species. On Western blots of Drosophila and Apis nervous tissue, antibody fb45 recognized an antigen with an apparent molecular weight higher than 180 kD. Application of the antibody to sections of Drosophila and Apis brain stained certain interneurons which conspicuously fasciculate in common tracts or neuropilar compartments. Both in Drosophila and in Apis, the antigen was also expressed on the perineural sheath and granular cell compartments in the majority of neuronal cell bodies. The antibody stained monopolar cells in the visual system of both species, and in Apis those fibers of the anterior superior optic tract which link the medulla with the mushroom bodies. In Drosophila, bundles of Kenyon cells of the mushroom bodies were stained. In worker bees and drones, the relay neurons of the median and lateral antennoglomerular tracts were labelled. Since the recognition of the antigen does not require fixation, the antibody can be employed to label selectively living neurons in dissociated cell culture. This opens up the possibility for future functional studies on the role of the antigen in vitro.
We have labeled the growth cones of retinal ganglion cell axons with HRP in intact mouse embryos. This has allowed us to visualize growth cone morphology during outgrowth along an entire CNS pathway from origin to target; to ask whether growth cone forms, and thus behaviors, differ at various points along the pathway; and to study the relationships of growth cones with the cellular environment. During the major period of axon outgrowth between embryonic day (E) 12 and 15, growth cones in the optic nerve are highly elongated (up to 40 microns) and have lamellopodial expansions, but the majority lack the microspikes or filopodia characteristic of many growth cones. Within the optic chiasm (E13-15), most growth cones shorten and spread, and project several short filopodia. In the optic tract, growth cones become more slender and again lack filopodia, resembling sleeker versions of optic nerve growth cones. Near the first target region (lateral geniculate nucleus), growth cones with filopodia arise from individual axon lengths and turn medially toward the target. Within target regions, the branches of immature axon arbors are tipped by minute swellings rather than by the enlarged growth cones prevalent during outgrowth toward targets. Electron-microscopic analysis of identified labeled growth cones in the optic nerve reveal intimate interactions between growth cones and glia or other growth cones in the form of invaginating contacts. In the optic nerve, growth cones contact immature glial (neuroepithelial) cells somewhere along their length, and also envelop bundles of neurites. In the chiasm, single growth cones simultaneously relate to many different profiles. These results demonstrate that in this single pathway from origin to targets, growth cone morphology varies systematically with position along the visual pathway. During outgrowth, simple growth cones are prominent when axons follow well-defined common pathways, and more elaborate filopodial forms appear when growth cones diverge, as they turn or come to decision regions. Together with observations in vitro and in nonmammalian nervous systems in situ, these data serve as reference points for testing to what extent growth cone form reflects intrinsic factors and interactions with the environment.
Although earlier experimental studies of the visual system in elasmobranch fishes suggested that these fishes possess fewer primary retino-recipient nuclei than other gnathostome vertebrates, recent studies utilizing more sensitive tracing methods indicate that most elasmobranch species possess ten primary retinofugal targets in addition to the optic tectum. Furthermore, many species appear to exhibit bilateral retinal projections to these nuclei. Similarly, initial claims that the organization of the visual thalamus of elasmobranchs is more primitive than that of most other gnathostomes--in that elasmobranchs possess only a single thalamic nucleus that receives both retinal and tectal inputs and that only a single thalamo-telencephalic projection exists to the telencephalon--have been refuted. Many, if not all, elasmobranchs possess a rostrally located dorsal thalamic nucleus (anterior thalamic nucleus), that receives retinal and tectal inputs and projects bilaterally to the dorsal and medial pallium, and a more caudally and dorsally located thalamic nucleus, the dorsal posterior thalamic nucleus, that receives bilateral tectal input and projects to the ventrolateral periventricular area and/or dorsal pallium of the telencephalon. Thus the thalamic organization of elasmobranch fishes is similar to that of other gnathostomes.
The koniocellular (K) pathway is one of three pathways from the lateral geniculate nucleus (LGN) to primate visual cortex (V1). K pathway projections to the cytochrome oxidase (CO) blobs of V1 suggest involvement in chromatic processing given reports that the CO blobs in diurnal primates contain cells selective for color. K LGN layers and CO blobs, however, are also well developed in nocturnal primates such as owl monkeys, which are likely to be color blind. Thus, the K pathway plays either different roles in different species or some as yet unidentified common role(s). Because synaptic arrangements underlie functional mechanisms, the purpose of this investigation was to compare the synaptic circuitry related to the K pathway within the CO blobs of two diurnal primates (macaque monkeys and squirrel monkeys) and one nocturnal primate (owl monkey). Presynaptic K axons were labeled with wheat germ agglutinin-HRP, and presynaptic and postsynaptic profiles in CO blobs were identified with post-embedding immunocytochemistry for GABA and glutamate. In all three species, K axon terminals are glutamatergic and larger than local axon terminals, suggesting that they have a greater impact on postsynaptic CO blob targets than signals arriving via layer IV from the P or M pathways. A greater proportion of K axons, however, synapse with larger glutamatergic shafts in the diurnal monkeys than in the nocturnal owl monkey, perhaps reflecting the importance of color within the K pathway of these diurnal species. Alternatively, the loss of color vision in the owl monkey could impact K pathway circuitry earlier in the pathway. The basic similarities between K axon circuitry within the CO blobs of the three primate species examined also could indicate that this pathway plays some common role or roles across species.