Patterns of corticocortical, corticotectal, and commissural connections in the opossum visual cortex.

Patterns of connections of the visual cortex of the South American opossum, Didelphis aurita, were revealed by using neuronal tracers to identify and characterize visual specializations of the peristriate cortex (PS). The visuotopy of corticotectal connections of the anterolateral portion of PS (PSal) is symmetrical to that of the striate cortex (ST or primary visual area [V1]). Three consecutive bands of commissural connections coincide, respectively, with the ST-PS border, the limit between the caudal and rostral PSal halves (PSc and PSr), and the border of PS with the parietal and temporal cortices. PSc and PSr contain regular commissural rings similar to those present in the peristriate cortex of eutherian mammals. ST projections define in PSc two strings of periodical foci consecutively concentric to V1 and a single focus in PSr. Although they were organized topographically, ascending, descending, and commissural connections between ST and PSal showed a high degree of convergence and divergence. These results conform to the model of a single area homologous to the second visual area (V2) bordering V1. Moreover, they suggest the possibility that PSal includes either one or two additional belt-like areas successively anterior to V2. Along with the finding of alternating bands of high and low cytochrome oxidase activity in PSal, the data further suggest that this region contains modular specializations similar to those of the peristriate cortex of primates and other eutherian mammals. The posterolateral peristriate cortex (PSpl) constitutes another visual area, since it consists of a distinct focus of reciprocal corticocortical and interhemispheric connections and a separate source of corticotectal projections. Finally, a visuomotor function for the orbital cortex is proposed based on its direct projections to optical tectal layers. The close cladistic relationship of opossums to mammalian ancestral forms suggests that the PSal parcelation into belt-like areas that contain modules reflects the primitive organization of the visual cortex. Moreover, a highly diffuse pattern of corticocortical connections may represent a requirement for a brain with few visual areas to perform global processing.

Patterns of interhemispheric and striate-peristriate connections in visual cortex of the South American marsupial Marmosa elegans (mouse opossum).

We have analyzed the distributions of interhemispheric and striate-peristriate connections in the South American marsupial, Marmosa elegans (mouse opossum). Following multiple injections of horseradish peroxidase (HRP) into one hemisphere, we found that anterogradely labeled terminations and retrogradely labeled perikarya are distributed unevenly in the contralateral hemisphere, forming a distinct tangential pattern in striate and peristriate cortex. This pattern delineates as many as eight peristriate areas relatively poor in commissural connections in lateral peristriate cortex, and in lateral and anterolateral portions of peristriate cortex. Single injections of HRP conjugated with wheat germ agglutinin into anterior or posterior regions of striate cortex produced as many as nine discrete ipsilateral fields of labeled perikarya, and terminations distributed over a broad cortical area in lateral and anterolateral peristriate cortex. Our observations of multiple areas with little or no HRP labeling in the interhemispheric pattern, and of multiple ipsilateral striate projection fields, indicate that the topography of visual cortex in Marmosa is highly elaborate, and suggest that extrastriate cortex is subdivided into several visual areas. Furthermore, by showing that the organization of visual cortex in this marsupial is as complex as in many placental mammals, our data support the view that a basic cortical plan, consisting of multiple visual areas, appeared early in mammalian evolution.

Patterns of cytochrome oxidase activity in the visual cortex of a South American opossum (Didelphis marsupialis aurita).

The normal pattern of cytochrome oxidase (CO) activity in the posterior cortical areas of the South American opossum (Didelphis marsupialis aurita) was assessed both in horizontal sections of flattened cortices and in transversal cortical sections. The tangential distribution of CO activity was uniformly high in the striate cortex. In the peristriate region alternating bands of dense and weak staining occupied all the cortical layers with the exception of layer I. This observation suggests the existence of a functional segregation of visual processing in the peristriate cortex of the opossum similar to that present in phylogenetically more recent groups.

Pattherns of cytochrome oxidase activity in the visual cortex of a South American opossum

The normal pattern of cytochrome oxidase (CO) activity in the posterior cortical areas of the South American opossum (Didelphis marsupialis aurita) was assessed both in horizontal section of flattened cortices and in transversal cortical sextions. the tangential distribution of CO activity was uniformly high in the strate cortex. In the peristriate region alternating bands of dense and weak staining occupied all the cortical layers with the exception of layer I. This observation suggests the existence of a functional segregation of visual processing in the peristriate cortex of the opossum similar to that present in phylogenetically more recent groups