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.