Organization and connections of V1 in Monodelphis domestica

We examined the internal organization and connections of the primary visual area, V1, in the South American marsupial Monodelphis domestica. Multiunit electrophysio logical recording techniques were used to record from neuron s at multiple sites. Receptive field location, size, progressions, and reve rsals were systematically examined to determine the visuotopic organization of V1 and its boundaries with adjacent visual areas. As in other mammals, a virtually complete representation of the visual hemifield was observed in V1, which was coextensive with a region of dense myelination. The vertical meridian was represented at the rostrolateral boundary of the field, the u pper visual quadrant was represented caudolaterally, whereas the lower visu al quadrant was represented rostromedially. Injections of fluorescent trace rs into V1 revealed dense connections with cortex immediately adjacent to t he rostrolateral boundary, in peristriate cortex (PS or V2). Connections we re also consistently observed with a caudotemporal area (CT), entorhinal co rtex (EC), and multimodal cortex (auditory/visual, AN). These results demon strate that M. domestica possess a highly differentiated neocortex with cle ar functional and architectonic cortical field boundaries, as well as discr ete patterns of cortical connections. Some connections of V1 are similar to those observed in eutherian mammals, such as connections with V2 and extra striate areas (e.g., CT), which suggests that there are general features of visual system organization that all mammals posses due to retention from a common ancestor. On the other hand, connections of V1 with EC and multimod al cortex may be a primitive feature of visual cortex that was lost in some lineages, may be a derived feature of marsupial neocortex, or may be a fea ture particular to mammals with small brains. (C) 2000 Wiley-Liss, Inc.