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 extra stria te 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 pro-jections 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. In this scheme, area 17 is at the first level, LM ranks higher and is likely at the second level, AL ranks above LM, and MX is at a still higher level. Sites in FLX receive columnar projections from LM, AL, and MX, and so by our criteria this complex spans the hierarchical levels that these areas occupy. The ordering of cortical areas is consistent with an ordering derived from the pattern of corticotectal projections where inputs from successively higher areas terminate in increasingly deeper layers.