Journal of Neuroscience, Vol 13, 3749-3772, Copyright © 1993 by Society for Neuroscience
Hierarchical organization of areas in rat visual cortex
TA Coogan and A Burkhalter
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110.
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 extrastriate 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 projections 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.(ABSTRACT TRUNCATED AT 400 WORDS)
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