Reciprocal projections of cat extrastriate cortex: I. Distribution and morphology of neurons projecting from posterior medial lateral suprasylvian sulcus to area 17.

Reciprocal projections between cortical areas have been subdivided into two functionally distinct component, "feedforward" and "feedback" (for review, see Felleman and Van Essen [1991] Cereb. Cortex 1:1-47). Some anatomical evidence, such as differences in the laminar distribution of the neurons of origin and of the terminations of their axons, has supported this division. However, very little is actually known about the distribution and morphology of the neurons of the feedback projections. In order to contribute further to our understanding of these two components of the corticocortical projections, I studied the distribution and morphology of a feedback projection, the reciprocal projection from the posterior medial lateral suprasylvian sulcus (PMLS), to primary visual cortex (area 17). Retrograde transport of horseradish peroxidase and fluorescent tracers in vivo combined with intracellular dye injections in lightly fixed cortical slices revealed many similarities between the feedforward and feedback projections: 1) They both emanate from all layers but layer 1; 2) each layer of origin contains a wide variety of standard and/or inverted pyramidal neurons; and 3) all of these, with the exception of a rare, large layer 5 neuron, have dendritic fields restricted principally to their layers of origin. There was, however, one major difference between the feedforward and feedback projections: In contrast to the projection from area 17 to PMLS, the projection from PMLS had a dense projection from layer 6 that compromised a striking abundance of spiny fusiform and inverted pyramidal neurons. These were morphologically distinct from other layer 6 neurons that project to the thalamus. Taken together, these data suggest that the reciprocal projections between area 17 and area PMLS, although not completely equivalent, share essential features that form a distinct population of neurons differing in morphology from corticothalamic projection neurons.