| Summary: | Using the double-labeling technique with fluorescent tracers local patch and callosal
neurons in area 18 of the cat were studied. The main goal was to determine whether single
cortical neurons participate in both the local patch networks and callosal projection. Callosal
and local patch neurons were found in overlapping and interdigitating patch networks.
Double-labeled cells were not found in the regions of overlap between the two populations
of labeled cells, or in any other locations in area 18, suggesting that the callosal and local
patch neurons form separate neuronal populations in the superficial layers of cat area 18.
The morphology of local patch neurons was next examined and compared to callosal
neurons in the superficial layers of area 18. The local patch neurons were mostly spiny
pyramidal neurons including modified, small to medium sized standard, and star pyramids.
Smooth, multipolar neurons, presumably of the basket cell type were also present. In the
tangential plane, the basal dendritic trees of about half of the local patch pyramidal neurons
in layer 2/3 had mediolaterally elongated fields and the rest had circular fields. The callosal
neurons in layer 2/3 were standard, fusiform, and star pyramids. Callosal standard
pyramids were, on average, larger than the local patch standard pyramids based on mean
values of somatic areal measurements. The mean dendritic field width of the callosal cells
was also larger than that of the local cells. Callosal cells also possesed a more complex
dendritic branching pattern, with numerous branch points. Both features appear to be
independent of cell size. Finally, almost all the callosal cells had closely circular dendritic
fields when viewed in the tangential plane. Thus, the local patch and callosal neurons can
be distinguished from each other by morphological and morphometric parameters.
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