Corticocortical connections of the cat sensory areas studied by means of retrograde axonal transport of horseradish peroxidase.

Afferent association connections to each functional cortex, i.e., the auditory, sensorimotor, visual, and "association" cortex, were investigated in the cat with the method of retrograde axonal transport of horseradish peroxidase (HRP). After injections of HRP in various parts of the cerebral cortex, a large number of labeled cells were found within the same functional areas, while a small number of cells were labeled in other cortical areas. This demonstrates that small parts within the functionally uniform cortical area are strongly interconnected by means of short association fibers. Labeled cells were found both in the cortex of the gyral surfaces and sulcal walls. Corticocortical neurons in the walls of some sulci were also shown to send their axons to various sensory areas. Cells of the origin of association fibers were mostly pyramidal in shape, occurring mainly in layer III of the various cortical areas (about 70% of the total number). Following injections of HRP suspension into the visual cortex, however, the majority of labeled cells (82%) were found in layers V and VI of the Clare-Bishop area, some of them were polygonal in shape. Somal diameters of the labeled cells were 15.8-17.2 micron (mean) +/- 2.5-3.0 (SD).