Pattern in the laminar origin of corticocortical connections.

The laminar origin of cortical projections to the frontal cortex was studied in 17 adult rhesus monkeys with the use of the retrograde transport of horseradish peroxidase (HRP). The frontal regions injected with HRP extended from the posterior periarcuate region to the frontal pole. The architectonic boundaries of areas containing HRP-labeled neurons were determined from matched sections stained for the visualization of cell bodies, myelin, or acetylcholinesterase. The results showed that the laminar origin of both nearby and distant corticocortical projections was correlated with the architectonic differentiation of the regions giving rise to the projecting afferent fibers. Frontally directed projections from limbic cortices, which show a rudimentary laminar organization, emanated mainly from deep layers. On the other hand, projections from increasingly more differentiated cortices arose progressively from the upper (or supragranular) layers. This pattern was observed for projections originating along the axis of architectonic differentiation of the visual, somatosensory, auditory, motor, and prefrontal cortical systems. Thus, as the cortical architecture within each system changes from limbic areas toward the primary cortices, the origin of frontally directed projections shifts from predominantly infragranular to predominantly supragranular layers.