Cytoarchitectural differences are a key determinant of laminar projection origins in the visual cortex.

Regularity of laminar origin and termination of projections appears to be a common feature of corticocortical connections. We tested three models of this regularity, originally formulated for primate cerebral cortex, using quantitative data on the relative supragranular layer origins (SGN%) of 151 projections from 19 areas ( approximately 145,000 neurons) to four areas of cat extrastriate cortex. Predictive variables in the models were: hierarchical level differences (Barone et al., 2000), structural type differences (Barbas, 1986), and distances (Salin and Bullier, 1995) between areas. Global and local hierarchies of cat visual cortex were used to evaluate the hierarchical model. Ranking of areas by their cytoarchitectural differentiation (e.g., relative prominence of layer IV) allowed testing of the structural model, while the distance model was tested for the number of borders separating areas. Laminar projection origins correlated moderately with hierarchical differences, and poorly with border distances, but were strongly and consistently correlated with area differences in cytoarchitectural rank. Moreover, projection densities were moderately and negatively correlated with area distances and structural differences. Our findings suggest that the relative cytoarchitectural differentiation of cortical areas is the main determinant of laminar projection origins in cat visual cortex, and may underlie a general laminar regularity of mammalian cortical connections.