Axonal distribution of Layers IV-VI Connections in Human Visual Cortex Revealed by in-vitro extracellular injections of Biocytin.

The organization of interlaminar axonal connections was studied in slices of adult human visual association cortex. Cortical tissue was obtained during surgical glioma treatment in three subjects. Slices were injected with the highly sensitive anterograde tracer biocytin. In some cases, tract tracing was combined with laminar recordings of electrically evoked potentials, employing current-source density analysis. This study is based on 23 injections placed at various cortical layers and analyzed in detail. The results reveal a striking columnar architecture of local axons organized in vertically oriented bundles as well as vertical columns of neuronal cells bodies. A high degree of specificity was observed in the connectional patterns of various layers. This specificity was reflected in the tendency of lateral connections to spread into different cortical layers, and in the large variations in the axonal bouton densities in different layers. More specifically. Layer IV injections resulted in a more circular projection pattern including massive projections into layer II-III. Layer V injections produced extensive intra-laminar lateral connections while layer VI injections resulted in a vertical projection leading up to layer IV and a lateral intra-laminar projection. Thus, our results reveal highly specific connectional patterns in the different cortical layers in adult human association visual cortex.