Functional specificity of a long-range horizontal connection in cat visual cortex: a cross-correlation study

Anatomical investigations of the visual cortex revealed a regular set of interlaminar connections and long-range horizontal connections. An important component of the intrinsic cortical circuit is the projection from layer 5 to layer 6 over long horizontal distances. Previous work has shown that when layer 5 is locally inactivated, layer 6 cells lose their response in a segment of their receptive fields corresponding to the blocked area in layer 5. It has therefore been proposed that the long receptive fields characteristic of layer 6 cells might be generated by concatenating layer 5 cells with similar orientation preferences. In the present study, we used cross-correlation analysis to examine both source and target cells of the interlaminar connection from layer 5 to layer 6. We found correlated firing between cells separated by up to 4.2 mm, the longest horizontal distance studied. The occurrence of correlated firing depended on the functional properties and on the topographic position of the cells in layer 5 and layer 6. Interactions were only observed if the cells had matching orientation tuning and similar eye preference and if the layer 5 cells lay within the summation area of the layer 6 cells. Both simple and complex cells in layer 6 fired in synchrony with layer 5 cells. In layer 5, mainly standard complex cells, but very few special complex cells, participated in correlated firing with layer 6 cells. These results suggest that cells in layer 6 receive their input from cooriented, coaxially aligned standard complex cells in layer 5. This topographic arrangement and the length summation properties of standard complex cells in layer 5 are well suited to generate long receptive fields in layer 6. Thus, our experiments provide evidence for the functional specialization of a particular component of cortical circuitry.