Orientation topography of layer 4 lateral networks revealed by optical imaging in cat visual cortex (area 18)

The functional specificity of corticocortical connections with respect to t he topography of orientation selectivity was studied by optical imaging of intrinsic signals and bulk injections of fluorescent latex beads (green and red) and biocytin into layer 4. The distributions of retrogradely labelled cells and anterogradely labelled axon terminals were histologically recons tructed from all cortical laminae, and the resulting anatomical maps compar ed with the optically imaged functional maps. Layer 4 injections produced e xtensive horizontal labelling up to 2-3 mm from the injection centres albei t without the clear patchy pattern described after layer 2/3 injections (Gi lbert & Wiesel 1989, J. Neurosci., 9, 2432-2442; Kisvarday et al. 1997, Cer ebral Cortex, 7, 605-618). The functional (orientation) distribution of the labelled projections was analysed according to laminar location and latera l spread. With regard to the former, no major difference in the orientation topography between supragranular- (upper tier), granular- (middle tier) an d infragranular (lower tier) layers was seen. Laterally, proximal and dista l projections were distinguished and further dissected into three orientati on categories, iso- (+/- 30 degrees), oblique- (+/- 30-60 degrees) and cros s-orientations (+/- 60-90 degrees) with respect to the orientation preferen ce at the injection sites. The majority of distal connections (retrograde a nd anterograde) was equally distributed across orientations (35.4% iso-, 33 .7% oblique-, and 30.9% cross-orientations) that are equivalent with a prep onderance to dissimilar orientations (oblique- and cross-orientations, 64.6 %). In one case, distal excitatory and inhibitory connections could be morp hologically distinguished. For both categories, a marked bias to dissimilar orientations was found (excitatory, 63.7%; inhibitory, 86.6%). Taken toget her, these results suggest that the long-range layer 4 circuitry has a diff erent functional role from that of the iso-orientation biased (52.9%, Kisva rday et al. 1997, Cerebral Cortex, 7, 605-618) layer 2/3 circuitry, and is perhaps involved in feature difference-based mechanisms, e.g. figure ground segregation.