Topography of orientation centre connections in the primary visual cortex of the cat

The functional topography of lateral connections to orientation-centre zone s was studied by optical imaging of intrinsic signals in combination with t racer injections (fluorescent beads and biocytin) and electrophysiological recordings. Three-dimensional reconstruction of anterogradely labelled axon terminals and retrogradely labelled somata revealed a uniform distribution across all orientations in a non-patchy manner. The overall lateral extent of the labelling was 3-4 mm in layer 3, that is about half of the extent o bserved for orientation domain connections in the same layer. These bulk in jection data are in contrast with the reportedly sharp orientation tuning o f neurons of centre zones and suggest that orientation specificity here doe s not require highly specific connections. Nonetheless, another plausible s cenario is that orientation centre connections are orientation specific but their specificity present at the single cell level cannot be revealed by b ulk labelling due to their large spatial overlap. NeuroReport 12:1693-1699. (C) 2001 Lippincott Williams & Wilkins.