The organization of visual corticocortical connections in early postnatal kittens.

We tested the hypothesis that, in newborn kittens, superficial layers of the extrastriate cortex receive more specific patterns of corticocortical innervation from the striate cortex than deep layers. First, we injected retrogradely transported tract-tracers at a range of depths in area 18 to label area 17. All injections were of similar tangential diameter and were in the same region of rostral area 18, where the visual field 10-20 degrees below the horizontal meridian is represented. Injections that involved only the superficial layers of area 18 labelled cells mainly in the superficial layers (future layers 2-4) of area 17, across a region that was 2-3 mm wider than the diameter of the injection site in the rostrocaudal direction. Injections that involved all layers of area 18 labelled cells in both superficial and deep layers (5 and 6) of area 17, across a region that was 6-9 mm wider than the diameter of the injection site in the rostrocaudal direction. These values demonstrate that, in neonates, the convergence of projections from area 17 to the superficial layers of area 18 is less than that to the deep layers of area 18. The lower values for convergence obtained by injecting only the superficial layers of area 18 in kittens were similar to those obtained by injecting all layers of area 18 in adult cats; the values obtained by injecting all layers of area 18 in kittens were much higher. Second, we injected the full depth of area 17 in newborn kittens with labels that travel anterogradely and retrogradely. Confirming the conclusions from the use of retrograde tracers, these focal injections produced very widespread labelling of the deep layers of area 18, but much more localized and topographically organized labelling of its superficial layers. These results indicate that there is a considerable postnatal improvement in the accuracy with which corticocortical cells in striate visual cortex target appropriate regions in extrastriate cortex, in agreement with previous findings. They also demonstrate that this change occurs mainly among those striate cortical neurons that innervate a wide region of the deep layers of extrastriate cortex at birth. The innervation of the superficial layers of extrastriate cortex is much more accurate from the outset.