THE INCIDENCE OF BIFURCATION AMONG CORTICOCORTICAL CONNECTIONS FROM AREA 17 IN THE DEVELOPING VISUAL-CORTEX OF THE CAT

In newborn kittens, cells in the striate cortex (visual area 17) that project to area 18 (part of extrastriate cortex) are distributed with uniform density in the superficial and in the deep layers. During postnatal weeks 2-3, some of these corticocortical connections are removed to generate an adult-like projection in which association cells are clustered mainly in the superficial layers of area 17. Axonal elimination, without cell death, is the major factor sculpting patches of corticocortical cells in superficial layers. In adult cats, few cells in area 17 (approximately 5%) have axons that bifurcate to multiple extrastriate areas. We have studied the possibility that the early exuberant innervation of area 18 by neurons in area 17 is largely from the transient collaterals of axons that also project to other visual areas. Kittens aged 2-21 days were each injected with a pair of retrogradely transported tracers, either diamidino yellow and fast blue. or diamidino yellow and a carbocyanine dye, at retinotopically corresponding points in area 18 and either area 19 or the posteromedial lateral suprasylvian cortex (PMLS). As for injections in area 18, those in area 19 and PMLS in kittens aged less-than-or-equal-to 5 days labelled cells in continuous bands in area 17; in older kittens neurons projecting from area 17 to extrastriate regions were in patches, mainly in superficial layers. In each animal, the labelling from the two injections overlapped by 51-92%. However, at all ages, never more than 4% of cells projecting to area 18 branched to PMLS; less-than-or-equal-to 6% of area 17-to-18 cells bifurcated to area 19 in kittens aged greater-than-or-equal-to 15 days, although slightly more (10-12%) did so at 3- 5 days. Thus, as in adults, we found no evidence of frequent collateralization among the axons of cells projecting from area 17 to other extrastriate areas in kittens.