Callosal connectivity of areas V1 and V2 in the newborn monkey.

The callosal connectivity of areas V1 and V2 in the newborn monkey has been investigated with the neuroanatomical tracers wheat germ agglutinin conjugated to horseradish peroxidase and free horseradish peroxidase. In the adult, callosal projecting neurons in cortex subserving the lower parafoveal visual field were found to extend from the V1/V2 border for a distance of 1-2.5 mm into V1 and 8 mm into V2. In the newborn, the tangential extent and total number of callosal projecting neurons were the same as in the adult. Within area V1, callosal projecting neurons in the adult and newborn were limited to supragranular layers. In the adult, axon terminals of callosal projections were located in layers 4B and 5 and were excluded from layer 4C. In the newborn, axon terminals were more extensively distributed than in the adult and invaded layer 4C. In area V2, the laminar distribution and the patchy location of callosal connections in regions of high cytochrome oxidase activity were similar in the newborn and adult animals. In both newborns and adults, the patchy distribution of callosal projections persisted when the neuroanatomical tracers were injected over extensive regions of the contralateral striate and extrastriate cortex. In the adult, area V1 and V2 project contralaterally to two heterotopic sites located in the fundus of the lunate sulcus and the superior temporal sulcus. This was also found to be the case in the newborn. In the adult the terminals of these heterotopic projections were focused in layer 4. This was not the case in the newborn, where after injection limited to the contralateral V1/V2 border they were more evenly distributed among the different cortical layers. Following extensive contralateral injection of tracer, terminals in cortex anterior to V2 were focused over layer 5 and the bottom of layer 4.