The maturational status of thalamocortical and callosal connections of visual areas V1 and V2 in the newborn monkey.

Cytochrome oxidase (CytOx) is known to preferentially stain those regions of the visual cortex which receive direct projections from the thalamus. The pattern of CytOx stain has been used to investigate the maturation of thalamic input to areas V1 and V2 in the newborn monkey. In both areas, the intensity of CytOx activity was similar in newborns and adults. The distribution of CytOx in area V2 was not found to vary with age. In area V1, the only difference in CytOx activity in newborns was a relative immaturity of staining in layer 4C. The callosal connections of visual areas V1 and V2 were investigated by the axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase and free horseradish peroxidase. In the adult, V1 was found to be reciprocally callosally connected for a distance of 1-2.5 mm from the V1/V2 border, whilst V2 was connected for a distance of 3-8 mm from the border. In both areas, callosal connections showed a certain degree of clustering, particularly in V2 which contained 97-98% of the total number of callosal connections of these two areas. In the newborn, the number, tangential extent and clustered distribution of callosal connections were as in the adult. In the newborn, as in the adult, callosal connections coincided with regions of high CytOx activity in area V2. The results showing a relative maturity of the tangential distribution of callosal projecting neurons on the one hand, and an immaturity of thalamic projections on the other, are discussed in terms of: (1) the maturational status of the newborn monkey compared to other mammals at the moment of birth and (2) the possible role of visual experience in shaping cortical connections.