Postnatal shaping of callosal connections from sensory areas.

Horseradish peroxidase (HRP) was injected unilaterally into the first and second visual areas (V1 and V2; areas 17 and 18) of 20 kittens aged between 2 and 90 days and into the second somatosensory area (S2) of 16 kittens aged between 1 and 52 days. The radial and tangential (normal and parallel to the pial surface, respectively) distributions of neurones giving origin to callosal axons (callosal neurones) were studied. In adult cats, callosal efferent zones (CZs) are defined by the distribution of callosal neurones. CZs occupy in the visual cortices, tangentially and radially restricted parts of areas 17, 18, 19 of the lateral suprasylvian gyrus and in the somatosensory cortices, parts of S1 and S2. At birth, callosal neurones are distributed throughout the tangential extent of visual and somatosensory areas; they are also more widespread in depth than in the adult. During the first postnatal month, as a result of the gradual disappearance of callosal neurones from parts of the visual and somatosensory areas, the adult CZs emerge. The CZ in areas 17 and 18 undergoes a further tangential reduction during the second and third postnatal months.