Transient projection from the superior temporal sulcus to area 17 in the newborn macaque monkey.

Area 17 in the neonate of numerous species receives projections from cortical areas that do not project to area 17 in the adult. To investigate if this were the case in the developing primate, we have made injections of retrograde tracers in area 17 of newborn monkeys (Macaca irus) and examined the areal distribution of labeled neurons. Neurons projecting to area 17 were found to be restricted to those cortical regions that project to area 17 in the adult. The projection to area 17 in the neonate did appear to be very different in that in the superior temporal sulcus there was a large contingent of labeled neurons in supragranular layers. This constitutes a transient projection because in the adult area 17 projecting neurons in this cortical region originate almost exclusively from infragranular layers. To test if a change in the laminar distribution of area 17 projecting neurons in extrastriate cortex is a general feature of postnatal development, we have computed in neonates and adults the proportion of area 17 afferent neurons in infra- and supragranular layers for each cortical region that projects to area 17. This revealed (i) that in the adult the laminar distribution of area 17 afferents is characteristic for each cortical area and (ii) that this distribution emerges during development from an immature state in which labeled neurons are more numerous in supragranular layers. These results show that there is an extensive remodeling of the neuronal circuitry connecting visual cortical areas during postnatal development in the monkey and that the transient connectivity of primate area 17 is very different from that observed in other mammalian species.