DEVELOPMENTAL REMODELING OF PRIMATE VISUAL CORTICAL PATHWAYS

The pre- and postnatal developmental changes of the cortical afferents to area 17 were studied in the macaque monkey. Paired injections of t he retrograde tracers fast blue and diamidino yellow were made in area 17. Quantitative techniques were used to examine the spatial patterns of labeling in three distinct locations of the extrastriate cortex th at correspond to known visual areas. In the adult, each cortical regio n has a characteristic laminar distribution. In the fetus the proporti on of supragranular layer neurons in all cortical regions was much hig her than in the adult. The present study shows that despite the very h igh levels of labeled supragranular layer neurons, there is some early areal specialization so that the adult configuration does not emerge from a uniform distribution. The developmental decline in the proporti on of labeled supragranular neurons is complete by 1 month after birth . Each injection of tracer gave rise in each cortical area to dense la beling in a restricted region (projection zone). Areal measurements of projection zones in the supra- and infragranular layers showed that t he developmental decrease in the proportion of labeled supragranular l ayer neurons is accompanied by a relative change of the dimensions of supra- and infragranular projection zones: the supragranular projectio n zone in the fetus is larger than the infragranular projection zone a nd vice versa in the adult. In the fetus, the two projection zones cor responding to each of the two tracers overlap in the supragranular lay ers whereas they are largely separated in the infragranular layers. Du ring development there is a progressive decrease in the overlap of the supragranular projection zones and an increase in the overlap in the infragranular layers. Again, the adult configuration is achieved 1 mon th after birth. This developmental inversion of the areal dimensions o f the projection zones in supra- and infragranular layers is accompani ed by a drastic decrease in the proportion of double-labeled neurons l ocated in supragranular layers. These results clearly show that early in development, axonal projections to area V1 are modified in very dif ferent ways according to whether they originate from supra- or infragr anular layers. This developmental process lasts for about 80 d. These findings show that in the primate there is a prolonged remodeling of a xonal projections that is a highly characteristic feature of this spec ies.