608 Child Development and associational fibers tend to terminate review,see Rakic Goldman-Rakic,1984) Ou focus has been on of axon eigf detal) y r yer areas that comprise the sys nce of the phic methods for studvine The sign ayering odoropetonted re t ml ets of inco of afferents have y in the dis ctive proje it w iographic ca vas the ava n in fig mad layer III pro fetal monkeys ore this metho eoiwgfst ng br ons for ause the e ca te silver) ade m 100 sbcortical structures;neu sin layer VE pp to de ping anim ect selectively to the tha (Fig. vival time aptcachihe ure and ngortiea d ser nt and system (Leo a 1973 lity of the Finally. the re because one could ation of con for ex aseat adult function ca It is wChodtdsooveredwihsihwerimpregna6e out that By cont radio phic tech sulcus an I that ga ioactive isot nt res ini cted size of re at the site begin the ds o th t6 irth or at any other arbi品 eins ancy,the processes and he proteins are rion of the to dg of cell bncamRoebe,ecd ceden logy,the precise survival times optimal for t in my labdy O of ten ina field liest embryor es that i dist on of target 6caeial r ev mpts tha s by whi the axo ns reach their targ e pla 6 at or a birth in smal rlier in the the brain rk:the Sodman-Rakic 1982).It here to develop nniques for 小12 pre nt any giv 608 Child Development and associational fibers tend to terminate selectively in layers IV and I, while associa￾tional fibers additionally terminate in layer VI (see Fig. 3 for details). The cortex also has a radial or horizontal geometry due to its distinct six-layered or￾ganization. The significance of the layering pattem is that cells in the specific layers that are targets of incoming callosal or associa￾tional afferents have distinctive projection targets in subcortical and/or other cortical structures. For example, as shown in Figure 3, pyramidal neurons in layer III project primarily to other cortical areas, bodi widiin the same (associational neurons) and opposite (callosal neurons) hemispheres. While over 80% of callosal and associational neurons originate in layer III, layer V is the msyor source of projections to the caudate nucleus and putamen, to the colliculus, and to other subcordcal structures; neurons in layer VI project selectively to the thalamus (Fig. 3). Knowing these facts, one can begin to ap￾preciate the synaptic architecture and wiring diagrams underlying a specific cortical func￾tion. Prenatal Development of Prefrontal Connections Background for prenatal studies.—It is reasonable to assume that adult function can emerge only if the proper connections and circuitry necessary to carry out that fimction are established. The principal sulcus and its interconnections provide a natural model sys￾tem for the study of the biological basis of die particular type of cognitive capacity indexed by delayed-response tasks, namely, the guid￾ance of behavior by stored representations. Although in theory it is possible to begin the developmental study of structure-function re￾lations at birth or at any other arbitrary point in infancy, the processes and mechanisms of development are complex, onierly, and syn￾chronized and can best be appreciated only by comprehensive analysis of antecedent causes. Accordingly, the study of prefrontal cortex development in my laboratory includes analysis of the earliest embryonic ages that it is feasible to study in primates. It is important to realize that the critical cellular events that take place at or after birth in small mammals, for example, the rat, occur much earlier in the primate, well before birth (for fuller discus￾sion of these events, see Goldman-Rakic, 1986; Rakic & Goldman-Rakic, 1982). It was therefore necessary to develop techniques for prenatal surgery and prenatal intervention that could be applied to the primate brain (for review, see Rakic & Goldman-Rakic, 1984). Our focus has been on the outgrowdi of axons and the establishment of synaptic connections between the key areas that comprise the sys￾tem underlying delayed response. Autoradiographic methods for studying prefrontal connections.—Very few studies have ever been conducted on the develop￾ment of connectivity in the primate brain, and it was the availability of the autoradiographic method in conjunction with prenatal surgery that made it feasible to trace connections in fetal monkeys. Before this method, connec￾tions were traced by performing rather large lesions of structures, and after an appropriate survival period, examining brain sections for degenerating axons and terminals (that could be visualized because they selectively im￾pregnate silver). This mediod had several drawbacks that made it particularly difficult to apply to developing animals. The optimal sur￾vival time for visualizing the products of de￾generation had to be worked out empirically and separately for every age point and neural system (Leonard, 1973). The reliability of the silver impregnation varied with investigators. Finally, the resolution of connections was in￾trinsically poor because one could describe projections only between large areas. Golum￾nar distribution of connections, for example, was not discovered with silver-impregnation methods. By contrast, the autoradiographic tech￾nique was a highly reliable method that gave clear and consistent results. Small quantities of radioactive isotopes are injected into desig￾nated areas; the size of the area injected can be controlled by the volume of isotope in￾jected. Neurons at the site of injection absorb die mdiolabeled amino acids into their cell bodies. In the cell body, the amino acids are converted into proteins by normal protein synthesis, and these proteins are transported from the cell dirough its axon to the terminal region of die cell. From knowledge of cell biology, the precise survival times optimal for transport to occur can be accurately predicted to optimize visualization of terminal fields (the distribution of labeled axons in target stmctures) and for determination of die path￾ways by which the axons reach their targets. Sections cut throu^ the brain are mounted on slides that are dipped in photographic emulsion in the dark; the emulsion is exposed only by the beta emissions radiating from labeled cells, axons, and terminals that may be present within any given section. After 8— 12 weeks, the slides are developed and the exposed areas appear as darkened grains