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Postnatal maturation of subcortical projections from the prefrontal cortex in the rhesus monkey

Orbital and dorsolateral prefrontal lesions were performed on a series of rhesus monkeys at 2, 6, or 24 months of age. The consequent degeneration in the efferent pathways from these cortical regions to the caudate nucleus, the dorsomedial nucleus of the thalamus and adjacent structures was studied at 5‐ and 15‐day survival times by a modification of the Nauta‐Gygax method for tracing degenerating fibers. Following dorsolateral lesions, considerable numbers of black‐impregnated degenerating fibers were found in the parvocellular division of the dorsomedial nucleus and in the fiber bundles of the internal capsule and the subcallosal fasciculus at all ages. In contrast, the degree of Nauta‐Gygax degeneration in the anterodorsal sector of the head of the caudate nucleus was age‐dependent: degenerating fibers were found in increasing numbers from two months of age, when virtually none could be detected, to 24 months of age, when they appeared in relatively dense concentration. Similar results were obtained following orbital prefrontal lesions at the same three ages: degeneration in the magnocellular division of the dorsomedial nucleus of the thalamus and in the internal capsule were not related to age, but anterograde degeneration in the ventral and lateral capsular region of the caudate increased substantially with advancing age. Negative results with the Nauta‐Gygax technique in very young animals may not signify an actual absence of the connection in question, but our findings, together with those of other studies employing silver degeneration methods, at least suggest that in the primate, certain cortical efferents undergo maturational changes during postnatal life, a fact which may be related to the differential behavioral consequences of cortical injuries at different stages of development.