Brain development in the neonatally decorticated rat.

The brain weights, cross-sectional areas of subcortical structures, gross morphology, and retrograde degeneration in the thalamus and basal ganglia were compared in rats with neonatal (1 or 5 days of age) or adult ablation of all of the neocortex. Neonatal decorticate and control rats also were given injections of fluorescent retrograde tracers. True blue or Nuclear yellow into the striatum, hippocampus, or subiculum in order to identify afferent pathways to these structures. Neonatal decortication resulted in morphological changes that included a dramatically shrunken and misshapen thalamus, a shrunken brainstem, distortion of the hippocampal structure and abnormal growth of the subiculum, none of which were observed following adult decortication. There was a complete absence of gliosis and calcification in the 1-day decorticates that was present in 5-day and adult decorticates. Although most striatal and hippocampal afferents from subcortical structures were present, label was not found in the ventral tegmental area or thalamus after True blue injections into the striatum and hippocampus, respectively. Acetylcholinesterase staining showed the normal striatal patchy organization. The absence of evidence for the development of new pathways or the reorganization of existing pathways is consistent with previous behavioral studies showing little sparing of function after neonatal decortication. The results suggest that the cortex plays little role in programming pattern formation and connections in subcortical structures and that cortical and subcortical structures provide unique contributions to the control of behavior.