Cognitive Memory formation and habit formation are two qualitatively different retention processes based on separate neural mechanisms. On the evidence that the limbic memory system is not fully developed in infant monkeys, we have prepared monkeys with neonatal removal of this system to see how emotional and social behavior develops in animals whose infantile global amnesia might persist through adulthood. Animals with neonatal removal of area TE, a higher-order station of the visual system, serve as controls. The results so far indicate that, at three months of age, neonatal ablation of area TE leads to a transient impairment of habit formation (compared to permanent impairment seen with the same lesion in adults), whereas limbic lesions in both infants and adults leave habit formation intact. Interestingly, data on both normal and operated infants suggest that development of the habit system is sexually dimorphic, and that this is due to the high testosterone levels present in male infants before and shortly after birth. At ten months of age, the infants with limbic lesions show impairment in memory formation, whereas the operated controls show significant functional sparing (compared to those that received the same lesions as adults). These findings point to greater compensatory potential after neonatal cortical than after neonatal limbic removals, indicating that association areas of the cortex are less mature at birth, and may thus possess greater plasticity than limbic structures. Direct evidence of neocortical immaturity in the macaque has been provided by our neurobiological studies on opiate and cholinergic receptor distribution and on metabolic activity. Finally, early dysfunction of the limbo-thalamic memory system produces symptoms that are similar to the behavioral syndrome seen in autistic children, providing an animal model of infantile autism.
