Cognitive memory and habit formation are two qualitatively different learning processes based on separate neural systems, a cortico-limbic and a cortico-nonlimbic system, respectively. To see how emotional and social behavior develop in animals whose infantile global amnesia might persist from infancy through adulthood, we have prepared monkeys with neonatal limbic lesions and followed their behavioral development. Animals with neonatal removal of cortical area TE, a higher-order visual station linked to both learning systems, serve as controls. Assessment of the effects of these neonatal lesions on the early development of habits and memories points to greater compensatory potential after neonatal cortical than after neonatal limbic removals. In addition, the long-term effects of these early lesions indicate that compensatory mechanisms operate early to promote permanent recovery from neonatal temporal cortical but not from neonatal limbic lesions. These findings suggest that association areas of the cortex are immature 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 opiatergic and cholinergic receptor distribution and on metabolic activity. Data on both normal and operated infants suggest that development of the nonlimbic habit system is sexually dimorphic, and that this is due to the high testosterone levels present in male infants before and shortly after birth. Interestingly, early damage to the limbic memory system produces later socio-emotional abnormalities. The developmental time course and the nature of these disturbances resemble those seen in autistic children. Finally, the studies in young adult monkeys together with those in normal aged animals are providing the anatomical and chemical basis for understanding the memory disorders in humans that accompany cerebrovascular and other cerebral accidents and diseases as well as the gradual decline in memory ability that occurs with normal aging.
