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 cognitive, 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 two and six months of age, monkeys with neonatal limbic lesions display abnormal social behavior, whereas the operated controls are essentially unimpaired relative to normal infants. 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). Our tentative conclusion is not only that early and late brain damage have different consequences on learning and memory but also that the direction of the difference depends on whether the locus of injury is cortical or subcortical, the task measures habit or memory formation, and the subject is male or female. Although visual recognition memory measured by problem solving develops late in ontogeny, it can be demonstrated in early infancy when measured by the preferential-viewing tasks. Even though this type of recognition memory is a primitive process, it is nevertheless markedly impaired by either early or late limbic-system damage though not by neonatal ablation of area TE.
NIH ExPorter
NIH ExPorter