DEVELOPMENT AND PLASTICITY OF THE NEURAL CIRCUITRY UNDERLYING VISUAL RECOGNITION MEMORY

In adult monkeys, visual recognition memory, as measured by the delaye d nonmatching to sample (DNMS) task, requires the interaction between inferior temporal cortical area TE and medial temporal lobe structures (mainly the entorhinal and perirhinal cortical areas). Ontogeneticall y, monkeys do not perform at adult levels of proficiency on the DNMS t ask until 2 years of age. Recent studies have demonstrated that this p rotracted development of visual recognition memory is due to an immatu rity of the association areas of the neocortex rather than the medial temporal lobe. For example, lesions of the medial temporal lobe struct ures in infancy or in adulthood yield profound and permanent visual re cognition loss, indicating that the medial temporal lobe structures op erate early in life to sustain visual memory. In contrast, early lesio ns of area TE, unlike late lesions, result in a significant and long-l asting sparing of visual memory ability. Further evidence for neocorti cal immaturity is provided by studies of the development of opiatergic and cholinergic receptors, of the maturation of metabolic activity, a nd of the connectivity between inferior temporal areas TE and TEO and cortical and subcortical structures. Together these results indicate g reater compensatory potential after neonatal cortical than after neona tal medial temporal removals. In support of this view, early damage to area TE leads to the maintenance of normally transient projections as well as to reorganization in cortical areas outside the temporal lobe . In addition, lesion studies indicate that, during infancy, visual re cognition functions are widely distributed throughout many visual asso ciation areas but, with maturation, these functions become localized t o area TE. Thus, the maintenance of exuberant projections together wit h reorganization in other cortical areas of the brain could account fo r the preservation of visual memories in monkeys that have had area TE removed in infancy.