Earlier we had assessed visual recognition in monkeys after neonatal damage either to the medial temporal lobe or to the cortical visual area TE, both of which are known to be essential for this mnemonic ability in adult monkeys. The results indicated substantial and long- lasting functional sparing after the early cortical but not after the early medial temporal removals. The findings suggested that area TE is immature early in infancy, and that, due to the plasticity of the developing brain, surrounding cortical visual areas can assume the functions of the damaged area TE. The assessment of visual recognition in monkeys requires that they first learn a rule, in this case the rule for delayed nonmatching-to-sample (DNMS). The ability to master the rule has a protracted ontogenetic development, which could be due to immaturity not only of area TE but also of the inferior prefrontal convexity, another area that is important for DNMS rule learning in adult monkeys. To test this possiblity, we examined the rule-learning effects of neonatal inferior prefrontal lesions and found that they yielded no impairment at 3 months of age and only a mild impairment at 2 years of age, a pattern of sparing closely resembling the one we had found earlier after the neonatal TE lesions. The results at 3 months suggest that the area TE-inferior prefrontal pathway is nonfunctional at this early age, resulting in a total dependency on medial temporal contributions to rule learning, a suggestion supported by a finding of even greater impairment after medial temporal lesions made in infancy than by those made in adulthood. With further development, the TE- prefrontal pathway begins to provide an alternative route for learning, at which point the ability begins to depend less on the medial temporal region and more on this neocortical pathway. Like the long-lasting sparing of rule-learning after early TE lesions, the sparing after early inferior prefrontal lesions is likely due to functional compensation by neighboring tissue, perhaps in this case by orbital frontal cortex, still another area known to be important for visual memory. - monkeys, visual recognition, rule learning, neonatal lesions, temporal visual cortical area TE, rhinal cortex, inferior prefrontal convexity
