Object and spatial relational memory in adult rhesus monkeys is impaired by neonatal lesions of the hippocampal formation but not the amygdaloid complex.

Adult rhesus monkeys with neonatal aspiration lesions of the hippocampal formation or the amygdaloid complex (including their respective subjacent cortices) and their age-matched controls were tested on the transverse patterning problem (A+ vs. B-, B+ vs. C- and C+ vs. A-) and a spatial version of the delayed nonmatching-to-sample (DNMS) task with delays of 10 s to 30 s, 60 s, 120 s, and 600 s. Monkeys with neonatal damage to the amygdaloid complex learned both tasks and did not differ from controls at any delay of the spatial DNMS task. Monkeys with neonatal hippocampal damage, however, were unable to learn transverse patterning, though they easily transferred to a linear series (A+ vs. B-, B+ vs. C-, and C+ vs. X-). Three of the four were also unable to reach criterion on the spatial DNMS task within the limits of testing, and the performance of all four monkeys deteriorated with increasing choice delays. The data suggest a role of the primate hippocampal region in both object and spatial relational learning.