Contrasting effects on discrimination learning after hippocampal lesions and conjoint hippocampal-caudate lesions in monkeys

Eighteen monkeys with lesions of the hippocampal region (the hippocampus pr oper, the dentate gyrus, and the subiculum) made by an ischemic procedure, radio frequency, or ibotenic acid were tested on a simple, two-choice objec t discrimination learning task that has been shown to be sensitive to large lesions of the medial temporal lobe. The monkeys were also tested on two o ther discrimination tasks (pattern discrimination and eight-pair concurrent discrimination) that can be learned normally by monkeys with large medial temporal lobe lesions. All of the lesion groups were impaired at learning t he simple object discrimination task. Seven of the monkeys who had sustaine d damage to the hippocampal region also sustained damage to the tail of the caudate nucleus. These seven monkeys, but not the other 11 monkeys with hi ppocampal lesions, were impaired on pattern discrimination and concurrent d iscrimination learning. The results suggest that the hippocampal region is important for learning easy, two-choice discriminations, whereas the caudat e nucleus is necessary for the normal learning of more difficult, gradually acquired discrimination tasks. The findings support the distinction betwee n declarative memory, which depends on the hippocampus and related medial t emporal lobe structures, and habit learning, which depends on the caudate n ucleus.