Damage to the hippocampus has been implicated in the permanent loss of memory in patients with medial temporal lobe resections. In two previous studies, it was established that bilateral ablations of the hippocampus in the monkey impaired performance on an associative learning task and on an object discrimination retention task. The two objectives of the present study were to assess the long term effects of hippocampal resections in the monkey and to extend the analysis of the effects of these resections to recognition memory.Therefore, the performance of monkeys with either hippocampal ablations or fornix transections, sustained 5 years earlier, was compared (1) on a concurrent discrimination task-a previously unencountered associative learning task-and (2) on a nonmatching-to-sample recognition task with either delays interposed between the presentation of the sample object and the recognition trial or with lists of either l-, 3-, 5-, or lo-object samples. Significant impairment on both tasks was found after hippocampal, but not after fornix, damage. Though monkeys in the hippocampal group were impaired on both delays and lists, the impairment was more severe on the lists, with abnormal sensitivity to pro-and retroactive interference as a possible source of difficulty. Thus, in parallel with clinical findings, ablations of the hippocampus in the nonhuman primate produce an enduring disruption of memory.When a severe and permanent loss of memory was found in patients after resections of medial temporal lobe structures which included the hippocampal formation, hippocampal gyrus, amygdala, and uncus (

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Ablations of anterior inferotemporal cortex in monkeys are known to impair learning when discriminations between members of several pairs of objects are taught concurrently. This deficit has been attributed to a loss of visual mnemonic functions. But ablations of hippocampus have also been shown to impair retention, and this impairment transcends the visual modality. Therefore, in the first of two experiments, we compared the behavioral effects of inferotemporal cortical lesions with those of either hippocampus, entorhinal area, or fornix, using a visual concurrent discrimination task. Monkeys with either hippocampal or entorhinal ablations were impaired, while those with fornix sections were not. However, ablations of hippocampus included inadvertent damage of the inferotemporal cortex. Therefore, in the second experiment, behavioral effects of inferotemporal lesions were compared with those of hippocampus (without additional inferotemporal damage) on the concurrent task in both visual and tactual modalities. In the visual mode, monkeys with hippocampal removals were as impaired as those with inferotemporal ablations. In the tactual mode, however, hippocampal, but no inferotemporal, ablations were followed by a deficit. Our results, taken together with other existing evidence, emphasize the role of the hippocampus in mediating associative learning in more than one modality. These results, obtained with non-human primates, are in line with clinical findings.

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