Lesions of perirhinal and parahippocampal cortex that spare the amygdala and hippocampal formation produce severe memory impairment.

In monkeys, bilateral damage to the medial temporal region produces severe memory impairment. This lesion, which includes the hippocampal formation, amygdala, and adjacent cortex, including the parahippocampal gyrus (the H+A+ lesion), appears to constitute an animal model of human medial temporal lobe amnesia. Reexamination of histological material from previously studied monkeys with H+A+ lesions indicated that the perirhinal cortex had also sustained significant damage. Furthermore, recent neuroanatomical studies show that the perirhinal cortex and the closely associated parahippocampal cortex provide the major source of cortical input to the hippocampal formation. Based on these 2 findings, we evaluated the severity of memory impairment in a group of monkeys that received bilateral lesions limited to the perirhinal cortex and parahippocampal gyrus (the PRPH lesion). The performance of the PRPH group was compared with that of monkeys with H+A+ lesions, who had been studied previously, and with a group of normal monkeys. Monkeys with PRPH lesions were severely impaired on 3 amnesia-sensitive tasks: delayed nonmatching to sample, object retention, and 8-pair concurrent discrimination. On pattern discrimination, a task analogous to ones that amnesic patients perform well, monkeys in the PRPH group performed normally. Overall, monkeys with PRPH lesions were as impaired or more impaired than the comparison group of monkeys with H+A+ lesions. These and other recent findings (Zola-Morgan et al., 1989b) suggest that the severe memory impairment in monkeys and humans associated with bilateral medial temporal lesions results from damage to the hippocampal formation and adjacent, anatomically related cortex, not from conjoint hippocampus-amygdala damage.