SEVERITY OF MEMORY IMPAIRMENT IN MONKEYS AS A FUNCTION OF LOCUS AND EXTENT OF DAMAGE WITHIN THE MEDIAL TEMPORAL-LOBE MEMORY SYSTEM

During the past decade, work with monkeys has helped identify the stru ctures in the medial temporal lobe that are important for memory: the hippocampal region (including the hippocampus proper, the dentate gyru s, and the subicular complex) and adjacent cortical areas that are ana tomically linked to the hippocampus, i.e., the entorhinal, perirhinal, and parahippocampal cortices. One idea that has emerged from this wor k is that the severity of memory impairment might increase as more com ponents of the medial temporal lobe are damaged. We have evaluated thi s idea directly by examining behavioral data from 30 monkeys (ten norm al monkeys and 20 monkeys with bilateral lesions involving structures within the medial temporal lobe) that have completed testing on bur st andard memory battery during the last 10 years. The main finding was t hat the severity of memory impairment depended on the locus and extent of damage to the medial temporal lobe. Specifically, damage limited t o the hippocampal region Produced a mild memory impairment. More sever e memory impairment was produced when the damage was increased to incl ude the adjacent entorhinal and parahippocampal cortices (the Hf lesio n). Finally, memory impairment was even more severe when the H+ lesion was extended forward to include the anterior entorhinal cortex and th e perirhinal cortex (H++ lesion). Taken together, these findings sugge st that, whereas damage to the hippocampal region produces measurable memory impairment, a substantial part of the severe memory impairment produced by large medial temporal lobe lesions in humans and monkeys c an be attributed to damage to entorhinal, perirhinal, and parahippocam pal cortices adjacent to the hippocampal region. (c) 1994 Wiley-Liss, Inc.