DAMAGE LIMITED TO THE HIPPOCAMPAL REGION PRODUCES LONG-LASTING MEMORYIMPAIRMENT IN MONKEYS

Research in humans and monkeys has demonstrated a system of anatomical ly related structures in the medial temporal lobe that is important fo r memory function. This system is comprised of the hippocampal region (i.e., the dentate gyrus, hippocampus proper and subicular complex) an d the entorhinal, perirhinal, and parahippocampal cortices, While the hippocampal region has long been thought to be important in memory, th ere are few systematic studies in primates of the effects on memory of damage limited to the hippocampal region. We have used magnetic reson ance imaging techniques, together with a stereotaxic approach, to prod uce bilateral lesions limited to the hippocampal region (the H lesion) . Damage to the adjacent perirhinal, entorhinal, and parahippocampal c ortex was minimal. Monkeys with the H lesion exhibited significant and long-lasting impairment on the delayed nonmatching to sample task. At the same time, on this and other amnesia-sensitive tasks, monkeys wit h the H lesion performed better overall than monkeys with lesions of t he hippocampal region that also included damage to the adjacent entorh inal and parahippocampal cortices (the H+ lesion). These findings show that, first, the hippocampal region itself is essential for normal me mory function; and second, the adjacent entorhinal and parahippocampal cortices, either alone or in combination, are also an essential compo nent of the medial temporal lobe memory system.