3 CASES OF ENDURING MEMORY IMPAIRMENT AFTER BILATERAL DAMAGE LIMITED TO THE HIPPOCAMPAL-FORMATION

Patient RB (Human amnesia and the medial temporal region: enduring mem ory impairment following a bilaterial lesion limited to field CA1 of t he hippocampus, S. Zola-Morgan, L. R. Squire, and D. G. Amaral, 1986, J Neurosci 6:2950-2967) was the first reported case of human amnesia i n which detailed neuropsychological analyses and detailed postmortem n europathological analyses demonstrated that damage limited to the hipp ocampal formation was sufficient to produce anterograde memory impairm ent. Neuropsychological and postmortem neuropathological findings are described here for three additional amnesic patients with bilateral da mage limited to the hippocampal formation. Findings from these patient s, taken together with the findings from patient RB and other amnesic patients, make three important points about memory. (1) Bilateral dama ge limited primarily to the CAI region of the hippocampal formation is sufficient to produce moderately severe anterograde memory impairment . (2) Bilateral damage beyond the CAI region, but still limited to the hippocampal formation, can produce more severe anterograde memory imp airment. (3) Extensive, temporally graded retrograde amnesia covering 15 years or more can occur after damage limited to the hippocampal for mation. Findings from studies with experimental animals are consistent with the findings from amnesic patients. The present results substant iate the idea that severity of memory impairment is dependent on locus and extent of damage within the hippocampal formation and that damage to the hippocampal formation can cause temporally graded retrograde a mnesia.