The neurology of memory: quantitative assessment of retrograde amnesia in two groups of amnesic patients

The phenomenon of retrograde amnesia has important implications for understanding normal memory as well as its neural organization. Using 6 tests of remote memory, we evaluated the extent and severity of retrograde amnesia in 2 groups of amnesic patients--7 patients with alcoholic Korsakoff's syndrome and 5 other patients with amnesia (anoxia or ischemia, N = 3; thalamic infarction, N = 1; unknown etiology, N = 1). Although there were individual differences, Experiment 1 showed that the severity and extent of retrograde amnesia was similar for the 2 groups. Retrograde amnesia was temporally graded across a period of about 15 years and was not detectable in more remote time periods. In Experiment 2, repeated testing during a 3 year period showed that amnesic patients and control subjects were similarly consistent in their responses. Amnesic patients did not catch up to control subjects by eventually accumulating as many correct answers as the control subjects. In Experiment 3, amnesic patients performed normally on a test of very difficult general information questions, which were based on material likely to have been learned long ago. In all 3 experiments, the 2 groups of amnesic patients performed similarly. The results support the following conclusions: (1) Extensive, temporally graded retrograde amnesia, which has been observed frequently in patients with Korsakoff's syndrome, occurs readily in other amnesic patients as well, even when their memory impairment appears well circumscribed; (2) patients with presumed damage to either the medial temporal or the diencephalic brain structures linked to memory functions can produce a similar kind of retrograde amnesia; (3) the impairment reflects a loss of usable knowledge, not simply difficulty accessing an intact memory store that can then be overcome given sufficient retrieval opportunities; (4) very remote memory, at least for factual information, can be intact in amnesia; (5) the structures damaged in amnesia support memory storage, retrieval, or both during a lengthy period of reorganization, after which representations in memory can become independent of these structures.