The medial temporal-lobe amnesic syndrome.

This article has attempted to show how early evidence of the existence of multiple memory systems in the brain arose from the study of a few patients with bilateral damage to the medial structures of the temporal lobe in the hippocampal region, as in the case of the now famous patient HM. Such patients exhibit a profound anterograde amnesia for the experiences of daily life, whereas previously acquired knowledge is well preserved and immediate or primary memory is intact, and other cognitive abilities, including language, perception, and reasoning also are unaffected by the lesion. Despite the seemingly global nature of HM's memory loss, it was possible to show by the appropriate choice of behavioral tasks that many implicit,procedural forms of learning were preserved, and these forms are now known to be mediated by different brain systems. The first major finding was the demonstration of normal acquisition of a motor skill by HM, although he remained unaware that he had done the task before. This finding was followed by the demonstration of preserved perceptual learning,and since then the examples of preserved learning in amnesia have multiplied. In addition, after many false starts, a convincing animal model has now been achieved, with convergent findings for human and nonhuman primates. Although considerable progress has been made since the early 1950s, many questions remain unanswered; particularly, the distinct contributions of the various medial temporal-lobe structures to memory processes and the interaction of these structures with other brain areas need to be clarified. As in the past, the solution of such problems will call for a multidisciplinary approach.