The hippocampus, memory, and space.

In the 1970s, there was considerable uncertainty about the function of the hippocampus. Study of the amnesic patient, H. M., who had sustained a bilateral medial temporal lobe resection, indicated that this region of the brain was important for memory functions (Scoville and Milner, 19.57). However , the medial temporal lobe is a large region that includes not only the hippocampus, but also the amygdala and adjacent cortex. The relevance of the hippocampus itself to H. M.'s memory impairment was uncertain. In addition, studies of rats and other animals with hippocampal lesions pointed in a number of directions other than memory, and there seemed to be little common ground in the work on different species (for reviews from this period evaluated the available data and proposed that the important brain region for memory was not the hippo-campus at all but temporal stem white matter, which lies adjacent to the hippocampus just above the lateral ventricle. This interpretation was subsequently ruled out by experiment , but the argument was cogently developed and demonstrated effectively that in 1978 the idea that the hippocam-pus is important for memory was not on firm ground. Second, it was reported that a large medial temporal lobe removal in the monkey. similar t o the surgical lesion sustained by patient H. M., caused severe memory impairment (Mishkin, 1978). Although there was more to do before the impairment was fully understood (Mishkin et al.. 1982: Squire and Zola-Mor-gan, 1983: Zola-Morgan and Squire, 1991), this publication was the first of a new era of work on the neuroanatomy of memory, and, in this sense, it signalled the successful establishment of an animal model of human amnesia in the non-human primate. The third event occurred in September 1978, when patient R. B. became amnesic a s the result of a post-operative ischemic event. After his death 5 years later, he was found to have selective bilateral damage to the CAI region of the hippocampus. This case thereby provided compelling evidence that damage limited to the hippocampus could cause clinically significant memory impairment (Zola-Morgan e t al., 1986). Finally, in the same year, O'Keefe and Nadel (1978) published their seminal book. Their view, which was influenced especially by work with rodents. was that the hippocampus is a cognitive map. a memory system that computes and stores information about allocentric (viewer-independent) space. The function of the hippocampus is much better understood now …