Studies of the anatomy and function of the brain system for memory in humans and animal models are reviewed from the Veterans Affairs Medical Center, San Diego and the Department of Psychiatry, University of California, San Diego, La Jolla, CA. Patients who underwent temporal lobe surgery developed memory impairment only when the removal extended far enough posteriorly to include the hippocampus and the parahippocampal gyrus. Horel proposed that memory functions were disrupted not by hippocampal damage, but by damage to temporal stem white matter adjacent to the hippocampus. Damage to both the hippocampus and the amygdala was required to produce severe amnesia in monkeys and humans. A circumscribed bilateral lesion involving the entire rostro-caudal extent of the CA1 field of the hippocampus caused memory impairment in the absence of other cognitive dysfunction. By use of the high resolution MRI the hippocampal region was found to be shrunken and atrophic in 4 amnesic patients while the temporal lobe was of normal size. Memory is classified as declarative (explicit) or non-declarative (implicit). Declarative memory refers to recollection of facts and events and depends on the integrity of the medial temporal lobe. Non-declarative memory refers to a collection of skills, habits, priming, conditioning and non-associated learning, all of which are non-conscious recollections, and is independent of the medial temporal lobe. The medial temporal lobe memory system is essential for establishing long-term memory for facts and events and is needed to bind together the distributed storage sites in neocortex that represent a whole memory. As time passes after learning, memory stored in the neocortex gradually becomes independent of the medial temporal lobe structures and the role of this system in memory is only temporary. More permanent memory develops presumably as a result of slow synaptic change and in concert with normal forgetting. The process by which the burden of long-term permanent memory storage is gradually assumed by the neocortex assures that the medial temporal lobe system is always available for the acquisition of new information. [1]

COMMENT: The article refers to a neuropsychological evaluation of a patient, H.M., with a profound and a selective impairment in human memory after bilateral surgical removal of the medial temporal lobe [2]. A reference to Marcel Proust’s novel of the mind “The Remembrance of Things Past,” is made in an editorial by Hilts PJ in The New York Times Sept 24, 1991. Proust’s memory of a town and gardens was triggered by the pleasures of a morsel of cake soaked in a spoonful of warm tea, a mechanism involving the amygdala.