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Of equal fascination to researchers is the persistence of memory, the ability not only to store but also to recall information and experiences. In Proust's Remembrance of Things Past, Marcel released a flood of memories by tasting a tea-soaked petite Madeleine. Others have found that a memory-jogging whiff of perfume, a word, a few notes of music can conjure up similar—and often realistic—recollections of events they experienced many years earlier. A landmark discovery was made by the great Canadian neurosurgeon, Wilder Penfield, when he found that he could stimulate memories electrically. Probing a patient's brain with an electrode in order to locate the source of her epileptic seizures, Penfield was amazed when the young woman recalled an incident from her childhood in vivid detail. Penfield continued his studies and found that touching various parts of his patients' cerebral cortices with an electrode could enable them to remember songs long forgotten and experiences they thought were lost forever.
Subsequent experiments have proved that though the cortex is involved with memory, it does not act like a computer's memory bank, in which each bit of information is stored in a single electronic "cell." Memory, it has been found, is "delocalized" or spread throughout the cortex, and perhaps throughout the higher brain. Removing half the cortex may cause a proportional loss of capacity to remember, but it does not destroy specific memories.
Experiments and observations now support a three-level theory of memory. The lowest level is short-term memory, lasting no more than a few seconds; every moment of life, hundreds of sensory impressions flow into the human brain and are promptly forgotten. On the next level is medium-term memory, which lasts from a few minutes to a few hours, and enables man to remember something like a telephone number just long enough to dial it or to cram for an examination. At the highest level is long-term memory, which is sifted out of all the impressions and information entering the brain and preserved because of its importance, usefulness or vividness.
Long-term memory takes time to register permanently on the brain. If rats are given an electric shock immediately after learning a new skill, memory of the skill is lost. If the shock is delayed for half an hour, the memory is impaired. But if 24 hours elapse between learning and shock, most of the memory remains. Human beings react in the same way.
Most researchers agree that the limbic, or feeling brain plays a key role in long-term memory. The limbic system is concerned with affects—strong emotional experiences, for example—which people obviously remember. One part of the limbic system, the hippocampus, is indisputably vital to memory. Patients whose hippocampi have been destroyed or partially removed cannot recall new information. Dr. Robert Livingston of the University of California at San Diego postulates that the structure plays the same role in memory as the "now store" button does on a computer, determining whether a particular bit of information is to be stored or discarded.