Memory

Memories are classified as short-term or long-term. Short-term memories, which sometimes last only a few seconds, are usually concerned with material of fleeting importance such as telephone numbers. Long-term memories, which may last as long as a lifetime, are concerned with material of great significance or possible future use.

Memories may be visual or verbal in content and reproduction. Verbal material includes ideas as well as poems, word lists, dates, and names. Memories acquired through motor activities, and which evoke motor responses—such as the ability to swim—are called skills. Emotional effects may also be stored and reproduced. A child frightened by a dog may retain the fear for many years. It was once thought that skills and emotional memories were retained longer than verbal memories, but experiments have not upheld this theory.

Memory is a complex mental activity. Under certain circumstances, parts of the activity may be made more efficient. Ability to retain increases until a person is about 20, after which it tends to decline. Illness, shock, or lack of sleep may impair memory.

Learning and memory are interrelated. Unless something is learned, whether intentionally or unintentionally, it cannot be retained. And unless mental material is retained for a measurable period of time, however brief, it has not been learned. Experiments have shown that the better the learning process, the longer and more completely the material is retained. For this reason, the principles of learning may be applied to memory.

A person learns more efficiently, and therefore retains better, when he understands the material, when it appeals to his interests, and when the material can be related to what he already knows. Practice is an aid to learning, especially if it is broken up with rest periods rather than massed into one “cramming” session.

Memory can be improved by the use of mnemonics, mental devices that relate material being memorized to something else—to previously learned material, a rhyme, or visual images, for example. A well-known mnemonic rhyme is the spelling rule “I before e, except after c, or when sounded as a as in neighbor and weigh.”

Remembering, or bringing memories to conscious thought, whether by recall or recognition, results from an appropriate stimulus. Hearing a certain song, for example, may stimulate a person to recall the first time he or she heard it, and under what circumstances. It is easier to recall a memory with many associations than one with a single association.

Forgetting is failure to retain stored mental material. The rate of forgetting is highest immediately after learning except for very young children. Psychologists believe that the most important cause of normal forgetting is the effect of new mental activity on what was previously learned. This effect is called retroactive inhibition. The more closely new mental activity resembles the old, the greater the influence of retroactive inhibition.

Some psychologists believe another cause of forgetting is repression. They maintain that if a person has emotional conflicts and is unaware of them, the person will tend to repress—that is, exclude from the conscious mind—memories that, if recalled, would produce anxiety by bringing the conflicts to consciousness.

Distortion is a kind of forgetting. If several persons witness an accident, each may recall it differently. This distortion may be in part because of false perception at the time of the accident. However, some distortion may occur later as wishes, fears, and prejudices affect the memory.

Some people have what is called eidetic imagery or “photographic memory.” Visual perception is retained as vivid images that are so sharp that they can be scanned and described in detail. Very few persons have eidetic imagery. It seems to be much more common in children than in adults. Persons said to have photographic memory are much more likely to be relying on ordinary visual memory than on eidetic imagery.

Hypermnesia is the exaggerated recall of details of experiences long past. It may occur when a person is under extreme tension, as when in sudden danger. Amnesia is partial or complete loss of memory. It is usually caused by brain damage or by emotional disturbances.

Experiments show that the higher intellectual processes—such as reasoning and memory—are based in the cerebral cortex (outer layer of the cerebral hemispheres of the brain). If certain parts of the cerebral cortex are stimulated electrically, there will be recall of experiences. If large areas of the cortex are destroyed, memory is impaired. Within the lateral ventricles of the cerebral hemispheres lies the limbic system. It consists of the hippocampus and the amygdala, which are responsible for storing specific memories. The hippocampus stores memories associated with places and facts; the amygdala stores memories associated with emotions and skills. Research indicates that glutamate—a neurotransmitter, or chemical messenger, in the hippocampus—plays an important role in memory retention. Glutamate serves two functions: (1) it activates certain receptors, called NMD A receptors, so they can activate calcium ions, which are crucial to forming memories, and (2) it permanently strengthens synapses, the connections between neurons, which are vital for memory formation. Learning depends on adding to or altering the strength of the synapses in the brain.

Exactly how information is retained, physiologically, has not been determined. One theory states that memory is localized— it is stored in specific regions in the brain. A particular memory is retrieved by a complex series of steps that activate a unique pattern of electrical activity in the brain. This theory is supported by the fact that certain brain injuries are specific in their effects on speech and memory. Another theory states that memory is diffuse—it is a collective action of countless cells. When a memory is retrieved, electrical activity spreads across the brain. The memory processes overlap and destruction of one area does not always erase the entire memory.