What is memory?

Introduction

Human memory is among the most complex phenomena in the universe. A Russian newspaper reporter once flawlessly recalled a list of fifty unrelated words he had studied for only three minutes fifteen years before. On the other hand, as everyone knows from personal experience, the memory system is also capable of losing information presented only seconds in the past. Errors in memory create so many problems that it seems imperative to know all that is possible about human memory. For that, a theory is needed.

A scientific theory is a systematic way to understand complex phenomena that occur in nature. A theory is judged to be useful insofar as its claims can be supported by the findings of empirical tests, especially experimentation, and insofar as it leads to further research studies. A theory is not right or wrong; it is simply a tool to describe what is known and to suggest what needs further study.

Three major forms of memory are generally described: short-term, long-term, and sensory memory. Short-term memory represents the temporary retention of newly acquired information. Generally, short-term memory lasts no longer than about twenty seconds. This is useful for short-term tasks, such as the recall of speech during discussions or discourse with another person. Short-term memory is rapidly lost, sometimes referred to as a process of decaying. Alan Baddeley, a major researcher in the field of memory, has suggested that a concept of working memory may be substituted for short-term. Repeat stimulation, or rehearsal, may transfer short-term memory into that of long-term.

Long-term memory involves storage of information over longer periods of time, potentially as long as the life of the individual. Some researchers into the subject consider long-term memory to include two major areas: episodic and semantic. Episodic memory addresses events that have a temporal relationship with a person’s life. This may include recall of when events or information appeared. Semantic memory represents the concepts or skills, represented in part by learning, that people acquire through the course of their lives.

Sensory memories are those that can be retrieved as a result of sensory stimuli. For example, a particular odor may result in the recall of events from the past. The unusual smell of a cleaning solution may cause the recall of a college dormitory from years past. This form of recall has been called olfactory memory. The image of a flower may result in the memory of a teenage boyfriend. Such a visual stimulus is sometimes referred to as iconic memory.

Theories of memory have been important to psychology for a long time, often occupying the time and interest of researchers throughout their careers. Memory, which is always connected to learning, is defined as the mental process of preserving information acquired through the senses for later use. The cognitive approach to memory places emphasis on mental processes, which result in the ability to comprehend or recall what is learned. The basis is found in changes that occur in the regions of the brain, such as the hippocampus, associated with memory. In a sense, memory is the record of the experiences of a lifetime. Without it, a person could not re-experience the past; everything at every moment would be brand-new. A person could not even recognize the face of a loved one or learn from any experience. A person would thus have a greatly reduced chance for survival and would have no sense of personal identity. Memory is, in short, critical to functioning as a human being.

Associationism, Cognitive Theory, and Neuropsychology

The goal of a theory of memory is to explain the structures (hardware) and the processes (software) that make the system work. Explaining how such a complex system works is a massive undertaking. The attempts have taken the form of large-scale theories, which seek to deal with all major operations of the memory systems. The major theories of memory are associationism and theories from cognitive psychology and neuropsychology. The theories differ primarily in views of the retention and retrieval functions of memory. They also differ in terms of their conception of memory as active or passive.

Associationism, the oldest of the three, is the theory that memory relies on forming links or bonds between two unrelated things. This theory stems from the work of Hermann Ebbinghaus, who started the use of laboratory methods in the study of memory in the late nineteenth century. According to this theory, the ability to remember depends on establishing associations between stimuli and responses (S-R). Establishing associations depends on the frequency, recency, and saliency of their pairing. If these bonds become very strong, the subject is said to have developed a habit. Associationism also assumes the existence of internal stimuli that produce behavioral responses. These responses then become stimuli for other unobservable internal responses, thus forming chains. In this way, complex physical behaviors and mental associations can be achieved. Associationists tend to view the memory system as essentially passive, responding to environmental stimuli.

Cognitive theory emphasizes studying complex memory in the real world; it is concerned with the ecological validity of memory studies. Most of this work stems from the research of Sir Frederic C. Bartlett, who was not satisfied with laboratory emphasis on “artificial memory,” but rather chose to study what he called meaningful memory. Meaningful memory, he said in his book Remembering: A Study in Experimental and Social Psychology (1932), is a person’s effort to make sense of the world and to function effectively in it. Cognitive psychology recognizes subjective experiences as inescapably linked to human behavior. It centers on internal representation of past experiences and assumes that intentions, goals, and plans make a difference in what is remembered and how well it is remembered. The focus in memory research is on semantic memory—the knowledge of words, categories, concepts, and meanings located in long-term memory. People have highly complex networks of concepts, which helps account for their behavior in the real world. These networks are called schemas. New experiences and new information are viewed in light of old schemas so that they are easier to remember. Cognitive theory emphasizes how the individual processes information, and it uses the computer as its working model of memory.

Neuropsychology has contributed the third major theory of memory. Although psychology has always recognized the connection between its concerns and those of biology and medicine, the technology now available has made neuropsychological analysis of brain structure and functioning possible. Karl Lashley was an early researcher who sought to find the location of memory in the brain. He ran rats through mazes until they had learned the correct pathway. His subsequent surgical operations on experimental rats’ brains failed to show localization of memory.

The search for the memory trace, the physiological change that presumably occurs as a result of learning, continued with Donald O. Hebb, who had assisted Lashley. The brain consists of billions of nerve cells, which are connected to thousands of other neurons. Hebb measured the electrical activity of the brain during learning, and he discovered that nerve cells fire repeatedly. He was able to show that an incoming stimulus causes patterns of neurons to become active. These cell assemblies discovered by Hebb constitute a structure for the reverberating circuits, a set of neurons firing repeatedly when information enters short-term memory. This firing seems to echo the information until it is consolidated in long-term memory. Other researchers have found chemical and physical changes associated with the synapses and in the neurons themselves during learning and when the learning is consolidated into long-term memory. The discovery of the memory trace, a dream of researchers for a long time, may become a reality. Neuropsychology sees memory as a neural function controlled by electrical and chemical activity.

Clinical Approaches to Memory Disorders

Human memory is so important to daily life that any theory that could explain its structures and processes and thus potentially improve its functioning would be invaluable. Memory is inextricably tied to learning, planning, reasoning, and problem solving; it lies at the core of human intelligence.

None of the three theories is by itself sufficient to explain all the phenomena associated with memory. Over the years, a number of ideas have been developed in the attempt to improve memory functioning through passive means. Efforts to induce learning during sleep and to assess memory of patients for events taking place while under anesthesia have had mixed results but on the whole have not succeeded. Memory enhancement through hypnosis has been attempted but has not been shown to be very effective or reliable. Pills to improve memory and thereby intelligence have been marketed but so far have not been shown to be the answer to memory problems. Research has begun on the possibility that certain drugs (such as tacrine) may interactively inhibit memory loss in people afflicted with certain kinds of dementia (for example, Alzheimer’s disease). Work in neuropsychology has shown the influence of emotion-triggered hormonal changes in promoting the memory of exciting or shocking events (such as one’s first kiss or an earthquake). This has led to an understanding of state-dependent memory: Things learned in a particular physical or emotional state are more easily remembered when the person is in that state again. This helps explain the difficulties in remembering events that took place when a person was intoxicated or depressed. In fact, heavy use of alcohol may result in significant memory loss. A person may not even remember having injured someone in a car accident. Although not fully researched, it may be that certain kinds of memory are mood-congruent. Perhaps memories of events that occurred when a person was in a certain mood may become available to the person only when that mood is again induced.

More active means for memory improvement have met with greater success. Associationist theory has demonstrated the value of the use of mnemonics, devices or procedures intentionally designed to facilitate encoding and subsequent recall. The use of rhymes, acronyms, pegwords, and the like enables people to recall factual information such as the number of days in each month (“Thirty days hath September . . . ”), the names of the Great Lakes (the acronym HOMES), and the colors of the visible spectrum (ROY G. BIV). Visual cues, such as tying a string around one’s finger or knotting one’s handkerchief, are traditional and effective ways to improve prospective memory. Cognitive psychology has demonstrated the importance of emotional factors—how and why something is learned—to the effectiveness of memory. It has provided the research base to demonstrate the effectiveness of study strategies such as the SQ3R (survey, question, read, recite, review) technique. Cognitive theory has also shown that metamemory, a person’s knowledge about how his or her memory works, may be important for the improvement of memory.

In clinical settings, much research has been concerned with memory impairment as a means to test the applicability of theories of memory. Head injuries are a common cause of amnesia in which events immediately prior to an accident cannot be recalled. Damage to the hippocampus, a part of the brain that is vital to memory, breaks down the transfer of information from short-term to long-term memory. One dramatic case concerns “H. M.,” a patient who had brain surgery to control epileptic seizures. After surgery, H. M.’s short-term memory was intact, but if he was momentarily distracted from a task, he could not remember anything about what he had just been doing. The information was never transferred to long-term memory. Such patients still remember information that was stored in long-term memory before their operation, but to them everyday experiences are always strangely new. They can read the same paragraph over and over, but each time the material will be brand-new. In H. M.’s case, it was discovered that his intelligence as measured by standardized tests actually improved, yet he was continually disoriented and unable to learn even the simplest new associations. Intelligence tests are made to measure general information, vocabulary, and grammatical associations; these things were stored in H. M.’s long-term memory and were apparently not affected by brain surgery. In cases less dramatic than H. M.’s, damage to particular areas of the brain can still have devastating effects on the memory. Damage can be caused by accidents, violent sports activity, strokes, tumors, and alcoholism. Alzheimer’s disease is another area to which research findings on memory may be applied. In this fatal disease, a patient’s forgetfulness increases from normal forgetting to the point that the patient cannot remember how to communicate, cannot recognize loved ones, and cannot care for his or her own safety needs.

Associationism, cognitive psychology, and neuropsychology can each explain some of the structures and processes involved in these and other real-world problems, but it seems as though none of the theories is sufficient by itself. Memory is such a complex phenomenon that it takes all the large-scale theories and a number of smaller-scale ones to comprehend it. The truth probably is that the theories are not mutually exclusive but rather are complementary to one another.

Physiological Basis of Memory

Theories of learning and memory have been of great concern to philosophers and psychologists for a long time. They have formed a major part of the history of psychology. Each of the theories has been ascendant for a time, but the nature of theory building requires new conceptions to compensate for perceived weaknesses in currently accepted theories and models. Associationism was the principal theory of memory of stimulus-response psychology, which was dominant in the United States until the mid-1950s. Cognitive psychology evolved from Gestalt psychology, from Jean Piaget’s work on developmental psychology, and from information-processing theory associated with the computer, and was extremely important during the 1970s and 1980s. Neuropsychology developed concurrently with advanced technology that permits microanalysis of brain functioning. It has resulted in an explosion of knowledge about how the brain and its systems operate.

Formation of memory seems to involve two individual events. Short-term memory develops first. Repeated rehearsal transfers this form of memory into long-term storage. At one time, it was believed both these forms of memory involved similar events in the brain. However, experimental models have shown such a theory to be incorrect. Two experimental approaches have addressed this issue: the separation of memory formation involving “accidental” or intentional interference with brain function, and development of an animal model for the study of memory.

Electroshock treatment of depression in humans has been shown to interfere with short-term memory formation. However, these persons are still perfectly able to recall the memory of earlier events stored within long-term memory. Accidental damage to temporal lobes of the brain does not appear to interfere with short-term memory but may inhibit the ability to recall events from the past.

The experimental use of an animal model in the study of memory formation was developed by Eric R. Kandel at Columbia University. Kandel has used the sea slug Aplysia in his study of memory. The advantage of such a model is its simplicity—instead of the approximately one trillion neurons that make up the nervous system of humans, Aplysia contains a “mere” twenty thousand.

Using a variety of stimuli on the animal and observing its response, Kandel has shown that the physiological basis for short-term memory differs from that of long-term. Specifically, short-term memory involves stimulus to only a small number of individual neurons. Long-term memory involves de novo (new) protein synthesis in the affected cells and formation of extensive neural circuits. Kandel was awarded the Nobel Prize in Physiology or Medicine in 2000 for this work.

Memory Retrieval

The basis for memory recall remains an active area of study. Memory retrieval can be of two types: recognition and recall. In recognition, the individual is presented with information that had been previously learned. The subject remembers he or she has already observed or learned that information. In effect, it is analogous to seeing a movie or book for the second time. In recall, information is reproduced from memory, as in response to a question. The physiological basis for retrieval probably involves the activation of regions of the brain that were involved in the initial encoding.

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