Memory (psychology)

B

Decay Theory of Forgetting

The oldest idea about forgetting is that it is simply caused by decay. That is, memory traces are formed in the brain when we learn information, and they gradually disintegrate over time. Although decay theory was accepted as a general explanation of forgetting for many years, most psychologists do not lend it credence today for several reasons. First, decay theory does not really provide an explanation of forgetting, but merely a description. That is, time by itself is not a causative agent; rather, processes operating over time cause effects. Consider a bicycle left out in the rain that has rusted. If someone asked why it rusted, he or she would not be satisfied with the answer of “time out in the rain.” A more accurate explanation would refer to oxidation processes operating over time as the cause of the rusty bicycle. Likewise, memory decay merely describes the fact of forgetting, not the processes that cause it.

The second problem for decay theory is the phenomenon of reminiscence, the fact that sometimes memories actually recover over time. Experiments confirm an observation experienced by most people: One can forget some information at one point in time and yet be able to retrieve it perfectly well at a later point. This feat would be impossible if memories inevitably decayed further over time. A final reason that decay theory is no longer accepted is that researchers accumulated support for a different theory—that interference processes cause forgetting.

C

Interference Theory of Forgetting

According to many psychologists, forgetting occurs because of interference from other information or activities over time. A now-classic experiment conducted in 1924 by two American psychologists, John Jenkins and Karl Dallenbach, provided the first evidence for the role of interference in forgetting. The experimenters enlisted two students to learn lists of nonsense syllables either late at night (just before going to bed) or the first thing in the morning (just after getting up). The researchers then tested the students’ memories of the syllables after one, two, four, or eight hours. If the students learned the material just before bed, they slept during the time between the study session and the test. If they learned the material just after waking, they were awake during the interval before testing. The researchers’ results are shown in the accompanying chart entitled, “Forgetting in Sleep and Waking.” The students forgot significantly more while they were awake than while they were asleep. Even when wakened from a sound sleep, they remembered the syllables better than when they returned to the lab for testing during the day. If decay of memories occurred automatically with the passage of time, the rate of forgetting should have been the same during sleep and waking. What seemed to cause forgetting was not time itself, but interference from activities and events occurring over time.

There are two types of interference. Proactive interference occurs when prior learning or experience interferes with our ability to recall newer information. For example, suppose you studied Spanish in tenth grade and French in eleventh grade. If you then took a French vocabulary test much later, your earlier study of Spanish vocabulary might interfere with your ability to remember the correct French translations. Retroactive interference occurs when new information interferes with our ability to recall earlier information or experiences. For example, try to remember what you had for lunch five days ago. The lunches you have had for the intervening four days probably interfere with your ability to remember this event. Both proactive and retroactive interference can have devastating effects on remembering.

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D

Repression

Another possible cause of forgetting resides in the concept of repression, which refers to forgetting an unpleasant event or piece of information due to its threatening quality. The idea of repression was introduced in the late 19th century by Austrian physician Sigmund Freud, the founder of psychoanalysis. According to Freudian theory, people banish unpleasant events into their unconscious mind. However, repressed memories may continue to unconsciously influence people’s attitudes and behaviors and may result in unpleasant side effects, such as unusual physical symptoms and slips of speech. A simple example of repression might be forgetting a dentist appointment or some other unpleasant daily activity. Some theorists believe that it is possible to forget entire episodes of the past—such as being sexually abused as a child—due to repression. The concept of repression is complicated and difficult to study scientifically. Most evidence exists in the form of case studies that are usually open to multiple interpretations. For this reason, many memory researchers are skeptical of repression as an explanation of forgetting, although this verdict is by no means unanimous. For further information on repressed memories, see the sidebar “Recovered Memories and False Memories” that accompanies this article.

VII

Biological Basis of Memory

One of the most exciting topics of scientific investigation lies in cognitive neuroscience: How do physical processes in the brain give rise to our psychological experiences? In particular, a great deal of research is trying to uncover the biological basis of learning and memory. How does the brain code experience so that it can be later remembered? Where do memory processes occur in the brain?

In the early and mid-1900s, psychologists engaged in the “search for the engram.” They used the term engram to refer to the physical change in the nervous system that occurs as a result of experience. (Today most psychologists use the term memory trace to describe the same thing.) The researchers hoped to find some particular location in the brain where memories were stored. This early work, conducted mostly with animals, failed to find a specific locus of memory in the brain. For example, American psychologist Karl Lashley trained rats to solve a maze, then surgically removed various parts of the rats’ brains. No matter what part of the brain he removed, the rats always retained at least some ability to solve the maze. From such research, psychologists concluded that memory is distributed across the brain, not localized in one place.

A

Brain Structures Involved in Memory

Modern research confirms the hypothesis that memories are not localized in one place in the brain, but rather involve interacting circuits operating across the brain. Many of the neural regions used in perceiving and attending to information seem also to be involved in the encoding and subsequent retrieval of information. Thus, although different brain regions perform different memory-related processes, the memories themselves do not appear to reside in any particular place.

The hippocampus is thought to be one of the most important brain structures involved in memory. The case of the patient H.M. (only his initials were used to preserve his anonymity), one of the most famous case studies in neuropsychology, strikingly demonstrates the importance of the hippocampus. In 1953, as a 27-year-old man, H.M. underwent brain surgery to control severe epileptic seizures. The surgeons removed his medial temporal lobes, which included most of the hippocampus, the amygdala, and surrounding structures. Although the operation successfully controlled H.M.’s seizures, it had an altogether unexpected and devastating side effect: H.M. was unable to form new long-term memories in a way that he could later retrieve them. That is, he could not remember anything that happened to him after the surgery. His memory of events prior to the surgery was mostly intact, and his reasoning and thinking skills remained strong. But he could not remember meeting new people or new experiences for more than a few minutes. Researchers concluded that the hippocampus and its surrounding structures in the medial temporal lobe play a critical role in the encoding of episodic memories, especially in binding elements of memories together to locate the memories in particular times and places.

Further evidence for the importance of the hippocampus and other regions of the brain in human memory has been provided by advanced brain imaging techniques, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Brain imaging methods allow researchers to see the activity of the living human brain on a computer screen as a person engages in different types of cognitive tasks, such as reading, solving math problems, or memorizing a list of words. These scanning methods take advantage of the fact that when a brain region becomes active, the rate at which neurons (brain cells) fire increases within this region. Increased neuronal firing in a region causes an increase in blood flow to that region, which the scanners can measure. Therefore, if a person is encoding new information into memory and the hippocampus is active during encoding, we would expect to see increased blood flow to the hippocampus. This is exactly the pattern observed in most studies.

Neuroimaging techniques have revealed other brain regions involved in memory. The frontal lobes play an important role in encoding and retrieving memories. For example, certain areas of the left frontal lobe seem especially active during encoding of memories, whereas those in the right frontal lobe are more active during retrieval. An area in the right anterior prefrontal cortex becomes active when a person is trying to retrieve a previously experienced episode. Some evidence indicates that this region may be even more active when the retrieval attempt is successful—that is, when the person not only attempts to remember but is able to remember some previous occurrence.

For more information on brain imaging methods, See also Brain: Brain Imaging.