Contents
“Apparently, we owe almost everything to memory that we have and who we are; our ideas and conceptions are her work, and our daily perceptions, thoughts and movements draw from her source. Memory collects the countless phenomena of our existence into a single whole; like our bodies, which would shatter into dust if their constituent atoms were not held together by the attraction of matter, so our consciousness would be broken into as many fragments as we lived for seconds, if not for the binding and unifying power of memory” (Hering, 1920 ).
The words of Goering, delivered by him at a lecture at the Vienna Academy of Sciences many years ago, testify to the importance of memory in the mental life of a person. As Hering’s remark on consciousness makes clear, it is memory that provides the sense of continuity on which the very concept of the self depends. In thinking about what it means to be human, one has to recognize the central role of memory. To understand the science of memory, you first need to understand how scientists divide the field into its subdivisions: short-term memory and long-term memory. After describing the main differences between them, we will look at the main data regarding short-term memory, long-term memory and the so-called implicit memory (this kind of memory is associated with the acquisition of skills). Then we will move on to the question that interests everyone: how to improve memory? We conclude by discussing the situations in which our memories are formed.
Three main sections of memory
Modern psychologists distinguish three main sections in memory. The first of these refers to the three stages of memory: encoding, storage, and reproduction. The second concerns the distinction between types of memory for short-term or long-term storage of information. The third section includes various types of memory, which are allocated depending on the content of the stored information (for example, one memory system is for facts, the other is for skills). For each of these divisions, there is data showing that distinct entities—say, short-term and long-term memory—are mediated (in part) by different brain structures.
Memory has three stages. The first stage, encoding, consists in placing the fact in memory; it takes place when we are learning. The second stage is storage, at this stage the fact is stored in memory. The third stage, recall, occurs when a fact is retrieved from memory, such as when taking an exam.
Three stages of memory
Suppose one morning you are introduced to a student who is told that her name is Barbara Cohn. That same afternoon, you see her again and say something like, “You are Barbara Cohn. We met this morning.» Obviously you remember her name, but how exactly did you remember it?
This small episode of memory can be divided into three stages (Fig. 8.1). First, when you were introduced, you somehow entered the name «Barbara Cohn» into memory; this is the coding stage. You have converted the physical input (sound waves) corresponding to the utterance of her name into a kind of code or representation suitable for her memory to accept, and then «put» that representation into memory. Secondly, you kept, or retained, that name during the time between these two meetings; this is the storage stage. Thirdly, at your next meeting, you retrieved her name from the vault; this is the reproduction stage.
Rice. 8.1. Three stages of memory. Memory theories explain forgetting by failures in one or more of three stages: encoding, storage, and recall (after Melton, 1963).
Memory can fail you at any of these three stages. If you couldn’t remember Barbara’s name on your second meeting, it could be due to bad encoding, storage, or reproduction. Much of today’s memory research aims to determine what mental operations are performed in each of these three stages and to explain what can go wrong in them, leading to a failed memory.
A number of recent studies have shown that different brain structures are involved in different stages of memory. The most impressive data comes from brain-scanning studies examining the neuroanatomical differences between the stages of encoding and reproduction. The experiments consisted of two parts. In part I, devoted to coding, the subjects memorized a set of verbal elements, for example, pairs consisting of the name of a category and its particular instance (furniture-sideboard); in part II, devoted to reproduction, the subjects had to recognize or recall these elements by the presented name of the category. In both parts of the experiment, brain activity was measured using a PET scan while the subjects were performing the task. The most notable result was that during encoding, most of the activated areas of the brain were in the left hemisphere, and during playback, most of these areas were in the right hemisphere (Shallice et al., 1994; Tulving et al., 1994). Thus, the distinction between encoding and reproduction has a clear biological basis.
Short and long term memory
In different situations, the three stages of memory proceed differently. Memory distinguishes situations when it is necessary to remember material for some seconds, and situations when material must be remembered for a long time — from several minutes to several years. It is said that in situations of the first kind, short-term memory works, and in situations of the second kind, long-term memory.
This distinction can be illustrated by changing the already familiar story of meeting Barbara Cohn. Suppose that the first time you meet, as soon as you hear her name, a friend comes up and you ask, «Do you know Barbara Cohn?» In this case, recalling Barbara’s name would be an example of short-term memory at work, since you recalled that name after only a second or two. Remembering her name on your second meeting would be an example of long-term memory at work, since the name was now recalled several hours after it was encoded.
When we recall a name immediately after having met it, it seems that the reproduction takes place effortlessly, as if the name was still sounding, still in our minds. But when we try to remember the same name hours later, recalling it is often difficult because it has already left our minds and, in a sense, has to be brought back.
It should be noted that there is another system of short-term storage that differs from working memory in that it retains detailed sensory images of any stimulus that has just been presented, but only for a few hundred milliseconds. For example, if a set of 12 letters briefly flashes on the screen, the individual will have a detailed visual image of all the letters for several hundred milliseconds (see, in particular: Sperling, 1960). It is believed that each sensory system has its own sensory memory (visual, auditory, gustatory, etc.), although only visual and auditory have been studied in detail. Sensory or iconic memory is certainly useful in extending the retention of short-term stimuli, but it plays a much smaller role in thinking and conscious recall than the memory system we focus on in this chapter.
It has long been known that short-term and long-term memory are realized by different brain structures. In particular, the hippocampus, a structure located under the cortex, near the middle of the brain, is critical for long-term memory, but not for short-term memory.
Many of the relevant data come from experiments with rats and other animal species. In some experiments, one group of rats was injured in the hippocampus and the surrounding cortex, while another group was damaged in a completely different area in the anterior cortex. Then both groups of rats had to perform a task with a delayed response: in each trial, first one stimulus (say, a square) was presented, and then, some time later, a second stimulus was presented (for example, a triangle); the animal had to respond only if the second stimulus was different from the first. How well the animal coped with this task depended on the nature of the brain damage it had suffered and the length of the delay interval between stimuli. With a long delay (15 seconds or more), animals with damaged hippocampus did poorly on the task, and with damage to the anterior part of the cortex, they were relatively normal. Since long-term memory is required to store the first stimulus when there is a long delay between stimuli, these results are consistent with the notion that the hippocampus plays a crucial role in long-term memory. With a short delay between two stimuli (only a few seconds), the opposite happens: now animals with cortical damage do poorly, while animals with hippocampal damage do relatively well. Since the first stimulus must be stored in short-term memory when there is a short delay between stimuli, these results indicate that areas of the frontal cortex are involved in short-term memory. This means that short-term and long-term memory are realized by different parts of the brain (see, for example: Gold-man-Rakic, 1987; Zola-Morgan & Squire, 1985).
Recent research shows that we use different forms of long-term memory for tasks such as storing skills, such as piano skills, and for tasks such as storing facts.
Is there evidence of such a distinction in humans? Patients who accidentally damage certain areas of the brain provide an opportunity to conduct a «natural experiment». So, some patients suffer damage to the hippocampus and the surrounding cortex, which causes them to have severe memory loss; because the hippocampus is located in the middle of the temporal lobe, these patients are said to have medial temporal amnesia. It is extremely difficult for such patients to remember material for a long time, but they have practically no difficulty remembering material for a few seconds. For example, a patient with medial-temporal amnesia may not recognize his doctor when he enters the room, despite having seen that doctor every day for years, and yet have no difficulty repeating that doctor’s full name when they are re-introduced ( Milner, Corkin & Teuber, 1968). This patient has severely “impaired long-term memory, but short-term memory is functioning normally.
Other patients, however, have the opposite problem. They cannot correctly repeat a sequence of just three words, but they do perfectly well on tests of long-term memory for words. In such patients, short-term memory is impaired, but long-term memory is not affected. And their brain damage is never localized to the medial temporal lobe (Shallice, 1988). Thus, in humans, as in other mammals, short-term and long-term memory are mediated by different brain structures.
Research done in recent years using brain scanning technologies shows that neurons in the frontal lobes, located just behind the frontal bone, hold information for short-term use, such as the phone number we need to dial now. Apparently, these neurons function as random access memory chips (random access memory, RAM. — Note. transl.), temporarily storing data for current use and switching to other data as needed. These cells are also able to take information from other parts of the brain and store it for as long as it takes to complete a particular task (Goldman-Rakic, cited in Goleman, 1995).
Different types of memory for different types of information
Even 15-20 years ago, psychologists generally believed that the same memory system is used for all types of material. It was assumed, for example, that the same long-term memory is used to store memories of the grandmother’s funeral, and to store the skills of riding a bicycle. New data has shown this to be false. In particular, different long-term memory is used to store facts (such as who is president now) and to store skills (such as how to ride a bicycle). Support for this difference, as usual, includes both psychological and biological evidence, but these will be discussed later in this chapter.
The best explained memory situation is one in which a person consciously recalls a past event, and this memory is experienced as occurring at a particular place and time. Such memory is called explicit memory, and we will focus on it in most sections of this chapter. The next two sections cover encoding, storage, and retrieval in short-term and long-term explicit memory. Then we’ll talk about what we know about another kind of memory that stores skills, called implicit memory.
Short-term memory
Short-term memory contains memories that are stored for only a few seconds. However, even in those situations where we need to remember information only for a short time, the memorization process includes three stages: encoding, storage and retrieval. Let’s take a closer look at each of these three stages in relation to working memory. See →
Long-term memory
Long-term memory is needed when information needs to be retained either for just a few minutes (for example, a remark in a conversation made earlier) or for a lifetime (for example, an adult’s memories of childhood). See →
implicit memory
There is a kind of memory that manifests itself in skills and is expressed in the perfect performance of some perceptual, motor or cognitive task without consciously recalling the experience that led to such perfection. See →
Improving memory
We will look at various methods for improving long-term memory; these methods are based on improving the efficiency of encoding and extracting information. See →
productive memory
When we hear a sentence or story, we often perceive it as an incomplete description of a real event and use our general knowledge of the world to construct a more detailed description. How do we do it? We add to the input sentences the propositions that are most likely to follow from them. See →
Are the memories stored in the subconscious real?
Recent studies have shown that suggested information can not only distort the details of a recent events, but and create completely false memories and beliefs in people’s minds. See →