II.

Types of Memory

Although the English language uses a single word for memory, there are actually many different kinds. Most theoretical models of memory distinguish three main systems or types: sensory memory, short-term or working memory, and long-term memory. Within each of these categories are further divisions.

A.

Sensory Memory

Sensory memory refers to the initial, momentary recording of information in our sensory systems. When sensations strike our eyes, they linger briefly in the visual system. This kind of sensory memory is called iconic memory and refers to the usually brief visual persistence of information as it is being interpreted by the visual system. Echoic memory is the name applied to the same phenomenon in the auditory domain: the brief mental echo that persists after information has been heard. Similar systems are assumed to exist for other sensory systems (touch, taste, and smell), although researchers have studied these senses less thoroughly.

American psychologist George Sperling demonstrated the existence of sensory memory in an experiment in 1960. Sperling asked subjects in the experiment to look at a blank screen. Then he flashed an array of 12 letters on the screen for one-twentieth of a second, arranged in the following pattern:

Sensory memory systems typically function outside of awareness and store information for only a very short time. Iconic memory seems to last less than a second. Echoic memory probably lasts a bit longer; estimates range up to three or four seconds. Usually sensory information coming in next replaces the old information. For example, when we move our eyes, new visual input masks or erases the first image. The information in sensory memory vanishes unless it captures our attention and enters working memory.

B.

Short-Term or Working Memory

Psychologists originally used the term short-term memory to refer to the ability to hold information in mind over a brief period of time. As conceptions of short-term memory expanded to include more than just the brief storage of information, psychologists created new terminology. The term working memory is now commonly used to refer to a broader system that both stores information briefly and allows manipulation and use of the stored information.

We can keep information circulating in working memory by rehearsing it. For example, suppose you look up a telephone number in a directory. You can hold the number in memory almost indefinitely by saying it over and over to yourself. But if something distracts you for a moment, you may quickly lose it and have to look it up again. Forgetting can occur rapidly from working memory. For more information on the duration of working memory, see the Rate of Forgetting section of this article.

Psychologists often study working memory storage by examining how well people remember a list of items. In a typical experiment, people are presented with a series of words, one every few seconds. Then they are instructed to recall as many of the words as they can, in any order. Most people remember the words at the beginning and end of the series better than those in the middle. This phenomenon is called the serial position effect because the chance of recalling an item is related to its position in the series. The results from one such experiment are shown in the accompanying chart entitled “Serial Position Effect.” In this experiment, recall was tested either immediately after presentation of the list items or after 30 seconds. Subjects in both conditions demonstrated what is known as the primacy effect, which is better recall of the first few list items. Psychologists believe this effect occurs because people tend to process the first few items more than later items. Subjects in the immediate-recall condition also showed the recency effect, or better recall of the last items on the list. The recency effect occurs because people can store recently presented information temporarily in working memory. When the recall test is delayed for 30 seconds, however, the information in working memory fades, and the recency effect disappears.

Working memory has a basic limitation: It can hold only a limited amount of information at one time. Early research on short-term storage of information focused on memory span—how many items people can correctly recall in order. Researchers would show people increasingly long sequences of digits or letters and then ask them to recall as many of the items as they could. In 1956 American psychologist George Miller reviewed many experiments on memory span and concluded that people could hold an average of seven items in short-term memory. He referred to this limit as “the magical number seven, plus or minus two” because the results of the studies were so consistent. More recent studies have attempted to separate true storage capacity from processing capacity by using tests more complex than memory span. These studies have estimated a somewhat lower short-term storage capacity than did the earlier experiments. People can overcome such storage limitations by grouping information into chunks, or meaningful units. This topic is discussed in the Encoding and Recoding section of this article.

Working memory is critical for mental work, or thinking. Suppose you are trying to solve the arithmetic problem 64 × 9 in your head. You probably would need to perform some intermediate calculations in your head before arriving at the final answer. The ability to carry out these kinds of calculations depends on working memory capacity, which varies individually. Studies have also shown that working memory changes with age. As children grow older, their working memory capacity increases. Working memory declines in old age and in some types of brain diseases, such as Alzheimer’s disease.

Working memory capacity is correlated with intelligence (as measured by intelligence tests). This correlation has led some psychologists to argue that working memory abilities are essentially those that underlie general intelligence. The more capacity people have to hold information in mind while they think, the more intelligent they are. In addition, research suggests that there are different types of working memory. For example, the ability to hold visual images in mind seems independent from the ability to retain verbal information.

C.

Long-Term Memory

The term long-term memory is somewhat of a catch-all phrase because it can refer to facts learned a few minutes ago, personal memories many decades old, or skills learned with practice. Generally, however, long-term memory describes a system in the brain that can store vast amounts of information on a relatively enduring basis. When you play soccer, remember what you had for lunch yesterday, recall your first birthday party, play a trivia game, or sing along to a favorite song, you draw on information and skills stored in long-term memory.

Psychologists have different theories about how information enters long-term memory. The traditional view is that that information enters short-term memory and, depending on how it is processed, may then transfer to long-term memory. However, another view is that short-term memory and long-term memory are arranged in a parallel rather than sequential fashion. That is, information may be registered simultaneously in the two systems.

There seems to be no finite capacity to long-term memory. People can learn and retain new facts and skills throughout their lives. Although older adults may show a decline in certain capacities—for example, recalling recent events—they can still profit from experience even in old age. For example, vocabulary increases over the entire life span. The brain remains plastic and capable of new learning throughout one’s lifetime, at least under normal conditions. Certain neurological diseases, such as Alzheimer’s disease, can greatly diminish the capacity for new learning.

Psychologists once thought of long-term memory as a single system. Today, most researchers distinguish three long-term memory systems: episodic memory, semantic memory, and procedural memory.

C.1.

Episodic Memory

Episodic memory refers to memories of specific episodes in one’s life and is what most people think of as memory. Episodic memories are connected with a specific time and place. If you were asked to recount everything you did yesterday, you would rely on episodic memory to recall the events. Similarly, you would draw on episodic memory to describe a family vacation, the way you felt when you won an award, or the circumstances of a childhood accident. Episodic memory contains the personal, autobiographical details of our lives.

C.2.

Semantic Memory

Semantic memory refers to our general knowledge of the world and all of the facts we know. Semantic memory allows a person to know that the chemical symbol for salt is NaCl, that dogs have four legs, that Thomas Jefferson was president of the United States, that 3 × 3 equals 9, and thousands of other facts. Semantic memories are not tied to the particular time and place of learning. For example, in order to remember that Thomas Jefferson was president, people do not have to recall the time and place that they first learned this fact. The knowledge transcends the original context in which it was learned. In this respect, semantic memory differs from episodic memory, which is closely related to time and place. Semantic memory also seems to have a different neural basis than episodic memory. Brain-damaged patients who have great difficulties remembering their own recent personal experiences often can access their permanent knowledge quite readily. Thus, episodic memory and semantic memory seem to represent independent capacities.

C.3.

Procedural Memory

Procedural memory refers to the skills that humans possess. Tying shoelaces, riding a bicycle, swimming, and hitting a baseball are examples of procedural memory. Procedural memory is often contrasted with episodic and semantic memory. Episodic and semantic memory are both classified as types of declarative memory because people can consciously recall facts, events, and experiences and then verbally declare or describe their recollections. In contrast, nondeclarative, or procedural, memory is expressed through performance and typically does not require a conscious effort to recall.

Could you learn how to tie your shoelaces or to swim through purely declarative means—say, by reading or listening to descriptions of how to do it? If it would be possible at all, the process would be slow, difficult, and unnatural. People best gain procedural knowledge by practicing the procedures directly, not via instructions given in words. Verbal coaching in sports is partly a case of trying to impart procedural knowledge through declarative means, although coaching by example (and videotape) may work better. Still, in most cases there is no substitution for practice. Procedural learning may take considerable effort, and improvements can occur over a long period of time. The accompanying chart, entitled “Practice and Speed in Cigar-Making,” shows the effect of practice on Cuban factory workers making cigars. The performance of the workers continued to improve even after they had produced more than 100,000 cigars.

C.4.

Interaction of Long-Term Memory Systems

Although long-term episodic, semantic, and procedural memory all represent independent systems, it would usually be wrong to think of a particular task as relying exclusively on one type. The examples used above (remembering yesterday’s events, knowing that Thomas Jefferson was president, or tying shoes) represent relatively pure cases. However, most human activities rely on the interaction of long-term memory systems. Consider the expression of social skills or, more specifically, table manners. If you know to set the dinner table with the fork to the left of each plate, is this an example of procedural memory, semantic memory, or even episodic memory from having witnessed a past example? Probably the answer is some blend of all three. In addition, procedural memory does not apply only to physical skills, as in the previous examples. Complex cognitive behavior, such as reading or remembering, also has a procedural component—the mental procedures we execute to perform these activities. Thus, the separation of procedural and declarative memory from one another is not clear-cut in all cases.