To explain events, scientists often propose explanatory models. A model is a depiction of structures and processes that caused an event to happen (Harré, 2002).

In the study of memory, the key event is that people retain information; they remember material presented previously. To explain this, psychologists propose explanatory models that depict mental structures and processes that enable people to remember material. One major model is the three-stage memory model (Atkinson & Shiffrin, 1968).

In the three-stage memory model, information makes its way from the environment to your permanent memory in three steps, or stages: (1) sensory memory, (2) short-term memory, and (3) long-term memory (Figure 6.1). Let’s examine each stage in detail.

Sensory memory is memory that is based on the workings of sensory systems, that is, the psychological systems that enable you to see, hear, and feel the world (see Chapter 5). You usually think of these systems as being involved only in perception, such as the perception of light (vision) or sound (hearing). But they contribute to memory, too. Different perceptual systems (e.g., vision, hearing, touch) produce different forms of sensory memory. Let’s first look at iconic memory, which is sensory memory of visual images.

BRIEF MEMORY FOR IMAGES: ICONIC MEMORY. In psychology’s first demonstration of iconic memory, George Sperling (1960) showed participants three rows of four letters for a very brief period: 50 milliseconds (1/20th of a second). The presentation was so brief that participants had no time to think about the material; if they subsequently could recall any letters, it would be thanks to iconic memory.

218

Participants did not do very well on this task; when asked to recall the 12 letters, they reported only about one-third, that is, four letters. What does this mean? Sperling recognized that there are two possibilities:

Sperling devised a clever test of these possibilities. After presenting the letters, he immediately sounded a tone indicating which row of letters participants should report (Figure 6.2). In this situation, participants performed excellently; they recalled most of the indicated information. Because participants did not know which row of letters would be signaled by the tone, their excellent performance meant that most of the letters were in iconic memory when the tone sounded. Sperling thus discovered that the capacity of iconic memory is large, but the duration for which it can store information is brief.

More recently, researchers have sought the biological basis of iconic memory. You might think it would be in the eye. However, findings indicate that the biological basis of iconic memory is a network of neural systems in the brain (Saneyoshi et al., 2011).

BRIEF MEMORY FOR SOUNDS: ECHOIC MEMORY. Iconic memory is not the only type of sensory memory. People also possess echoic memory, which is sensory memory for sound.

You are probably already familiar with echoic memory from the so-called cocktail party effect: At a party, you overhear someone mention your name and wonder, “What was he saying about me?” If you quickly concentrate on what you heard, you usually can remember what the person said 1 to 2 seconds earlier; the sound is very briefly stored in the sensory system of hearing. This brief storage system is your echoic memory.

Sensory memory, then, is the first stage in getting information such as “the exam is next Tuesday” from the classroom projection screen into your mind. The second stage is short-term memory.

219

The second stage in the three-stage model of memory is short-term memory, which is a memory system that enables people to keep a limited amount of information actively in mind for brief periods of time (Jonides et al., 2008).

You already have some intuitive knowledge of how short-term memory works. If someone says that his phone number is 312-555-2368 and you want to remember it, you know three things:

You know intuitively, then, that (1) you possess a mental system that holds information for only a limited amount of time; (2) you can increase the retention time by repeating information to yourself; yet (3) there is a size limitation; even repetition does not work if there is too much information. This system is short-term memory. Psychologists have explored four main questions about short-term memory:

GETTING INFORMATION INTO SHORT-TERM MEMORY: ENCODING. Much of the information that reaches sensory memory fades rapidly and can never be recalled, as you learned earlier. But some of it makes its way from sensory to short-term memory. The process of getting information from sensory memory to short-term memory is called encoding.

Encoding is any process that transforms information from one format to another. Recall that in sensory memory, the information format is physical stimulation (e.g., light waves that reach the eye; sound waves at the ear). The contents of short-term memory, though, are generally not physical sensations. They are ideas. In our exam-next-Tuesday example, you didn’t remember physical stimuli, such as “a dark vertical line with a horizontal bar on top” (the letter T in “Tuesday”). You remembered ideas: The exam is on Tuesday and the chapters to study are 7 through 9. This means that your mind encoded the physical input, converting it into ideas that are meaningful.

What information is encoded into short-term memory? There are two types: information to which we devote attentional effort and information that “leaps into mind” without effort because we are biologically attuned to it.

Attentional effort is a focusing of attention on something in the environment that is relevant to a goal you are trying to achieve (Sarter, Gehring, & Kozak, 2006). In everyday terms, it’s called “concentrating.” If, right now, you look up from your reading and glance around the room, many objects are in view: walls, furniture, books, an old poster, scraps of paper, candy wrappers, a roommate, and so on. You might think all this information would be overwhelming, but it isn’t. What normally happens is that you concentrate, or focus your attention, on one item at a time. If your goal is to clean your room, you might focus on the candy wrapper. If you want to redecorate, you might focus on the poster. Sometimes focusing attention requires effort; if there’s a big stack of books and papers on your desk, you might have to concentrate to notice some scattered candy wrappers. Similarly, if you’re surrounded by a variety of sounds—people talking, a radio playing, cars outside your window, other people chatting—you can “pick out” one stream of sound by exerting a bit of concentrated effort. This is attentional effort (Kahneman, 1973).

220

Information receiving attentional effort is the information most likely to move from sensory memory to short-term memory. This is what Sperling’s experiments demonstrated. When participants heard a tone, they directed attentional effort to a specific row of letters. Those letters then were the ones encoded into short-term memory.

Attention, therefore, determines which information is encoded into short-term memory from sensory memory. Interestingly, this path from attentional effort to short-term memory is a two-way street. The contents of short-term memory influence what you pay attention to. If you were thinking about some object in the recent past, that object is more likely to grab your attention if it appears in the present environment (Downing, 2000).

Although attentional effort is needed often, sometimes objects simply grab your attention. For instance, if you’re on a hike and encounter a long, thin object slithering on the ground, you would notice it immediately, even if you had not been focusing attentional effort on snakes. If there is a burning smell in your home, you would notice it, even if you hadn’t been concentrating on the possibility of a fire. Or if a particularly attractive person walks by, you would notice automatically (Maner, DeWall, & Gailliot, 2008).

These examples illustrate a second type of information we encode into short-term memory: information about objects that grab our attention automatically. These objects usually have evolutionary significance; they have been relevant to survival and reproduction throughout human history. People who failed to notice them—who didn’t automatically pay attention to a threatening animal, a fire, or an attractive member of the opposite sex—were less likely to survive and reproduce. The human mind, then, has evolved to automatically notice these evolutionarily significant people, objects, and events (Öhman & Mineka, 2001; Schaller, Park, & Kenrick, 2007). Evidence of this comes from research on the speed with which information fades from iconic memory. Emotionally threatening stimuli (e.g., a scorpion, a weapon) fade relatively slowly (Kuhbandner, Spitzer, & Pekrun, 2011), which raises their chances of being transferred from iconic to short-term storage.

In sum, information goes from sensory memory to short-term memory in two ways: (1) through attentional effort and (2) automatically. Now let’s turn to our second question about short-term memory: its capacity.

THE SPACE LIMITS OF SHORT-TERM MEMORY: CAPACITY. What is the capacity of short-term memory; that is, how many pieces of information does it hold? More than a half-century ago, George Miller summarized existing research and concluded that the answer is “seven, plus or minus two” (Miller, 1956, p. 81). This number was found across a variety of tasks:

221

Miller therefore concluded that the best estimate of the capacity of short-term memory was seven pieces of information.

Not long after Miller’s work, other psychologists began to recognize that his classic estimate was an overestimate (Cowan, 2001). It turned out that, in some of the research Miller reviewed, participants likely had time, during the experiment, to transfer some information from short-term memory into long-term memory. The estimate of seven thus overestimated the capacity of short-term memory per se (Jonides et al., 2008). Contemporary evidence suggests that the classic estimate of seven is merely a “legend” (Cowan, Morey, & Chen, 2007, p. 45) and that short-term memory’s capacity is actually only four pieces of information (Cowan, 2001).

Four pieces of information may not seem like much. Yet if you try to think of more than four different things at a time, you’ll realize that it’s a lot. Try to think simultaneously about: (1) a fact about psychology (e.g., the capacity of short-term memory), (2) a fact about history (e.g., the Magna Carta was written in 1215); (3) an upcoming social plan (“We’re going to see High School Musical IX on Saturday”); (4) a mathematical formula (e.g., the area of a circle); (5) a fact about the solar system (Jupiter has the largest number of moons); plus. … “Hey, wait a minute,” you might be thinking, “Once I got to #5, I forgot the date of the Magna Carta.” Keeping your attention focused on more than four things at once is extraordinarily difficult.

FORGETTING INFORMATION IN SHORT-TERM MEMORY: DECAY AND INTERFERENCE. Let’s again follow the fate of our psych exam information: Day of exam is Tuesday; Chapters on exam are 7, 8, and 9. “Tuesday,” “7,” “8,” and “9” are four pieces of information, so all of it should fit into short-term memory. But even if you paid attention in class and all the information was encoded in short-term memory at the time, you still might forget it. Why? Two factors contribute to forgetting: decay and interference.

Decay is the fading of information from memory. Like a skywriter’s message on a windy day, the contents of short-term memory decay rapidly. How rapidly? In other words, what is the rate at which information in short-term memory decays?

Answering this question is difficult; a seemingly attractive research strategy is limited. To measure decay, one could (1) present information to participants, (2) wait for varying periods of time, and then (3) ask them to recall it. But there’s a problem: During the waiting period, some people might devise strategies for enhancing their memory (e.g., if the information included the three-letter sequence KFJ, they might note that the letters are the initials of a famous U.S. president, only backward). These strategies would counteract the natural effects of decay.

222

Can you think of a way to determine how rapidly information decays from short-term memory in the absence of memory-enhancement strategies? One approach is to give participants a task to perform during the waiting period, such as counting backward by 3s from a large number (Brown, 1958; Peterson & Peterson, 1959). The task is hard enough that people cannot do it while also devising memory strategies.

Research using this method shows that information in short-term memory decays quite rapidly. On average, people forget about 90% of the information they learn after only 18 seconds of counting backward (Peterson & Peterson, 1959). That’s a lot of forgetting! Information decays so rapidly that, in less than half a minute, most of it fades away.

After discovering this rapid decay of information, psychologists performed more research that revealed something interesting: Some information does not decay rapidly (Neath, 1998). Information presented in the first few trials of the task decays slowly (Keppel & Underwood, 1962). In fact, on the very first trial, there is no evidence of short-term memory decay at all; even after counting backward for 18 seconds, people remember information presented on the first trial (Figure 6.3). Why would this be so?

The explanation is interference. Interference is a failure to retain information in short-term memory that occurs when material learned earlier or later prevents its retention. There are two types of interference: proactive and retroactive. Proactive interference occurs when material learned earlier impairs memory for material learned later. Conversely, retroactive interference occurs when material learned later impairs memory for material learned earlier.

As an example of these two types of interference, imagine that you sign up for a class in which 20 people are enrolled, and on the first day everyone introduces themselves in class. What would your memory of the 20 names be like? You’d probably remember the names of the first few people because when you heard them, no other names were yet stored in your short-term memory—there was no proactive interference. By the time the fifth or sixth person is introduced, however, your head is so full of names that it’s hard to remember any more. Proactive interference from the first few names impairs memory for the later ones.

Results shown in Figure 6.3 demonstrate this process. On the first few trials of a task, people’s memory is excellent because there is no proactive interference. On later trials, proactive interference begins, and information decays rapidly.

This example also illustrates retroactive interference. When the last few people (i.e., the 18th, 19th, and 20th classmates) introduce themselves, you probably would remember their names. Because there are no further introductions after these people, there is no retroactive interference. You’d be more likely to forget the names of the students introduced just prior to that (e.g., the 15th, 16th, and 17th classmates to speak). Retroactive interference created by the names of the last few students interferes with memory for the names that preceded them.

Combining what you’ve learned about proactive and retroactive interference, you can make some predictions about memory performance. Whenever people try to recall a list of items, there is:

223

Items in the middle of the list are subject to both proactive and retroactive interference. Memory for those items thus should be relatively poor.

TRY THIS!

This discussion of primacy and recency effects should remind you of something: the Try This! activity you attempted earlier in this chapter.

The activity tested your memory for a long list of words. Importantly, it didn’t report merely the overall number of words you remembered; instead, you learned the number of words you remembered from different parts of the list—beginning, middle, and end.

If you were like most people, you remembered relatively few words from the middle of the list. In that part of the list, memory can be impaired by both proactive and retroactive interference.

Figure 6.4 shows the results obtained when researchers run the Try This! experiment with a large group of participants and plot results for each word position. The finding is known as the serial position effect, which is people’s tendency to display better recall for items positioned at the beginning or end of a list than in the middle.

RETAINING INFORMATION THAT IS IN SHORT-TERM MEMORY: REHEARSAL AND DEEP PROCESSING. You have seen how information in short-term memory may be forgotten. Forgetting, though, is not your goal; if “Exam next Tuesday, Chapters 7, 8, and 9” is announced, you want to remember it. What mental strategies might help you retain the information in short-term memory?

One strategy is to repeat the information to yourself over and over. This strategy is similar to rehearsing lines for a performance—where the “performance” is correctly recalling information a few days later—and therefore is called rehearsal. Rehearsal is the strategy of repeating information to retain it in short-term memory (Atkinson & Shiffrin, 1968).

Rehearsal does maintain information in short-term memory; as long as you keep repeating the information, it will be retained there. Yet, as a memory strategy, rehearsal has two big limitations. One is obvious: It’s inconvenient. You surely have things to do other than repeating “Tuesday, Chapters 7, 8, and 9” until you can find someplace to write it down.

The other limitation is that merely repeating information is not particularly effective. Psychologists have explored the effectiveness of rehearsal by varying the amount of time that people rehearsed items prior to a memory test. After rehearsal times of varying lengths, they asked participants to recall the words. What did they find? In a sense, nothing. Repeating a word for a longer time period had no effect on later memory for it (Craik & Watkins, 1973).

Because rehearsal is not as effective a strategy as one might like, if you want to remember information, you need another strategy. One is to process information “deeply.”

224

The idea of “deep”—versus “shallow”—processing describes variations in how people think about information when it is presented. Deep processing is thinking about something meaningful, such as what a word means. Shallow processing is thinking about something superficial, such as the color or case (upper or lower) of the printed word. Depth of processing, then, is the degree to which people think about meaningful rather than superficial aspects of presented information (Craik & Tulving, 1975).

In depth of processing studies, researchers ask participants to think about information in different ways (Figure 6.5). They find that deeper thinking improves memory. Fergus Craik and Endel Tulving (1975) found that the ability to recall information is more than four times greater when information is processed in a deep, rather than shallow, manner.

THIS JUST IN

FROM “SHORT-TERM MEMORY” TO “WORKING MEMORY.” You’ve just read that short-term memory is a limited-capacity memory system from which information is lost due to interference and decay, and in which information can be retained through rehearsal and deep processing. There are two more things to learn about short-term memory. They are so significant that they expand the conception of this memory system from short-term memory to something psychologists call working memory.

To answer these questions, psychologists expanded the original conception of short-term memory. In the new conception, rather than short-term memory, psychologists propose working memory, which is a set of interrelated systems that both store and manipulate information (Baddeley, 2003; Baddeley & Hitch, 1974; Repovš & Baddeley, 2006). Working memory has three components (Figure 6.7):

Like the phonological loop and visuospatial sketchpad, the central executive’s capacity is limited. In practice, this means that your ability to concentrate can be frustratingly small. Even if you have to finish this chapter right now to prepare for an upcoming exam, you may find your mind “wandering” (Feng, D’Mello, & Graesser, 2013); you become distracted rather than focusing on your reading. People’s ability to control their thoughts, emotions, and behavior relies on working-memory capacity (Hofmann et al., 2008). Physical and mental fatigue can reduce capacity, making self-control more difficult (Muraven & Baumeister, 2000).

When working on complex tasks, people combine information from different systems into one mental workspace. An architect, for example, might design a structure using her visuospatial sketchpad but simultaneously evaluate it and consider alternatives using her phonological loop (thinking, “It might be better if we knock out this wall”). A person can make decisions, then, based on a combination of visuospatial and verbal information.

227

So far, you’ve seen how information travels from sensory memory to short-term or working memory. (In practice, psychologists often use the terms short-term memory and working memory interchangeably.) You’ve also learned about factors that influence whether that information is remembered for long periods of time, so you can use it later. If it is remembered, where does that information go? It gets stored in long-term memory, which is the mental system that stores knowledge for extended periods of time. When, in an example above, deeply processed material was remembered for a long period of time, the depth of processing caused the information to be stored in long-term memory.

228

LONG-TERM MEMORY BASICS: DURATION, CAPACITY, AND TYPES OF MEMORY. One question about long-term memory that psychological science wants to answer is “How long does a long-term memory last?”

Think back to your childhood: a favorite teacher; a day when you got a great present; a “bad day” when you did something embarrassing. It’s not hard to remember them, despite the passage of time. Memories of some experiences last a lifetime.

Another type of memory that lasts is memory for how to do things. Even if you haven’t ridden a bike for years, you can hop on now and pedal away. If you hadn’t read anything for years, you would still be able to read material instantly if somebody put a book in front of you. Similarly, memories for many facts do not fade away. You won’t forget the name of the first U.S. president or the number of sides of a triangle. They are stored in your memory, permanently.

Long-term memory, then, may never fade away. Information in long-term memory can reside there permanently.

A second question concerns long-term memory’s capacity. You’re storing lots of information in long-term memory during your college years. Will you eventually “run out of space”?

No, you won’t—long-term memory has no space limitation. Consider two historical examples. Some Jewish scholars of the Talmud, an ancient work of law and ethics that is thousands of pages long, have been able to remember not only the entire text but where, on each printed page, each passage of text appears (all editions include the same number of pages and the same material on each page; Stratton, 1917). The Italian symphony conductor Arturo Toscanini had such an exceptional memory for musical works that he knew “every note of every instrument of about 250 symphonic works and the words and music of about 100 operas” (Marek, 1975, p. 414).

How can long-term memory be infinite? It’s puzzling if you think that memories are “stored” in long-term memory. The idea of “storage” makes it sound as if memory is a container that eventually will fill up. But this isn’t the only way to think about long-term memory. Some theorists argue that we should think of remembering as an activity (e.g., Stern, 1991). To remember something is to do something. A person “remembers the year World War II ended” when she does something: She says “1945.” A person “remembers a personal experience” when he does something: He forms a mental image that corresponds to the event. Because there is no limit to the number of things we can do using our minds, there is no limit to long-term memory.

A third question about long-term memory concerns the different types of information people recall. Consider some of the examples above: remembering the name of a U.S. president, a personal experience from childhood, and how to ride a bike. Each one of these examples is a different type of long-term memory (Tulving, 1972): semantic memory, episodic memory, and procedural memory, respectively.

This shoe-tying example raises another way of classifying memories; some are explicit and others implicit (Schacter, 1987). Explicit memory is conscious recall of previously encountered information or experience. Implicit memory is task performance that is affected by previous information or experience, even if that prior material is not explicitly remembered. The case of HM, presented in this chapter’s opening, illustrates the distinction clearly. Researchers taught HM a difficult mirror-tracing task, in which one traces an image using a pencil while being able to see his hand only in a mirror. With practice, HM improved on the task. Yet, being HM, he could not remember any of the occasions on which he had practiced. HM thus had implicit memory from his practice sessions—an improvement in performance—while not having any conscious, explicit memory of the experiences.

The different types of memory are distinct, yet they influence one another. Consider semantic and episodic memory. Evidence at both mind and brain levels of analysis supports the distinction between them. Research, however, also shows that if you have more of one type of memory, you are likely to acquire more of the other (Greenberg & Verfaellie, 2010). Increased semantic memory, for example, can enhance episodic memory. If you read a book about a travel location, you will acquire semantic knowledge. If you then visit the location, the increased semantic memory will tend to increase the detail and richness of the episodic memories you form.

GETTING INFORMATION INTO LONG-TERM MEMORY PERMANENTLY: CONSOLIDATION. Earlier, we said that psychologists have compared the mind to a computer. In many ways, the comparison is apt; for example, the mind’s short-term and long-term memory are like a computer’s random access memory and its permanent, hard-drive storage. However, minds and computers differ when it comes to the process of storing information permanently.

230

In a computer, permanent storage takes only a fraction of a second: the time needed to save information onto a hard drive. But in the mind, it can take hours or days. Information in long-term memory gradually consolidates. Consolidation is a process in which information in long-term memory changes from a fragile state, in which the information can be lost, to a fixed state in which it is available relatively permanently (McGaugh, 2000).

Cases of head trauma—for instance, people hitting their heads in a traffic accident or a sports injury—can disrupt the process of consolidation. You might expect that a surprising and painful accident would be unforgettable. But, instead, people often do not remember the accident at all. A head injury can disrupt biological processes in the brain that are required in order for a memory to consolidate (McAlister, 2011). As a result, people might not remember what they were doing before the accident occurred. (We’ll explore biological processes in consolidation later in this chapter, in a section on memory and the brain.)

Consolidation processes occur not only when people acquire a new memory, but also when they are reminded of an old one. Reminders trigger a period of reconsolidation during which memories are once again temporarily fragile and unstable, as they were when being consolidated originally (Lee, 2009). During the reconsolidation period, the unstable long-term memories can be substantially altered; in fact, in some cases, they can be erased. In one study conducted across a three-day period, researchers first presented a geometric figure along with a mild electric shock across a series of experimental trials (Schiller et al., 2010). On Day 2, they conducted two key experimental conditions:

On Day 3, the researchers presented the geometric figure again and measured participants’ reactions. In the first experimental condition, participants reacted fearfully; their reactions showed that they remembered that the geometric figure had been paired with shock. In the second condition, participants did not react at all; their reactions suggested that they had completely forgotten the connection between the figure and shock, even though information about the connection previously existed in long-term memory. Memory thus was significantly altered during the previous day’s reconsolidation period.

The findings illustrate that activating a memory is not like opening a book on a shelf, where the information in the book remains unchanged. Instead, during a reconsolidation period, activating a memory is more like opening a word-processing file in edit mode, where information can be altered.

RETRIEVING INFORMATION FROM LONG-TERM MEMORY: CUES AND CONTEXT. Information held in long-term memory is not useful unless you can get it out of there. You need to retrieve the information (“Hmm, when was that exam?”) from long-term memory and transfer it back into working memory (“Aha, I remember: Tuesday!”), where you can use it. The process of accessing information stored in long-term memory is called retrieval.

Two factors can help you retrieve information from long-term memory. One is retrieval cues, which are environmental stimuli related to the information you are trying to recall. When you encounter a cue, its relation to information stored in memory helps you remember that information. Suppose you are strolling about town and someone asks you to name one of the three highest-grossing films of all time, and you happen to walk past a travel agency with cruise ship posters. Bingo! Titanic springs to mind. The posters are retrieval cues that helped you recall Titanic.

231

The second factor is context, that is, the overall situation or environment you are in when learning information and then trying to recall it. If the contexts at the time of learning and recall match, memory is better. A study of deep-sea divers illustrates the point. Divers learned lists of words in one of two contexts: on dry land or under water (Godden & Baddeley, 1975). Later, when they tried to recall the information in each of the two contexts, divers’ memory was better when the contexts matched (e.g., learned under water and recalled under water) than when they mismatched (e.g., learned under water and recalled on dry land). Later research tested divers’ memory for information in decompression tables, which indicate how to avoid adverse effects from changing depth under water. Again, memory was impaired when the learning and recall contexts differed (Martin & Aggleton, 1993).

You may think of the word “context” as referring to external conditions, such as a classroom or coffee shop. However, people’s emotional states also are contexts that affect memory. People’s memory is better when their emotional state at recall matches their emotional state at the time they encoded the information. Gordon Bower (1981) put research participants into either a happy or sad mood and then gave them some information to learn. Later, he again induced a happy or sad mood and asked them to recall the information. Bower’s experimental design thus included four conditions: learn when happy/recall when sad; learn when happy/recall when happy; learn when sad/recall when sad; learn when sad/recall when happy. The participants’ memory was best when the recall context matched the learning context (Figure 6.8), that is, in the learn-when-happy/recall-when-happy and learn-when-sad/recall-when-sad conditions (Bower, 1981). Mood, then, is a contextual cue that can enhance memory.

In summary, retrieval cues and context help you retrieve information from long-term memory. And, once you’ve retrieved it, where does it go? Having concluded our coverage of the three-stage memory model (sensory, short-term or working, and long-term), answering this question should be easy. The information goes back to your working-memory system, where you can use it to answer questions, make decisions, and solve problems.

232