Encoding is the process of turning perceptions into memories; storage is the process of maintaining information in memory over time. There are three major kinds of memory storage: sensory, short-term, and long-term. As these names suggest, the three kinds of storage are distinguished primarily by the amount of time over which a memory is retained.

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Sensory memory is a type of storage that holds sensory information for a few seconds or less. In a classic study, participants viewed three rows of four letters each, as shown in FIGURE 6.5. The researcher flashed the letters on a screen for just 1/20th of a second. When asked to remember all 12 of the letters they had just seen, participants recalled fewer than half (Sperling, 1960). There were two possible explanations for this: Either people simply couldn’t encode all the letters in such a brief period of time, or they had encoded the letters but forgot them faster than they could recall them all.

To test the two ideas, the researcher relied on a clever trick. Just after the letters disappeared from the screen, a tone sounded that cued the participants to report the letters in a particular row. A high tone cued participants to report the contents of the top row, a medium tone cued participants to report the contents of the middle row, and a low tone cued participants to report the contents of the bottom row. When asked to report only a single row, people recalled almost all of the letters in that row! Because the tone sounded after the letters disappeared from the screen and because participants had no way of knowing which of the three rows would be cued, the researcher inferred that virtually all the letters had been encoded. In fact, if the tone was substantially delayed, participants couldn’t perform the task because the information had slipped away from their sensory memories. Like the after image of a flashlight, the 12 letters flashed on a screen are visual icons, a lingering trace stored in memory for a very short period.

Because we have more than one sense, we have more than one kind of sensory memory. Iconic memory is a fast-decaying store of visual information. A similar storage area serves as a temporary warehouse for sounds. Echoic memory is a fast-decaying store of auditory information. When you have difficulty understanding what someone has just said, you probably find yourself replaying the last few words—listening to them echo in your “mind’s ear,” so to speak. When you do that, you are accessing information that is being held in your echoic memory store. The hallmark of both the iconic and echoic memory stores is that they hold information for a very short time. Iconic memories usually decay in about 1 second or less, and echoic memories usually decay in about 5 seconds (Darwin, Turvey, & Crowder, 1972). These two sensory memory stores are a bit like doughnut shops: The products come in, they sit briefly on the shelf, and then they are discarded. If you want one, you have to grab it fast.

A second kind of memory storage is short-term memory, which holds nonsensory information for more than a few seconds but less than a minute. For example, if someone tells you a telephone number, you can usually repeat it back with ease—but only for a few seconds. In one study, research participants were given consonant strings to remember, such as DBX and HLM. After seeing each string, participants were asked to count backward from 100 by 3 for varying amounts of time and were then asked to recall the strings (Peterson & Peterson, 1959). As shown in FIGURE 6.6, memory for the consonant strings declined rapidly, from approximately 80% after a 3-second delay to less than 20% after a 20-second delay. These results suggest that information can be held in the short-term memory store for about 15 to 20 seconds.

What if 15 to 20 seconds isn’t enough time? What if we need the information for a while longer? We can use a trick that allows us to get around the natural limitations of our short-term memories. Rehearsal is the process of keeping information in short-term memory by mentally repeating it. If someone gives you a telephone number and you can’t put it immediately into your cell phone or write it down, you say it over and over to yourself until you can. Each time you repeat the number, you are reentering it into short-term memory, giving it another 15 to 20 seconds of shelf life.

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Short-term memory is limited in how long it can hold information, and it is also limited in how much information it can hold. Most people can keep approximately seven items in short-term memory, but if they put more new items in, then old items begin to fall out (Miller, 1956). Those items can be numbers, letters, or even words or ideas. Therefore, one way to increase storage is to group several letters into a single meaningful item. Chunking involves combining small pieces of information into larger clusters or chunks that are more easily held in short-term memory. Waitresses who use organizational encoding (p. 174) to organize customer orders into groups are essentially chunking the information, giving themselves less to remember.

Short-term memory was originally conceived of as a kind of “place” where information is kept for a limited amount of time. More recently, researchers developed and refined a more dynamic model of a limited-capacity memory system, working memory, which refers to active maintenance of information in short-term storage (Baddeley & Hitch, 1974). Working memory includes subsystems that store and manipulate visual images or verbal information, as well as a central executive that coordinates the subsystems (Baddeley, 2001). If you wanted to keep the arrangement of pieces on a chessboard in mind as you contemplated your next move, you’d be relying on working memory. Working memory includes the visual representation of the positions of the pieces, your mental manipulation of the possible moves, and your awareness of the flow of information into and out of memory, all stored for a limited amount of time. Brain imaging studies indicate that the central executive component of working memory depends on regions within the frontal lobe that are important for controlling and manipulating information on a wide range of cognitive tasks (Baddeley, 2001).

Can working memory skills be trained? Some studies suggest yes. In one study, elementary school students who were trained on several working memory tasks (about 35 minutes/day for at least 20 days over a 5- to 7-week time period) showed improvement on other working memory tasks (Holmes, Gathercole, & Dunning, 2009). These gains were evident even when the children were tested 6 months after training. However, other studies suggest that working memory training improves performance—but only on the specific working memory task that was trained, not on other cognitive tasks (Redick et al., 2013). More research will be needed to determine whether working memory training produces any general improvements in cognitive performance (Shipstead, Redick, & Engle, 2012).

In contrast to the time-limited sensory memory and short-term memory stores, long-term memory is a type of storage that holds information for hours, days, weeks, or years. In contrast to both sensory and short-term memory, long-term memory has no known capacity limits (see FIGURE 6.7). For example, most people can recall 10,000 to 15,000 words in their native language, tens of thousands of facts (The capital of France is Paris and 3 × 3 = 9), and an untold number of personal experiences. Just think of all the song lyrics you can recite by heart, and you’ll understand that you’ve got a lot of information tucked away in long-term memory!

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The Role of the Hippocampus as Index

Where is long-term memory located in the brain? The clues to answering this question come from individuals who are unable to store long-term memories. In 1953, a young man, known then by the initials HM, suffered from intractable epilepsy (Scoville & Milner, 1957). In a desperate attempt to stop the seizures, HM’s doctors removed parts of his temporal lobes, including the hippocampus and some surrounding regions (FIGURE 6.8). After the operation, HM could converse easily, use and understand language, and perform well on intelligence tests, but he could not remember anything that happened to him after the operation. HM could repeat a telephone number with no difficulty, suggesting that his short-term memory store was just fine (Corkin, 2002, 2013; Hilts, 1995; Squire, 2009). But after information left the short-term store, it was gone forever. For example, he would often forget that he had just eaten a meal or fail to recognize the hospital staff who helped him on a daily basis. Studies of HM and others have shown that the hippocampal region of the brain is critical for putting new information into the long-term store. When this region is damaged, individuals suffer from a condition known as anterograde amnesia, which is the inability to transfer new information from the short-term store into the long-term store.

Some individuals with amnesia also suffer from retrograde amnesia, which is the inability to retrieve information that was acquired before a particular date, usually the date of an injury or surgery. The fact that HM had much worse anterograde than retrograde amnesia suggests that the hippocampal region is not the site of long-term memory. Indeed, research has shown that different aspects of a single memory—its sights, sounds, smells, emotional content—are stored in different places in the cortex (Damasio, 1989; Schacter, 1996; Squire & Kandel, 1999). Some psychologists have argued that the hippocampal region acts as a kind of “index” that links together all of these otherwise separate bits and pieces so that we remember them as one memory (Schacter, 1996; Squire, 1992; Teyler & DiScenna, 1986). Over time, this index may become less necessary.

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You can think of the hippocampal region index like a printed recipe. The first time you make a pie, you need the recipe to help you retrieve all the ingredients and then mix them together in the right amounts. As you bake more and more pies, though, you don’t need to rely on the printed recipe anymore. Similarly, although the hippocampal region index is critical when a new memory is first formed, it may become less important as the memory ages. Scientists are still debating the extent to which the hippocampal region helps us to remember details of our old memories (Bayley et al., 2005; Kirwan et al., 2008; Moscovitch et al., 2006; Squire & Wixted, 2011; Winocur, Moscovitch, & Bontempi, 2010), but the notion of the hippocampus as an index explains why people like HM cannot make new memories and why they can remember old ones.

Memory Consolidation

The idea that the hippocampus becomes less important over time for maintaining memories is closely related to the concept of consolidation, the process by which memories become stable in the brain (McGaugh, 2000). Shortly after encoding, memories exist in a fragile state in which they can be easily disrupted; once consolidation has occurred, they are more resistant to disruption. One type of consolidation operates over seconds or minutes. For example, when someone experiences a head injury in a car crash and later cannot recall what happened during the few seconds or minutes before the crash—but can recall other events normally—the head injury probably prevented consolidation of short-term memory into long-term memory. Another type of consolidation occurs over much longer periods of time—days, weeks, months, and years—and likely involves transfer of information from the hippocampus to more permanent storage sites in the cortex. The operation of this longer-term consolidation process is why patients like HM can recall memories from childhood relatively normally, but they are impaired when recalling experiences that occurred just a few years prior to the time they became amnesic (Kirwan et al., 2008; Squire & Wixted, 2011).

How does a memory become consolidated? The act of recalling a memory, thinking about it, and talking about it with others probably contributes to consolidation (Moscovitch et al., 2006). As explained in the Hot Science box (p. 181), mounting evidence indicates that sleep also plays an important role in memory consolidation.

Many researchers have long believed that a fully consolidated memory becomes a permanent fixture in the brain, more difficult to get rid of than a computer virus. But even seemingly consolidated memories can become vulnerable to disruption when they are recalled, thus requiring them to be consolidated again. This process is called reconsolidation (Dudai, 2012; Nader & Hardt, 2009). Evidence for reconsolidation mainly comes from experiments with rats showing that when animals are cued to retrieve a new memory that was acquired a day earlier, giving the animal a drug (or an electrical shock) that prevents initial consolidation will cause forgetting (Nader, Shafe, & LeDoux, 2000; Sara, 2000). In fact, each time they are retrieved, memories become vulnerable to disruption and have to be reconsolidated.

Might it be possible one day to eliminate painful memories by disrupting reconsolidation? Recent research with traumatized individuals suggests it could be: When a traumatic event was reactivated after administration of a drug that reduces anxiety, there was a subsequent reduction in traumatic symptoms (Brunet et al., 2008, 2011). Related work indicates that disrupting reconsolidation can seemingly eliminate a conditioned fear memory in a part of the brain called the amygdala, which we will learn, later in this chapter, plays a key role in emotional memory (Agren et al., 2012). Reconsolidation thus appears to be a key memory process with many important implications.

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We’ve already discussed parts of the brain that are related to memory storage, but we haven’t said much about how memories are stored. Much of what we know about the neurological basis for long-term memory comes from the sea slug Aplysia, which has a simple nervous system consisting of only 20,000 neurons (compared to roughly 100 billion in the human brain). When an experimenter stimulates Aplysia’s tail with a mild electric shock, the slug immediately withdraws its gill, and if the experimenter does it again a moment later, Aplysia withdraws its gill even more quickly. If the experimenter comes back an hour later and shocks Aplysia, the withdrawal of the gill happens as slowly as it did the first time, as if Aplysia can’t “remember” what happened an hour earlier (Abel et al., 1995). But if the experimenter shocks Aplysia over and over, it does develop an enduring “memory” that can last for days or even weeks. Research suggests that this long-term storage involves the growth of new synaptic connections between neurons (Abel et al., 1995; Kandel, 2006; Squire & Kandel, 1999). You’ll recall from the Neuroscience and Behavior chapter that a synapse is the small space between the axon of one neuron and the dendrite of another, and neurons communicate by sending neurotransmitters across these synapses. As it turns out, the act of sending actually changes the synapse. Specifically, it strengthens the connection between the two neurons, making it easier for them to transmit to each other the next time. This is why researchers sometimes say, “cells that fire together wire together” (Hebb, 1949).

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If you’re something more complex than a slug—say, a chimpanzee or your roommate—a similar process of synaptic strengthening happens in the hippocampus, which we’ve seen is an area crucial for storing new long-term memories. In the early 1970s, researchers applied a brief electrical stimulus to a neural pathway in a rat’s hippocampus (Bliss & Lømo, 1973). They found that the electrical current produced a stronger connection between synapses that lay along the pathway and that the strengthening lasted for hours or even weeks. They called this long-term potentiation (more commonly known as LTP), a process whereby communication across the synapse between neurons strengthens the connection, making further communication easier. Drugs that block LTP can turn rats into rodent versions of patient HM: The animals have great difficulty remembering where they’ve been recently and become easily lost in a maze (Bliss, 1999; Morris et al., 1986).