There is something fiendishly frustrating about piggy banks. You can put money in them, you can shake them around to assure yourself that the money is there, but you can’t easily get the money out. If memories were like pennies in a piggy bank, stored but inaccessible, what would be the point of saving them in the first place? Retrieval is the process of bringing to mind information that has been previously encoded and stored, and it is perhaps the most important of all memory processes (Roediger, 2000; Schacter, 2001a).

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One of the best ways to retrieve information from inside your head is to encounter information outside your head that is somehow connected to it. The information outside your head is called a retrieval cue, external information that is associated with stored information and helps bring it to mind. Retrieval cues can be incredibly effective. How many times have you said something like, “I know who starred in Trouble with the Curve, but I just can’t remember her name?” At such moments, did a friend give you a hint (“Wasn’t she in Julie & Julia?”), which instantly brought the answer to mind (“Amy Adams!”)? Such incidents suggest both that information is sometimes available in memory even when it is momentarily inaccessible and also that retrieval cues help us bring inaccessible information to mind.

Hints are one kind of retrieval cue, but they are not the only kind. The encoding specificity principle states that a retrieval cue can serve as an effective reminder when it helps re-create the specific way in which information was initially encoded (Tulving & Thomson, 1973). External contexts often make powerful retrieval cues (Hockley, 2008). For example, in one study, divers learned some words on land and some other words underwater; they recalled the words best when they were tested in the same dry or wet environment in which they had initially learned them because the environment itself served as a retrieval cue (Godden & Baddeley, 1975). Similarly, recovering alcoholics often experience a renewed urge to drink when visiting places in which they once drank because those places serve as retrieval cues. There may even be some wisdom to finding a seat in a classroom, sitting in it every day, and then sitting in it again when you take a test because the feel of the chair and the sights you see may help you remember the information you learned while you sat there.

Retrieval cues need not be external contexts—they can also be inner states. State-dependent retrieval is the tendency for information to be better recalled when the person is in the same state during encoding and retrieval. For example, retrieving information when you are in a sad or happy mood increases the likelihood that you will retrieve sad or happy episodes (Eich, 1995), which is part of the reason it is so hard to “look on the bright side” when you’re feeling low. If the person’s state at the time of retrieval matches the person’s state at the time of encoding, the state itself serves as a retrieval cue—a bridge that connects the moment at which we experience something to the moment at which we remember it. Retrieval cues can even be thoughts themselves, as when one thought calls to mind another, related thought (Anderson et al., 1976).

The encoding specificity principle makes some unusual predictions. For example, you learned earlier that making semantic judgments about a word usually produces more durable memory for the word than does making rhyme judgments. So if you were shown a cue card of the word brain and if your friend were asked to think about what brain means while you were asked to think of a word that rhymes with brain, we would expect your friend to remember the word better the next day if we asked you both, “Hey, what was that word you saw yesterday?” However, suppose we asked you both, “What was that word that rhymed with train?” In this case, the retrieval cue would match your encoding context better than your friend’s, and we would expect you to remember it better than your friend did (Fisher & Craik, 1977). The principle of transfer-appropriate processing is the idea that memory is likely to transfer from one situation to another when the encoding and retrieval contexts of the situations match (Morris, Bransford, & Franks, 1977; Roediger, Weldon, & Challis, 1989).

Human memory differs substantially from computer memory. Simply retrieving a file from my computer doesn’t have any effect on the likelihood that the file will open again in the future. Not so with human memory. Retrieval doesn’t merely provide a readout of what is in memory; it also changes the state of the memory system in important ways.

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Retrieval Can Improve Subsequent Memory

The simple act of retrieval can strengthen a retrieved memory, making it easier to remember that information later (Bjork, 1975). For example, in one experiment, participants studied brief stories and then either studied them again or were given a test that required retrieving the stories (Roediger & Karpicke, 2006). Participants were then given a final recall test for the stories either 5 minutes, 2 days, or 1 week later. As shown in FIGURE 6.9, at the 5-minute delay, studying the stories twice resulted in slightly higher recall than studying and retrieving them. Critically, the opposite occurred at the 2-day and 1-week delays: Retrieval produced much higher levels of recall than did extra study exposure. These findings have potentially important implications for learning in educational contexts (Karpicke, 2012), which we will explore further in the Learning chapter (p. 239).

Retrieval Can Impair Subsequent Memory

As much as retrieval can help memory, that’s not always the case. Retrieval-induced forgetting is a process by which retrieving an item from long-term memory impairs subsequent recall of related items (Anderson, 2003; Anderson, Bjork, & Bjork, 1994). For example, when a speaker selectively talks about some aspects of memories shared with a listener and doesn’t mention related information, both the speaker and the listener later have a harder time remembering the omitted events (Cuc, Koppel, & Hirst, 2007; Hirst & Echterhoff, 2012). Retrieval-induced forgetting can even affect eyewitness memory. When witnesses to a staged crime are questioned about some details of the crime scene, their ability to later recall related details that they were not asked about is impaired compared with witnesses who were not questioned at all initially (MacLeod, 2002; Shaw, Bjork, & Handal, 1995). These findings suggest that initial interviews with eyewitnesses should be as complete as possible in order to avoid potential retrieval-induced forgetting of significant details that are not probed during an interview (MacLeod & Saunders, 2008).

Retrieval Can Change Subsequent Memory

In addition to improving and impairing subsequent memory, the act of retrieval can also change what we remember from an experience. In a recent experiment, participants went to a museum, where they took tours in which they viewed designated exhibits; each individual tour contained several different stops (St. Jacques & Schacter, 2013). The participants took a tour while wearing a camera that, every 15 seconds, automatically took pictures of what was in front of them. Two days later, participants visited the memory laboratory (in a separate building) for a “reactivation session.” After memories of some of the stops were reactivated by looking at photos of them, participants were asked to rate, on a 1–5 scale, how vividly they reexperienced what had happened at each stop. Next, the participants were shown novel photos of unvisited stops within the exhibit; then they were asked to judge how closely these novel photos were related to the photos of the stops that they had actually seen in that exhibit. Finally, the participants were given a memory test 2 days after the reactivation session.

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Participants sometimes incorrectly remembered that the stop shown in a novel photo had been part of the original tour. Most important, participants who tended to make this mistake also tended to have more vivid recollections during the reactivation session. In other words, retrieving and vividly reexperiencing memories of what participants actually did see at the museum led them to incorporate into their memories information that was not part of the original experience. This finding may be related to the phenomenon of reconsolidation that we discussed earlier (p. 180), where reactivating a memory temporarily makes it vulnerable to disruption and change. At the very least, this finding reinforces the idea that retrieving a memory involves far more than a simple readout of information.

Before leaving the topic of retrieval, let’s look at how the process actually works. There is reason to believe that trying to recall an incident and successfully recalling one are fundamentally different processes that occur in different parts of the brain (Moscovitch, 1994; Schacter, 1996). For example, regions in the left frontal lobe show heightened activity when people try to retrieve information that was presented to them earlier (Oztekin, Curtis, & McElree, 2009; Tulving et al., 1994). This activity may reflect the mental effort of struggling to dredge up the past event (Lepage et al., 2000). However, successfully remembering a past experience tends to be accompanied by activity in the hippocampal region (see FIGURE 6.10; Eldridge et al., 2000; Giovanello, Schnyer, & Verfaellie, 2004; Schacter, Alpert, et al., 1996). Furthermore, successful recall also activates parts of the brain that play a role in processing the sensory features of an experience. For instance, recall of previously heard sounds is accompanied by activity in the auditory cortex (the upper part of the temporal lobe), whereas recall of previously seen pictures is accompanied by activity in the visual cortex (in the occipital lobe; Wheeler, Petersen, & Buckner, 2000). Although retrieval may seem like a single process, brain studies suggest that separately identifiable processes are at work.

This sheds some light on the phenomena we just discussed: retrieval-induced forgetting. Recent fMRI evidence indicates that during memory retrieval, regions within the frontal lobe that are involved in retrieval effort play a role in suppressing competitors (Benoit & Anderson, 2012; Kuhl et al., 2007; Wimber et al., 2009). When hippocampal activity during retrieval signals successful recall of an unwanted competitor, frontal lobe mechanisms are recruited that help to suppress the competitor. Once the competitor is suppressed, the frontal lobe no longer has to work as hard at controlling retrieval, ultimately making it easier to recall the target item (Kuhl et al., 2007). In addition, successful suppression of an unwanted memory causes reduced activity in the hippocampus (Anderson et al., 2004). These findings make sense once we understand the specific roles played by particular brain regions in the retrieval process.

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