In 1977, neurologist Oliver Sacks interviewed a young man named Greg who had a tumor in his brain that wiped out his ability to remember day-to-day events. One thing Greg could remember was his life during the 1960s, when Greg’s primary occupation seemed to be attending rock concerts by his favorite band, The Grateful Dead. Greg’s memories of those concerts stuck with him over the following years, when he was living in a long-term care hospital. In 1991, Dr. Sacks took Greg to a Dead concert at New York’s Madison Square Garden, wondering whether such a momentous event might jolt his memory into action. “That was fantastic,” Greg told Dr. Sacks as they left the concert. “I will always remember it. I had the time of my life.” But when Dr. Sacks saw Greg the next morning and asked him whether he recalled the previous night’s concert, Greg drew a blank: “No, I’ve never been to the Garden” (Sacks, 1995, pp. 76–77).

Although Greg was unable to make new memories, some of the new things that happened to him seemed to leave a mark. For example, Greg did not recall learning that his father had died, but he did seem sad and withdrawn for years after hearing the news. Similarly, HM could not make new memories after his surgery, but if he played a game in which he had to track a moving target, his performance gradually improved with each round (Milner, 1962). Greg could not consciously remember hearing about his father’s death, and HM could not consciously remember playing the tracking game, but both men showed clear signs of having been permanently changed by experiences that they so rapidly forgot. In other words, they behaved as though they were remembering things while claiming to remember nothing at all. This suggests that there must be several kinds of memory, some that are accessible to conscious recall, and some that we cannot consciously access (Eichenbaum & Cohen, 2001; Schacter & Tulving, 1994; Schacter, Wagner, & Buckner, 2000; Squire & Kandel, 1999).

187

The fact that people can be changed by past experiences without having any awareness of those experiences suggests that there must be at least two different classes of memory (FIGURE 6.11). Explicit memory occurs when people consciously or intentionally retrieve past experiences. Recalling last summer’s vacation, incidents from a novel you just read, or facts you studied for a test all involve explicit memory. Indeed, anytime you start a sentence with “I remember …,” you are talking about an explicit memory. Implicit memory occurs when past experiences influence later behavior and performance, even without an effort to remember those experiences or an awareness of the recollection (Graf & Schacter, 1985; Schacter, 1987). Implicit memories are not consciously recalled, but their presence is “implied” by our actions. Greg’s persistent sadness after his father’s death, even though he had no conscious knowledge of the event, is an example of implicit memory. So is HM’s improved performance on a tracking task that he didn’t consciously remember doing. So is the ability to ride a bike or tie your shoelaces or play guitar: You may know how to do these things, but you probably can’t describe how to do them. Such knowledge reflects a particular kind of implicit memory called procedural memory, which refers to the gradual acquisition of skills as a result of practice, or “knowing how” to do things. The fact that people who have amnesia can acquire new procedural memories suggests that the hippocampal structures that are usually damaged in these individuals may be necessary for explicit memory, but they aren’t needed for implicit procedural memory. In fact, it appears that brain regions outside the hippocampal area (including areas in the motor cortex) are involved in procedural memory. The Learning chapter discusses this evidence further, where you will also see that procedural memory is crucial for learning various kinds of motor, perceptual, and cognitive skills.

188

Not all implicit memories are procedural or “how to” memories. For example, priming refers to an enhanced ability to think of a stimulus, such as a word or object, as a result of a recent exposure to the stimulus (Tulving & Schacter, 1990). In one experiment, college students were asked to study a long list of words, including items such as avocado, mystery, climate, octopus, and assassin (Tulving, Schacter, & Stark, 1982). Later, explicit memory was tested first by showing participants some of these words along with new ones they hadn’t seen and then by asking them which words were on the list. To test implicit memory, participants received word fragments and were asked to come up with a word that fit the fragment. Try the test yourself:

You probably had difficulty coming up with the answers for the first and third fragments (chipmunk, bogeyman) but had little problem coming up with answers for the second and fourth (octopus, climate). Seeing octopus and climate on the original list made those words more accessible later during the fill-in-the-blanks test. Just as priming a pump makes water flow more easily, priming the memory system makes some information more accessible. In the fill-in-the-blanks experiment, people showed priming for studied words even when they failed to consciously remember that they had seen them earlier. This suggests that priming is an example of implicit, not explicit, memory.

A truly stunning example of this point comes from a study by Mitchell (2006), in which participants first studied black-and-white line drawings depicting everyday objects. Later, the participants were shown fragmented versions of the drawings that are difficult to identify; some of them depicted objects that had been studied earlier in the experiment, whereas others depicted new objects that had not been studied. Mitchell found that participants correctly identified more fragmented drawings of studied than new objects, and the participants also identified more studied objects than did participants in a control group who had never seen the pictures—a clear demonstration of priming (see FIGURE 6.12). Here’s the stunning part: The fragmented drawing test was given 17 years after presentation of the study list! By that time, participants had little or no explicit memory of having seen the drawings, and some had no recollection that they had ever participated in the experiment! “I’m sorry—I really don’t remember this experiment at all,” said one 36-year-old man who showed a strong priming effect. A 36-year-old woman who showed even more priming stated simply, “Don’t remember anything about it” (Mitchell, 2006, p. 929). These observations confirm both that priming is an example of implicit memory and also that priming can persist over very long periods of time.

As such, you’d expect amnesic individuals such as HM and Greg to show priming. In fact, many experiments have shown that amnesic individuals can show substantial priming effects—often as large as healthy, nonamnesic individuals—even though they have no explicit memory for the items they studied. Priming, like procedural memory, does not require the hippocampal structures that are damaged in cases of amnesia (Schacter & Curran, 2000).

If the hippocampal region isn’t required for priming, what parts of the brain are involved? When research participants are shown the word stem mot___ or tab___ and are asked to provide the first word that comes to mind, parts of the occipital lobe involved in visual processing and parts of the frontal lobe involved in word retrieval become active. But if people perform the same task after being primed by seeing motel and table, there’s less activity in these same regions (Buckner et al., 1995; Schott et al., 2005). Priming seems to make it easier for parts of the cortex that are involved in perceiving a word or object to identify the item after a recent exposure to it (Schacter, Dobbins, & Schnyer, 2004; Wiggs & Martin, 1998). This suggests that the brain saves a bit of processing time after priming (see FIGURE 6.13).

189

Consider these two questions: (1) Why do Americans celebrate on July 4th? and (2) What is the most spectacular Fourth of July celebration you’ve ever seen? Every American knows the answer to the first question (we celebrate the signing of the Declaration of Independence on July 4, 1776), but we all have our own answers to the second. Although both of these questions require you to search your long-term memory and explicitly retrieve information that is stored there, one requires you to dredge up a fact that every American schoolchild knows and that is not part of your personal autobiography, and one requires you to revisit a particular time and place—or episode—from your personal past. These memories are called semantic and episodic memories, respectively (Tulving, 1972, 1983, 1998). Semantic memory is a network of associated facts and concepts that make up our general knowledge of the world, whereas episodic memory is the collection of past personal experiences that occurred at a particular time and place.

Episodic memory is special because it is the only form of memory that allows us to engage in mental time travel, projecting ourselves into the past and revisiting events that have happened to us. This ability allows us to connect our pasts and our presents and construct a cohesive story of our lives. People who have amnesia can usually travel back in time and revisit episodes that occurred before they became amnesic, but they are unable to revisit episodes that happened later. For example, Greg couldn’t travel back to any time after 1969 because that’s when he stopped being able to create new episodic memories. But can people with amnesia create new semantic memories?

Researchers have studied three young adults who suffered damage to the hippocampus during birth as a result of difficult deliveries that interrupted oxygen supply to the brain (Brandt et al., 2009; Vargha-Khadem et al., 1997). Their parents noticed that the children could not recall what happened during a typical day, had to be constantly reminded of appointments, and often became lost and disoriented. In view of their hippocampal damage, you might also expect that these children would perform poorly in school. Remarkably, however, all three children learned to read, write, and spell; developed normal vocabularies; and acquired other kinds of semantic knowledge that allowed them to perform well in school. Based on this evidence, researchers have concluded that the hippocampus is not necessary for acquiring new semantic memories.

190

Episodic Memory and Imagining the Future

We’ve already seen that episodic memory allows us to travel backward in time, but it turns out that episodic memory also plays a role in allowing us to travel forward in time. An amnesic man known by the initials K.C. provided an early clue to this insight about traveling forward in time. K.C. could not recollect any specific episodes from his past, and when asked to imagine a future episode—such as what he might do tomorrow—he reported a complete “blank” (Tulving, 1985). Consistent with this observation, more recent findings from individuals with hippocampal amnesia reveal that some of them have difficulty imagining new experiences, such as sunbathing on a sandy beach (Hassabis et al., 2007), or events that might happen in their everyday lives (Race, Keane, & Verfaellie, 2011). Something similar happens with aging. When asked either to recall episodes that actually occurred in their pasts or imagine new episodes that might occur in their futures, older adults provided fewer details about what happened or what might happen than did college students (Addis, Wong, & Schacter, 2008; Schacter, Gaesser, & Addis, 2012). Consistent with these findings, neuroimaging studies reveal that a network of brain regions known to be involved in episodic memory—including the hippocampus—shows similarly increased activity when people remember the past and imagine the future (Addis, Wong, & Schacter, 2007; Okuda et al., 2003; Schacter, Addis, et al., 2012; Szpunar, Watson, & McDermott, 2007; see FIGURE 6.14).

Taken together, these observations strongly suggest that we rely heavily on episodic memory to envision our personal futures (Schacter, Addis, & Buckner, 2008; Szpunar, 2010). Episodic memory is well-suited to the task, because it is a flexible system that allows us to recombine elements of past experience in new ways so that we can mentally try out different versions of what might happen (Schacter, 2012; Schacter & Addis, 2007; Suddendorf & Corballis, 2007). For example, when you imagine having a difficult conversation with a friend that will take place in a couple of days, you can draw on past experiences to envisage different ways in which the conversation might unfold, and hopefully, you will then avoid saying things that, based on past experience, are likely to make the situation worse. As we’ll discuss later, however, this flexibility of episodic memory might also be responsible for some kinds of memory errors (see p. 202).

191

Social Influences on Remembering: Collaborative Memory

So far, we’ve focused mainly on memory in individuals functioning on their own. But remembering also serves important social functions, which is why we get together with family to talk about old times or share our memories with friends by posting our vacation photos on Facebook. Sharing memories with others can strengthen those memories (Hirst & Echterhoff, 2012), but we’ve already seen that talking about some aspects of a memory but omitting other related events can also produce retrieval-induced forgetting (see p. 184; Coman, Manier, & Hirst, 2009; Cuc, Koppel, & Hirst, 2007). Psychologists have become increasingly interested in how people remember in groups, which is now referred to as collaborative memory (Rajaram, 2011).

In a typical collaborative memory experiment, participants first encode a set of target materials, such as a list of words, on their own (just like in the traditional memory experiments that we’ve already considered). Things start to get interesting at the time of retrieval when participants work together in small groups (usually two or three participants) to try to remember the target items. The number of items recalled by this group can then be compared with the number of items recalled by individuals who are trying to recall items on their own without any help from others. The collaborative group typically recalls more target items than any individual (Hirst & Echterhoff, 2012; Weldon, 2001), suggesting that collaboration benefits memory. For example, Tim might recall an item that Emily forgot, and Eric might remember items that neither Tim nor Emily recalled, so the sum total of the group will exceed what any one person can recall.

But things get really interesting when we compare the performance of the collaborative group to the performance of several individuals recalling target items on their own. For example, let’s assume that after studying a list of eight words, and recalling the items on their own, Tim recalls items 1, 2, and 8; Emily recalls items 1, 4, and 7; and Eric recalls items 1, 5, 6, and 8. Adding them all together, Tim, Emily, and Eric recalled in combination seven of the eight items that were presented (nobody recalled item 3). The surprising finding is that when they remember together as a group, Tim, Emily, and Eric will typically come up with fewer total items than when they remember on their own (Basden et al., 1997; Hirst & Echterhoff, 2012; Rajaram, 2011; Rajaram & Pereira-Pasarin, 2010; Weldon, 2001). This negative effect of group recall on memory is known as collaborative inhibition: The same number of individuals working together recall fewer items than they would on their own.

192

What’s going on here? One possibility is that the retrieval strategies used by some members of the group disrupt those used by others whenever the group members are recalling items together (Basden et al., 1997; Hirst & Echterhoff, 2012; Rajaram, 2011). For example, suppose that Tim goes first and recalls items in the order that they were presented. This retrieval strategy may be disruptive to Emily, who prefers to recall the last item first and then work backward through the list. So, next time you are sharing memories of a past activity with friends, you will be shaping your memories for both better and worse. (Can you rely on your computer for collaborative remembering? See The Real World: Is Google Hurting Our Memories?)

193