Читать книгу The Invisible Gorilla: And Other Ways Our Intuition Deceives Us - Christopher Chabris - Страница 18

This chapter is about this illusion of memory: the disconnect between how we think memory works and how it actually works. But how, exactly, do we think it works? Before answering this question, we’d like you to try a brief memory test. Read through the following list of words: bed, rest, awake, tired, dream, wake, snooze, blanket, doze, slumber, snore, nap, peace, yawn, drowsy. We’ll get back to them in a few paragraphs.

Most of us cannot remember a fifteen-digit number, and we know that we cannot, so we do not even try. We all sometimes forget where we put our car keys (or our car), we fail to recall a friend’s name, or we neglect to pick up the dry cleaning on the way home from work. And we know that we often make these mistakes—our intuitive beliefs about such everyday memory failures are reasonably accurate. Our intuitions about the persistence and detail of memory are a different story.

In the national survey of fifteen hundred people we commissioned in 2009, we included several questions designed to probe how people think memory works. Nearly half (47%) of the respondents believed that “once you have experienced an event and formed a memory of it, that memory doesn’t change.” An even greater percentage (63%) believed that “human memory works like a video camera, accurately recording the events we see and hear so that we can review and inspect them later.” People who agreed with both statements apparently think that memories of all our experiences are stored permanently in our brains in an immutable form, even if we can’t access them. It is impossible to disprove this belief—the memories could in principle be stored somewhere—but most experts on human memory find it implausible that the brain would devote energy and space to storing every detail of our lives (especially if that information could never be accessed).²

Just as the illusion of attention leads us to think that important and distinctive events capture our attention when they don’t, the illusion of memory reflects a basic contrast between what we think we remember and what we actually remember. Why do people easily grasp the limitations of short-term memory, but misunderstand the nature of long-term memory? This chapter is about how our memories can mislead us and how our beliefs about the workings of memory are mistaken. The illusion of attention happens when what we notice is different from what we think we notice. The illusion of memory happens when what we remember is different from what we think we remember.

Now we’d like you to try to recall all of the words from the list you read. Do your best to recall as many as you can. Write them down on a piece of paper before you continue reading.

What could be simpler than recalling a list of words that you read only moments ago? Not much, but even a task as simple as this reveals systematic distortions in memory. Look at the list you wrote down. How do you think you did? Most likely, you didn’t recall all fifteen words. When we use this task as a classroom demonstration, most students recall a few words from the beginning of the list and a few from the end of the list.³ They often recall fewer than half of the words from the middle of the list, though, and on average, they tend to recall only about seven or eight of the fifteen words correctly. Stop to think about this for a moment. Those words were all utterly common and familiar, you were not under any special stress (we hope) when you read them, and there was no time pressure when you had to recall them. Computers built in the 1950s were able to perfectly store fifteen words in memory, but despite our magnificent cognitive abilities, we cannot remember with precision what we read just minutes ago.

If you ask a small child to remember a short list of words for a few minutes, you will notice that as late as age four kids still don’t appear to realize that they need to exert special effort to keep the words in memory. As adults, though, we have learned that there are limits to how much we can maintain in memory for a short time. When we have to remember a phone number long enough to dial it, we repeat it to ourselves, either silently or out loud, as long as necessary. Once an arbitrary list is longer than the “magic number” of about seven items, most people have trouble holding it in their short-term memories.⁴ That is why license plates have only about seven letters and numbers and why phone numbers historically only required seven numbers (and why the three-digit prefix often began with the first two letters of the town or neighborhood’s name; where Chris grew up, in Armonk, New York, some old signs and advertisements still listed the numbers of local businesses as starting with AR-3 instead of 273). When we have to remember anything more than this, we use memory crutches (notepads, voice recorders, and so on) to help.

The reason your difficulty recalling all fifteen words in our list illustrates the illusion of memory is not that it reveals limits on how much we can remember. People generally understand those limits. It reflects the illusion of memory because it highlights how we remember what we do. Take a look at the list of words you recalled. Does it contain the word “sleep”? About 40 percent of the people reading this book will recall having seen the word “sleep.” If you are one of those people, you are probably as confident about having seen “sleep” as you are about any of the other words you remembered. You might even have a distinct recollection of seeing it on the list—but it wasn’t there. You fabricated it.

Memory depends both on what actually happened and on how we made sense of what happened. The list you read was designed to produce just this type of false memory. All of the words are closely associated with the missing word “sleep.” As you read the words on the list, your mind made sense of them, automatically processing the connections among them. At some level, you knew that they were all related to sleep, but you didn’t take special note of the fact that “sleep” was not on the list. Then, when you recalled the words, your mind reconstructed the list as best it could, based on both your specific memory for the words you saw and on your knowledge of how the words were generally related.

When we perceive something, we extract the meaning from what we see (or hear, or smell…) rather than encode everything in perfect detail. It would be an uncharacteristic waste of energy and other resources for evolution to have designed a brain that took in every possible stimulus with equal fidelity when there is little for an organism to gain from such a strategy. Likewise, memory doesn’t store everything we perceive, but instead takes what we have seen or heard and associates it with what we already know. These associations help us to discern what is important and to recall details about what we’ve seen. They provide “retrieval cues” that make our memories more fluent. In most cases, such cues are helpful. But these associations can also lead us astray, precisely because they lead to an inflated sense of the precision of memory. We cannot easily distinguish between what we recall verbatim and what we construct based on associations and knowledge. The word-list example, originally devised in the 1950s by psychologist James Deese and then studied extensively by Henry Roediger and Kathleen McDermott in the 1990s,⁵ is a simple way to demonstrate this principle, but memory distortions and the illusion of memory extend well beyond arbitrary lists of words.

Just as the gorilla experiment showed that people see what they expect to see, people often remember what they expect to remember. They make sense of a scene, and that interpretation colors—or even determines—what they remember about it. In a dramatic demonstration of this principle, psychologists William Brewer and James Treyens conducted a clever experiment using a simple ruse.⁶ Subjects in their study were led to a graduate student office and asked to wait there for a minute while the experimenter made sure the previous subject was finished. About thirty seconds later, the experimenter returned and led the subjects to another room, where they unexpectedly were asked to write down a list of everything that they had seen in the waiting room. In most respects, the waiting room was a typical graduate student office, with a desk, chairs, shelves, and so on. Almost all of the subjects recalled such common objects. Thirty percent of them also recalled seeing books, and 10 percent recalled seeing a file cabinet. But this office was unusual—it contained no books or file cabinets.

In the same way that people tended to recall having seen the word “sleep” when remembering a list of words associated with sleep, their memory reconstructed the contents of the room based both on what actually was there and on what should have been there. (If you look at a picture of the office, it will probably seem perfectly normal until someone points out what’s missing, and then it will suddenly start to look strange.) What is stored in memory is not an exact replica of reality, but a re-creation of it. We cannot play back our memories like a DVD—each time we recall a memory, we integrate whatever details we do remember with our expectations for what we should remember.