Why We Forget and How to Remember Better, page 7
Real Memories . . . or Stories?
So, consolidated memories have their connections strengthened . . . but is the hippocampus still needed to retrieve that memory? Yes and no. Most researchers believe that in order to have that subjective sense of “time-traveling” when you relive a memory, the connection to the hippocampus must still be present in the consolidated memory.3 Consolidated memories that have lost the connection to the hippocampus become more of a “story” that you remember of what happened to you, rather than a true reliving of the actual event. You may recall both the general outline and quite a lot of facts of what happened during that event of your life, but it isn’t quite the same as retrieving a memory in which you can relive the event.
Do you have any childhood memories for events that you retell in roughly the same way each time? Does it no longer feel to you like you’re transported back to that past moment when you tell that story? Even if you try, are you unable to see the world from your child-height vantage point, or to bring to mind other sights or sounds or smells that aren’t commonly included in your story? While there is no litmus test, if your answer to all these questions is “yes,” then you may have a childhood memory that, over time, has lost its connection with the hippocampus.
Ribot’s Law Explained
With the knowledge you have learned in this chapter, you are now ready to understand the basis of Ribot’s law and the answer to the question posed by the daughter of the patient at the beginning of this chapter: How can my father’s memory be so good for things that happened 60 years ago when he can’t remember what happened yesterday or last month? We now understand that when the hippocampus is damaged by head trauma, Alzheimer’s, or other disorders, there are a number of consequences:
• The formation of new memories will be compromised because of impaired hippocampal binding and indexing.
• Any recent memories that are solely dependent upon the hippocampus will be difficult or impossible to retrieve.
• Some older, consolidated memories, whose components are directly linked together, may be partially independent of the hippocampus and thus can still be retrieved. These older memories, however, will be closer to stories that are remembered than actual episodic autobiographical memories.
You now understand the basics about how you acquire, store, and retrieve autobiographical memories—memories for the events that make up your life. We will continue to explore autobiographical event memories in Part 2. But next we turn to how you are able to learn, retain, and retrieve knowledge about the world. As we will discuss, it is with this type of memory that the full power of consolidation is realized.
Ways to Improve Your Episodic Memory
• Pay attention to your sensations, thoughts, and emotions.
◦ Focusing on the sights, sounds, smells, tastes, thoughts, and feelings while you are experiencing them will help you form long-lasting memories.
• Focus on the differences between two similar events if you wish to remember each of them distinctly.
◦ To recall each distinctly, concentrate on the distinguishing features of different people, puppies, or parking spots.
• To enhance your retention, review an event after it has concluded.
◦ To remember your evening out, think about each aspect of the night as you are traveling home: how you looked in the mirror before you left, the sounds you heard when you opened the door, how others were dressed, the tastes of the food, the faces of the people you met, and the topics you discussed.
5
Semantic memory
What you know
A college professor was referred to the clinic for memory problems. Although she admitted to trouble with names, she reported her memory as “pretty good.” She then went on to speak about current events with a fair amount of detail. She was only 63 years old and, at this point in the interview, Andrew was thinking, “Well, perhaps her memory is normal after all.” He asked about her other medical problems, and she began to speak about a past surgery. She rolled up her left pants leg and explained that “this one” was done, showing him a long scar across her knee, and then proceeded to roll up her right pants leg, saying that “this one” needed to be done, flexing the right knee and wincing a bit as she did so.
“What do you call that joint?” Andrew asked.
“Oh, I don’t know the technical name,” she responded.
“What’s the ordinary name?”
She shook her head and said, “I don’t know.”
“It’s your knee, right?”
“Knee?” she replied, shaking her head, “What’s a knee?”
“You know, it’s like your elbow,” Andrew said, showing her his own, “except for your leg.”
“Elbow, elbow . . . I know the word, but I don’t know what it means.”
On further questioning it became clear that she had lost the meaning of many words that referred to body parts and articles of clothing.
Facts and Other Things You Know = Your Knowledge Base = Your Semantic Memory
How can this college professor remember news events and what she did last week but not remember what the words “knee” and “elbow” mean? It is, in part, from such individuals that we know there must be a separate memory system that stores our knowledge of the world, including the names we use for things.
We call this system semantic memory. The word “semantic” refers to the meaning of something and, in fact, some individuals with semantic memory impairment lose not only the words that signify items but also the meaning and use of those items. For example, some patients lose not only the words “fork” and “remote control” but also the meaning of what those items are and their ability to use them, as if they grew up in a culture that did not use forks or remote controls.
Semantic memory constitutes your knowledge of the world that is not related to a specific event that you recall—in other words, unrelated to a particular episodic memory. For example, you probably remember who Queen Victoria and Leonardo da Vinci are, but not where you were and what you were doing when you first learned about them. Similarly, you likely know that the primary colors are red, blue, and yellow, and that if you mix blue and yellow you make green, but not how you know this information. And, of course, you know what to do with forks, screwdrivers, straws, and remote controls even though you probably cannot recall how you learned this information.
Acquiring New Facts: Building Your Semantic Memory
Building new semantic memories is essentially the same thing as learning new facts. Sometimes this means learning information from a textbook, such as who fought in the War of 1812 or what the French word l’amour means. Other times it comes from your experience, such as if you hold very still, a parakeet may eat out of your hand, or if you arrive at the cinema to watch the new movie 10 minutes before it starts, it will likely be sold out. As these examples suggest, you need to use your episodic memory to efficiently get information into sematic memory. So, to learn new semantic information, typically the same processes take place as when you are learning new episodic information.
For example, a few days or weeks after you first learn the conjugation of a French verb, you can recall exactly where you sat in the classroom, how the teacher wrote the words on the board, and how she pronounced each one. You can also recall how the sun streamed in through the open window and how you were perspiring slightly on that early September day. Thus, you learned the verb by forming a new episodic memory. Over the next 9 months, however, you proceed to study that verb in your textbook, listen to it being pronounced on videos, and speak it in conversation with classmates. By May, you’ve studied that verb hundreds of times. You know how to use it perfectly, but no longer recall your initial learning of that verb in the classroom nor the details of your other sessions in which you were studying it. The information has moved from being tied to a specific event to information that is unrelated to any particular episode of your life—it is now part of your semantic memory.
From Episodic to Semantic Memory
For a memory to move from an event of your life to part of your knowledge base (in other words, from a hippocampally-dependent episodic memory to a semantic memory), two things generally happen in your brain.
Consolidation
One thing that turns events of your life into knowledge is that process of consolidation that we discussed in Chapter 4. For example, the first time you heard the word “lightsaber” was probably when you were watching one of the Star Wars movies. Shortly after seeing that movie you might even be able to recall the scene—perhaps the one in which Obi Wan gave a lightsaber to Luke. You can remember the bright blue shimmer it had and the characteristic sounds it made when it was turned on, turned off, and swished through the air. Over time, when you are sleeping, the process of consolidation takes place, directly linking together the word “lightsaber” (stored with your vocabulary in the outer and lower parts of your temporal lobe) with its image (stored with other images in your occipital lobe) and its sounds (stored with other sounds in the upper part of your temporal lobe).
After the memory has been consolidated, if you happen to run into the word, image, or sound of a lightsaber, its other attributes will immediately spring to mind—but you will have lost that personal, autobiographical, time-traveling aspect of the memory back to when you watched that scene in the movie. Note that this doesn’t necessarily mean you will have forgotten that scene, just that your experience won’t be as vivid or life-like.
Is it a bad thing to lose the episodic nature of your memory for facts? Not at all. If you’re helping your friend assemble a piece of furniture and she says, “Pass me the screwdriver,” you don’t need to be reliving the experience of every prior time you’ve used a screwdriver, you just need to know what it looks like so you can find it from the array of tools scattered on the floor and hand it to her.
Got the Gist?
Another thing that happens to semantic memories is that they become generalized, such that you are able to discern a variety of different-looking smartphones, car models, can openers, tulips, and tigers even if you have never seen those particular ones before. Together, the hippocampus and the lobes of the brain are, in fact, quite good at extracting the general concept, idea, or gist of a collection of items, and storing that gist away for later retrieval. For example, because of your experience with beach balls, you know the gist of them is that they are filled with air, very light, at least a foot or two in diameter, often brightly colored, and float in water. Your gist of a baseball is very different; it is small enough to be held in your hand, quite hard and solid, heavy for its size, and will sink in water. The gist of your experience with beach balls and baseballs can be consolidated, just like particular features of episodic memories.
This ability to extract and retain the gist of something in your semantic memory is incredibly powerful and quite useful. If you see something that looks like a house cat but has stripes and is about 100 times larger, you don’t need to have seen that exact tiger for you to know that it is dangerous and you should stay away. And when your friend returns from the salon you don’t say, “Who are you?” (although you have never before seen her with her new hairstyle), but rather, “Nice haircut.”
Constantly Updating
This last example, of how you can recognize your friend after her haircut, brings up another important feature of semantic memory, which is that it constantly updates the attributes of things with the latest representations. In other words, even though you may have grown up in the 1970s and developed the concept of “cars” with Ford Pintos and Pontiac Firebirds, your concept of cars has been updated over the last 50 years such that if someone says “car” today you’re more likely to think of a Tesla or Toyota Prius.
Episodic and Semantic People
Our concepts of individual people generally have both episodic and semantic components. If you think of your family members, you will likely have a number of very specific episodic memories that you can use to travel back in time and relive experiences with them in your mind. You also have a semantic memory concept of each member of your family that is not related to any particular event. This semantic concept includes attributes such as how tall they are, what color their hair is, how they generally dress, and what they like to eat. It also includes their visual appearance, the sound of their voice, and even their mannerisms and how they perform activities.
For example, you might be able to recognize your spouse, children, or friends by the way each of them skis down a slope—very handy when you’re trying to meet up with them at the lift. Because people have a semantic component and it is constantly updating, we can now understand why parents cannot usually detect their children growing—the semantic attribute of their height is updated daily. But when your Aunt Marge comes to visit, she notices because—not having seen the children for a year—her semantic image of them is a year out of date. (In fact, Aunt Marge’s semantic image may be more than a year out of date, as her image is an average of a number of prior experiences—including when the children were even younger.)
Retrieving Semantic Knowledge
When you are trying to retrieve semantic information, sometimes it comes to mind readily and without effort. In this case what generally happens is that a cue from the environment—perhaps the visual image of the word “Bahnhof” on your German vocabulary quiz—activates attributes that are related by their meaning and have been linked together through consolidation, such as the German word for train (Zug) and the English word “station,” and you easily write down “train station” on the exam sheet.
At other times the information you are looking for does not spontaneously come to mind. You’re sure you know the person’s name, vocabulary word, mathematical formula, or biological process but you’re drawing a blank. When this happens, your brain’s central executive needs to use a strategy to find the missing information (see Chapter 3 for more on the central executive). One good strategy is to think about other attributes that you can bring to mind regarding the information you’re seeking. For example, you see your friend’s face, but this visual image is not automatically bringing up her name. Think about her career, children, hometown, favorite foods, and other information that you can recall about her. It is likely that one of these pieces of information will be linked together with her name, allowing you to retrieve it. We will discuss other retrieval strategies in Chapter 9 (Part 2) and Chapters 22 through 25 (Part 5).
Where Are Words in the Brain?
Now that you know what semantic memory is and how it works, you might be wondering where it is stored in the brain. Hanna and Antonio Damasio, wife-and-husband neurologists who combine their clinical work with neuroscience research, conducted a pair of relevant studies published in 1996 while they were at the University of Iowa. They demonstrated that retrieving words for specific items depends upon the left temporal lobe, including its tip (often called the “temporal pole”) just behind your left eye, and its lower and outer extent as it stretches backward toward your occipital lobe in the back of your head.
In the first experiment, they studied over 100 individuals who had strokes in the outer part of their left temporal lobe. They found that those individuals who had strokes in their temporal pole had the most difficulty naming people. Those who had strokes slightly farther back, in the middle of the temporal lobe by the ear, had the most difficulty naming animals. And those who had strokes in the portion of the temporal lobe just behind the ear had the most difficulty naming tools and other manmade objects.
In the second experiment, they used a positron emission tomography (PET) scan to measure brain activity when healthy young individuals were naming either people, animals, or tools. They found that the activity was greatest in the temporal pole when they were naming people, greatest in the back of the temporal lobe when they were naming tools, and more in the middle of the temporal lobe when they were naming animals.
These experiments—along with other work in humans and animals—suggest that not only is your vocabulary stored in your left temporal lobe, but different parts of your temporal lobe are somewhat specialized for remembering different types of words.
Where Are Other Attributes in the Brain?
OK, so your words are in the outer part of the left temporal lobe. But what about the image of Cleopatra, the opening sounds of Beethoven’s Fifth Symphony, or the taste and texture of an orange wedge as you bite into it? We and most researchers believe that these parts of your knowledge are located close to where these visual, auditory, gustatory, and tactile sensations are formed. Thus, your store of visual images would be in your occipital lobes (which enables vision), your memory of sounds would be in the top part of your temporal lobes (which enables hearing), your memory for tactile sensations would be in your parietal lobes (which enables touch), and so forth. As mentioned earlier in our lightsaber example, if a particular semantic memory has multiple modalities—such as the color, taste, and texture of an orange—the brain representations of these different attributes will be directly connected to each other and to the word “orange” in your temporal lobe, such that you could close your eyes, feel the smooth yet uneven surface of an orange, and immediately have its name, color, and taste spring to mind from your semantic memory.
Normal Aging and Semantic Memory
Many older adults have trouble coming up with the names of people and other proper nouns—so many that it is considered part of normal aging. Why do older adults have trouble with names? Although no one is entirely sure and there are several competing theories, it may be due to frontal lobe dysfunction—common in normal aging—that makes retrieval of this semantic information difficult. It is also interesting to note that brain shrinkage in the temporal pole is also so prevalent in older adults that it is considered “normal.” Recall that we learned from the Damasios that retrieving the names of people depends upon the proper function of the temporal pole. It is, therefore, at least plausible that healthy older adults experience trouble with names because of the shrinkage in the temporal pole. Future research may reveal whether the shrinkage and naming difficulties are truly connected or whether it’s just a coincidence. (Why, you may ask, does the temporal pole shrink with normal aging? One speculation is that it might be a side effect of walking upright.)
