As you read this book, your brain is processing information, and if all goes well, you'll remember it for many years. But where, exactly, does this information go? One of the most enduring myths about memories is that they're stored in one place in the brain: a memory bank. Years ago, scientists assumed that you formed memories by depositing them in this bank and that you remembered things by withdrawing or borrowing them. Once you were finished using these memories, you returned them to the memory bank.

Although it was long suspected that this was not really the case, it was only about twenty years ago that scientists were able to prove that this assumption was wrong. Imaging technology became available that enabled us to get our first glimpse of the brain at work. The technology, called functional brain imaging, includes single photon emission computed tomography (SPECT), positron emission tomography (PET), and functional magnetic resonance imaging (fMRI). These methods allow researchers to see the brain at work by quantifying blood flow and tracking the brain's metabolism of certain substances in order to identify the regions that are most active during a particular type of mental activity. In other words, researchers can now see which part of the brain a person is using for different types of mental activity. This research has greatly increased our understanding of how memory works. J3.

Your Brain's Memory Networks

When researchers used these imaging devices to study people's brains during their investigations of learning and memory, they observed that memories aren't stored in a single location, or memory bank, but rather are widely distributed in different networks of neurons (brain cells) throughout the brain. Most of these networks reside in the cerebral cortex, which is the outer layer of the brain's two hemispheres and the most highly developed part of the human nervous system. The cortex contains about twenty billion neurons that make possible all of the complex thinking and activity that you do. The anatomy of a neuron is shown in Figure 2.1.

Most of your neurons are highly specialized, responding selectively to only certain types of input. For example, some neurons become activated, or "fire," only in response to the movement of an object across your visual field in a particular direction. So these neurons would respond to a dog running from your neighbor's yard into yours but not going in the opposite direction. Other neurons respond only to sounds of a particular tonal pitch, while still others respond to salty tastes but not sweet tastes. Neurons in other brain regions control your voluntary movements—everything from walking to starting the car to playing piano. And other groups of neurons enable you to speak, write, and make decisions.

We've long known that different areas of the brain specialize in processing different kinds of information. For example, in more than 90 percent of people, language skills are concentrated in the left frontal and temporal lobes of the brain, and "seeing" (really, the brain's registration of visual images relayed from the eyes) occurs in the occipital lobes in the back of the brain. Other data, for example, derived from hearing, smell, and the analysis of spatial information (things like finding your way around town or playing chess), are processed in other regions.

What does all this have to do with memory? It suggests that a single memory is not stored in a single place, like a book on a shelf. Instead, your brain breaks down a memory into its informational components and routes each type of information to the figure 2.1 Anatomy of a Neuron






Axon terminal

Receptors I



area of the brain that is specialized for processing it. Let's take an apple, for example. Your memory of an apple consists of how it looks, how it tastes, how it sounds when you bite into it, and so on. Each of these qualities of an apple is stored in a different place in your brain. Its visual form is stored in the occipital lobe. How it tastes is stored in the gustatory cortex in the insula and the amygdala. The sound of its crunch is stored in the temporal lobes. And its name is stored in the left temporal and parietal lobes. When you want to retrieve your memory of an apple, all of these brain regions become activated and work in concert to recombine the "experience" of apple into a complete whole, like instruments in an orchestra playing together to produce a symphony.

That's not all. Each memory is connected to many related memories. For instance, if you associate apple with your mother's apple pies and Thanksgiving, each of these memories will be cross-referenced. With such a vast network for storing memories, your brain is like the Internet. Calling up memories is like doing an Internet search, with one or two words activating dozens or even hundreds of hyperlinks.

To get a sense of how far ranging these hyperlinks can be, try playing the game Six Degrees of Kevin Bacon. The basic objective is to connect another actor (or any well-known person) to Kevin Bacon through a series of no more than six links. This game turns out to be quite easy for movie buffs, who are able to connect the most obscure film producer from the 1930s to Kevin Bacon through a series of links.

Memory works a bit like this game. Your individual memories are situated within an amazingly dense network of associations and connections in which a tiny bit of experience can release a virtual cascade of recollection. We've all had the experience of remembering something that seems to come out of the blue, only to realize after reflecting that some seemingly trivial figment of a thought or an image triggered the memory.

Three Stages of Memory

But just how does the information that you see, hear, and learn on a daily basis get filed away? We're not entirely sure, but a few models have been proposed. The first one, devised in the 1960s, proposed that new learning proceeded in three distinct stages. First, the brain would register the sensory experiences (what you see, hear, smell, and so on), then the memory would go into a short-term storage system, and finally, the memory would either be transferred into a long-term memory store or discarded because it was not deemed important enough for long-term stor-16 age. Studies of people with amnesia have focused on stages of learning, and these stages proceed in a sequence of acquisition (registration), consolidation (storage), and retrieval. Current clinical tests of memory also use these three stages. A problem in any of them can interfere with memory function.

Stage 1: Acquisition

To remember anything, you first have to "acquire" it. When you are learning something, it is initially encoded in the form of temporary pathways of neuronal activation in your brain. Neuronal activation refers to a pattern of nerve cell "firing" in which nerve cells, or neurons, communicate with each other. The paths are forged by the communication of one neuron with the next.

The location of these pathways depends on the nature of the information being processed. For example, if you're studying a map to figure out how to get somewhere, the pathway will probably recruit neurons from the right parietal lobe, an area of the cortex that processes spatial information. If you're listening to someone speak, a pathway will form in the left temporal lobe, which processes language.

Keep in mind that the pathways that represent what you've just experienced are temporary, which means that the information is part of your short-term memory system. Most of this information will quickly fade away. This is why you can look up a phone number, remember it on your trip from the phone book to the phone, but then forget it as soon as you've placed the call. The information that makes its way into your long-term memory is the information that you encoded most completely in the first place and that is strengthened over time through a process known as consolidation, which I'll discuss shortly. One of the most important things that determines how completely you encode new information is how well you focused your attention when you were initially acquiring it. So when you have trouble remembering something, it's often because you weren't concentrating and didn't acquire it very well in the first place.

One reason that many people have more trouble remembering things as they age is that they have more trouble concentrating. V7_

As we age, we are (for a variety of reasons) more easily distracted by background noises and other interruptions. Though some people are naturally better than others at tuning out distractions, as a rule, the ability to maintain focused attention declines with age.

Think back to when you were a teenager. Could you study for an exam effectively with the stereo on and people talking in the next room? Today, when you're reading a book and then someone turns on music, do you have trouble concentrating? If so, that's because the words in the book and the music drifting into the room are competing for your attention and making it difficult for you to focus on either of them very well. In clinical neuropsy-chology, this is what we refer to as a divided attention task, which is the basis for some of the most notoriously difficult cognitive tests. Your attentional focus is in some ways like the lens of a video camera; it can see only one field of view at a time. You can shuttle back and forth between two locations, but the result will be that you will capture only partial information from each of them.

Fortunately, there are highly effective strategies that you can use to improve your ability to concentrate and acquire information. I've taught them to my patients with great success. You'll learn what they are and how to use them in Chapter 10.

Stage 2: Consolidation

Closely concentrating when you read or listen to someone speak increases the probability that you'll remember the information over the long term, but it's not a guarantee. For the information to become secured in long-term memory, the initial neural pathway must be strengthened. The strengthening process is called consolidation.

The consolidation process occurs over a period of minutes, hours, days, or even longer, depending on the nature and complexity of the information. Chemical and structural changes strengthen the neuronal pathways that were initially created during acquisition, making them more durable. These chemical and structural changes also bolster the information so that it resists 18 interference from other information and disruptive influences.

Ultimately, the consolidation of newly acquired memories leads to the creation of new synapses, the junctions between two neurons across which neurotransmitters (chemicals that regulate neuronal communication) carry messages. Increases in this chemical connectivity lead, in turn, to the sprouting of the billions of infin-itesimally small branchlike neuronal projections, the axons and dendrites, which send and receive these chemical messages.

What determines if a short-term memory will be effectively consolidated into a longer-term memory? Several factors come into play, including how well you sleep, as you'll see in the following sections.

Consolidation of Declarative Memories. The consolidation of declarative memories—such as names and faces—is mediated by the hippocampus, a seahorse-shaped structure deep within the lim-bic system of the brain. The hippocampus becomes activated during the consolidation of important information. Consolidation entails the replay and rehearsal of the sequence of events to be remembered, thereby strengthening the pattern of neuronal activation. The hippocampus and other limbic system structures that play a role in memory consolidation are shown in Figure 2.2.

The hippocampus is selective with regard to the information that it consolidates. Several factors influence whether the hippocampus responds to new information and gives the signal to store it as a long-term memory. For one thing, you're more likely to retain new information if it relates to long-term memories that are already established. If you follow professional baseball, for example, you will have an easier time remembering details about recent team statistics and players than someone who's not interested in the sport. Another factor that influences consolidation is the emotional impact of the information. You're far more likely to remember a photograph or story that's disturbing (an image or a description of an anguished victim of war, for example) or joyful (two lovers embracing) than one that's bland (a newspaper ad for a vacuum cleaner). The part of the limbic system that reacts most directly to emotionally powerful information is the amyg-

figure 2.2 Limbic System figure 2.2 Limbic System

dala, situated right next to the hippocampus. Research using PET scans shows that information that activates the amygdala is more likely to be retained over the long term.

A good night's sleep appears to be important for memory consolidation. Several studies have shown that people remember word lists, spatial information, and visual and motor tasks better if they sleep after acquiring the information. Sleep might also help you recover new memories that faded during the previous day.

In a study published in the journal Nature in 2003, people learned new words and word sounds and were tested periodically throughout the day to see how well they retained the information. Predictably, they recalled most of the novel words and sounds soon after learning them, but as the day wore on, they 20, recalled fewer and fewer of them. After awakening the following morning, however, the people's memories had rebounded—they recalled as many of the words and sounds as they had immediately after learning them the previous day. Clearly, something happened during sleep to render the memories more accessible and stable.

Why would sleep make such a big difference in how we remember? It seems that the connections between neurons, which support each memory, are strengthened when we sleep. In research with rodents, researchers at the Massachusetts Institute of Technology have shown that during the nondreaming stage of sleep, the pattern of neuronal firing in the hippocampus is similar to the activity that appeared earlier during the learning episode. This finding suggests that during nondreaming sleep, the hippocampus strengthens the pattern by playing it over and over again. The replay of the newly learned information is thought to be a key component of the consolidation process.

Dreaming may also play a role in memory consolidation. Research shows that during the stage of sleep when dreaming occurs—called rapid eye movement, or REM, sleep—there's increased activity in areas of the neocortex where most memories are thought to be stored. We all dream occasionally about events or experiences from the day before. Generating such dreams may well be a strategy that the brain uses to strengthen the neuronal pathways that make a memory remain with you. Dr. Robert Stickgold and his colleagues from Harvard Medical School, who study dreaming and memory, believe that when you dream, the hippocampus and the cortex are shuttling information back and forth—in essence, transferring information from the brain region (the hippocampus) that's first involved in learning to areas of the brain (the cortex) that will store the information over the long term.

Consolidation of Procedural Memories. Skills you acquire—like learning how to serve a tennis ball, playing a computer game, or coordinating the left and right hands to play the piano—are considered procedural memories and are consolidated differently from declarative memories. ¿1

Although research has revealed less about the consolidation of procedural memories than the consolidation of declarative memories, we do know that procedural memory doesn't depend on the hippocampus. People who have amnesia and have damage to the hippocampus have trouble forming new declarative memories but are capable of learning new skills, procedural memories, through practice. This phenomenon—improved behavioral performance even when a person can't remember learning the skill—has been dubbed learning without awareness.

We also know that procedural memory is distributed widely throughout the brain in regions including the frontal lobes, the cerebellum, and the basal ganglia. These structures are important for motor function (your ability to move) and communicate with your muscles to coordinate your body's movements. Because these brain structures are less vulnerable to the aging process and degenerative disorders such as Alzheimer's disease, procedural memory remains relatively intact across the life span. The hippocampus, on the other hand, does change with age and is devastated in the setting of Alzheimer's disease. It stands to reason, then, that the memories that become more difficult to recall when you get into your forties and fifties are the memories that are mediated by the hippocampus: people's names, appointments you've made, and other declarative memories. Procedural memories, on the other hand, are relatively robust. You are less likely to forget how to ride a bicycle or play the piano.

Recent studies strongly suggest that sleep also helps consolidate procedural memory and that a good night's sleep is essential for you to learn to perform any motor tasks well. Researchers have divided sleep into a number of stages, largely based upon their respective patterns of brain electrical activity. Sufficient amounts of specific stages of sleep appear to be critical for the consolidation of procedural memory.

Dr. Stickgold and his colleagues at Harvard conducted an experiment in which students played a computer game and then, over the following days, were tested on how well they remem-22, bered the game. Students who had more than six hours of sleep the night after they learned the game remembered it better the next day than did students who had less sleep. Again, specific phases of sleep were critical for effective learning. Even two days to a week later, students who were well rested outperformed those who hadn't slept as well.

Stage 3: Retrieval

Retrieval is the act of recalling something. As I mentioned earlier, each memory resides in a unique pattern of neuronal activation in your brain. To retrieve information, your brain must reactivate the pattern.

Similar memories have partially overlapping patterns of neuronal activation. Occasionally when you try to retrieve one memory, a similar memory comes to mind and blocks the information you want. You might try to remember the name of a song, but instead you remember the name of the singer who recorded the song or the name of a movie that featured it.

It takes less than a second to reactivate a neuronal pathway that holds simple or highly familiar information, like your phone number or the image of your father's face. In studies of face processing in which people are asked to decide whether photographed faces are familiar or unfamiliar, it takes about a fifth of a second for the image to reach the area of the brain that processes visual information and another fifth of a second for the person to decide whether the image is familiar.

If it always took just a fraction of a second to remember something, you wouldn't worry about your memory. But as we all know, it often takes much longer. Even if there's nothing wrong with your memory, it can take several seconds or more to recall complex information. See how long it takes you to determine the square root of 169. Depending on your proficiency with math, you might first need to activate the neuronal pathway that holds the definition of square root and then activate pathways that enable you to calculate the answer, 13. You can survey your recall of information from other domains, such as the number of U.S. presidents with the last name of Johnson, books written by Jane 223

Austen, or films that won the Academy Award for best picture over the last five years.

The more often you retrieve a piece of information, the easier it is for you to find it the next time. Information that you haven't retrieved lately might take a while to recall, or you may not be able to retrieve it without the help of a cue, a bit of information that triggers the recollection of other information. As you age, you accumulate more and more information that remains "unrecalled" for longer and longer periods of time. In the process of retrieving a fact such as a specific word or someone's name, you might struggle for seconds or minutes, feeling that the answer is on the tip of your tongue. If the neuronal pathways in your brain leading to the answer are still intact, chances are that you'll eventually retrieve it.

It's tempting to think of the process of retrieving memories as akin to taking a book off a shelf, but it's not. A book's content remains the same, but your memories don't. Memories change somewhat over time in response to new experiences. That's because your brain itself is also an ongoing work in progress. Each time you have a conversation or learn something or go somewhere, neural pathways in your brain are reconfigured. Some connections are strengthened and others weakened; these changes tweak and embellish and, in some cases, erode the memories that have been stored in your brain.

Researchers used to assume that once a memory was consolidated into the long-term memory system, it had become so durable that it couldn't be lost or altered by subsequent experience the way short-term memories are. However, new research suggests that when you recall consolidated memories, they temporarily become fragile again. In this fragile state, they are vulnerable to being altered or partially lost by interruptions and intrusions from other data that are swirling around your brain, such as sensory input, thoughts, feelings, and other memories.

Some memory scientists now believe that after you retrieve a memory, your brain has to consolidate it again. If this is true, it 24, could be yet another factor that explains age-related memory loss.

As the hippocampus ages, it becomes less adept not only at consolidating new memories but also perhaps at reconsolidating old ones.

Test Your Visual Memory

This exercise assesses your immediate recall of visual information and how much visual detail you retain over a longer period. First, study the design in Figure 2.3 for fifteen seconds. Then cover the page and draw the design from memory. Don't look at the original design again just yet. After doing something else for thirty minutes, draw the design again. Now compare your two drawings to the i figure 2.3 Visual Memory Test i figure 2.3 Visual Memory Test


Test Your Visual Memory, continued original. How accurate was your first drawing? How accurate was the second?

Although neuropsychologists rely on standardized scoring and interpretive criteria to score this type of test, you can get a general assessment of your visual memory by seeing how much of the design you remember over time. Most people forget small details of the figure the second time around but remember the large features. The more detail you can remember, the better your visual memory is.

Memories That Last

Most of my patients don't come to see me because they've forgotten how to make a pot of coffee or ride a bicycle. These skills are forms of procedural memory and, as I mentioned earlier, procedural memory remains relatively intact as you age. Patients tend to come see me when they begin to forget things like important dates, tasks they have to do, and even names of people they know well. It's these kinds of declarative memories that can become elusive when you're middle-aged or older. But some declarative memories are more durable than others. Remember, there are two subtypes of declarative memory: semantic (factual) and episodic (event-related).

Specifically, information that is part of your semantic memory (the strictly factual knowledge that you call up over and over again, such as your spouse's name, what 5 times 4 equals, and the knowledge base that you draw on to do your job) is more resilient than episodic memories (the unique event memories that are linked to a point in time, such as the dinner party you attended last month). Even though both episodic and semantic memory initially depend on the hippocampus, there are important differences. Because we tend to access and use semantic memories more often, they tend to 26 be more durable. Certainly, many semantic memories do fade, like

Recipe for a Lasting Memory


• Begin with a unique experience—something that stands out from mundane everyday life.

• Have the experience engage all of the senses:






• Make it meaningful—something that you connect with and that engages your interest.

• Add a dash of emotional salience—not too much and not too little. (Too much emotion can interfere with your memory, whereas too little emotion can make information so dull that it is unmemorable.)


1. Focus your attention fully and single-mindedly.

2. Actively process the experience as it is occurring; create an association between the new experience and an established memory.

3. Rehearse and replay the experience at different times and in different places. Talk about it with others and think about it on your own immediately afterward and from time to time thereafter.

4. Get a good night's sleep.

Enjoy for a lifetime!

the plots of novels and movies that you've seen, but these tend to be the memories that you don't use very much.

Episodic memories are more fragile because you're not as likely to recall them as often as information that you use every day. An episodic memory is far more specific than semantic infor- l27

mation, being a one-of-a-kind event that occurred in a singular place and time. For example, you can probably remember that a city named Burlington is located in Vermont (a semantic memory). This is general knowledge; you don't need to remember who first told you about Burlington, Vermont, or where you were when you first learned of it. You just know it. But you're more apt to struggle to remember a lunch you had at a McDonald's on June 8, 1995, in Burlington (an episodic memory). Why? Because this type of event overlaps with other similar events in the category of "lunches at McDonald's." Chances are you weren't straining to absorb all of the details around you at that McDonald's. And how often would you have recalled this specific episode over the years?

Certain subtypes of semantic memory appear to be particularly durable. There's some truth to the adage that you never forget a face. Some research shows that older people recall faces nearly as well as younger people do, although other studies suggest a significant decline in the ability of older people to remember other types of visual information, such as images or scenes.

Perhaps one reason that we tend to remember faces more easily than other visual information is that faces are linked to our emotions. You pay more attention to information that resonates with you emotionally than to information that leaves you cold. You also call emotionally linked information into conscious thought more often, and therefore, the neural network that supports it is more elaborate. It's no coincidence that the structures that govern memory and the structures that govern emotion are close together in the limbic system of the brain and share millions of reciprocal connections for facilitating two-way communication.

Your capacity for acquiring, consolidating, and retrieving memories is a wondrously complex and dynamic process. Next, you'll see what sorts of problems can gum up the works to distort some memories and erase part or all of others. The following chapters describe the differences between normal and abnormal memory loss, and tell you what you can do about them.


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