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THE FIRST THINGS TO GET STRAIGHT

How Attention and Memory Work

We live in a world of illusions. We think we’re aware of everything going on around us. We look out and see an uninterrupted, complete picture of the visual world, composed of thousands of little detailed images. We may know that each of us has a blind spot, but we go on day to day blissfully unaware of where it actually is because our occipital cortex does such a good job of filling in the missing information and hence hiding it from us. Laboratory demonstrations of inattentional blindness (like the gorilla video of the last chapter) underscore how little of the world we actually perceive, in spite of the overwhelming feeling that we’re getting it all.

We attend to objects in the environment partly based on our will (we choose to pay attention to some things), partly based on an alert system that monitors our world for danger, and partly based on our brains’ own vagaries. Our brains come preconfigured to create categories and classifications of things automatically and without our conscious intervention. When the systems we’re trying to set up are in collision with the way our brain automatically categorizes things, we end up losing things, missing appointments, or forgetting to do things we needed to do.

Have you ever sat in an airplane or train, just staring out the window with nothing to read, looking at nothing in particular? You might have found that the time passed very pleasantly, with no real memory of what exactly you were looking at, what you were thinking, or for that matter, how much time actually elapsed. You might have had a similar feeling the last time you sat by the ocean or a lake, letting your mind wander, and experiencing the relaxing feeling it induced. In this state, thoughts seem to move seamlessly from one to another, there’s a merging of ideas, visual images, and sounds, of past, present, and future. Thoughts turn inward—loosely connected, stream-of-consciousness thoughts so much like the nighttime dream state that we call them daydreams.

This distinctive and special brain state is marked by the flow of connections among disparate ideas and thoughts, and a relative lack of barriers between senses and concepts. It also can lead to great creativity and solutions to problems that seemed unsolvable. Its discovery—a special brain network that supports a more fluid and nonlinear mode of thinking—was one of the biggest neuroscientific discoveries of the last twenty years. This network exerts a pull on consciousness; it eagerly shifts the brain into mind-wandering when you’re not engaged in a task, and it hijacks your consciousness if the task you’re doing gets boring. It has taken over when you find you’ve been reading several pages in a book without registering their content, or when you are driving on a long stretch of highway and suddenly realize you haven’t been paying attention to where you are and you missed your exit. It’s the same part that took over when you realized that you had your keys in your hand a minute ago but now you don’t know where they are. Where is your brain when this happens?

Envisioning or planning one’s future, projecting oneself into a situation (especially a social situation), feeling empathy, invoking autobiographical memories also involve this daydreaming or mind-wandering network. If you’ve ever stopped what you were doing to picture the consequence of some future action or to imagine yourself in a particular future encounter, maybe your eyes turned up or down in your head from a normal straight-ahead gaze, and you became preoccupied with thought: That’s the daydreaming mode.

The discovery of this mind-wandering mode didn’t receive big headlines in the popular press, but it has changed the way neuroscientists think about attention. Daydreaming and mind-wandering, we now know, are a natural state of the brain. This accounts for why we feel so refreshed after it, and why vacations and naps can be so restorative. The tendency for this system to take over is so powerful that its discoverer, Marcus Raichle, named it the default mode. This mode is a resting brain state, when your brain is not engaged in a purposeful task, when you’re sitting on a sandy beach or relaxing in your easy chair with a single malt Scotch, and your mind wanders fluidly from topic to topic. It’s not just that you can’t hold on to any one thought from the rolling stream, it’s that no single thought is demanding a response.

The mind-wandering mode stands in stark contrast to the state you’re in when you’re intensely focused on a task such as doing your taxes, writing a report, or navigating through an unfamiliar city. This stay-on-task mode is the other dominant mode of attention, and it is responsible for so many high-level things we do that researchers have named it “the central executive.” These two brain states form a kind of yin-yang: When one is active, the other is not. During demanding tasks, the central executive kicks in. The more the mind-wandering network is suppressed, the greater the accuracy of performance on the task at hand.

The discovery of the mind-wandering mode also explains why paying attention to something takes effort. The phrase paying attention is well-worn figurative language, and there is some useful meaning in this cliché. Attention has a cost. It is a this-or-that, zero-sum game. We pay attention to one thing, either through conscious decision or because our attentional filter deemed it important enough to push it to the forefront of attentional focus. When we pay attention to one thing, we are necessarily taking attention away from something else.

My colleague Vinod Menon discovered that the mind-wandering mode is a network, because it is not localized to a specific region of the brain. Rather, it ties together distinct populations of neurons that are distributed in the brain and connected to one another to form the equivalent of an electrical circuit or network. Thinking about how the brain works in terms of networks is a profound development in recent neuroscience.

Beginning about twenty-five years ago, the fields of psychology and neuroscience underwent a revolution. Psychology was primarily using decades-old methods to understand human behavior through things that were objective and observable, such as learning lists of words or the ability to perform tasks while distracted. Neuroscience was primarily studying the communication among cells and the biological structure of the brain. The psychologists had difficulty studying the biological material, that is, the hardware, that gave rise to thought. The neuroscientists, being stuck down at the level of individual neurons, had difficulty studying actual behaviors. The revolution was the invention of noninvasive neuroimaging techniques, a set of tools analogous to an X-ray that showed not just the contours and structure of the brain but how parts of the brain behaved in real time during actual thought and behavior—pictures of the thinking brain at work. The technologies—positron emission tomography, functional magnetic resonance imaging, and magnetoencephalography—are now well known by their abbreviations PET, fMRI, and MEG.

The initial wave of studies focused primarily on localization of brain function, a kind of neural mapping. What part of the brain is active when you mentally practice your tennis serve, when you listen to music, or perform mathematical calculations? More recently, interest has shifted toward developing an understanding of how these regions work together. Neuroscientists have concluded that mental operations may not always be occurring in one specific brain region but, rather, are carried out by circuits, networks of related neuron groups. If someone asked, “Where is the electricity kept that makes it possible to operate your refrigerator?” where would you point? The outlet? It actually doesn’t have current passing through it unless an appliance is plugged in. And once one is, it is no more the place of electricity than circuits throughout all the household appliances and, in a sense, throughout the house. Really, there is no single place where electricity is. It is a distributed network; it won’t show up in a cell phone photo.

Similarly, cognitive neuroscientists are increasingly appreciating that mental function is often spread out. Language ability does not reside in a specific region of the brain; rather, it comprises a distributed network—like the electrical wires in your house—that draws on and engages regions throughout the brain. What led early researchers to think that language might be localized is that disruption to particular regions of the brain reliably caused loss of language functions. Think of the circuits in your home again. If your contractor accidentally cuts an electrical wire, you can lose electricity in an entire section of your home, but it doesn’t mean that the electricity source was at the place that was cut—it simply means that a line necessary for transmission was disrupted. In fact, there is almost an infinity of places where cutting the wires in your house will cause a disruption to service, including cutting the wire at the source, the circuit breaker box. From where you stand in your kitchen with a blender that won’t mix your smoothie, the effect is the same. It begins to look different only when you set out to repair it. This is how neuroscientists now think of the brain—as a set of intricate overlapping networks.

The mind-wandering mode works in opposition to the central executive mode: When one is activated, the other one is deactivated; if we’re in one mode, we’re not in the other. The job of the central executive network is to prevent you from being distracted when you’re engaged in a task, limiting what will enter your consciousness so that you can focus on what you’re doing uninterrupted. And again, whether you are in the mind-wandering or central executive mode, your attentional filter is almost always operating, quietly out of the way in your subconscious.

For our ancestors, staying on task typically meant hunting a large mammal, fleeing a predator, or fighting. A lapse of attention during these activities could spell disaster. Today, we’re more likely to employ our central executive mode for writing reports, interacting with people and computers, driving, navigating, solving problems in our heads, or pursuing artistic projects such as painting and music. A lapse of attention in these activities isn’t usually a matter of life or death, but it does interfere with our effectiveness when we’re trying to accomplish something.

In the mind-wandering mode, our thoughts are mostly directed inward to our goals, desires, feelings, plans, and also our relationship with other people—the mind-wandering mode is active when people are feeling empathy toward one another. In the central executive mode, thoughts are directed both inward and outward. There is a clear evolutionary advantage to being able to stay on task and concentrate, but not to entering an irreversible state of hyperfocus that makes us oblivious to a predator or enemy lurking behind the bushes, or to a poisonous spider crawling up the back of our neck. This is where the attentional network comes in; the attentional filter is constantly monitoring the environment for anything that might be important.

In addition to the mind-wandering mode, the central executive, and the attentional filter, there’s a fourth component of the attentional system that allows us to switch between the mind-wandering mode and the central executive mode. This switch enables shifts from one task to another, such as when you’re talking to a friend at a party and your attention is suddenly shifted to that other conversation about the fire in the kitchen. It’s a neural switchboard that directs your attention to that mosquito on your forehead and then allows you to go back to your post-lunchtime mind-wandering. In a 2010 paper, Vinod Menon and I showed that the switch is controlled in a part of the brain called the insula, an important structure about an inch or so beneath the surface of where temporal lobes and frontal lobes join. Switching between two external objects involves the temporal-parietal junction.

The insula has bidirectional connections to an important brain part called the anterior cingulate cortex. Put your finger on the top of your head, just above where you think the back of your nose is. About two inches farther back and two inches below that is the anterior cingulate. Below is a diagram showing where it is, relative to other brain structures.

The relationship between the central executive system and the mind-wandering system is like a see-saw, and the insula—the attentional switch—is like an adult holding one side down so that the other stays up in the air. This efficacy of the insula-cingulate network varies from person to person, in some functioning like a well-oiled switch, and in others like a rusty old gate. But switch it does, and if it is called upon to switch too much or too often, we feel tired and a bit dizzy, as though we were see-sawing too rapidly.

Notice that the anterior cingulate extends from the orbital and prefrontal cortex in front (left on the drawing) to the supplementary motor area at the top. Its proximity to these areas is interesting because the orbital and prefrontal areas are responsible for things like planning, scheduling, and impulse control, and the supplementary motor area is responsible for initiating movement. In other words, the parts of your brain that remind you about a report you have due, or that move your fingers across the keyboard to type, are biologically linked to the parts of your brain that keep you on task, that help you to stay put in your chair and finish that report.

This four-circuit human attentional system evolved over tens of thousands of years—distinct brain networks that become more or less active depending on the situation—and it now lies at the center of our ability to organize information. We see it every day. You’re sitting at your desk and there is a cacophony of sounds and visual distractions surrounding you: the fan of the ventilation unit, the hum of the fluorescent lights, traffic outside your window, the occasional glint of sunlight reflecting off a windshield outside and streaking across your face. Once you’ve settled into your work, you cease to notice these and can focus on your task. After about fifteen or twenty minutes, though, you find your mind wandering: Did I remember to lock the front door when I left home? Do I need to remind Jeff of our lunch meeting today? Is this project I’m working on right now going to get done on time? Most people have internal dialogues like this going on in their heads all the time. It might cause you to wonder who is asking the questions inside your head and—more intriguingly—who is answering them? There isn’t a bunch of miniature you’s inside your head, of course. Your brain, however, is a collection of semidistinct, special-purpose processing units. The inner dialogue is generated by the planning centers of your brain in the prefrontal cortex, and the questions are being answered by other parts of your brain that possess the information.

Distinct networks in your brain can thus harbor completely different thoughts and hold completely different agendas. One part of your brain is concerned with satisfying immediate hunger, another with planning and sticking to a diet; one part is paying attention to the road while you drive, another is bebopping along with the radio. The attentional network has to monitor all these activities and allocate resources to some and not to others.

If this seems far-fetched, it may be easier to visualize if you realize that the brain is already doing this all the time for cellular housekeeping purposes. For example, when you start to run, a part of your brain “asks” the question, “Do we have enough oxygen going to the leg muscles to support this activity?” while in tandem, another part sends down an order to increase respiration levels so that blood oxygenation is increased. A third part that is monitoring activity makes sure that the respiration increase was carried out per instructions and reports back if it wasn’t. Most of the time, these exchanges occur below the level of consciousness, which is to say, we’re not aware of the dialogue or signal-response mechanism. But neuroscientists are increasingly appreciating that consciousness is not an all-or-nothing state; rather, it is a continuum of different states. We say colloquially that this or that is happening in the subconscious mind as though it were a geographically separate part of the brain, somewhere down deep in a dank, dimly lit basement of the cranium. The more accurate neural description is that many networks of neurons are firing, much like the network of telephones simultaneously ringing in a busy office. When the activation of a neural network is sufficiently high, relative to other neural activity that’s going on, it breaks into our attentional process, that is, it becomes captured by our conscious mind, our central executive, and we become aware of it.

Many of us hold a folk view of consciousness that is not true but is compelling because of how it feels—we feel as though there is a little version of ourselves inside our heads, telling us what is going on in the world and reminding us to take out the trash on Mondays. A more elaborated version of the myth goes something like this: There’s a miniature version of us inside our heads, sitting in a comfortable chair, looking at multiple television screens. Projected on the screens are the contents of our consciousness—the external world that we see and hear, its tactile sensations, smells, and tastes—and the screens also report our internal mental and bodily states: I’m feeling hungry now, I’m too hot, I’m tired. We feel that there is an internal narrator of our lives up here in our heads, showing us what’s going on in the outside world, telling us what it all means, and integrating this information with reports from inside our body about our internal emotional and physical states.

One problem with the account is that it leads to an infinite regress. Is there a miniature you sitting in a theater in your head? Does that miniature you have little eyes and ears for watching and listening to the TV screens? And a little brain of its own? If so, is there an even smaller miniature person inside its brain? And another miniature person inside the brain of that miniature person? The cycle never ends. (Daniel Dennett showed this explanation to be both logically and neurally implausible in Consciousness Explained.) The reality is more marvelous in its way.

Numerous special-purpose modules in your brain are at work, trying to sort out and make sense of experience. Most of them are running in the background. When that neural activity reaches a certain threshold, you become aware of it, and we call that consciousness. Consciousness itself is not a thing, and it is not localizable in the brain. Rather, it’s simply the name we put to ideas and perceptions that enter the awareness of our central executive, a system of very limited capacity that can generally attend to a maximum of four or five things at a time.

To recap, there are four components in the human attentional system: the mind-wandering mode, the central executive mode, the attentional filter, and the attentional switch, which directs neural and metabolic resources among the mind-wandering, stay-on-task, or vigilance modes. The system is so effective that we rarely know what we’re filtering out. In many cases, the attentional switch operates in the background of our awareness, carrying us between the mind-wandering mode and the central executive mode, while the attentional filter purrs along—we don’t realize what is in operation until we’re already in another mode. There are exceptions of course. We can will ourselves to switch modes, as when we look up from something we’re reading to contemplate what is said. But the switching remains subtle: You don’t say, “I’m switching modes now”; you (or your insula) just do it.

The Neurochemistry of Attention

The last twenty years in neuroscience have also revealed an enormous amount about how paying attention actually happens. The mind-wandering network recruits neurons within the prefrontal cortex (just behind your forehead and eyes) in addition to the cingulate (a couple of inches farther back), joining them to the hippocampus, the center of memory consolidation. It does this through the activity of noradrenaline neurons in the locus coeruleus, a tiny little hub near the brainstem, deep inside the skull, which has evolved a dense mass of fibers connected to the prefrontal cortex. Despite the similarity of names, noradrenaline and adrenaline are not the same chemical; noradrenaline is most chemically similar to dopamine, from which it is synthesized by the brain. To stay in the mind-wandering mode, a precise balance must be maintained between the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter GABA (gamma-Aminobutyric acid). We know dopamine and serotonin are components of this brain network, but their interactions are complex and not yet fully understood. There is tantalizing new evidence that a particular genetic variation (of a gene called COMT) causes the dopamine and serotonin balance to shift, and this shift is associated both with mood disorders and with responsiveness to antidepressants. The serotonin transporter gene SLC6A4 has been found to correlate with artistic behaviors as well as spirituality, both of which appear to favor the mind-wandering mode. Thus a connection among genetics, neurotransmitters, and artistic/spiritual thinking appears to exist. (Dopamine is no more important than glutamate and GABA and any number of other chemicals. We simply know more about dopamine because it’s easier to study. In twenty years, we’ll have a far more nuanced understanding of it and other chemicals.)

The central executive network recruits neurons in different parts of the prefrontal cortex and the cingulate, plus the basal ganglia deep inside the center of the brain—this executive network is not exclusively located in the prefrontal cortex as popular accounts have tended to suggest. Its chemical action includes modulating levels of dopamine in the frontal lobes. Sustained attention also depends on noradrenaline and acetylcholine, especially in distracting environments—this is the chemistry underlying the concentration it takes to focus. And while you’re focusing attention on the task at hand, acetylcholine in the right prefrontal cortex helps to improve the quality of the work done by the attentional filter. Acetylcholine density in the brain changes rapidly—at the subsecond level—and its release is tied to the detection of something you’re searching for. Acetylcholine also plays a role in sleep: It reaches a peak during REM sleep, and helps to prevent outside inputs from disturbing your dreaming.

In the last few years, we’ve learned that acetylcholine and noradrenaline appear to be integrated into the brain’s circuitry via heteroreceptors—chemical receptors inside a neuron that can accept more than one type of trigger (as distinguished from the more typical autoreceptors that function like a lock and key, letting only one specific neurotransmitter into the synapse). Through this mechanism, acetylcholine and noradrenaline can influence the release of each other.

The attentional filter comprises a network in the frontal lobes and sensory cortices (auditory and visual cortex). When we’re searching for something, the filter can retune neurons to match the characteristics of the thing we’re searching for, such as the red and white stripes of Waldo, or the size and shape of your car keys. This allows search to be very rapid and to filter out things that are irrelevant. But due to neural noise, it doesn’t always work perfectly—we sometimes look right at the thing we’re searching for without realizing what it is. The attentional filter (or Where’s Waldo? network) is controlled in part by neurons with nicotinic receptors located in a part of the brain called the substantia innominata. Nicotinic receptors are so named because they respond to nicotine, whether smoked or chewed, and they’re spread throughout the brain. For all the problems it causes to our overall health, it’s well established that nicotine can improve the rate of signal detection when a person has been misdirected—that is, nicotine creates a state of vigilance that allows one to become more detail oriented and less dependent on top-down expectations. The attentional filter also communicates closely with the insula, so that it can activate the switch there in order to pull us out of the mind-wandering mode and into the stay-on-task mode when necessary. In addition, it’s strongly coupled to the cingulate, facilitating rapid access to the motor system to make an appropriate behavioral response—like jumping out of the way—when a dangerous object comes at you.

Recall from earlier that the attentional filter incorporates a warning system so that important, life-altering signals can break through your mind-wandering or your focused-task mode. If you’re driving along and your thoughts start to wander, this is the system that snaps to when a large truck suddenly crosses over into your lane, and gives you a shot of adrenaline at the same time. The warning system is governed by noradrenaline in the frontal and parietal lobes. Drugs, such as guanfacine (brand names Tenex and Intuniv) and clonidine, that are prescribed for hypertension, ADHD, and anxiety disorders can block noradrenaline release, and in turn block your alerting to warning signals. If you’re a sonar operator in a submarine, or a forest ranger on fire watch, you want your alerting system to be functioning at full capacity. But if you are suffering from a disorder that causes you to hear noises that aren’t there, you want to attenuate the warning system, and guanfacine can do this.

The attentional switch that Vinod Menon and I located in the insula helps to turn the spotlight of attention from one thing to another, and is governed by noradrenaline and cortisol (the stress hormone). Higher levels of dopamine here and in surrounding tissue appear to enhance the functioning of the mind-wandering network. The locus coeruleus and noradrenaline system also modulate these behavioral states. The noradrenaline system is evolutionarily very old and is found even in crustaceans, where some researchers believe it serves a similar role.

Where Memory Comes From

The way neuroscientists talk about these attentional systems, you might think they are modes that affect the whole brain in an all-or-none fashion: You’re either in the central executive mode or see-sawing into the mind-wandering mode. You’re either awake or asleep. After all, we know when we’re awake, don’t we? And when we’re asleep, we are completely off-line, and realize we’ve been asleep only after we wake up. This is not the way it works.

In stark contrast to this misperception, neuroscientists have recently discovered that parts of the brain can fall asleep for a few moments or longer without our realizing it. At any given moment, some circuits in the brain may be off-line, slumbering, recouping energy, and as long as we’re not calling on them to do something for us, we don’t notice. This applies just as well to the four parts of the attentional system—any or all of them can be partially functioning. This is likely responsible for a great proportion of things we misplace or lose: The part of our brain that should be attending to where we put them is either asleep or distracted by something else. It’s what happens when we miss something we’re searching for or look right at it and don’t recognize it; it happens when we’re daydreaming and it takes us a beat to shift back to alertness.

Thus many things get lost when we are not attending to the moment of putting them down. The remedy is to practice mindfulness and attentiveness, to train ourselves to a Zen-like focus of living in the moment, of paying attention whenever we put things down or put things away. That little bit of focus goes a long way in training the brain (specifically the hippocampus) to remember where we put things, because we’re invoking the central executive to help with encoding the moment. Having systems like key hooks, cell phone trays, and a special hook or drawer for sunglasses externalizes the effort so that we don’t have to keep everything in our heads. Externalizing memory is an idea that goes back to the Greeks, and its effectiveness has been confirmed many times over by contemporary neuroscience. The extent to which we do it already is astounding when you think about it. As Harvard psychologist Dan Wegner noted, “Our walls are filled with books, our file cabinets with papers, our notebooks with jottings, our homes with artifacts and souvenirs.” The word souvenir, not coincidentally, comes from the French word for “to remember.” Our computers are filled with data records, our calendars with appointments and birthdays, and students scribble answers to tests on their hands.

One current view among some memory theorists is that a very large number of the things you’ve consciously experienced in your life is encoded in your brain—many of the things you’ve seen, heard, smelled, thought, all those conversations, bicycle rides, and meals are potentially in there somewhere, provided you paid attention to them. If it’s all in there, why do we forget? As Patrick Jane of The Mentalist described it, rather eloquently, “Memory is unreliable because the untrained brain has a crappy filing system. It takes everything that happens to you and throws it all willy-nilly into a big dark closet—when you go in there looking for something, all you can find are the big obvious things, like when your mom died, or stuff that you don’t really need. Stuff that you’re not looking for, like the words to ‘Copacabana.’ You can’t find what you need, but don’t panic, because it’s still there.”

How is this possible? When we experience any event, a unique network of neurons is activated depending on the nature of the event. Watching a sunset? Visual centers that represent shadows and light, pink, orange, and yellow are activated. That same sunset a half hour earlier or later looks different, and so invokes correspondingly different neurons for representing it. Watching a tennis game? Neurons fire for face recognition for the players, motion detection for the movements of their bodies, the ball, the rackets, while higher cognitive centers keep track of whether they stayed in bounds and what the score is. Each of our thoughts, perceptions, and experiences has a unique neural correlate—if it didn’t, we would perceive the events as identical; it is the difference in neuronal activations that allows us to distinguish events from one another.

The act of remembering something is a process of bringing back on line those neurons that were involved in the original experience. The neurons represent the world to us as the thing is happening, and as we recall it, those same neurons re-present the thing to us. Once we get those neurons to become active in a fashion similar to how they were during the original event, we experience the memory as a lower-resolution replay of the original event. If only we could get every one of those original neurons active in exactly the same way they were the first time, our recollections would be strikingly vivid and realistic. But the remembering is imperfect; the instructions for which neurons need to be gathered and how exactly they need to fire are weak and degraded, leading to a representation that is only a dim and often inaccurate copy of the real experience. Memory is fiction. It may present itself to us as fact, but it is highly susceptible to distortion. Memory is not just a replaying, but a rewriting.

Adding to the difficulty is the fact that many of our experiences share similarities with one another, and so when trying to re-create them in memory, the brain can get fooled by competing items. Thus, our memory tends to be poor most of the time, not because of the limited capacity of our brains to store the information but because of the nature of memory retrieval, which can easily become distracted or confounded by other, similar items. An additional problem is that memories can become altered. When they are retrieved they are in a labile or vulnerable state and they need to be reconsolidated properly. If you’re sharing a memory with a friend and she says, “No, the car was green, not blue,” that information gets grafted onto the memory. Memories in this labile state can also vanish if something interferes with their reconsolidation, like lack of sleep, distraction, trauma, or neurochemical changes in the brain.

Perhaps the biggest problem with human memory is that we don’t always know when we’re recalling things inaccurately. Many times, we have a strong feeling of certainty that accompanies an incorrect, distorted memory. This faulty confidence is widespread, and difficult to extinguish. The relevance to organizational systems is that the more we can externalize memory through physical records out-there-in-the-world, the less we must rely on our overconfident, underprecise memory.

Is there any rhyme or reason about which experiences we’ll be able to remember accurately versus those that we won’t? The two most important rules are that the best-remembered experiences are distinctive/unique or have a strong emotional component.

Events or experiences that are out of the ordinary tend to be remembered better because there is nothing competing with them when your brain tries to access them from its storehouse of remembered events. In other words, the reason it can be difficult to remember what you ate for breakfast two Thursdays ago is that there was probably nothing special about that Thursday or that particular breakfast—consequently, all your breakfast memories merge together into a sort of generic impression of a breakfast. Your memory merges similar events not only because it’s more efficient to do so, but also because this is fundamental to how we learn things—our brains extract abstract rules that tie experiences together. This is especially true for things that are routine. If your breakfast is always the same—cereal with milk, a glass of orange juice, and a cup of coffee for instance—there is no easy way for your brain to extract the details from one particular breakfast. Ironically, then, for behaviors that are routinized, you can remember the generic content of the behavior (such as the things you ate, since you always eat the same thing), but particulars to that one instance can be very difficult to call up (such as the sound of a garbage truck going by or a bird that passed by your window) unless they were especially distinctive or emotional. On the other hand, if you did something unique that broke your routine—perhaps you had leftover pizza for breakfast and spilled tomato sauce on your dress shirt—you are more likely to remember it.

A key principle, then, is that memory retrieval requires our brains to sift through multiple, competing instances to pick out just the ones we are trying to recollect. If there are similar events, it retrieves many or all of them, and usually creates some sort of composite, generic mixture of them without our consciously knowing it. This is why it is difficult to remember where we left our glasses or car keys—we’ve set them down in so many different places over so many years that all those memories run together and our brains have a difficult time finding the relevant one.

On the other hand, if there are no similar events, the unique one is easily distinguished from others and we are able to recollect it. This is in direct proportion to how distinctive the event was. Having pizza for breakfast may be relatively unusual; going out for breakfast with your boss may be more unusual. Having breakfast served to you in bed on your twenty-first birthday by a new, naked, romantic partner is even more unusual. Other unusual events that are typically easy for people to remember include life cycle events such as the birth of a sibling, a marriage, or the death of a loved one. As an amateur bird-watcher, I remember exactly where I was when I saw a pileated woodpecker for the first time, and I remember details about what I was doing a few minutes before and after seeing him. Similarly, many of us remember the first time we saw identical twins, the first time we rode a horse, or the first time we were in a thunderstorm.

Evolutionarily, it makes sense for us to remember unique or distinctive events because they represent a potential change in the world around us or a change in our understanding of it—we need to register these in order to maximize our chances for success in a changing environment.

The second principle of memory concerns emotions. If something made us incredibly frightened, elated, sad, or angry—four of the primary human emotions—we’re more likely to remember it. This is because the brain creates neurochemical tags, or markers, that accompany the experience and cause it to become labeled as important. It’s as though the brain took a yellow fluorescent highlighter to the text of our day, and selectively marked up the important parts of the day’s experiences. This makes evolutionary sense—the emotionally important events are probably the ones that we need to remember in order to survive, things like the growl of a predator, the location of a new freshwater spring, the smell of rancid food, the friend who broke a promise.

These chemical tags, tied to emotional events, are the reason we so readily remember important national events such as the assassination of President Kennedy, the space shuttle Challenger explosion, the attacks of 9/11, or the election and inauguration of President Obama. These were emotional events for most of us, and they became instantly tagged with brain chemicals that put them in a special neural status facilitating access and retrieval. And these neurochemical tags work for personal memories as well as national ones. You might not be able to remember when you last did your laundry, but you probably remember the person with whom you had your first kiss and exactly where it took place. And even if you are sketchy on some of the details, it is likely you’ll remember the emotion associated with the memory.

Unfortunately, the existence of such emotional tags, while making memory retrieval quicker and easier, does not guarantee that the memory retrieval will be more accurate. Here is an example. If you are like most Americans, you remember right where you were when you first learned that the World Trade Center Twin Towers in New York City had been attacked on September 11, 2001. You probably remember the room you were in, roughly the time of day (morning, afternoon, evening), and perhaps even who you were with or who you spoke to that day. You probably also remember watching the horrifying television images of an airplane crashing into the first tower (the North Tower), and then, about twenty minutes later, the image of a second plane crashing into the second tower (the South Tower). Indeed, according to a recent survey, 80% of Americans share this memory. But it turns out this memory is completely false. The television networks broadcasted real-time video of the South Tower collision on September 11, but video of the North Tower collision wasn’t available and didn’t appear on broadcast television until the following day, on September 12. Millions of Americans saw the videos out of sequence, seeing the video of the South Tower impact twenty-four hours earlier than the video of the North Tower impact. But the narrative we were told and knew to be true, that the North Tower was hit about twenty minutes before the South Tower, causes our memory to stitch together the sequence of events as they happened, not as we experienced them. This caused a false memory so compelling that even President George W. Bush falsely recalled seeing the North Tower get hit on September 11, although the television archives show this to be impossible.

As a demonstration of the fallibility of memory, try this exercise. First, get a pen or pencil and a piece of paper. Below, you’ll see a list of words. Read each one out loud at a rate of one word per second. That is, don’t read as quickly as you can, but take your time and focus on each one as you read it.

REST

TIRED

AWAKE

DREAM

SNORE

BED

EAT

SLUMBER

SOUND

COMFORT

PILLOW

WAKE

NIGHT