[Editor's note: This article is from 2007. Please also see our listings on BrainWeb for learning and memory.]
Sections include: the many modes of memory, the stages of memory and learning, the anatomy of memory, memory, aging and disease
When we speak of a person with “brainpower,” we often refer to the ability to take in new information and to recall it quickly. The value of such memory skills goes well beyond television quiz shows. Our ability to learn and retrieve knowledge is crucial to recognizing friends, following directions, and even turning a doorknob smoothly. Most of how we interact with the world has been shaped by our experiences, whether or not we remember them. Compared with other animals, our instincts tend to be a less important influence on our behavior. Being able to acquire new knowledge has allowed humans to remain biologically the same for several hundred thousand years yet build the civilization we know today. That is the power of memory.
The Many Modes of Memory
Memory is not in fact a single function but a collection of mental abilities that depend on different systems within the brain. Careful studies can isolate memory from perception and other intellectual abilities and examine each kind of memory separately. The most important distinction between forms of memory divides our conscious recall of facts and events (declarative memory) from various skills, habits, and reactions we remember without conscious effort (nondeclarative memory).
Declarative memory is our storehouse of detailed information about the world and our experience of it: a story, an image, a mathematical relationship, and so on. Memory researchers group the information in declarative memory into two categories:
■ Episodic memory, for specific experiences in our lives (your first kiss, last night’s dinner, where you found this book)
■ Semantic memory, for facts (when the Declaration of Independence was signed, where your supermarket stocks cooking oil, 12 × 12 = 144).
Episodic memory always includes information about where, when, and how you experienced an event (“The first time I met Aunt Millie’s dog”). In contrast, you may not recall when and how you learned about a fact in your semantic memory, but nevertheless you feel sure of the fact (“That’s Aunt Millie’s dog”).
Declarative memory is fast, specialized in learning things quickly. It makes connections among different stimuli, helping us model the world around us: what things are, how they work, what events we have personally observed or participated in. The capacity for declarative memory takes a while to develop, which is why it is rare to truly remember experiences from before the age of 3 or so.
Amnesia is an impairment of declarative memory, affecting the ability to retrieve past memories, the ability to form new memories, or both. A memory loss affecting someone’s knowledge of what he or she once knew is called an agnosia.
When we use nondeclarative memory we are remembering reflexively, but this is not the same as instinct. We had to learn the information stored in nondeclarative memory at some point, although, like instinct, nondeclarative memory is responsible for a great range of automatic human behavior. Nondeclarative memory handles three types of memory activity that we need in order to function from moment to moment:
■ Procedural memory, or knowing how to do something, is the most important task of nondeclarative memory. It is the basis of our mental and physical skills. We usually acquire such skills gradually over time. Though we may experience sudden breakthroughs, repeated practice is key.
■ People with amnesia usually retain most of their skills: they may not recall where they bought their shoes, but they remember how to tie those shoes. Amnesic patients can also usually learn new skills. Those facts underscore how declarative and procedural memories rely on different systems within the brain. A person who has lost the knowledge of how to perform particular acts is said to have an “apraxia”.
■ Conditioning is the process of acquiring the kind of information that the brain sends to the body for an automatic response. The information and response are generally the same every time this form of memory is triggered. That is, a particular stimulus—a bell, in Ivan Pavlov’s classic experiments with dogs, or the sight of a clock’s hands approaching lunchtime—cues a physical reaction. A conditioned response can occur through our involuntary autonomic nervous system (faster heartbeat, salivation, and so on) or our voluntary muscles (getting up from our desks).
■ Priming is the nondeclarative memory function that improves the brain’s ability to detect, identify, or respond to a stimulus that it has processed recently. For instance, it takes about .8 seconds for the average person to name each object in a series of drawings, but only .7 seconds for a drawing he or she has seen shortly before.
■ People with difficulty forming new declarative memories still display the phenomenon of priming. They can, for example, read material aloud more quickly if they have seen it before, even though they do not remember having seen it.
An example of emotional memory is a phobia. This kind of memory depends on the amygdala. The amygdala also modulates declarative and nondeclarative memory. Thus we remember arousing (declarative) events better than boring ones.
Experiences that are novel or emotionally affecting are especially memorable, and emotions can color all forms of memory. Emotional meaning gives more weight to particular details or episodes. Conversely, strong emotion can cause difficulty in recalling certain episodes or facts. Sometimes an emotional memory can produce a feeling, such as fear, even when a person cannot dredge up from his or her declarative memory any reason to be frightened; in extreme cases, this can become a phobia.
In everyday life, we draw on all these memory systems in tandem to learn. A basketball player preparing for a game, for example, mentally replays previous games (episodic memory), reviews facts about the upcoming opponent (semantic memory), and practices the coach’s new plays with other team members (procedural memory, conditioning, and priming). Especially if the game is a big one, the player’s practice may be associated with thoughts of excitement or apprehension, due to previous experiences stored in memory.
The Stages of Memory and Learning
Not only do we constantly take in and store new information, but to help do so, we retrieve, use, and re-store existing memory information. For example, you overhear a coworker introducing a new employee, a tall, thin woman with red hair, as “Sheila.” Your declarative memory says she resembles Sheila who carpooled with you to school in fifth and sixth grade. You go over to shake hands, asking if she is the same person, and then store the new information that she grew up in another city. In a large firm with many employees, you may remember her better because of this momentary link to your past. In an example like this, you called on both declarative (childhood experience) and nondeclarative (the handshake) memory functions, here with an emotional (nostalgia) overlay. (Conditioning being what it is, you might also find yourself wondering for a few mornings how she is getting to work.) These memories, both new and old, seem to go through short-term and long-term stages, though they rely on different systems.
When information enters our eyes, ears, or other sensory channels, the nervous system creates a very brief but thorough record of all those stimuli. This sensory memory can hold a great deal of information, but only for a short period. Our visual systems seem to retain images for about a tenth of a second, while our hearing retains sounds for one or two seconds.
One study that demonstrated the power of sensory memory involved showing volunteers 12 letters, arranged as three rows of four. A researcher flashed these rows on a screen and asked the volunteers to report the letters they had seen; typically, people recalled about 40 percent. Then the researcher told the volunteers they would hear a high, medium, or low tone to indicate which single row they should try to remember. The tone sounded just as the letters disappeared, so the volunteers could not focus their attention on the single row while the letters were still visible. Despite that timing, most people named at least three of the four letters—more than 75 percent—in the designated row. The volunteers’ sensory memories must therefore have held the information about all the rows, enabling the volunteers to retrieve the designated row when they heard the tone.
Sensory memory gives the brain’s cognitive areas time to choose which parts of the stream of incoming information are worthy of further processing. This choice relies on what the brain identifies as important. For example, the areas responsible for attention may be looking for the information, those responsible for pain may register it. In the case of the new employee, Sheila, her resemblance to a childhood acquaintance prompted the brain to get more information.
The next stage of the learning process is shortterm memory, which we can also think of as working memory. Our brains temporarily store the information we need at the moment, whether we are reading a sentence, solving a problem, or planning an action. Short-term memory handles both new information, such as a telephone number someone has just given you to call, and old information, retrieved to compare with something new (“Is this the same person I went to school with?”). Short-term memory is powerful but not infinite, and limits to its capacity can cause us to feel overwhelmed with information.
There are, in addition, different forms of shortterm memory, determined by the data being processed and thus the parts of the brain involved. A sort of sketch pad called the visuospatial loop stores images and patterns, speech-based information travels the phonological loop, and so on. Indeed, some research suggests that each of the brain areas that process specialized information might have its own working memory system.
Learning information for longer than a few minutes means encoding it in a stable way somewhere in the brain. We humans have a vast capacity for such long-term memory, but retrieving information in a useful and timely manner depends on how it is stored.
During the storage phase, the mind performs two essential actions that make learning more efficient. First, it does not attend to some of the data the brain has processed. To learn valuable information, we usually have to grasp the gist of an experience, or its most important aspects, not every detail. To recognize patterns in various objects or experiences, we need to set aside their differences. If we didn’t exclude extraneous information, our minds might easily become crowded, like an e-mail in-box.
The second important phase of storing new information is consolidation: organizing the new material alongside what the mind has already remembered. To store all the data usefully, our brains organize material into networks based on conceptual categories. Your memory of how to use an eggbeater is probably linked to your knowledge of other kitchen utensils, memories of particular recipes, recollections of learning to cook, and so on. That style of organization explains why people who suffer injuries to small parts of the brain can lose very specific knowledge: of animals, for instance, which are often learned about by knowing how they look, but not of tools, which are often learned about by knowing how they are used.
Consolidating a memory also involves stabilizing the information, which can take several years. During that time, new data can cause a memory to change, strengthen, or grow weak and confused. Once a memory has been consolidated, it becomes robust and difficult to revise. That does not mean, however, that a consolidated memory is complete, or completely accurate. Memory does not work like a video camera, capturing every detail of an experience. Rather, it seems to retain snatches of our experiences, enough to allow the mind to reconstruct events when needed. When our minds try to “make sense” of our memories, they sometimes fill in or alter details. Retrieving a memory is seldom exact, therefore, and the act of retrieval itself can even change what we remember.
The Anatomy of Memory
Learning a fact, skill, or habit depends on making structural changes in the brain. That does not mean each new memory creates new neurons. Rather, the process of forming a memory changes the way existing neurons connect and communicate with each other.
Short-term memory involves temporary changes in neurons’ electrical activity and the chemicals that they exchange through their synapses. Depending on what a person has experienced, specific connections become stronger or weaker. Animal studies indicate that the process of forgetting can involve weakening of the synapses between cells. To store information in long-term memory, the brain must make more permanent changes, which take two forms:
■ Nerve cells can extend their axons, thus allowing more connections to other cells.
■ Cells can increase their ability to release chemical neurotransmitters through their synapses, thus increasing the power of each connection.
What determines the “content” of a memory is the location in the brain where synaptic changes occur.
The brain stores information about various aspects of the world in the same parts of the cortex that process that information when it first arrives. Thus, one part of the brain deals with smells, another musical sounds, another faces, and so on. If we think of memories as perceptions that the brain has chosen to store, it makes sense for them to be encoded in the areas that deal with similar new experiences. Usually the memory of an event, or even the memory of an object, is broken into component parts and stored in several areas of the brain at once. Thus, when the brain processes new stimuli in any of these areas, the record of what a person has already learned is nearby.
The crucial step of transforming a perception into a long-term memory occurs not in the cortex, however, but in other parts of the brain. Declarative memory depends on the medial temporal lobe (parahippocampal cortex, entorhinal cortex, perirhinal cortex, and the hippocampus) and the diencephalon. The various regions of the frontal lobes contribute the crucial information about an event, including information to link the experience with a time and place. Information from all these structures is distributed to the appropriate networks in the cortex for storage.
Different long-term storage systems are involved in learning skills, priming, and conditioning. To learn habits, for example, we depend on the caudate nucleus and putamen. Emotional memories—recollections of fear, sadness, pleasure, and other states—involve the amygdala.
Learning thus causes significant, permanent changes in the brain, even in areas once believed to be immutable. For instance, scientists have long known that we all have representations of our bodies in the sensory cortex. Research has shown, however, that learning can alter how much of that band of brain tissue is devoted to the various parts of the body. If people learn to use the ring finger on the left hand unusually often, as violinists do, the corresponding area in the cortex expands. That effect is especially notable when people start such learning early in life.
Memory, Aging, and Disease
Our memory functions decline naturally as we age, usually starting in our mid-30s. Common memory problems in normal aging include trouble recalling where or when we learned something or in what order events occurred and difficulty remembering the need to do particular tasks at scheduled times. Sometimes we may not even perceive a problem with memory as such. Older people often report difficulty following conversations in noisy rooms and assume the problem is hearing loss, but studies indicate that usually the real challenge is mentally sorting the most significant information from all the other sounds flowing into their ears.
These ordinary memory slippages seem to be related to anatomical changes. As we age, we tend to lose neurons in the dentate gyrus and subiculum, which are connected to the hippocampus. Fortunately, most people can compensate for these problems in daily life. Furthermore, individuals retain a wide range of abilities. On some standard memory tests, 20 percent of septuagenarians outscore the average 30-year-old.
It is important to distinguish normal agerelated memory loss from that caused by injury or illness. For example, the neurons in a particular portion of the hippocampus called CA1 remain intact as we age, but this region is especially vulnerable to damage from loss of blood (ischemia) during surgery or stroke. Damage to the CA1 region disrupts the flow of information into and out of the hippocampus, thus causing amnesia and other memory problems. In addition, an early hallmark of Alzheimer’s disease is the death of nerve cells in CA1 and the nearby entorhinal cortex, which impairs the brain’s input and output for long-term memories. And losing part of our memories or our ability to remember means, in some profound ways, losing part of ourselves.
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