What is Memory?
Memory is often used as a catchall phrase referring to a person’s general thinking (cognitive) abilities. Actually, memory is just one — albeit very important — aspect of cognition. It refers specifically to all that you remember as well as your capacity for remembering. Not all memories are created equal. Some memories are meant to be retained for a short period and then discarded. For example, you remember the telephone number of the local pizza place only long enough to make the call. But memories that are more important are stored in the brain and can be retrieved at will: the names of close friends and relatives, the multiplication tables, your phone number, and other information you use regularly. Certain kinds of information can be memorized only if you concentrate, whereas other kinds of memories, such as the faces of people you see regularly and the steps of simple everyday routines like brushing your teeth, are absorbed without conscious effort. The process of learning new information, storing it, and recalling it involves a complex interplay of brain functions (see Figure 1).
Researchers and neuroscientists have devised several classification systems to describe the various forms of memory. One major system relies on duration, making a distinction between short-term memories, which are fleeting, and long-term memories, which can persist for a lifetime. Another scheme breaks memories down according to the type of information they contain, such as whether they are straight facts, specific events, or learned procedures for doing something.
Forming and storing a memory is a multistep process that involves several parts of the brain. The memory of an event is not a single entity, like a book on a shelf. Instead, it is the aggregation of multiple streams of sensory information, filtered through the perception of the person observing or participating in the event. Each of the different components of a memory is stored and processed in a different region of the brain. It is not uncommon for a person to have problems with one type of memory, such as recalling specific events, but to function normally in other areas, such as remembering routes to different locations.
Figure 1: Anatomy of memory processing
Deep within the brain, a structure known as the hippocampus plays a crucial role in acquiring and consolidating new memories. The nearby amygdala is the part of the brain that reacts to emotionally powerful information, helping the brain retain information that has emotional impact. Once a memory is established (consolidated), it is stored mainly in areas of the cerebral cortex, the large, domed outer layer of the brain.
This is information that the mind stores temporarily, encompassing what you need to remember in the next few seconds or minutes. Short-term memories include, for example, the name of the person who just spoke at a dinner party (as well as what that person said), and the date and time of the appointment you just made — and must remember only until you write it in your date book.
Working memory is a form of short-term memory that involves actively holding information and manipulating it. For example, working memory comes into play when you remember prices at the supermarket while at the same time performing a computation with them so you can compare costs between different brands or quantities.
Short-term memories are supposed to be fleeting. They turn over at a high rate because new ones are continually replacing them, and there are only so many short-term memories you can keep in mind. Research shows that the average person can hold only about seven (plus or minus two) unrelated “bits” of information in mind at one time. That’s why it’s easier to remember a seven-digit phone number than a longer number such as the identification number on a driver’s license.
The relatively transient nature of your short-term memory is actually beneficial because it allows you to discard unnecessary information. Imagine what life would be like if you kept every short-term memory — the name of the telemarketer who called your house an hour ago, the price of each dish you ordered from a Chinese restaurant, what color tie your friend wore yesterday. Your mind would be so overloaded with trivia that you’d have trouble focusing on the things that really are important. It would be as if you kept all your junk mail and let it bury your personal letters, bank statements, and other important documents.
Short-term memory has another limitation. It’s fragile and easily disturbed by interruptions. If you’re trying to remember a phone number and someone walks into the room and asks you a question, chances are you’ll forget the number and have to look it up again. That additional bit of information (the question) “bumps” the short-term memory out of your awareness.
Although most unimportant short-term memories quickly decay, the brain stores the important ones — those that are emotionally compelling or personally meaningful. That stored information is long-term memory. It is the total of what you know: a compendium of data ranging from your name, address, and phone number and the names of friends and relatives to more complex information, such as the sounds and images of important events that happened decades ago. It also includes the routine information you use every day, like how to make coffee, operate your computer, and carry out all of the intricate behavioral sequences involved in performing your job or running your household.
Your long-term memory and short-term memory are not distinguished merely by how long the memories last. Another difference is the amount of information each memory system and its associated brain regions can handle. Although the brain can juggle only a relatively small number of short-term memories at a time, it can store an enormous number of long-term memories. Barring disease or injury, you can always learn and retain something new. Furthermore, long-term memories are less fragile than short-term memories, which means they’re not lost when something interrupts your train of thought. Some types of previously learned long-term memories even tend to remain intact in the early stages of dementia, when patients have trouble learning new information (see “Dementia”).
Maintaining a long-term memory often requires that you periodically “revisit” it. Some long-term memories that go unused or become irrelevant fade or become distorted over time. Have you ever read a book that you loved, but years later found yourself unable to recall much more than the title? That’s probably because you hadn’t thought of the plot and characters in a long time. On the other hand, some long-term memories are amazingly persistent, no matter how infrequently you use them. For example, many adults are surprised by their ability to remember minute details of their youth — an unjustified punishment they received, a fifth-grade science project, their first date. Interestingly, research demonstrates that although long-term memory is more durable than short-term memory, it is also changeable. For example, the way you remember your first romance can evolve over time in response to experiences and information you acquire years later.
Long-term memory can be divided into two categories: declarative memory and implicit memory. (We will discuss one form of implicit memory, called procedural memory.)
Memory researchers use the term “flashbulb memory” to describe a vivid memory of an unexpected, emotionally charged public event. The assassination of President Kennedy and the destruction of the World Trade Center are examples of compelling public events that became ingrained in the memories of many who witnessed them, either directly or through television. Flashbulb memories tend to include numerous minute details associated with your experience of the event — where you were standing, what you were doing, who was around you, and so on. It is likely that the combination of profound meaningfulness and emotional impact surrounding the event serves to inscribe it intensively in long-term memory.
Experts used to assume that flashbulb memories remained more accurate over time than ordinary memories, but research has shown that they are vulnerable to the same biases and distortions as memories of less dramatic events.
This theory has been corroborated in studies of different groups of people in the aftermath of the Sept. 11, 2001, terrorist attack. In a 2009 study in The Journal of Experimental Psychology, researchers questioned more than 3,000 people from seven U.S. cities about their memories of the event one week, 11 months, and 35 months after the assault. Participants answered questions about where they were, what they were doing, and how they felt when they first heard the news, as well as specific facts about the attack, such as the number of planes involved (referred to as event memory). The researchers found that the rate of forgetting was about 20% or more the first year and between 5% and 10% thereafter — a rate that’s similar to ordinary autobiographical memories.
Also known as explicit memory, declarative memory is information that requires a conscious effort to recall. There are two types of declarative memory: semantic memory and episodic memory. Semantic memory is factual knowledge, such as the names of the continents, the color of your spouse’s eyes, or what winter is. Much of the basic information you acquired during your school days falls into this category. In addition to being factual, semantic memory has another key characteristic: it is not bound to a specific point in time. You can’t point to the exact moment when you learned that George Washington led the Revolutionary War, for example. And even if you can remember the specific day when you learned the multiplication tables or other facts in school, the timing isn’t important to your knowledge of them.
By contrast, episodic memory contains the images and details of experiences you have had. Episodic memories are personal memories tied to specific times and places. The party you attended last weekend, the vacation you took last summer, and your children’s birthday celebrations are all episodic memories. An episodic memory is more fragile than a semantic memory because it is more specific; it has a smaller network of associated connections in the brain. Throughout your lifetime, you’ve probably thought about George Washington being the first president of the United States a hundred or more times, in many different contexts — maybe during social studies class, then during a trip to see his historic house, then when you saw his head on a quarter, etc. Episodic memories, on the other hand, are probably brought up in fewer contexts and less often.
Although patients with Alzheimer’s are frequently able to recall events from many years ago, they have profound difficulty acquiring new episodic memories. That’s partly because a brain region called the hippocampus (see Figure 1), which plays a central role in memory encoding, is particularly vulnerable to degenerative disorders such as Alzheimer’s disease. The frontal lobes and their connections also seem to be particularly vulnerable to age-related changes. The frontal lobes are essential in focusing attention and ignoring distractions, initiating strategies for the effective acquisition of new information, activating and retrieving stored memories, recollecting the source of information, and keeping track of the timing and order of specific events. The frontal lobes have been likened to the “file clerk” of the episodic memory system, the hippocampus and medial temporal lobes to the “recent memory file cabinet,” and other cortical regions to the “remote memory file cabinet” (see Figure 2). Different anatomical nodes in the memory network are connected via “white matter” fiber bundles, which are commonly disrupted by age-related conditions including high blood pressure, diabetes, high cholesterol, and small vessel disease. Disruption of these pathways slows processing speed and undermines memory capacity.
Procedural memory refers to memory for skills and routines. It is a form of implicit memory that allows your performance on a task to improve without conscious awareness of previous experiences. You draw on procedural memories automatically to perform actions like getting dressed or driving your car. How to ride a bicycle, write in cursive, operate a video recorder — each of these skills required effort and practice at one time, but once you mastered it, you were able to perform it without remembering how you learned it or the separate steps involved. When you take out your bike for a ride, you don’t say to yourself, “Okay, first I straddle the seat, then I put my left foot on the left pedal, and then I push off the ground with my right foot….” You just get on and go. It’s as though your body does the remembering for you.
In contrast to declarative memory, procedural memory is more resistant to aging and illness. Individuals with Alzheimer’s can perform many routine tasks well into the disease process. This is because procedural memory is supported by different brain systems than declarative memory (the basal ganglia is one). These parts of the brain are relatively well-preserved in Alzheimer’s disease. In contrast, patients with impairment of basal ganglia functioning (as in Parkinson’s disease) often have impaired procedural memory, but relatively well-preserved episodic memory.
Similarly, studies of patients with amnesia who spend time each day practicing new activities, such as playing computer games, suggest that they can learn new skills. Although the amnesiac patients usually can’t recall ever having played or even seen the computer games, their performance improves over time and with practice, indicating that they are capable of acquiring new procedural memories.