by Posit Science, AARP, www.aarp.org
Memory may be the most important skill you have.
It doesn’t just allow you to reminisce about your experiences — it’s what enables you to do everything that you had to learn at one point, from holding up your head to speaking your native language.
Memory actually takes many different forms. We know that when we store a memory, we are storing information. But what that information is and how long we retain it determines the type of memory it is.
The broadest categories of memory are short-term memory and long-term memory, based on the length of time the memory is stored.
Short-Term Memory: This refers to the very short time that you keep something in mind before either dismissing it or transferring it to long-term memory.
Long-Term Memory: This is our brain’s system for storing, managing and retrieving information. We store different types of information (procedures, life experiences, language, etc.) with separate memory systems.
Short-term memory is like a receptionist for the brain. It’s responsible for registering information temporarily and determining whether it will be dismissed or transferred on to long-term memory. Although it can sound complicated, this sorting function takes less than a minute to complete. For example, it is helping you right now make sense of this sentence by storing information from the start of it until you’ve finished reading the end.
More recently, scientists have begun to dive a little deeper into “short-term” brain functions and have added a separate (but similar) type of memory, known as “working” memory. Although the two are often used interchangeably, they are different. Working memory emphasizes the brain’s manipulation of information it receives (using it, storing it and so on), while short-term memory is a more passive concept. Working memory is often thought of as the brain’s “scratch pad,” which keeps information — such as a number or name — on hand just long enough to use it.
As we grow older, the length of time our short-term memory can store information becomes shorter. This makes us more likely to have trouble keeping up with certain tasks, like remembering which button to push in a bank’s phone menu. It also gives our brains less time to successfully move new information to long-term memory, making us more likely to forget details of recent events. Memory lapses and cognitive decline are a normal part of aging, but not an inevitable one, you can work to slow down the process by maintaining a brain-healthy lifestyle and keeping your memory active.
A long-term memory is anything you remember that happened more than a few minutes ago. Long-term memories can last for just a few days or for many years. Long-term memories aren’t all of equal strength. Stronger memories enable you to recall an event, procedure or fact on demand — for example, that Paris is the capital of France. Weaker memories often come to mind only through prompting or reminding.
Long-term memory isn’t static. You do not imprint a memory and leave it as if untouched. Instead, you may revise the memory over time — perhaps by merging it with another memory or incorporating into it what others tell you about the memory. As a result, your memories are not always reliable.
The two major subdivisions of this sort of memory are explicit memory (also called declarative memory) and implicit memory (nondeclarative memory).
It’s the type of memory that doesn’t require conscious thought. It enables us to carry out commonly learned tasks without consciously thinking about them. It’s our “how to” knowledge — what allows us to ride a bike, tie a shoe and wash up the dishes after dinner. Even the things we think of as natural tasks, such as walking, require this kind of memory.
Retrieving an explicit memory requires conscious thought — such as recalling who came to dinner last night or naming animals that live in the rain forest. It’s what most people have in mind when they think of “memory” and whether theirs is good or bad. Explicit memory is often associative; your brain links these kinds of memories together. For example, when you think of a word like “car,” a whole host of associated memories may come to mind — from rearview mirrors to your commute to a family road trip.
There are two kinds of explicit memory: episodic and semantic.
Semantic memory accounts for our “textbook learning” or general knowledge about the world. It’s what enables us to say, without knowing exactly when and where we learned it, that a zebra is a striped animal, or that Vermont is in New England. Scientists aren’t sure where semantic memory happens; some say in the hippocampus and related areas, while others think it’s widely spread throughout the brain. Semantic memory is better sustained over time than episodic memory. We are often able to retain a highly functioning semantic memory into our 60s — after which it begins slowly declining.
Episodic memory, by contrast, is autobiographical: It provides us with a crucial record of our personal experiences. It is our episodic memory that allows us to remember the trip we took to the British Virgin Islands, what we had for dinner last night or who told us that our friend Maryann was expecting. Any past event in which we played a part, and which we remember as an “episode” (a scene of events), constitutes an episodic memory.
This form of memory appears to be centered in the brain’s hippocampus — with considerable help from the cerebral cortex. Read more about this type of autobiographical memory and take a test to see if your episodic memories center in the same time of life as the average person’s.
From the moment of birth, each of us is exposed to a world full of sensations and information. All of these experiences — first kisses, familiar places, sad farewells — have the potential to end up as autobiographical memories.
Not all of them do, of course. Scientists have long been interested in understanding what we remember about our past and why we remember it. But figuring out a way to study this presents problems. Other kinds of memory are tested fairly easily in the laboratory by way of experiments in which people are asked to do things like memorize lists of related and unrelated words. But this approach doesn’t work so well for autobiographical memories, which are acquired over time and in various places along the way.
The 19th-century English psychologist Sir Francis Galton pioneered a simple method to study autobiographical memory. He decided to go fishing, as it were, for memories associated with a list of common, everyday words. Four times he threw out his net of words, using the same cues to catch his recollections.
One of Galton’s findings was that it was difficult to pinpoint when the events he remembered had occurred. Another finding was that his brain tended to pull up the same associations over and over again. “This shows much less variety in the mental stock of ideas than I had expected,” he wrote, “and makes us feel that the roadways of our minds are worn into very deep ruts.”
In the 1970s, researchers adapted Galton’s cue-word method to study the distribution of autobiographical memories over time. They found that the college students they tested reported many more memories from the recent than the distant past. In fact, researchers found that a graph of the relationship between forgetting and time resembles a steep slide. Still, over time, the rate at which one forgets things does level off, leaving us with small but stable cores of knowledge.
As life expectancy increased, psychologists began studying autobiographical memories reported by middle-aged and older people. Imagine their surprise when, instead of encountering a simple slide, they found something resembling a bumpy roller coaster track, with a pronounced “bump” of memories from adolescence and young adulthood. Researchers haven’t yet agreed on the causes of this “bump.” One (now discredited) theory says the bump, or “reminiscence effect,” simply reflects the fact that the brain functions best during adolescence and young adulthood.
Another theory: Many situations are new to younger people, so memories from this time are more durable because they were laid down without significant interference from other, similar experiences.