Forgetting: What’s normal?

(This article was first printed in the Harvard Health Publications Special Health Report “Improving Memory: Understanding and Preventing Age-Related Memory Loss”. For more information or to order, please go to http://health.harvard.edu/IM.)

It’s normal to forget some things, and it’s normal to become somewhat more forgetful as you age, but it’s not normal to forget too much. The question is, how much is too much? Regardless of age, some people’s memories are better than others — just as some people are better at math or more physically coordinated. How can you tell whether your memory lapses are within the scope of normal aging or are a symptom of something more serious?

Neuroscientists and physicians have not fully answered that question, but they have identified some key differences between normal memory lapses and those that occur with more serious cognitive deficits, such as dementia.

Seven types of normal memory problems

Regardless of age, healthy people can experience memory loss or memory distortion. In a book, Daniel Schacter, a professor of psychology at Harvard University, describes seven common “sins” of memory, discussed below. Some of these memory flaws become more pronounced with age, but — unless they are extreme and persistent — they are not considered indicators of Alzheimer’s or other memory-impairing illnesses.

Transience

This is the tendency to forget facts or events over time. You are most likely to forget information soon after you learn it; as time passes, the likelihood of forgetting decreases. However, memory has a use-it-or-lose-it quality: Memories that are called up and used frequently are least likely to be forgotten. Although transience might seem like a sign of memory weakness, brain scientists regard it as beneficial because it clears the brain of unused memories, making way for newer, more useful ones. In this sense, transience is akin to cleaning the junk out of your closets or clearing the temporary files from your computer’s hard drive.

Although everyone experiences transience of memory, it is extreme and debilitating in people with particular kinds of brain damage. For instance, people with amnesia from damage to the hippocampus and related structures have normal short-term memory, but they are unable to form new long-term memories (see “Amnesia”). They forget information soon after they learn it.

Absentmindedness

This type of forgetting occurs when you don’t pay close enough attention. You forget where you just put your pen because you didn’t focus on where you put it in the first place. You were thinking of something else (or, perhaps, nothing in particular), so your brain didn’t encode the information securely. Absentmindedness also involves forgetting to do something at a prescribed time, like taking your medicine or keeping an appointment. In this case, the problem is that you didn’t focus on things that can serve as cues to remind you to do something — like putting your vitamin pills at your place at the table so you’ll remember to take one when you eat breakfast.

For example, if the doctor told you to take your medicine at bedtime and you forgot to do so, it could be that you didn’t pay close enough attention to the key word: bedtime. If you had, chances are that certain details of your bedtime routine (like brushing your teeth or watching a particular show on TV) would have served as cues to remind you to take your medicine.

Blocking

Someone asks you a question and the answer is right on the tip of your tongue — you know that you know it, but you just can’t think of it. This tip-of-the-tongue experience is perhaps the most familiar example of blocking, the temporary inability to retrieve a memory. Blocking doesn’t occur because you weren’t paying attention or because the memory you’re looking for has faded from your brain. On the contrary, blocking occurs when a memory is properly stored in your brain, but something is keeping you from finding it.

In many cases, the memory block is a memory similar to the one you’re looking for but one that you retrieve instead. This other memory is so intrusive that you can’t think of the memory you want. A common example is if you call your older daughter by your younger daughter’s name, or vice versa. Scientists call blocking memories “ugly stepsisters” because they’re domineering, like the stepsisters in Cinderella. Scientists have used ugly stepsisters experimentally to block memories. In one such experiment, people were asked to pick out the definitions of uncommon words from a selection of possible definitions. When definitions that were similar in sound or in meaning were given along with the accurate definition, more people had memory blocks than when unrelated ones were given.

Brain-imaging studies suggest how blocking might work in the brain. When a person is retrieving a memory, some regions of the brain become more active and others become less active. Scientists interpret this finding to mean that the active regions actually inhibit the other regions. When the right regions are activated, the inhibition of other regions can work in your favor by keeping your brain from calling up irrelevant information. But when you call up an ugly stepsister by mistake, the brain regions that encode it may suppress the regions needed to retrieve the memory you really want.

Scientists think that memory blocks become more common with age and that they account for the trouble older people have remembering other people’s names. But it remains unclear whether tip-of-the-tongue experiences are more common than other age-related memory problems. Nor have researchers determined whether memory blocks are simply caused by the overall slowing of memory retrieval that occurs with age. In any case, there’s encouraging news about blocking. Research shows that people are able to retrieve about half of the blocked memories within just a minute.

Misattribution

Consider the following scenario: You’re asked who “John Smith” is, and you remember quite clearly not only who he is, but also what he’s done lately that’s been in the news. Then you’re asked where you learned these details. You think for a moment and reply that it was on the evening TV news. However, there was no report about John Smith on TV. Instead, you got your information from the friend you had lunch with yesterday.

Right memory, wrong source — that’s one example of misattribution. Misattribution occurs when you remember something accurately in part, but misattribute some detail, like the time, place, or person involved. Another kind of misattribution occurs when you believe a thought you had was totally original when, in fact, it came from something you had previously read or heard but had forgotten about. This sort of misattribution explains cases of unintentional plagiarism, in which a writer passes off some information as original when he or she actually read it somewhere before.

Misattribution happens to everyone. Usually it’s harmless, but it can have profound consequences, particularly in the criminal justice system. In some cases, misattribution on the part of eyewitnesses is responsible for the arrest and conviction of individuals for crimes they didn’t commit.

The older the memory, the more likely it is to suffer misattribution. One study documented how misattribution crept into people’s recollections of the explosion of the space shuttle Challenger. Immediately after the Challenger tragedy, researchers at Emory University asked students to write down what they were doing when they heard the news. A year later, the researchers asked the same students to write down their memories of the event again. These later descriptions were riddled with misattributions. But misattributions can be startlingly strong. When told of the discrepancies, the students had trouble believing that their memories were inaccurate. Indeed, three years later, when the students were once again asked to recall the Challenger explosion, their recollections were closer to their second accounts.

As with several other kinds of memory lapses, misattribution becomes more common with age. Age matters in at least two ways. First, as you age, you absorb fewer details when acquiring information because you have somewhat more trouble concentrating. Second, as you grow older, your memories grow older as well. And old memories are especially prone to misattribution.

Suggestibility

Imagine that you saw someone fleeing from a car as its antitheft alarm was blaring. You didn’t get a good look at the thief, but another person on the street insisted that it was a man wearing a green plaid jacket. Later, when the police show you photos of possible suspects, you’re confused until you see a man dressed in green plaid. Then you point to him.

Suggestibility refers to false memories that you develop because someone or something gives you some key information at the same time that you’re trying to retrieve a memory. Although little is known about exactly how suggestibility occurs in the brain, the suggestion fools your mind into thinking it’s a real memory. Suggestibility is the culprit in memories that adults have of incidents from their childhood that never really happened (see “Are recovered memories real?”).

In several studies, people have been induced to recall false events from their childhood after a researcher planted ideas in their minds. In one study, college students’ parents were asked to complete a questionnaire that asked whether certain events happened to the students. Then the students themselves were asked whether they recalled several events — some of which had actually happened (according to the parents’ responses) and some of which hadn’t. At first, 80%–90% of the students accurately recalled the events that had and hadn’t happened to them. But in later interviews, if a researcher suggested that a false event had actually happened to them when they were children, 20%–40% of the students described some memory of it.

No one knows whether people become more vulnerable to suggestibility as they age, but studies have demonstrated that many children experience it when asked to recollect alleged incidents of sexual abuse. Several studies with preschoolers indicate that suggestive questioning by the police or other adults can lead children to assert that certain events occurred when in fact they didn’t.

Bias

One of the biggest myths about memory is that it works like a camera, recording what you learn with complete accuracy. But even the sharpest memory isn’t a flawless snapshot of reality. In your memory, reality is filtered by your personal biases — experiences, beliefs, prior knowledge, and even your mood at the moment. Your biases affect your memories when they’re being encoded in your brain. And your mood and other biases at the moment when you retrieve a memory can influence what information you actually call up.

Bias can affect all sorts of memories, but among the most interesting examples are people’s recollections of their romantic relationships. In one study, couples who were dating were asked to evaluate themselves, their partners, and their relationships initially and then two months later. During the second session, participants were asked to recall what they had said initially. The people whose feelings for their partners and their relationships had become more negative recalled their initial evaluations as more negative than they really were. On the other hand, people whose feelings for their partners and their relationships had become more loving recalled their initial evaluations as more positive than they really were. Although everyone’s attitudes and preconceived notions bias their memories, there’s been virtually no research on the brain mechanisms behind memory bias or whether they become more common with age.

Persistence

Most people worry about forgetting things. But in some cases people wish they could forget, but can’t. The persistence of memories about traumatic events, negative feelings, and chronic fears is another form of memory problem. Some of these memories accurately reflect horrifying events, while others may be negative distortions of reality.

Two groups of people are especially prone to having persistent, upsetting memories. One group is individuals with depression. Research has shown that depressed people are given to ruminating over unpleasant events in their lives or mistakes that they believe they have made. Dwelling on such negatives also fuels a vicious cycle of increasing depression. The other group with persistent, unwanted memories includes people with post-traumatic stress disorder (PTSD). PTSD is a condition that can result from many different sorts of traumatic events — for example, sexual abuse or wartime experiences. Flashbacks, which are persistent, intrusive memories of the traumatic event, are a core feature of PTSD.

Research has shown that persistent memories depend on the activation of those parts of the brain that respond to fear, anxiety, and emotionally charged information. Brain-imaging studies have shown that when people with PTSD recall a persistent, intrusive memory, there is activity in the amygdala as well as in other regions associated with the experience of fear and anxiety. Many people learn to control persistent memories through therapy that involves guided imagery, or visualization. With this technique, a therapist helps the patient learn gradually to envision the traumatic incident without intense fear, eventually lessening PTSD symptoms.