Forgetfulness is at least as strong a force, of course. The number of experiences, people, and images we forget during our lives is mind-boggling. "Certainly you don't remember the vast amount of information you take in over the course of a day," says Paul Bays, a neuroscientist at University College London. Forgetting where you left the car keys is inconvenient. But dispensing with memories has uses, too: Do you really want to recall in detail every stubbed toe and dentist visit? "Marital therapists can tell you it would be advantageous to have a pill that would accelerate forgetfulness," says Scott Small, a neurologist and memory researcher at Columbia University.
Forgetfulness may even be essential to maintaining memories, since it frees up space in the brain.
"Its hard to imagine your identity without your memory," says Sandra Aamodt, a coauthor of Welcome to Your Brain (Bloomsbury; 2009). The experiences we have, the people we know, and the skills we learn are essential components of who we are and would be impossible for us to process without memory. Even when memories are unclear or flat-out wrong, they color our present selves. "Our memories are important independent of how accurate they are," says Sam Wang, an associate professor of molecular biology and neuroscience at Princeton University and the coauthor, with Aamodt, of Welcome to Your Brain.
Human memory has several distinct aspects, which scientists examine individually because they are each maintained by a different system within the brain. We use motor memory, for example, to learn a dance step, ride a bike, or play the flute; spatial memory helps us get around without getting lost; and fear conditioning (or associative memory) keeps us from being burned by fire.
But the two forms of memory that get the most attention from researchers are working memory and long-term memory. Working memory holds on to small amounts of information for a short time, just long enough to carry on a conversation or dial a phone number. In fact, this short-term memory is called working memory because its the memory we use to achieve immediate goals, Bays says.
Long-term memory is what you retain: "It's pretty much anything you can remember the next day," Wang says. And that certainly includes any memories strong enough to last from childhood. Working memories become long-term memories if your brain deems them important enough.
We can compare this process, Small says, to the operation of a computer: Working memory is whats on screen and long-term memory is the hard drive. In the brain, both groups of memories are located in the parietal and temporal lobes at the back of the cerebral cortex. The hippocampus manages the click-save function, placing certain short-term memories into long-term storage. The frontal lobes, the seat of thinking and reasoning, are in charge of recalling memories -- the click-open function.
This computer analogy is not exact. After all, if you had a computer that recalled saved information as imperfectly as our brains do, you'd replace it, Small says. Our brains are selective about what they remember. "Two people may experience the same event but remember it differently, because they're holding on to the things they think are important and throwing away the things they think are unimportant, Wang says.
"Nobody remembers nearly as much as they think they remember," Aamodt says. You remember some of the details, and you fill in the rest with what makes sense." Absentmindedness, bias, suggestibility, misattribution, and the fleeting nature of memory all prevent us from keeping track accurately, as Daniel Schacter, a professor of psychology at Harvard University, says in his book The Seven Sins of Memory (Houghton Mifflin; 2002). The most irritating kind of lapse is absentmindedness, Schacter says, citing the example of Yo-Yo Ma leaving a $2.5 million cello in the trunk of a New York City taxi.
Studies have shown that many creatures, including fruit flies, mice, and humans, remember things better after resting. Debate is ongoing about how sleep works to preserve memories. One idea is that during sleep the brain reruns experiences from the day and reinforces connections between neurons. Another theory is that sleep does the opposite: It causes neural connections to dissolve, ridding the brain of unimportant memories. When you reduce the noise of weak memories, "the stronger ones are the ones that pop," says Paul Shaw, a neurobiologist at Washington University School of Medicine, in Saint Louis. He compares it to tuning in to a radio station; reception is better if there are no other stations near the same frequency.
What makes one memory weaker than another? Practice makes a difference. Something that is repeated becomes a stronger memory, as anyone who has studied for an exam knows. Emotion also plays a part. It's why people remember where they were when President John F. Kennedy was shot or terrorists attacked New York and Washington, D.C. Although interestingly, studies have shown that such memories are not always accurate when tested years later.
Sleep and emotion actually work together, says Jessica Payne, a memory researcher at the University of Notre Dame. "During sleep, the brain is making calculations about what to remember and what to get rid of," she says. "Sleep selectively preserves the emotional scenes over neutral ones."
By the time people reach their 30s, memories become less sharp, but typically they don't notice this until they are in their 40s or 50s. "It's mostly the tip-of-the-tongue phenomenon: What's the name of that flower?" says Aamodt, the coauthor of Welcome to Your Brain. It's not that brain cells die, but they shrink and their connections with one another begin to retract.
Researchers look for evidence that certain foods might help, and there is some suggestion that antioxidants -- in blueberries, for example -- might be beneficial. But scientists have not found a reliable memory food. A deficiency in vitamin B12 is bad for memory, but if you are not deficient, taking more B12 doesn't help, says Small, the neurologist at Columbia University.
An active social life helps delay memory loss, according to a recent Harvard University survey. As for brainteasers and puzzles, their effects are limited. "Brain training makes you better at the things you practice," Aamodt says. "If what you want is to remember your grocery list, then if you practice remembering lists, you'll get better at that. But what most people really want is to be more sharp and intellectually agile than they feel."
The best way to preserve memory, many studies suggest, is to stay fit. Exercise seems to preserve function in the hippocampus, the brain area essential to long-term memory.
Memory Loss: Common Causes
Aging: Ordinary forgetfulness increases with age, but for most people it's no more than a nuisance. It gets harder to remember details big and small, especially people's names.
Alzheimer's disease: This progressive disease is well known for its devastating effects on memory. Some 5.3 million Americans are afflicted, and it is the seventh leading cause of death in the United States. In Alzheimer's disease, neurofibrillary tangles and plaque build up in and between brain cells, damaging a person's ability to learn and remember.
Depression: People who are clinically depressed often experience memory loss. This may be because the sadness causes a lack of attention and focus. But doctors have also observed that people with depression often have elevated levels of cortisol, the so-called stress hormone secreted by the adrenal glands as part of the fight-or-flight response. And it may be that the elevated cortisol causes cognitive problems, including memory loss.
Head injury: A blow to the head can cause immediate memory loss, but the problem is usually temporary.
Heavy alcohol use: Overimbibing can alter chemicals in the brain that affect memory, and it can also lead to deficiencies in vitamin B1 (thiamine), which can hurt memory.
Stroke-related dementia: The occurrence of multiple strokes, some of which are too small to be noticeable, can cause memory loss in older people. Each disrupts blood flow to the brain, damaging tissue, and over time this impairs memory. Maintaining good cardiovascular health -- by exercising, eating well, and avoiding smoking and excess alcohol -- can help reduce the risk of stroke.
Text by Mary Duenwald