Illustration by Jon Krause
“Was it unpleasant?” asked a friend’s well-bred mother, arching one brow. “Then why would I remember it?”
Had it been unpleasant, she would remember it, because unless she deliberately swept aside its traces, that unpleasant memory would be stuck inside her brain. Neutral events rinse away, but any event that carries an emotional charge—positive or negative—has staying power.
“The birth of a child, the death of a loved one,” murmurs Tony Buchanan, an assistant professor of psychology at Saint Louis University. “We remember the central details, what we call the gist. We may pick up a few peripheral details, too, like the color of the scrubs in the operating room. But it’s the gist that persists over time.”
Why?
Because before we worked in temperature-controlled offices and ordered Chinese takeout, we needed to learn certain things to survive. If we were in danger or distress, a jolt of fear guaranteed we’d never forget how we got ourselves into that situation. Joy gave us a similar advantage: “We remember positive events—where we got our food, where we met our mate—because they’re rewards,” Buchanan adds. “And we might need to repeat them.”
The part of the brain responsible for processing intense emotion is the amygdala. It’s a very old structure, shaped like an almond and not much bigger, that is found in all complex vertebrates. In other words, it’s primal, and not exclusively human. It evaluates emotions that might be cues to fight, flee, mate, or forage, so we can react instantly.
Not coincidentally, it’s also a gatekeeper for long-term memory.
Scientists started to realize the amygdala’s power a few decades ago, when they began studying “flashbulb memories.” We all have them: sharp, bright, stop-action snapshots of moments when we were elated or grief-stricken or terrified. Their vividness told researchers that we must process and store emotionally charged life events differently than we process and store, say, Gödel’s theorem.
Now, Buchanan and his colleagues are pushing our understanding further. The real memory booster is the adrenaline (epinephrine) or cortisol that accompanies strong emotion, releasing during or just after the event. Shoot stress hormones into a mouse who’s just learned to tap dance, and he’ll still remember the steps two days later. Or, in the more prosaic terms of real-life lab experiments, stress out a college student right after he’s learned new information, and he’ll be more likely to remember it on his final exam. Stress hormones excite the amygdala, which then strengthens the long-term memory of the event.
Stress during the final exam, however, has the opposite effect. “The current thinking is that retrieval is an active process,” Buchanan says. “You have to re-collect the memory, and if your resources are drained by stress, you won’t retrieve it as easily.”
Not all stress is problematic. “We need challenge; we need a certain amount of arousal just to stay awake,” Buchanan says. “But what might be a challenge for one person might be overwhelming stress for another.”
Buchanan’s hunting for clues to resilience, because if we can figure out how to make ourselves less vulnerable to stress, all sorts of health benefits may follow. “Researchers are asking adults for early childhood experiences and even looking in utero, at the mother’s stress levels,” he says. One thing he does know is that people we call “neurotic”—emotionally unstable; temperamentally tense, moody, and anxious—are highly vulnerable to stress. If researchers can figure out why others are more resilient, it might suggest treatments for post-traumatic stress disorder, depression, phobias, and anxiety, all of which can involve horrific memories; irrational, amygdala-driven reactions; and distorted ideas about the world.
BACK WHEN SCIENTISTS first began to study flashbulb memories, they assumed that because the images were so vivid, they must be accurate.
Now we know our brain Photoshops them.
“Memories are very malleable,” Buchanan says. “Your memory system can be tricked, and is tricked, on a regular basis.” That’s because memories are stored, and elaborated, over time. “It takes a while for memories to be consolidated,” he says. “We don’t know how long. But we do know there’s a time lag, because in cases of amnesia, you get the bonk on the head, and you forget what happened right before the bonk.”
We consolidate our memories over time.
Then we reconsolidate them, editing and refining again and again.
“What we represent in our brains is not a photograph of the world,” Buchanan notes. “It’s our understanding of the world at a given time. But our understanding changes as we change. Each time you talk about an event, someone else might add something new, and that changes your memory.”
That’s unsettling, because Buchanan defines memory as “the thread that keeps our identity together. It lets us remember who we are, what we have experienced, and who we have learned to be over the years. At a very basic level, if you don’t have memory, you have lost your identity.”
Not only are our memories unreliable, but they worsen with age. That’s because there’s a wee bit of shrinkage in the hippocampus, which is necessary for new learning, and the part of the prefrontal cortex that orders, organizes, and retrieves our memories.
Could our worsening memories be a capacity problem, like a computer that needs more hard-drive space? “I doubt there’s an upper limit,” Buchanan says dryly. “At least not as far as storage. The problem is getting to the memories.”
On the bright side, we’ll remember the bright side better. “As we get older,” Buchanan says, “we have more ready access to positive memories than to negative ones. The positive memories are easier to recollect, and people seem to dwell more on them.” They’re not just repressing the unpleasant? “I don’t think so. These positive memories seem to recur spontaneously to older people. They are not having to search for them. In brain studies, older people were shown positive pictures—kitty cats and babies—and negative pictures—burn victims and sick people. There was more reactivity to the positive pictures, more activation of the amygdala.”
Maybe that explains nostalgia: We’re convinced the world was better in our youth because we’re remembering so selectively.
But what about people tormented by negative memories, suffering traumatic flashbacks for the rest of their lives? Psychiatrists have long yearned for a way to erase traumatic memories. Now it’s looking like that might not be such a good idea.
An article in the April 12 edition of Proceedings of the National Academy of Sciences described a study in which patients with amnesia were shown happy, funny videos, then tested for recall. They remembered little or nothing, yet their elevated mood persisted just as long as the mood of people who could recall the humor. Now, here’s the really interesting part: When both groups were shown sad movies, those with amnesia stayed sad longer than those who remembered the content. Feeling sad and not knowing why, they felt compelled to search for a cause.
Erase a bad memory, and the experience’s emotional residue could last even longer.
