Co-Authored with Jeremy Manning
We all have memories we’ve tried to forget. The time you dressed as the Oregon Trail wagon for Halloween. Your last tequila shot, and all the ones before it. Most of your early teens.
Each year, we spend billions of dollars trying to prevent people from forgetting. We research normal aging, Alzheimer’s Disease, and esoteric dementias. Much less is known about how we clear out memories we don’t need or don’t want. Cognitively speaking, how do we clean house?
Studying What’s Not There
It’s difficult to study forgetting, because we can’t pinpoint when forgetting actually happens. We might remember something one day and not remember it the next, but there’s no timestamp for when the information was actually lost. Forgetting is also complicated because it isn’t a behavior, like remembering, but rather a lack of behavior. Like a dark matter of the mind, forgetting is fundamental to who we are. Yet how can we study what’s not there?
For years, scientists suspected that when we intentionally forget an experience, we also forget the context in which the experience occurred. We erase the whole kit and caboodle.
Consider how scientists might begin to prove this theory - it’s a big challenge. Real-life thoughts are constantly evolving. Trying to forget an especially awkward moment during a recent date might dredge up thoughts about a song that played in the restaurant, the shirt you wore, the piece of spinach stuck in your teeth, your date’s staccato laugh, and that ill-fated shot of tequila. The theory says that the act of forgetting will purge not only the specific memory you’re trying to forget, but also the rich tapestry of contextual thoughts that goes along with that moment. Yet the thoughts are inherently too complex, multifaceted, and fleeting to track. Like Heraclitus’s river, we never revisit precisely the same sequence of thoughts twice. So how could we know when (or if, or how) that tapestry of thoughts gets pushed from our minds?
A Thought Experiment
New tools developed by Jeremy Manning (co-author of this article) and colleagues have finally enabled us to track the elusive act of forgetting.
The team used functional MRI, an imaging technique that highlights when different parts of the brain are active. Earlier work showed that particular parts of the brain are active when people look at pictures of scenes (forests, mountains, fields, etc.). Jeremy and colleagues asked people to memorize a list of random words that were displayed on a screen, interspersed with photographs of scenes. The act of studying each word was like a mini “event” in the person’s life, and the scenes served as an artificial context. The two became automatically linked in people's’ minds.
Each person brought a unique blend of experiences and thoughts into the experiment, but the photographs flashing by provided a common context that researchers could follow over time. In contrast to the impossibly complex and fleeting context of real-life thoughts, the manufactured scene image “contexts” were tightly controlled, and therefore traceable.
Researchers told people to forget the words they had studied and carefully watched to see whether activity in the scene-processing areas of the brain decreased. After a distraction, people were (unexpectedly) asked to recall as many of the forgotten words as they could manage.
People who were better at forgetting the words were also better at deactivating the scene areas of their brains. In other words, they were better at erasing not only the words, but also the context in which the words had been learned. For the first time, scientists were able to measure part of that rich contextual tapestry being expunged from people’s thoughts as they tried to forget the associated experiences.
I Know What You’re Not Thinking
Yet the implications go a step further. By measuring the decrease in activity in the brain’s scene-processing areas, Jeremy’s team could actually watch people forget. It is a step beyond classical mind-reading; not “I know what you’re thinking,” but rather “I know what you’re not thinking.”
As you are reading or listening to this text, you’re likely also aware of the room you’re in, the people you’re with, music playing in the background, smells, sounds, your emotional state, feeling hungry, whatever you did before you picked out this article, and so on. Each of these individual elements forms a part of the full set, but our total sense of being in a particular moment is reflected only by the gestalt. Injecting traceable thoughts into this gestalt is useful, not just for understanding forgetting, but for helping scientists to test theories about the fundamental nature of our thoughts. Just as molecular biologists use fluorescent proteins to track the behavior of molecules, neuropsychologists can now inject traceable thoughts into the brain to track the behavior of the mind.
Rest assured, we are not headed for Tommy Lee Jones’ and Will Smith’s neuralyzer, at least not anytime soon. The details remain ambiguous, like whether contexts disappear because we are actively pushing them out or because we are focusing on remembering something new. While we can now quantify forgetting, we still don’t know how we can control it (think Eternal Sunshine of the Spotless Mind).
So: next time you chose the most embarrassing Halloween costume in the room, rest assured - you’ll probably forget the entire party, anyways.
Jeremy Manning is an Assistant Professor of Psychological and Brain Science at Dartmouth College. Sara Manning Peskin, his cousin, is a neurology resident at the University of Pennsylvania and Chief Blogger at Borderwise.