What You Just Forgot May Be ‘Sleeping’
Can’t remember what you were just thinking about? A new study amends our understanding of how memory works
If you’ve ever forgotten why you just entered a room, you know how fickle memory can be. One moment it’s obvious why you walked down the hall, and the next moment you’re standing there befuddled.
Here today, gone in a millisecond. At least that’s how we used to think about short-term, or working, memory. But a study just published in the journal Science tells a different story. A recent idea or word that you’re trying to recall has not, in fact, gone AWOL, as we previously thought. According to new brain-decoding techniques, it’s just sleeping.
“Earlier experiments show that a neural representation of a word disappeared,” said the study’s lead author, Brad Postle, a professor of psychology and psychiatry at the University of Wisconsin-Madison. But by using a trio of cutting-edge techniques, Dr. Postle and his team have revealed just where the neural trace of that word is held until it can be cued up again.
Their study amends the long-standing view of how memory works. Until now, psychologists thought that short-term memory evaporates when you stop thinking about something, while long-term memory permanently rewires neural connections. The new research reveals a neural signature for a third type of memory: behind-the-scenes thoughts that are warehoused in the brain.
In the study’s four experiments, a total of 65 students viewed a pair of images—some combination of a word, a face or a cloud of moving dots—on a screen. During a 10-second period, the students were prompted to think about one of the two images they had seen. After a brief delay, they had to confirm whether a picture they saw matched one of the first two images.
Throughout the experiment, the Postle team monitored the students’ pattern of neural activity. When they prompted the students to remember a particular image, a unique 3-D display of neural activity corresponding to that idea popped up. What really interested the experimenters, though, was what the brain was doing with the image it had effectively set aside. Where did that memory go while it was waiting in the wings?
To find out, the team used a new technique to see what happened when the participants were warned that they would be tested on the set-aside image. This novel approach created a dynamic 3-D display of electroencephalogram (brain wave) and brain-imaging data that let the researchers see beyond what part of the brain “lights up” and zoom in on the pattern of activity within a region. That’s how the team learned that the students’ brain activity had indeed shifted to the “on hold” image’s distinctive pattern—which until then had been invisible.
To confirm that the memory still existed even while a person was not thinking about it, the scientists used another recent technique, transcranial magnetic stimulation, or TMS. They positioned a wand over a participant’s scalp and delivered a harmless magnetic pulse to the brain areas that held the images. The pulse made the distinctive neural signature of those fleeting memories visible to the scientists and triggered their recall in the students.
Dr. Postle compared working memory to paper inscribed with invisible ink. Words written in lemon juice are initially imperceptible, but by passing a hot cup of coffee over the paper, “you can see the part of the message that was heated up…. Our TMS is like the coffee cup.” In this way the team activated a memory that was not only temporary but below the student’s level of consciousness.
Using Dr. Postle’s new trifecta of brain-imaging and brain-stimulation techniques to reactivate forgotten memories has enticing—though still remote—therapeutic possibilities. It is neuroscience’s most faithful reading yet of the real-time content of our thoughts—about as close as we have ever come to mind-reading.
“Our study suggests that there’s information in the penumbra of our awareness. We are not aware that it’s there, but it’s potentially accessible,” said Dr. Postle.