Scientists Bring Back People’s ‘Forgotten’ Memories

The research could overturn a previous theory on how short-term memory works

Illustration: Tara Jacoby
Dec 01, 2016 at 2:00 PM ET

Without our short-term memory, where would we be as a species? Constantly forgetting our keys, that’s where. But given its importance to our daily functioning, it’s surprising that scientists still don’t quite understand how short-term memory works.

Now researchers from the University of Wisconsin, Madison may have upended the prevailing theory on the brain’s method of retaining and recalling short-term memories. In the process, they were able to bring back memories that study participants seemed to have forgotten.

There’s a distinct process for storing long-term memories, and that process takes time. So the brain can retain things that you might need to know immediately, like a phone number or address, in the short-term or working memory. Scientists have long believed that neurons had to be constantly firing in order to keep information in working memory. But several studies in recent years have called that theory into question, suggesting that maybe the neurons didn’t have to be active all the time and could be stored in passive neurons, too.

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In a series of experiments in this study, published Thursday in the journal Science, the researchers put the theory to the test. Participants were asked to remember just a few faces, words, or directions of movement (up, down) in a series. The researchers recorded brain activity as participants tried to remember the stimuli. They saw that the neurons preserving the other stimuli they thought to be less important would stop activating as the experiment continued. Just by looking at their brains, it was possible to see how the subjects forgot what they weren’t supposed to remember.

But the researchers wanted to see if they could revive those memories later. Using machine learning software, they pinpointed exactly where those memories were stored in the brain, then blasted the spot with a non-invasive technique called transcranial magnetic stimulation (TMS). The stimulation reactivated the neurons, and after, the participants were able to remember the words or faces they thought they had forgotten. That suggests that working memory might have two different levels: the “active” level in which the neurons are firing and an “activity-silent” level where memories can be recalled given the proper stimulus.

The experiments were small, with between six and 20 participants in each, but with hundreds of trials. That’s because it’s really difficult to get the baseline measurements for each person’s brain and pinpoint exactly where to direct the TMS, Nathan Rose, now a psychology professor at the University of Notre Dame and the lead author of the study, tells Vocativ.

“This study opens up a lot more questions than it answers,” Rose adds. The researchers aren’t sure exactly how TMS is working on the silent neurons, or which parts of the brain may be the most important for passive working memory, or how the different levels of working memory could affect the process by which those memories are stored for the long term.

Rose and his collaborators hope to keep studying the basic mechanisms that govern the function of working memory. Other researchers, he notes, may be more interested in the clinical applications, since people with different neurological conditions such as schizophrenia, attention disorders, or age-related disorders might fundamentally process memories differently, which could aid in diagnosis or treatment.

Correction: An earlier version of this story stated incorrectly that Nathan Rose was a research assistant at the University of Wisconsin. It has been corrected to indicate that he is now a professor at Notre Dame.