Forgotten Memories Linger in the Brain

When it comes to memories, it seems the old adage is reversed: They’re forgotten, but not gone. Even when a memory can’t be accessed — that is, when it’s forgotten — new research suggests it may persist in the brain.

In a study published September 10 in Neuron, researchers found that when college students couldn’t remember the specifics of an event, they still showed patterns of brain activation corresponding to the forgotten information.

“Even though the subject couldn’t tell us any details they remembered, we could still pick those details out in the brain activity,” said cognitive neuroscientist Jeff Johnson of the University of California, Irvine, the study’s lead author.

Researchers divide memories of events into two types: recollection and familiarity. During recollection, a memory includes concrete details — not only do you recognize people, but you know their name or where you met them. With familiarity, those details are missing, but you have a strong feeling you’ve met that person before.

Brain areas that are active during learning are reactivated when something is recollected, in a process known as reinstatement.  Now, Johnson and his colleagues have shown that those same patterns of activity are generated during familiarity.

Mark Wheeler, a memory researcher at University of Pittsburgh not involved in the study, hopes this finding will allow researchers to go beyond the black-or-white, recollection-or-familiarity divisions they often make in the lab. “This is a really nice method of trying to look past that and extract more information than is otherwise available,” he said.

Each of the 16 students in the study was asked to lie in an fMRI machine, which measures blood flow in the brain, while the researchers showed them lists of words. As they viewed a word, the students performed one of three tasks: considering how an artist might draw it, listing uses for it or pronouncing it backwards.

After 20 minutes, the students were shown the words again and asked to recall them as best they could.

Reinstatement was strongest when students remembered the word and the task they performed while they viewed it – stronger than when they only remembered the word and said it was familiar. But the reinstatement signal for familiar words still contained useful information. By comparing that signal to patterns of activity seen during each task, researchers could deduce which task the student had initially done with that word.

“It’s interesting that we’re able to learn more about the subject’s brain than what they’re able to tell us,” Johnson said. But, he wondered, how long do those patterns of activity endure? He hopes that, by examining patterns of reinstatement, researchers can gain insight into how long such “forgotten” memories last in the brain.