Researchers find that memory consists of recreating past brain states.

Earlier this year, students at MIT held the first and last Time Traveler Convention, to disappointing results: No confirmed time-travelers showed up. This latest setback followed centuries of logical, philosophical and physical blows to the idea of time travel. Will we never be able to turn back the clock? Are we doomed to live always in the present?

Perhaps not. Recent research shows that we regularly venture back in time—or at least our brains do.

In a new study published in Science, University of Pennsylvania postdoc Sean Polyn and a team of Princeton researchers showed that when people remember past events, they actually recreate their brain state as it was at the time the event occurred.

“What I believe is that our experiences in the world are kind of coded by our brain into a pattern of cortical activity, and to some extent, that pattern of cortical activity is our experience,” Polyn said. “The job of the memory system is to bring back patterns that were once present but are now gone, and to the extent that we can do that, we are able to revisit past experiences.”

Polyn and his team devised an experiment to examine subjects’ brains both when a stimulus was presented as well as when it was recalled in memory. The subjects’ brains were scanned with fMRI as they looked at pictures of celebrities’ faces, well-known locations and common objects. The subjects then made a judgment about the item in each picture, such as how much they liked the celebrity featured.

Polyn, who performed this research as a doctoral student, said they chose faces, objects and locations because prior research has shown that each of these categories corresponds to a distinct pattern of activity in the human brain.

Using computer analysis on the first round of data, the researchers determined what pattern of brain activity corresponded to each category of stimulus for each individual participant. Then, using fMRI, the researchers then scanned the brains of the subjects while they freely recalled as many of the items as possible.

The researchers found that about five seconds before a subject started to name items from one of the three categories, the pattern corresponding to the category would show up in the brain scan. The researchers could, therefore, predict what kinds of items a subject was going to name before the subject started naming them.

This delay, Polyn said, shows that the participants started to recreate their internal experience of seeing faces, locations or objects before they were able to remember specific items.

“It’s not a very abstruse idea,” said Ken Norman, Polyn’s adviser at Princeton. “If you want to remember an event, you make your brain like the way it was during that event.”

Past research has strongly indicated that whether or not a person finds a desired memory depends largely on what information is included in a retrieval cue. Memory researchers, however, put a sharp distinction between the information contained in the cue and the information retrieved. Norman said this study indicates that retrieved details become part of the cue and can aid in the recovery of more details.

“When we think back to the past, each detail that we remember triggers another until a memory returns completely,” Polyn said. “I guess in that sense, memory retrieval is like revisiting the past.

“Brain patterns that are long gone can be revived by the memory system.”

Originally published December 22, 2005

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