Mental time travel

Remembering is like a form of mental time travel – our brain reinstates the patterns of neural activity that existed at the time of the recalled experience. That’s the suggestion from a new brain imaging study that compared participants’ neural activity when they studied photographs, with later activity that occurred when they subsequently attempted to recall those photographs.

Sean Polyn (pictured) at the University of Pennsylvania and colleagues scanned the brains of nine participants while they were shown thirty photographs from three categories: famous faces, including Halle Berry and Madonna; famous places such as the Taj Mahal; and everyday objects, like tweezers or spray paint. To give each category a distinguishing mental context, participants were asked to make a different judgement for each category – they had to say whether they loved or hated each celebrity; how much they would like to visit each location; and how often they came across the objects. Later on, the participants were given three minutes to recall as many of the 90 photographs they had seen as possible.

Polyn’s team found that when the participants successfully recalled a photo, their brain activity resembled the pattern of activity shown when they originally looked at the photo. In fact, this similarity emerged several seconds before the participants made each recollection, so that the researchers could predict in advance what category of photo a participant was about to recall.

Sean Polyn said “This study shows that, as you search for memories of a particular event, your brain state progressively comes to resemble the state it was in when you initially experienced the event”.

Co-researcher Ken Norman explained how the research might have practical benefits: "Our method gives us some ability to see what cues participants are using, which in turn gives us some ability to predict what participants will recall. We are hopeful that, in the long run, this kind of work will help psychologists develop better theories of how people strategically cue memory, and also will suggest ways of making these cues more effective".
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Polyn, S.M., Natu, V.S., Cohen, J.D. & Norman, K.A. (2005). Category-specific cortical activity precedes retrieval during memory search. Science, 310, 1963-1966.

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