Neural activity gets fired up in different parts of the brain when we recall a memory
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Even memories that we have forgotten seem to guide our actions, which could tell us more about how they are stored in the brain.
“People intuitively think of memory as something we can recall and wax poetic about,” says Nick Turk-Browne at Yale University, who wasn’t involved in the work. “But we don’t spend most of our days sitting around remembering the past. We’re working, and being parents and having fun, and our memory has this ever-present influence on our behaviour. I would guess 95 per cent of our mind operates in the shadows like this.”
Our memories can be defined in different ways. One is based on what people report, such as recalling what they ate for dinner last night or what happened on their seventh birthday. Another way is in terms of an enduring pattern or circuit of cells and connections in the brain, known as an engram, that constitutes the biological representation of a remembered experience.
It has been thought by many researchers that when you forget something, the engram related to that memory vanishes. However, research in mice suggests that forgotten memories can persist, they just can’t be consciously recalled.
To see if forgotten memories are also detectable in human brains, Tom Willems at the University of Bern in Switzerland and his colleagues got 40 people to quickly look at 96 pairs of images, made up of a human face and an object, such as a guitar or stapler.
The researchers then used high-resolution functional magnetic resonance imaging to observe the participants’ brain activity during tests where they were asked whether they had seen two images paired up before, carried out around 30 minutes later and 24 hours later. The participants also said whether they remembered that two images went together, were unsure whether or not they remembered, or thought they were completely guessing.
When they said they could remember, they chose the co