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 correct pairing 87 per cent of the time during both tests. Those who said they had forgotten which image went with which got about half right.
The participants who were unsure if they remembered guessed correctly 57 per cent of the time after 30 minutes and 54 per cent after 24 hours. The fact that these results were slightly higher than would be expected by chance suggests that this group may actually have remembered on these occasions.
When the guessers chose the right answer, the same brain activation patterns were seen in the right hippocampal region of the brain as in those who remembered, which implies that the engrams of forgotten memories remained and were influencing their choices.
In the tests done 24 hours later, the engrams of memories among people who said they had forgotten stayed within the hippocampus, while those who said they could remember had both these and new engrams in the anterior cingulate gyrus in the neocortex.
The movement of memories to the neocortex is correlated with their recall, so we don’t know for sure if they are the cause or consequence of things being remembered, says Amy Milton at the University of Cambridge.
However, the findings are in line with the leading description of the workings of memory, known as standard consolidation theory, says Turk-Browne, which says that memories are initially made in the hippocampus. Then during sleep, they are replayed and stored for the long-term in the neocortex, and some other brain regions.
The work shows there can be a dissociation between the memory we consciously access and the related engram in the brain, says Turk-Browne. “This is a really exciting demonstration of the subtle automatic and pervasive ways that memories in the hippocampus can influence behaviour.”
“Essentially, what they’re trying to argue is that [some] memories don’t necessarily need to be consciously retrieved in order for those memories to subsequently influence behaviour,” says Milton.
She isn’t surprised that there could be a memory trace that is strong enough to influence behaviour without us being consciously aware of it. She mentions the phenomenon of priming, in which seeing or hearing something can prompt us to react in a certain way without us realising why.
Priming presents in different parts of the brain, though, such as the prefrontal cortex, says Turk-Browne, and tends to have a transient effect lasting just a few seconds or minutes.
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