Remember? What?

Certain memories such as what you dined on last night are easier to recall than others. But even memories you have apparently forgotten may be guiding your everyday doings. One way of defining memory is based on what people report, another is using a circuit of connections and cells in the brain called an engram.

It had been widely accepted by scientists that this engram vanishes when something is forgotten, but research in mice suggests that these memories remain, they just can’t be consciously retrieved. To delve into this further, researchers at the University of Bern in Switzerland asked 40 people to briefly look at 96 pairs of images with a human face and an object such as a stapler or a guitar to see if forgotten memories are still detectable in the human brain. High-resolution functional magnetic resonance imaging was then used to observe the brain activity of the participants during tests conducted some 30 minutes and 24 hours later. The participants were asked if they had seen two images paired up before, and whether they were unsure or guessing.

Those who said they remembered went for the correct pairing 87 per cent of the time in both tests, participants who said they had forgotten got about half right. The same activation patterns were seen in the right hippocampus region of the brain among those who guessed the right answer as those who remembered, implying that the engrams of forgotten memories were still there and influencing choices. Tests 24 hours later showed that participants who guessed right had engrams remaining in the hippocampus, while those who remembered had engrams now in the neocortex.

This is in line with other findings that memories are created in the hippocampus, then during sleep are re-run and stored for the long term in the neocortex. The research hence strongly suggests that there is a disconnect between the memory we consciously access and the related engram in the brain.