There has long been convincing evidence that recalling a memory can cause changes in that memory, potentially weakening it, strengthening it, or otherwise altering it. A new study, published in Nature Neuroscience by researchers from the University of Birmingham, the MRC Cognition and Brain Sciences Unit, and the Behavioral and Clinical Neurosciences Institute, presents a convincing argument that the weakening of old memories may be an adaptive function, one that helps the brain integrate new memories with existing ones.

The researchers were able to watch subjects’ brain activity as memories were recalled using a functional Magnetic Resonance Imaging (fMRI) machine. An fMRI machine allows scientists to see changes in brain activity by producing images that show where oxygen-rich blood is flowing within the brain.

Subjects were shown 144 pairs of pictures/words while in the fMRI machine, and their brain activity was observed. Then, the subjects entered a learning phase, in which they were trained on 72 picture-word pairs, a subset of the initial 144. They were asked to construct a highly detailed association between the pictures and the words, using intricate mental imagery. They were then asked to learn a second set of associations for each word; these served as competition for the first set.

Brain activity is measured in discrete regions called voxels, a 3D pixel. To analyze the effects of the competing memories, the brain activity displayed during recall were analyzed to look for differences in which voxels showed activity.

The subjects were scanned in the fMRI machine four times. Once came after training with the first picture association, another after training with the second association. Then, they were scanned while being asked to recall the first association after having been trained on the second, which should interfere with it. As a control, they were scanned one final time when they were asked to distinguish the pictures they had studied from similar pictures they had not studied.

These experiments led the researchers to five findings that indicate they may have isolated the adaptive forgetting mechanism.

Focusing on the process itself, the researchers found that the selective retrieval of the second association caused forgetting of the first association. When subjects were asked to recall the second associations, the competing first associations were no longer recognized at the same levels. This effect remained true for faces, objects, and scenes, indicating it is not specific to certain stimuli.

The data also showed that the competing memories were progressively more repressed. This gradual suppression of competing patterns was expected based on past studies.

Other indications came from the brain activity. When the subjects were shown pictures and asked to recall their first association after training on a second, their activation levels gradually became lower than the activation levels for images that were not included in the training set at all. The researchers conclude that these associations are getting actively inhibited.

The researchers also found that engagement of regions in the midventrolateral prefrontal cortex predicted this memory suppression. This region, known as the mid-VLPFC, had previously been linked to adaptive forgetting. This finding supports the idea that the mid-VPLFC is involved in a top-down adaptive suppression of previous memory—determining when to suppress memories, and which ones.

Finally, the participants who showed the strongest average reduction in activation of the pathway linked to the initial association showed the most forgetting. This supports the hypothesis that suppressing existing memories is important to adaptive forgetting—formation of a new memory is not the only factor involved.

When these five findings are considered together, they provide strong support for the hypothesis that cortical pattern suppression have a role in human adaptive forgetting. This article presents convincing evidence that new memories may overwrite existing ones through adaptive forgetting, and contributes substantially to the growing literature showing that forgetting may be nearly as important as remembering in humans.

doi:10.1038/nn.3973