Homer Simpson wasn’t a neuroscientist, but he evidently had some insight into how the brain works. In one episode of the long-running series, he explains why education is wasted on him: “Every time I learn something new, it pushes some old stuff out of my brain,” he tells Marge. “Remember when I took that home wine-making course and I forgot how to drive?”

Researchers say they have now observed this Homer Simpson Effect* in the brain for the first time. Using functional magnetic resonance imaging (fMRI) combined with behavioural tests, they show that the retrieval of visual memories causes forgetting of related memory traces. The work, published in the journal Nature Neuroscience, suggests that the act of remembering past experiences might hinder our ability to recall similar events, or perhaps make our memories of them completely inaccessible.

We knew, from earlier work, that remembering can cause forgetting, and this is thought to occur because of an inhibitory signal from the frontal cortex that suppresses similar memories and prevents them from interfering with retrieval. Until now, though, evidence for this has been hard to come by, one reason being that it is difficult to distinguish between the brain activity patterns associated with related memories using fMRI.

In this latest study, Maria Wimber and her colleagues recruited 24 healthy participants and trained them on a visual memory task in which they were shown a series of words, each associate with a pair of images. For example, the word ‘sand’ was shown with an image of Marilyn Monroe and then one of a hat. The researchers then scanned the participants’ brains while they performed a selective retrieval task, in which they saw some of the words again, and had to indicate the first image they associated with it.

The researchers wanted to track and compare changes in the participants’ memories for images they had been asked to recall and those they had not. They assumed that seeing each of the ‘cue’ words again would reactivate activity patterns for both of the associated images, and predicted that recalling one image would engage inhibitory mechanisms that actively diminish the memory of the other, competing image.

To test this, they scanned the participants’ brains one more time, while showing them half of the images they had seen earlier, mixed randomly with other similar images. Thus, using specially designed fMRI data analysis software, they were able to identify ‘signature’ patterns of brain activity for some of the recalled images, as well as for the competing images paired with them, and those that were shown just once at the beginning of the experiment.

Analysis of the data from the earlier scans confirmed their predictions. During the retrieval task, the words were displayed multiple times, forcing the participants to recall the correct images repeatedly and strengthen these memories. Seeing the reminder word ‘sand’ reactivated the signatures of both images associated with it in the visual and memory areas of the brain, and the neural signature for the image of Marilyn Monroe grew stronger with each successive presentation of the word.

At the same time, the signature of the competing image (the hat) grew smaller each time, so that by the fourth presentation it was weaker than ‘baseline’ signatures for images that were shown only once.

The researchers then looked to the prefrontal cortex, parts of which have been identified as the likely source of a top-down control mechanism that detects and suppresses interfering memories. In support of this idea, they identified a small region of the ventro-lateral prefrontal cortex associated with suppression of the competing activity patterns. During the memory retrieval tasks, activity in this area decreased gradually with repeated showing of the reminder words, presumably because the memories grew stronger each time, so that suppressing the competing memory became less demanding.

Wimber and her colleagues say their findings strongly suggest that remembering induces forgetting by actively suppressing the brain activity encoding similar, interfering memories. They believe this is an adaptive mechanism that increases the likelihood that we will successfully encode and remember only the most relevant information. Top-down prefrontal control of memory seems to break down in schizophrenia and Attention Deficit Hyperactivity Disorder (ADHD), so follow-up work that provides more details about these processes could advance our understanding of such conditions.

Reference: Wimber, M., et al. (2015). Retrieval induces adaptive forgetting of competing memories via cortical pattern suppression. Nat. Neurosci., 18: 582–589. DOI: 10.1038/nn.3973

*The term ‘Homer Simpson Effect’ was coined by David Hawkins: