We know that memories are changed by up-dating details, consolidating similar memories, and forgetting some altogether. In a recent paper, researchers have shown that forgetting a memory can be due to recall of other memories (citation and abstract below). Remembering a memory enhances that memory but can suppress similar memories that interfere with its recall. This ‘adaptive forgetting’ strengthens often recalled memories and causes forgetting of interfering memories.
The New York Times has a report on this paper (here) giving details of the method.
Wimber and others, using scans and pattern analysis were able to observe the activity of memories in the visual cortex. First the subjects were trained to associate words with unrelated pictures – each word was associated with two different pictures. Then they were given a word and asked to remember the first picture they were trained to associate with that word. The pattern analysis showed the extent of the pattern for the first picture and for the second picture. This trial was repeated several times amongst other trials. The pattern for the first picture grew stronger over the repeated trials and the pattern of the second picture grew weaker. To see what had happened to the second picture, the subjects were shown each picture along with a similar one and asked which picture they had been trained and tested on in each pair. They knew the correct first picture but not had more trouble identifying the correct second picture – in other words, the memory of the word and second picture association was being destroyed.
This has implications for witness testimony after repeated questioning – the questioning may have destroyed some memories by adaptive forgetting. It also weakens the theory that memories are not forgotten but overlaid and hidden by newer memories.
Here is the abstract of the paper (Wimber, Alink, Charest, Kriegeskorte, Anderson; Retrieval induces adaptive forgetting of competing memories via cortical pattern suppression. Nature Neuroscience, 2015) “Remembering a past experience can, surprisingly, cause forgetting. Forgetting arises when other competing traces interfere with retrieval and inhibitory control mechanisms are engaged to suppress the distraction they cause. This form of forgetting is considered to be adaptive because it reduces future interference. The effect of this proposed inhibition process on competing memories has, however, never been observed, as behavioral methods are ‘blind’ to retrieval dynamics and neuroimaging methods have not isolated retrieval of individual memories. We developed a canonical template tracking method to quantify the activation state of individual target memories and competitors during retrieval. This method revealed that repeatedly retrieving target memories suppressed cortical patterns unique to competitors. Pattern suppression was related to engagement of prefrontal regions that have been implicated in resolving retrieval competition and, critically, predicted later forgetting. Thus, our findings demonstrate a cortical pattern suppression mechanism through which remembering adaptively shapes which aspects of our past remain accessible.”
Could this have anything to do with the urban myth about the professor who complained that every time he remembered a student’s name, he forgot the name of another fish?
The BBC report: “Dr Wimber told the BBC the implications of the new findings were not as simple as a “one in, one out” policy for memory storage. “It’s not that we’re pushing something out of our head every time we’re putting something new in. The brain seems to think that the things we use frequently are the things that are really valuable to us. So it’s trying to keep things clear - to make sure that we can access those important things really easily, and push out of the way those things that are competing or interfering.” The idea that frequently recalling something can cause us to forget closely related memories is not new; Dr Wimber explained that it had “been around since the 1990s“.
This probably is only be one of the ways we forget our memories.