Recall of a particular memory often involves its selection from several memories which are similar to it. Although these other memories are irrelevant, their similarity to the “target” memory leads to competition for retrieval, placing demands on the cognitive mechanisms by which the correct memory is selected.
A new study by researchers at Stanford University now shows that forgetting may be part of the process of remembering. Anthony Wagner and his colleagues at the Stanford Memory Laboratory provide evidence that the active suppression by the brain of competing memories is essential for proper memory function. Their findings – that to remember something involves forgetting something similar – have been published online in Nature Neuroscience.
In the study, 20 participants performed a simple memory test. They were first asked to study and remember a series of three word pairs. Two of these word pairs (ATTIC-junk and ATTIC-dust) were similar and could easily be confused with each other, while the third (MOVIE-reel) was completely unrelated. The participants were asked to examine one of the two similar word pairs (ATTIC-dust) again. They were then presented with retrieval cues (the capitalised words) to help them recall all three pairs.
The researchers had hypothesized that during retrieval the similarity of the two word pairs containing the word ‘ATTIC’ would lead to competition between the two, and that the presentation of one of the pairs a second time would cause the memory of the other to be suppressed. This was indeed the case – the participants had greater difficulty retrieving the ATTIC-dust word pair than in recalling the other two word pairs. For example, the unrelated word pair, MOVIE-reel, was recalled 15% more of the time than was ATTIC-dust.
With practice, the participants became more adept at retrieving one word pair and suppressing the memory of the other. Using functional magnetic resonance imaging (fMRI) the researchers showed that this “increased forgetting” of the competing word pair was closely correlated with changes in the activity of a region in the prefrontal cortex called the anterior cingulate cortex (ACC), is known to be involved in detecting similarities in, and conflict between, similar (or competing) information. It was found that activity in the ACC decreased with each successive trial.
This retrieval-induced forgetting has its benefits – the suppression of competing but irrelevant memories reduces the demands placed on the cognitive control mechanisms mediating selection of memories for retrieval. The process by which the relevant memory is selected for retrieval therefore becomes more efficient. Thus, forgetting may be crucial to remembering.
Kuhl, B. A., et al (2007). Decreased demands on cognitive control reveal the neural processing benefits of forgetting. Nat. Neurosci. doi:10.1038/nn1918. [Abstract]