Decreased demands on cognitive control reveal the neural processing benefits of forgetting

Remembering often requires the selection of goal-relevant memories in the face of competition from irrelevant memories. Although there is a cost of selecting target memories over competing memories (increased forgetting of the competing memories), here we report neural evidence for the adaptive benefits of forgetting—namely, reduced demands on cognitive control during future acts of remembering. Functional magnetic resonance imaging during selective retrieval showed that repeated retrieval of target memories was accompanied by dynamic reductions in the engagement of functionally coupled cognitive control mechanisms that detect (anterior cingulate cortex) and resolve (dorsolateral and ventrolateral prefrontal cortex) mnemonic competition. Strikingly, regression analyses revealed that this prefrontal disengagement tracked the extent to which competing memories were forgotten; greater forgetting of competing memories was associated with a greater decline in demands on prefrontal cortex during target remembering. These findings indicate that, although forgetting can be frustrating, memory might be adaptive because forgetting confers neural processing benefits. Remembering the past is fraught with competition. Given the associative nature of memory, remembering a goal-relevant memory often involves selecting it against several competing memories, placing demands on effortful cognitive control mechanisms that detect and resolve competition 1–3 . Fortunately, memory can be adaptive 4–7 , as acts of selective remembering seem to regulate mnemonic competition. Specifically, although selective remembering facilitates future retrieval of the same target memories 8,9 , it also produces a cost—forgetting—for selected-against competing memories (a phenomenon termed ‘retrievalinduced forgetting’) 1,10,11 . Such forgetting of competing memories is hypothesized to reflect their weakening (or suppression) 1,5,10,12 . A crucial question is whether forgetting is indeed adaptive, such that it confers processing benefits precisely because it reduces competition during future attempts to retrieve target memories 4,5 . To the extent that this is the case, it predicts that the forgetting (suppression) of competing memories will be associated with a beneficial decline in demands on the cognitive control mechanisms that are typically required for remembering in the face of competition. Our functional magnetic resonance imaging (fMRI) experiment tested this hypothesis by examining the relationship between the engagement of prefrontal cognitive control mechanisms during repeated acts of selective retrieval and later behavioral evidence that competing memories were forgotten. The experiment was divided into three phases: study, selective retrieval practice and test (Fig.1a). During the study phase, participants encoded a series of cue-associate word pairs, encoding multiple associates of each cue word. Next, participants engaged in selective retrieval practice, repeatedly retrieving some of the associates of some of the cues. Crucially, of the associates that were not practiced, some competed during retrieval practice (that is, they shared cues with practiced associates) whereas others did not. Finally, about 15 min after retrieval practice, memory for all of the initially encoded cue-associate pairs was tested to assess the consequences of selective retrieval practice for both practiced and unpracticed memories. To test our hypothesis, we examined the relationship between fMRI measures of prefrontal cortical (PFC) activation during repeated selective retrieval practice and a behavioral measure of competitor forgetting. First, we predicted that unpracticed memories that competed with targets during selective retrieval practice would suffer a greater rate of forgetting than would unpracticed memories that did not compete 10 . Second, we predicted that repeated selective retrieval would yield benefits for practiced memories, reflected in both behavioral measures of retrieval efficiency and neural measures of reduced demands on PFC-mediated cognitive control mechanisms during repeated retrieval. Finally, and most important, we predicted that the behavioral measure of long-term competitor forgetting (which putatively reflects memory suppression) would correlate with fMRI measures of reduced demands on PFC-mediated control mechanisms during repeated selective retrieval. The data support each of these predictions, providing functional neurobiological evidence that mnemonic suppression occurs when competing memories conflict with target memories during retrieval, and that the successful suppression of competing memories yields immediate benefits—namely, reduced demands on neural mechanisms that detect conflict (in the anterior cingulate cortex; ACC) 13–15 and overcome competition through selection and inhibition (right dorsolateral and ventrolateral PFC) 2,3,16–24 .