Abstract
Reminders of the past can trigger the recollection of episodes that we would rather forget, and indeed repeatedly preventing an unwanted memory from coming to mind can lead to its forgetting (Anderson & Green, 2001). This study demonstrates two mechanisms that can support such voluntary forgetting. Both mechanisms, direct suppression (DS) and thought substitution (TS), impair long-term retention by limiting momentary awareness of the memories, yet they operate in opposite ways. To scrutinize these mechanisms, 36 participants first learned multiple cue-target associations (e.g., GLOW - GHOST). In the subsequent suppression phase they were presented with the cues and either recalled the respective targets (Recall condition) or prevented the targets from coming to mind (Suppress condition) while they were scanned by functional MRI. Critically, one group accomplished this by engaging in a task designed to recruit a putative mechanism of retrieval inhibition (DS), i.e., they suppressed the targets without focusing on any distracting thought. By contrast, the other group prevented awareness of the unwanted targets by retrieving substitute memories instead (TS; e.g., GLOW - SMILE). Following this phase, subjects were required to recall all targets irrespective of their previous condition (i.e., recall, suppress, and baseline targets that were not cued during the suppression phase). Both DS and TS induced forgetting of the unwanted targets, as indicated by lower recall rates than for baseline items. Critically, these forgetting effects were mediated by dissociable neural networks. As predicted, only DS was associated with decreased activation in the hippocampus (HC), a region important to episodic recollection, and increased activation in right dorsolateral prefrontal cortex (DLPFC). DLPFC recruitment predicted the degree of below-baseline forgetting. Importantly, using dynamic causal modeling, we establish that DLPFC exerts a negative influence over HC activation, thus effectively suppressing episodic retrieval processes. By comparison, TS is achieved in opposite fashion, by engaging retrieval processes to occupy the limited focus of awareness with a substitute memory. It is mediated by interactions between left caudal and mid-ventrolateral PFC that support the selective retrieval of substitutes in the context of pre-potent, unwanted memories. The coupling between these regions varies with the difficulty to substitute the unwanted target, and it is weaker when less unwanted targets interfere with substitute recall. These findings demonstrate that unwanted memories can be voluntarily forgotten by two opposite mechanisms of mnemonic control.
