Moderate levels of activation lead to forgetting in the think/no-think paradigm

Behavioral and EEG Evidence for Auditory Memory Suppression

The neural basis of motivated forgetting using the Think/No-Think (TNT) paradigm is receiving increased attention with a particular focus on the mechanisms that enable memory suppression. However, most TNT studies have been limited to the visual domain. To assess whether and to what extent direct memory suppression extends across sensory modalities, we examined behavioral and electroencephalographic (EEG) effects of auditory TNT in healthy young adults by adapting the TNT paradigm to the auditory modality. Behaviorally, suppression of memory strength was indexed by prolonged response time (RTs) during the retrieval of subsequently remembered No-Think words. We examined task-related EEG activity of both attempted memory retrieval and inhibition of a previously learned target word during the presentation of its paired associate. Event-related EEG responses revealed two main findings: (1) a centralized Think > No-Think positivity during auditory word presentation (from approximately 0-500 ms); and (2) a sustained Think positivity over parietal electrodes beginning at approximately 600 ms reflecting the memory retrieval effect which was significantly reduced for No-Think words. In addition, word-locked theta (4-8 Hz) power was initially greater for No-Think compared to Think during auditory word presentation over fronto-central electrodes. This was followed by a posterior theta increase indexing successful memory retrieval in the Think condition. The observed event-related potential pattern and theta power analysis are similar to that reported in visual TNT studies and support a modality non-specific mechanism for memory inhibition. The EEG data also provide evidence supporting differing roles and time courses of frontal and parietal regions in the flexible control of auditory memory.

Post-learning Hippocampal Dynamics Promote Preferential Retention of Rewarding Events

Reward motivation is known to modulate memory encoding, and this effect depends on interactions between the substantia nigra/ventral tegmental area complex (SN/VTA) and the hippocampus. It is unknown, however, whether these interactions influence offline neural activity in the human brain that is thought to promote memory consolidation. Here we used fMRI to test the effect of reward motivation on post-learning neural dynamics and subsequent memory for objects that were learned in high- and low-reward motivation contexts. We found that post-learning increases in resting-state functional connectivity between the SN/VTA and hippocampus predicted preferential retention of objects that were learned in high-reward contexts. In addition, multivariate pattern classification revealed that hippocampal representations of high-reward contexts were preferentially reactivated during post-learning rest, and the number of hippocampal reactivations was predictive of preferential retention of items learned in high-reward contexts. These findings indicate that reward motivation alters offline post-learning dynamics between the SN/VTA and hippocampus, providing novel evidence for a potential mechanism by which reward could influence memory consolidation.

The representational consequences of intentional forgetting: Impairments to both the probability and fidelity of long-term memory

We investigated whether intentional forgetting impacts only the likelihood of later retrieval from long-term memory or whether it also impacts the fidelity of those representations that are successfully retrieved. We accomplished this by combining an item-method directed forgetting task with a testing procedure and modeling approach inspired by the delayed-estimation paradigm used in the study of visual short-term memory (STM). Abstract or concrete colored images were each followed by a remember (R) or forget (F) instruction and sometimes by a visual probe requiring a speeded detection response (E1-E3). Memory was tested using an old-new (E1-E2) or remember-know-no (E3) recognition task followed by a continuous color judgment task (E2-E3); a final experiment included only the color judgment task (E4). Replicating the existing literature, more "old" or "remember" responses were made to R than F items and RTs to postinstruction visual probes were longer following F than R instructions. Color judgments were more accurate for successfully recognized or recollected R than F items (E2-E3); a mixture model confirmed a decrease to both the probability of retrieving the F items as well as the fidelity of the representation of those F items that were retrieved (E4). We conclude that intentional forgetting is an effortful process that not only reduces the likelihood of successfully encoding an item for later retrieval, but also produces an impoverished memory trace even when those items are retrieved; these findings draw a parallel between the control of memory representations within working and long-term memory.

Functional (dissociative) retrograde amnesia

Retrograde amnesia is described as condition which can occur after direct brain damage, but which occurs more frequently as a result of a psychiatric illness. In order to understand the amnesic condition, content-based divisions of memory are defined. The measurement of retrograde memory is discussed and the dichotomy between "organic" and "psychogenic" retrograde amnesia is questioned. Briefly, brain damage-related etiologies of retrograde amnesia are mentioned. The major portion of the review is devoted to dissociative amnesia (also named psychogenic or functional amnesia) and to the discussion of an overlap between psychogenic and "brain organic" forms of amnesia. The "inability of access hypothesis" is proposed to account for most of both the organic and psychogenic (dissociative) patients with primarily retrograde amnesia. Questions such as why recovery from retrograde amnesia can occur in retrograde (dissociative) amnesia, and why long-term new learning of episodic-autobiographic episodes is possible, are addressed. It is concluded that research on retrograde amnesia research is still in its infancy, as the neural correlates of memory storage are still unknown. It is argued that the recollection of episodic-autobiographic episodes most likely involves frontotemporal regions of the right hemisphere, a region which appears to be hypometabolic in patients with dissociative amnesia.

Adaptive top-down suppression of hippocampal activity and the purging of intrusive memories from consciousness

When reminded of unwanted memories, people often attempt to suppress these experiences from awareness. Prior work indicates that control processes mediated by the dorsolateral prefrontal cortex (DLPFC) modulate hippocampal activity during such retrieval suppression. It remains unknown whether this modulation plays a role in purging an intrusive memory from consciousness. Here, we combined fMRI and effective connectivity analyses with phenomenological reports to scrutinize a role for adaptive top-down suppression of hippocampal retrieval processes in terminating mnemonic awareness of intrusive memories. Participants either suppressed or recalled memories of pictures depicting faces or places. After each trial, they reported their success at regulating awareness of the memory. DLPFC activation was greatest when unwanted memories intruded into consciousness and needed to be purged, and this increased engagement predicted superior control of intrusive memories over time. However, hippocampal activity was decreased during the suppression of place memories only. Importantly, the inhibitory influence of the DLPFC on the hippocampus was linked to the ensuing reduction in intrusions of the suppressed memories. Individuals who exhibited negative top-down coupling during early suppression attempts experienced fewer involuntary memory intrusions later on. Over repeated suppressions, the DLPFC-hippocampus connectivity grew less negative with the degree that they no longer had to purge unwanted memories from awareness. These findings support a role of DLPFC in countermanding the unfolding recollection of an unwanted memory via the suppression of hippocampal processing, a mechanism that may contribute to adaptation in the aftermath of traumatic experiences.

Initial retrieval shields against retrieval-induced forgetting

Testing, as a form of retrieval, can enhance learning but it can also induce forgetting of related memories, a phenomenon known as retrieval-induced forgetting (RIF). In four experiments we explored whether selective retrieval and selective restudy of target memories induce forgetting of related memories with or without initial retrieval of the entire learning set. In Experiment 1, subjects studied category-exemplar associations, some of which were then either restudied or retrieved. RIF occurred on a delayed final test only when memories were retrieved and not when they were restudied. In Experiment 2, following the study phase of category-exemplar associations, subjects attempted to recall all category-exemplar associations, then they selectively retrieved or restudied some of the exemplars. We found that, despite the huge impact on practiced items, selective retrieval/restudy caused no decrease in final recall of related items. In Experiment 3, we replicated the main result of Experiment 2 by manipulating initial retrieval as a within-subject variable. In Experiment 4 we replicated the main results of the previous experiments with non-practiced (Nrp) baseline items. These findings suggest that initial retrieval of the learning set shields against the forgetting effect of later selective retrieval. Together, our results support the context shift theory of RIF.

Retrieval induces adaptive forgetting of competing memories via cortical pattern suppression

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 adaptive because it reduces future interference. The impact of this proposed inhibition process on competing memories has, however, never been observed both because behavioural methods are “blind” to retrieval dynamics and because neuroimaging methods have not isolated retrieval of individual memories. Here we introduce 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 implicated in resolving retrieval competition, and, critically, predicted later forgetting. We thus demonstrate a cortical pattern suppression mechanism through which remembering adaptively shapes which aspects of our past remain accessible.

The impact of intermediate-term alcohol abstinence on memory retrieval and suppression

Background: The nature of episodic memory deficit in intermediate-term abstinence from alcohol in alcohol dependence (AD) is not yet clarified. Deficits in inhibitory control are commonly reported in substance use disorders. However, much less is known about cognitive control suppressing interference from memory. The Think/No-think (TNT) paradigm is a well established method to investigate inhibition of associative memory retrieval.

Methods: Thirty-six unmedicated patients with AD and 36 healthy controls (HCs) performed the TNT task. Thirty image–word pairs were trained up to a predefined accuracy level. Cued recall was examined in three conditions: Think (T) for items instructed to-be-remembered, No-think (NT) assessing the ability to suppress retrieval and Baseline (B) for general relational memory. Premorbid IQ, clinical variables and impulsivity measures were quantified.

Results: AD patients had a significantly increased demand for training. Baseline memory abilities and effect of practice on retrieval were not markedly different between the groups. We found a significant main effect of group (HC vs. AD) × condition (B, T, and NT) and a significant difference in mean NT–B scores for the two groups.

Discussion: AD and HC groups did not differ essentially in their baseline memory abilities. Also, the instruction to focus on retrieval improved episodic memory performance in both groups. Crucially, control participants were able to suppress relational words in the NT condition supporting the critical effect of cognitive control processes over inhibition of retrieval. In contrast to this, the ability of AD patients to suppress retrieval was found to be impaired.

Competition between items in working memory leads to forgetting

Switching attention from one thought to the next propels our mental lives forward. However, it is unclear how this thought-juggling affects our ability to remember these thoughts. Here we show that competition between the neural representations of pictures in working memory can impair subsequent recognition of those pictures. We use pattern classifiers to decode functional magnetic resonance imaging (fMRI) data from a retro-cueing task where participants juggle two pictures in working memory. Trial-by-trial fluctuations in neural dynamics are predictive of performance on a surprise recognition memory test: trials that elicit similar levels of classifier evidence for both pictures (indicating close competition) are associated with worse memory performance than trials where participants switch decisively from thinking about one picture to the other. This result is consistent with the non-monotonic plasticity hypothesis, which predicts that close competition can trigger weakening of memories that lose the competition, leading to subsequent forgetting.

Memory reactivation during rest supports upcoming learning of related content

Although a number of studies have highlighted the importance of offline processes for memory, how these mechanisms influence future learning remains unknown. Participants with established memories for a set of initial face-object associations were scanned during passive rest and during encoding of new related and unrelated pairs of objects. Spontaneous reactivation of established memories and enhanced hippocampal-neocortical functional connectivity during rest was related to better subsequent learning, specifically of related content. Moreover, the degree of functional coupling during rest was predictive of neural engagement during the new learning experience itself. These results suggest that through rest-phase reactivation and hippocampal-neocortical interactions, existing memories may come to facilitate encoding during subsequent related episodes.

Briefly cuing memories leads to suppression of their neural representations

Previous studies have linked partial memory activation with impaired subsequent memory retrieval (e.g., Detre et al., 2013) but have not provided an account of this phenomenon at the level of memory representations: How does partial activation change the neural pattern subsequently elicited when the memory is cued? To address this question, we conducted a functional magnetic resonance imaging (fMRI) experiment in which participants studied word-scene paired associates. Later, we weakly reactivated some memories by briefly presenting the cue word during a rapid serial visual presentation (RSVP) task; other memories were more strongly reactivated or not reactivated at all. We tested participants' memory for the paired associates before and after RSVP. Cues that were briefly presented during RSVP triggered reduced levels of scene activity on the post-RSVP memory test, relative to the other conditions. We used pattern similarity analysis to assess how representations changed as a function of the RSVP manipulation. For briefly cued pairs, we found that neural patterns elicited by the same cue on the pre- and post-RSVP tests (preA-postA; preB-postB) were less similar than neural patterns elicited by different cues (preA-postB; preB-postA). These similarity reductions were predicted by neural measures of memory activation during RSVP. Through simulation, we show that our pattern similarity results are consistent with a model in which partial memory activation triggers selective weakening of the strongest parts of the memory.

Pruning of memories by context-based prediction error

The capacity of long-term memory is thought to be virtually unlimited. However, our memory bank may need to be pruned regularly to ensure that the information most important for behavior can be stored and accessed efficiently. Using functional magnetic resonance imaging of the human brain, we report the discovery of a context-based mechanism for determining which memories to prune. Specifically, when a previously experienced context is reencountered, the brain automatically generates predictions about which items should appear in that context. If an item fails to appear when strongly expected, its representation in memory is weakened, and it is more likely to be forgotten. We find robust support for this mechanism using multivariate pattern classification and pattern similarity analyses. The results are explained by a model in which context-based predictions activate item representations just enough for them to be weakened during a misprediction. These findings reveal an ongoing and adaptive process for pruning unreliable memories.

Neural mechanisms of motivated forgetting

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Motivated forgetting of unwanted memories shapes what we retain of our personal past.

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Motivated forgetting is achieved in part by inhibitory control over encoding or retrieval.

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Prefrontal cortex reduces hippocampal and cortical activity to suppress memories.

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Electrophysiological activity during motivated forgetting implicates active inhibition.

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A neurobiological model of memory control can inform disordered control over memory.

Not all memories are equally welcome in awareness. People limit the time they spend thinking about unpleasant experiences, a process that begins during encoding, but that continues when cues later remind someone of the memory. Here, we review the emerging behavioural and neuroimaging evidence that suppressing awareness of an unwelcome memory, at encoding or retrieval, is achieved by inhibitory control processes mediated by the lateral prefrontal cortex. These mechanisms interact with neural structures that represent experiences in memory, disrupting traces that support retention. Thus, mechanisms engaged to regulate momentary awareness introduce lasting biases in which experiences remain accessible. We argue that theories of forgetting that neglect the motivated control of awareness omit a powerful force shaping the retention of our past.

Suppressing unwanted memories reduces their unconscious influence via targeted cortical inhibition

Suppressing retrieval of unwanted memories reduces their later conscious recall. It is widely believed, however, that suppressed memories can continue to exert strong unconscious effects that may compromise mental health. Here we show that excluding memories from awareness not only modulates medial temporal lobe regions involved in explicit retention, but also neocortical areas underlying unconscious expressions of memory. Using repetition priming in visual perception as a model task, we found that excluding memories of visual objects from consciousness reduced their later indirect influence on perception, literally making the content of suppressed memories harder for participants to see. Critically, effective connectivity and pattern similarity analysis revealed that suppression mechanisms mediated by the right middle frontal gyrus reduced activity in neocortical areas involved in perceiving objects and targeted the neural populations most activated by reminders. The degree of inhibitory modulation of the visual cortex while people were suppressing visual memories predicted, in a later perception test, the disruption in the neural markers of sensory memory. These findings suggest a neurobiological model of how motivated forgetting affects the unconscious expression of memory that may be generalized to other types of memory content. More generally, they suggest that the century-old assumption that suppression leaves unconscious memories intact should be reconsidered.

Classification aided analysis of oscillatory signatures in controlled retrieval

Control processes are critical for both facilitating and suppressing memory retrieval, but these processes are not well understood. The current work, inspired by a similar fMRI design (Detre et al., in press), used a modified Think/No-Think(TNT) paradigm to investigate the neural signatures of volition over enhancing and suppressing memory retrieval. Previous studies have shown memory enhancement when well-learned stimulus pairs are restudied in cued recall ("Recall or think of studied pair item"), and degradation when restudied with cued suppression ("Avoid thinking of studied pair item"). We used category-based (faces vs. scenes) multivariate classification of electroencephalography signals to determine if individual target items were successfully retrieved or suppressed. A logistic regression based on classifier output determined that retrieval activation during the cued recall/suppression period was a predictor for subsequent memory. Labeling trials with this internal measure, as opposed to their nominal Think vs. No-Think condition, revealed the classic TNT pattern of enhanced memory for successful cued-retrieval and degraded memory for cued-suppression. This classification process enabled a more selective investigation into the time-frequency signatures of control over retrieval. Comparing controlled retrieval vs. controlled suppression, results showed more prominent Theta oscillations (3 to 8 Hz) in controlled retrieval. Beta oscillations (12 to 30 Hz) were involved in high levels of both controlled retrieval and suppression, suggesting it may have a more general control-related role. These results suggest unique roles for these frequency bands in retrieval processes.

To think or not to think, that is the question: individual differences in suppression and rebound effects in autobiographical memory

Two studies explored the effects of forget instructions on autobiographical memory at immediate test and following delays of either 12-13 months, or 3-4 months. Using the Autobiographical Think/No-Think procedure (cf., Noreen & MacLeod, 2013), 24 never-depressed participants (Study 1) first generated 12 positive and 12 negative autobiographical memories and associated cues. Participants were then asked to recall the memory associated with some of the cues (i.e., 'think' condition), or to avoid saying or thinking about the memory associated with other cues (i.e., 'no-think' condition). Participants were then asked to recall the memories associated with all the cues at immediate test and following a delay of 12-13 months. Participants were found to be successful at forgetting both positive and negative autobiographical memories following 'no-think' instructions at immediate test but this forgetting effect did not persist following a 12-13 month delay. This pattern of remembering and forgetting was replicated in a second study (using 27 never-depressed participants) following a 3-4 month delay. Participants who had been less successful at forgetting 'no-think' memories at immediate test, were more likely to show rebound effects for those memories following a delay compared to memories which received neither 'think' nor 'no-think' instructions. Individual differences in inhibitory control and the efficacy of potential therapeutic interventions of 'no-think' instructions are considered.

Strength of Coupling within a mnemonic control network differentiates those who can and cannot suppress memory retrieval

The ability to direct our thought processes influences not only what we do, but also what we remember later. Here we sought to identify the brain network that supports the ability to control memory retrieval and to understand the neural basis of age-related changes and individual differences in the capacity for mnemonic control. To this end, we collected functional MRI data from 43 children and young adults while they attempted to retrieve or suppress retrieval of previously learned associations. Seed-based functional connectivity analyses revealed a largely right-lateralized dorsolateral prefrontal cortex-cingulate-parietal-hippocampal network that exhibited strongly correlated activity during retrieval suppression. Regardless of age, individuals who were able to suppress memory retrieval exhibited tighter coupling between key nodes in this dorsolateral prefrontal cortex-cingulate-parietal-hippocampal network than individuals who did not. Further, only those capable of mnemonic control exhibited tighter coupling during successful retrieval suppression (intentional forgetting) than during unsuccessful retrieval (unintentional forgetting). Across both children and adults, individual differences in retrieval suppression were best explained by the strength of these network interactions.