Visual perspective in autobiographical memories of self-incongruent episodes.

It is widely assumed that autobiographical memory relies on an integration of episodic memory with the self-model. We hypothesise that self-memory integration depends critically on self-congruence. More specifically, self-incongruent experiences such as those that elicit shame or guilt may be more difficult to integrate. Self-incongruence may affect both the semantic reports of memories and their phenomenological characteristics, in particular their visual perspective (1PP or 3PP, i.e., field or observer perspective), their affective valence, and their perceived centrality. Diary based memories were assigned to 4 categories (shame, guilt, negative, neutral) and were rated for the different phenomenological dimensions. We used a deep neural network, univariate and multilevel models to assess differences and relationships between different variables. We found that memories that elicited shame (but not guilt) showed more pronounced 3PP as compared to other experiences. Shameful episodes also elicited the most pronounced negative affect. A multilevel analysis revealed that the amount of shame that an episode elicited, and its semantic similarity with shame episodes, predicted higher 3PP, while affective valence did not. Our results show that self-incongruence affects memories both at the level of their semantic reports and their phenomenology, and thus contributes to a mechanistic understanding of self-memory integration.

Tracking Selective Rehearsal and Active Inhibition of Memory Traces in Directed Forgetting.

Selectively remembering or forgetting newly encountered information is essential for goal-directed behavior. It is still an open question, however, whether intentional forgetting is an active process based on the inhibition of unwanted memory traces or whether it occurs passively through reduced recruitment of selective rehearsal [1, 2]. Here, we show that intentional control of memory encoding relies on both, enhanced active inhibition and decreased selective rehearsal, and that these two processes can be separated in time and space. We applied representational similarity analysis (RSA [3]) and time-frequency analysis to EEG data during an item-method directed forgetting experiment [4]. We identified neural signatures of both, the intentional suppression and the voluntary upregulation of item-specific representations. Successful active forgetting was associated with a downregulation of item-specific representations in an early time window 500 ms after the instruction. This process was initiated by an increase in oscillatory alpha (8-13 Hz) power, a well-established signature of neural inhibition [5, 6], in occipital brain areas. During a later time window, 1500 ms after the cue, intentional forgetting was associated with reduced employment of active rehearsal processes, as reflected by an attenuated upregulation of item-specific representations as compared to intentionally encoded items. Our data show that active inhibition and selective rehearsal are two separate mechanisms whose consecutive employment allows for a voluntary control of memory formation.

The neural dynamics of deficient memory control in heavily traumatized refugees.

Victims of war, torture and natural catastrophes are prone to develop posttraumatic stress disorder (PTSD). These individuals experience the recurrent, involuntary intrusion of traumatic memories. What neurocognitive mechanisms are driving this memory disorder? Here we show that PTSD symptoms in heavily traumatized refugees are related to deficits in the effective control of memory retrieval. In a think/no-think task, PTSD patients were unable to forget memories that they had previously tried to suppress when compared to control participants with the same trauma history but without PTSD. Deficits in voluntary forgetting were clinically relevant since they correlated with memory intrusions in everyday life. Magnetoencephalography (MEG) recorded during suppression attempts revealed that PTSD patients were unable to downregulate signatures of sensory long-term memory traces in the gamma frequency band (70-120 Hz). Thus, our data suggest that the inability to suppress unwanted memories through modulation of gamma activity is related to PTSD symptom severity.

Stress Elevates Frontal Midline Theta in Feedback-based Category Learning of Exceptions.

Adapting behavior based on category knowledge is a fundamental cognitive function, which can be achieved via different learning strategies relying on different systems in the brain. Whereas the learning of typical category members has been linked to implicit, prototype abstraction learning, which relies predominantly on prefrontal areas, the learning of exceptions is associated with explicit, exemplar-based learning, which has been linked to the hippocampus. Stress is known to foster implicit learning strategies at the expense of explicit learning. Procedural, prefrontal learning and cognitive control processes are reflected in frontal midline theta (4-8 Hz) oscillations during feedback processing. In the current study, we examined the effect of acute stress on feedback-based category learning of typical category members and exceptions and the oscillatory correlates of feedback processing in the EEG. A computational modeling procedure was applied to estimate the use of abstraction and exemplar strategies during category learning. We tested healthy, male participants who underwent either the socially evaluated cold pressor test or a nonstressful control procedure before they learned to categorize typical members and exceptions based on feedback. The groups did not differ significantly in their categorization accuracy or use of categorization strategies. In the EEG, however, stressed participants revealed elevated theta power specifically during the learning of exceptions, whereas the theta power during the learning of typical members did not differ between the groups. Elevated frontal theta power may reflect an increased involvement of medial prefrontal areas in the learning of exceptions under stress.

Episodic Memory Retrieval Functionally Relies on Very Rapid Reactivation of Sensory Information.

Episodic memory retrieval is assumed to rely on the rapid reactivation of sensory information that was present during encoding, a process termed "ecphory." We investigated the functional relevance of this scarcely understood process in two experiments in human participants. We presented stimuli to the left or right of fixation at encoding, followed by an episodic memory test with centrally presented retrieval cues. This allowed us to track the reactivation of lateralized sensory memory traces during retrieval. Successful episodic retrieval led to a very early (∼100-200 ms) reactivation of lateralized alpha/beta (10-25 Hz) electroencephalographic (EEG) power decreases in the visual cortex contralateral to the visual field at encoding. Applying rhythmic transcranial magnetic stimulation to interfere with early retrieval processing in the visual cortex led to decreased episodic memory performance specifically for items encoded in the visual field contralateral to the site of stimulation. These results demonstrate, for the first time, that episodic memory functionally relies on very rapid reactivation of sensory information. SIGNIFICANCE STATEMENT: Remembering personal experiences requires a "mental time travel" to revisit sensory information perceived in the past. This process is typically described as a controlled, relatively slow process. However, by using electroencephalography to measure neural activity with a high time resolution, we show that such episodic retrieval entails a very rapid reactivation of sensory brain areas. Using transcranial magnetic stimulation to alter brain function during retrieval revealed that this early sensory reactivation is causally relevant for conscious remembering. These results give first neural evidence for a functional, preconscious component of episodic remembering. This provides new insight into the nature of human memory and may help in the understanding of psychiatric conditions that involve the automatic intrusion of unwanted memories.

Brain Oscillations Mediate Successful Suppression of Unwanted Memories.

To avoid thinking of unwanted memories can be a successful strategy to forget. Studying brain oscillations as measures of local and inter-regional processing, we shed light on the neural dynamics underlying memory suppression. Employing the think/no-think paradigm, 24 healthy human subjects repeatedly retrieved (think condition) or avoided thinking of (no-think condition) a previously learned target memory upon being presented with a reminder stimulus. Think and no-think instructions were delivered by means of a precue that preceded the reminder by 1 s. This allowed us to segregate neural control mechanisms that were triggered by the precue from the effect of suppression on target memory networks after presentation of the reminder. Control effects were reflected in increased power in the theta (5-9 Hz) frequency band in the medial and dorsolateral prefrontal cortex and higher long-range alpha (10-14 Hz) phase synchronization. Successful suppression of target memories was reflected in a decrease of theta oscillatory power in the medial temporal lobes and reduced long-range theta phase synchronization emerged after presentation of the reminder. Our results suggest that intentional memory suppression correlates with increased neural communication in cognitive control networks that act in down-regulating local and inter-regional processing related to memory retrieval.

Intentional suppression can lead to a reduction of memory strength: behavioral and electrophysiological findings.

Previous research has shown that the intentional suppression of unwanted memories can lead to forgetting in later memory tests. However, the mechanisms underlying this effect remain unclear. This study employed recognition memory testing and event-related potentials (ERPs) to investigate whether intentional suppression leads to the inhibition of memory representations at an item level. In a think/no-think experiment, participants were cued to either suppress (no-think condition) or retrieve (think condition) previously learned words, 18 or 0 times. Performance in a final recognition test was significantly reduced for repeatedly suppressed no-think items when compared to the baseline, zero-repetition condition. ERPs recorded during the suppression of no-think items were significantly more negative-going in a time window around 300 ms when compared to ERPs in the think condition. This reduction correlated with later recognition memory impairment. Furthermore, ERPs to no-think items from 225 to 450 ms were more negative-going in later phases of the experiment, suggesting a gradual reduction of memory strength with repeated suppression attempts. These effects were dissociable from correlates of recollection (500-600 ms) and inhibitory control (450-500 ms) that did not vary over the time-course of the experiment and appeared to be under strategic control. Our results give strong evidence that the no-think manipulation involves inhibition of memory representations at an item level.

α/ß oscillations indicate inhibition of interfering visual memories.

Selective retrieval of a specific target memory often leads to the forgetting of related but irrelevant memories. Current cognitive theory states that such retrieval-induced forgetting arises due to inhibition of competing memory traces. To date, however, direct neural evidence for this claim has not been forthcoming. Studies on selective attention suggest that cortical inhibition is mediated by increased brain oscillatory activity in the alpha/beta frequency band. The present study, testing 18 human subjects, investigated whether these mechanisms can be generalized to selective memory retrieval in which competing memories interfere with the retrieval of a target memory. Our experiment was designed so that each cue used to search memory was associated with a target memory and a competitor memory stored in separate brain hemispheres. Retrieval-induced forgetting was observed in a condition in which the competitor memory interfered with target retrieval. Increased oscillatory alpha/beta power was observed over the hemisphere housing the sensory representation of the competitor memory trace and predicted the amount of retrieval-induced forgetting in the subsequent memory test. These results provide the first direct evidence for inhibition of competing memories during episodic memory retrieval and suggest that competitive retrieval is governed by inhibitory mechanisms similar to those employed in selective attention.

Trait anxiety, working memory capacity, and the effectiveness of memory suppression.

We aimed at replicating the finding that humans are able to suppress unwanted memories, and tested whether this ability varies with individual differences in working memory capacity, trait anxiety and defensiveness. In a think/no-think experiment, participants either recalled or suppressed previously learned words for 0, 8 or 16 times. Suppression did not have an overall detrimental effect on later recall performance. However, higher recall rates after repeated suppression were exclusively predicted by higher trait anxiety. These results are discussed in relation to current theories on anxiety and executive control.

ERP correlates of intentional forgetting.

In this study we investigated the neurocognitive processes underlying the control of memory retrieval. In a Think/No-Think paradigm, adopted for the use in an event-related potential (ERP) experiment, participants learned word pairs and were subsequently presented with cue words and asked to either suppress or to recall the target word. During final cued recall tests for all initially learned targets, memory for the to-be-suppressed or to be-recalled items were tested. Memory for to be-recalled items was enhanced but no forgetting of to-be-suppressed items was obtained. The ERPs in the test phase were separated on the basis of prior learning success and failure, allowing separate analyses of strategic memory control, i.e. attempts to retrieve or to avoid retrieval and the outcome of these processes, i.e. successful retrieval and retrieval avoidance. An early P2 component and a parietal positivity were related to retrieval attempts and a centro-parietal N2 component was associated with attempts to avoid memory retrieval. The parietal positivity was attenuated for No-Think trials on learned items, for which item-specific memories exist. However, under the present testing conditions and in contrast to prior studies (Bergström, Velmans, de Fockert, Richardson-Klavehn, 2007) the parietal positivity was also sensitive to mere retrieval attempts. To examine whether similar neural systems are involved in the inhibitory control of unwanted memories and prepotent motor responses, a motor stopping experiment using a stop signal task was conducted with the same participants. Successful stopping was associated with an enhanced stop signal N2 that showed a similar centro-parietal scalp distribution as the aforementioned N2 to No-Think trials. As both components were significantly correlated, we assumed that some of the systems recruited to override prepotent motor responses are also involved to suppress memory retrieval.