Involuntary retrieval of unwanted memories is a common symptom in several clinical disorders, including post-traumatic stress disorder. With an aim to track the temporal dynamics of such memory intrusions, we recorded electrophysiological measures of brain activity while participants engaged in a Think/No-Think task. We presented the left hand word (the cue) of previously encoded word pairs in green or red font. We asked participants to think of the associated right hand word (the associate) when the cue appeared in green (Think condition) and to avoid thinking of the associate when the cue appeared in red (No-Think condition). To isolate cases when participants experienced an intrusive memory, at the end of each trial, participants judged whether the response had come to mind; we classified memories that came to mind during No-Think trials, despite efforts to stop retrieval, as intrusions. In an event-related potential (ERP) analysis, we observed a negative going slow wave (NSW) effect that indexed the duration of a trace in mnemonic awareness; whereas voluntary retrieval and maintenance of the associate was related to a sustained NSW that lasted throughout the 3-s recording epoch, memory intrusions generated short-lived NSWs that were rapidly truncated. Based on these findings, we hypothesize that the intrusion-NSW reflects the associate briefly penetrating working memory. More broadly, these findings exploit the high temporal resolution of ERPs to track the online dynamics of memory intrusions.

The very act of retrieval modifies the accessibility of memory for knowledge and past events and can also cause forgetting. A prominent theory of such retrieval-induced forgetting (RIF) holds that retrieval recruits inhibition to overcome interference from competing memories, rendering these memories inaccessible. The present study tested a fundamental tenet of the inhibitory-control account: The competition-dependence assumption. Event-related potentials (ERPs) were recorded while participants engaged in a competitive retrieval task. Competition levels were manipulated within the retrieval task by varying the cue-item associative strength of competing items. In order to temporally separate ERP correlates of competitor activation and target retrieval, memory was probed with the sequential presentation of 2 cues: A category cue, to reactivate competitors, and a target cue. As predicted by the inhibitory-control account, competitors with strong compared with weak cue-competitor association were more susceptible to forgetting. Furthermore, competition-sensitive ERP modulations, elicited by the category cue, were observed over anterior regions and reflected individual differences in ensuing forgetting. The present study demonstrates ERP correlates of the reactivation of tightly bound associated memories (the competitors) and provides support for the inhibitory-control account of RIF.

We investigated the duration of suppression-induced forgetting (SIF), and the extent to which retrieval suppression differs between negative and neutral memories. We further examined if SIF was differently affected by sleep versus wake during the delay interval between retrieval suppression and re-test. Fifty participants first learned to associate neutral words with either neutral or negative images. Then, a subset of the words was shown again, and participants were asked to either recall (Think), or to suppress retrieval of (No-Think) the associated images. Finally, a memory test for all items was performed either immediately after the Think/No-Think (T/NT) phase (No Delay), or after a 3.5 h delay interval containing either sleep or wake. Results revealed a SIF effect only in the No Delay group, indicating that this forgetting effect dissipates already after a 3.5 h delay interval. Negative items were experienced as more intrusive than neutral ones during the T/NT phase.

Sometimes, we intentionally evaluate stimuli to assess if we recognise them, whereas other times, stimuli automatically elicit recognition despite our efforts to ignore them. If multiple stimuli are encountered in the same environment, intentional recognition judgements can be biased by unintentional recognition of to-be-ignored stimuli. Aging is associated with increased distractibility and impaired intentional retrieval processes, which can make older adults more susceptible to distraction-induced recognition biases. We measured recognition memory performance, ERPs and EEG oscillations in old (60-74) and young (18-24) adults to investigate how aging affects unintentional and intentional memory processes, and how these processes interact over time to produce distraction-induced recognition biases. Older participants had poorer intentional recognition memory, but the biasing effect of unintentional distractor recognition was similar across age groups. ERP effects related to intentional and unintentional recognition that were strongly expressed in the younger group were reduced or absent in the older group. Furthermore, the older group showed qualitatively different ERP activity during intentional recognition compared to the younger group. However, similar patterns of theta and alpha oscillations were found in both age groups, who showed theta power increases for both intentional and unintentional recognition, whereas alpha power was enhanced for intentional recognition but reduced for unintentional recognition. Overall, the findings show that unintentional and intentional recognition involve multiple dissociable memory processes that have different time-courses and functional characteristics, and are differentially affected by aging. Whereas aging has strong effects on the neurocognitive mechanisms underlying intentional recognition memory, unintentional recognition mechanisms are less affected.