Episodic memory formation in unrestricted viewing

The brain systems of episodic memory and oculomotor control are tightly linked, suggesting a crucial role of eye movements in memory. But little is known about the neural mechanisms of memory formation across eye movements in unrestricted viewing behavior. Here, we leverage simultaneous eye tracking and EEG recording to examine episodic memory formation in free viewing. Participants memorized multi-element events while their EEG and eye movements were concurrently recorded. Each event comprised elements from three categories (face, object, place), with two exemplars from each category, in different locations on the screen. A subsequent associative memory test assessed participants' memory for the between-category associations that specified each event. We used a deconvolution approach to overcome the problem of overlapping EEG responses to sequential saccades in free viewing. Brain activity was time-locked to the fixation onsets, and we examined EEG power in the theta and alpha frequency bands, the putative oscillatory correlates of episodic encoding mechanisms. Three modulations of fixation-related EEG predicted high subsequent memory performance: (1) theta increase at fixations after between-category gaze transitions, (2) theta and alpha increase at fixations after within-element gaze transitions, (3) alpha decrease at fixations after between-exemplar gaze transitions. Thus, event encoding with unrestricted viewing behavior was characterized by three neural mechanisms, manifested in fixation-locked theta and alpha EEG activity that rapidly turned on and off during the unfolding eye movement sequences. These three distinct neural mechanisms may be the essential building blocks that subserve the buildup of coherent episodic memories during unrestricted viewing behavior.

Phase separation of competing memories along the human hippocampal theta rhythm

Competition between overlapping memories is considered one of the major causes of forgetting, and it is still unknown how the human brain resolves such mnemonic conflict. In the present magnetoencephalography (MEG) study, we empirically tested a computational model that leverages an oscillating inhibition algorithm to minimise overlap between memories. We used a proactive interference task, where a reminder word could be associated with either a single image (non-competitive condition) or two competing images, and participants were asked to always recall the most recently learned word–image association. Time-resolved pattern classifiers were trained to detect the reactivated content of target and competitor memories from MEG sensor patterns, and the timing of these neural reactivations was analysed relative to the phase of the dominant hippocampal 3 Hz theta oscillation. In line with our pre-registered hypotheses, target and competitor reactivations locked to different phases of the hippocampal theta rhythm after several repeated recalls. Participants who behaviourally experienced lower levels of interference also showed larger phase separation between the two overlapping memories. The findings provide evidence that the temporal segregation of memories, orchestrated by slow oscillations, plays a functional role in resolving mnemonic competition by separating and prioritising relevant memories under conditions of high interference.

An event-related potential study of the testing effect: Electrophysiological evidence for context-dependent processes changing throughout repeated practice

The testing effect refers to a special form of performance improvement following practice. Specifically, repeated retrieval attempts improve long-term memory. In the present study we examined the underlying mechanisms of the testing effect as a function of time by investigating the electrophysiological correlates of repeated retrieval practice. We additionally investigated the ERP waveforms of the repeated practice phase as a function of the accuracy on the final test in a "difference due to memory" (Dm) analysis. We found a parietally distributed, increased positive amplitude between 500-700 ms, and a more positive parietal wave between 700 and 1000 ms in the later relative to the early phases of retrieval practice. We found parietal Dm effects in the same two time windows in the retrieval practice condition with a more positive amplitude predicting retrieval success on the final test. We interpret the earlier waveform as a component associated with episodic recollection and the later ERP as a component related to post-retrieval evaluation processes. Our results demonstrate the important role of these retrieval-related processes in the facilitating effect of retrieval practice on later retrieval, and show that the involvement of these processes changes throughout practice.

Electrophysiological evidence for cross-language interference in foreign-language attrition

Foreign language attrition (FLA) appears to be driven by interference from other, more recently-used languages (Mickan et al., 2020). Here we tracked these interference dynamics electrophysiologically to further our understanding of the underlying processes. Twenty-seven Dutch native speakers learned 70 new Italian words over two days. On a third day, EEG was recorded as they performed naming tasks on half of these words in English and, finally, as their memory for all the Italian words was tested in a picture-naming task. Replicating Mickan et al. recall was slower and tended to be less complete for Italian words that were interfered with (i.e., named in English) than for words that were not. These behavioral interference effects were accompanied by an enhanced frontal N2 and a decreased late positivity (LPC) for interfered compared to not-interfered items. Moreover, interfered items elicited more theta power. We also found an increased N2 during the interference phase for items that participants were later slower to retrieve in Italian. We interpret the N2 and theta effects as markers of interference, in line with the idea that Italian retrieval at final test is hampered by competition from recently practiced English translations. The LPC, in turn, reflects the consequences of interference: the reduced accessibility of interfered Italian labels. Finally, that retrieval ease at final test was related to the degree of interference during previous English retrieval shows that FLA is already set in motion during the interference phase, and hence can be the direct consequence of using other languages.

Strategic retrieval prevents memory interference: The temporal dynamics of retrieval orientation

Resolving interference between overlapping memories is crucial to remember the past. This study tests the novel prediction that orienting search focus benefits goal-relevant retrieval by reducing competition from unwanted memories. In a modified retrieval-practice paradigm, participants encoded word-pairs in one of two encoding tasks. Critically, to evaluate whether this retrieval orientation (RO) reduces memory interference, target and competitor memories were always related to different encoding tasks. At retrieval, instructions were provided for half of the blocks with the intention to bias remembering towards items encoded with one of the ROs. Behavioural data show that adopting an RO improved target accessibility, strengthened the testing effect, and reduced retrieval-induced forgetting (RIF) of competitors. Specifically, RIF - typically attributed to inhibitory control of memory interference - was prominent when no retrieval orientation (NRO) instruction was provided. Furthermore, a neural correlate of RO was calculated by training a linear discriminant analysis (LDA) to discriminate the electroencephalographic (EEG) spatial brain patterns correspondent to the two ROs over the time course of selective retrieval. RO was characterised by increases in the theta and decreases in the beta frequency band, evident both before and after category-cue onset. While the pre-cue RO reinstatement effect predicted both immediate retrieval-practice success and later target accessibility, the post-cue effect predicted disengagement of inhibitory control, such that participants showing a stronger RO reinstatement effect showed lower levels of RIF. These data suggest that strategically orienting search focus during retrieval both increases target memory accessibility and reduces memory interference, which consequently protects related memories from inhibition and later forgetting. Furthermore, they also highlight the roles of theta and beta oscillations in establishing and maintaining a task-relevant bias towards target memory representations during competitive memory retrieval.

Active Forgetting: Adaptation of Memory by Prefrontal Control

Over the past century, psychologists have discussed whether forgetting might arise from active mechanisms that promote memory loss to achieve various functions, such as minimizing errors, facilitating learning, or regulating one's emotional state. The past decade has witnessed a great expansion in knowledge about the brain mechanisms underlying active forgetting in its varying forms. A core discovery concerns the role of the prefrontal cortex in exerting top-down control over mnemonic activity in the hippocampus and other brain structures, often via inhibitory control. New findings reveal that such processes not only induce forgetting of specific memories but also can suppress the operation of mnemonic processes more broadly, triggering windows of anterograde and retrograde amnesia in healthy people. Recent work extends active forgetting to nonhuman animals, presaging the development of a multilevel mechanistic account that spans the cognitive, systems, network, and even cellular levels. This work reveals how organisms adapt their memories to their cognitive and emotional goals and has implications for understanding vulnerability to psychiatric disorders.

Increases in theta CSD power and coherence during a calibrated stop-signal task: implications for goal-conflict processing and the Behavioural Inhibition System

Psychologists have identified multiple different forms of conflict, such as information processing conflict and goal conflict. As such, there is a need to examine the similarities and differences in neurology between each form of conflict. To address this, we conducted a comprehensive electroencephalogram (EEG) analysis of Shadli, Glue, McIntosh, and McNaughton’s calibrated stop-signal task (SST) goal-conflict task. Specifically, we examined changes in scalp-wide current source density (CSD) power and coherence across a wide range of frequency bands during the calibrated SST (n = 34). We assessed differences in EEG between the high and low goal-conflict conditions using hierarchical analyses of variance (ANOVAs). We also related goal-conflict EEG to trait anxiety, neuroticism, Behavioural Inhibition System (BIS)-anxiety and revised BIS (rBIS) using regression analyses. We found that changes in CSD power during goal conflict were limited to increased midfrontocentral theta. Conversely, coherence increased across 23 scalp-wide theta region pairs and one frontal delta region pair. Finally, scalp-wide theta significantly predicted trait neuroticism but not trait anxiety, BIS-anxiety or rBIS. We conclude that goal conflict involves increased midfrontocentral CSD theta power and scalp-wide theta-dominated coherence. Therefore, compared with information processing conflict, goal conflict displays a similar EEG power profile of midfrontocentral theta but a much wider coherence profile. Furthermore, the increases in theta during goal conflict are the characteristic of BIS-driven activity. Therefore, future research should confirm whether these goal-conflict effects are driven by the BIS by examining whether the effects are attenuated by anxiolytic drugs. Overall, we have identified a unique network of goal-conflict EEG during the calibrated SST.

Theta oscillations show impaired interference detection in older adults during selective memory retrieval

Seemingly effortless tasks, such as recognizing faces and retrieving names, become harder as we age. Such difficulties may be due to the competition generated in memory by irrelevant information that comes to mind when trying to recall a specific face or name. It is unknown, however, whether age-related struggles in retrieving these representations stem from an inability to detect competition in the first place, or from being unable to suppress competing information once interference is detected. To investigate this, we used the retrieval practice paradigm, shown to elicit memory interference, while recording electrophysiological activity in young and older adults. In two experiments, young participants showed Retrieval-Induced Forgetting (RIF), reflecting the suppression of competing information, whereas older adults did not. Neurally, mid-frontal theta power (~4-8 Hz) during the first retrieval cycle, a proxy for interference detection, increased in young compared to older adults, indicating older adults were less capable of detecting interference. Moreover, while theta power was reduced across practice cycles in younger adults, a measure of interference resolution, older adults did not show such a reduction. Thus, in contrast with younger adults, the lack of an early interference detection signal rendered older adults unable to recruit memory selection mechanisms, eliminating RIF.

Dopamine Modulates Delta-Gamma Phase-Amplitude Coupling in the Prefrontal Cortex of Behaving Rats

Dopamine release and phase-amplitude cross-frequency coupling (CFC) have independently been implicated in prefrontal cortex (PFC) functioning. To causally investigate whether dopamine release affects phase-amplitude comodulation between different frequencies in local field potentials (LFP) recorded from the medial PFC (mPFC) of behaving rats, we used RuBiDopa, a light-sensitive caged compound that releases the neurotransmitter dopamine when irradiated with visible light. LFP power did not change in any frequency band after the application of light-uncaged dopamine, but significantly strengthened phase-amplitude comodulation between delta and gamma oscillations. Saline did not exert significant changes, while injections of dopamine and RuBiDopa produced a slow increase in comodulation for several minutes after the injection. The results show that dopamine release in the medial PFC shifts phase-amplitude comodulation from theta-gamma to delta-gamma. Although being preliminary results due to the limitation of the low number of animals present in this study, our findings suggest that dopamine-mediated modification of the frequencies involved in comodulation could be a mechanism by which this neurotransmitter regulates functioning in mPFC.

Competitors or Teammates

Research has documented that proper names are more difficult to learn and remember than other types of words. Various causes of this difficulty have been proposed to better understand how proper names are represented in memory and the degree to which names compete with each other. In the retrieval of names, some studies show competition, whereas other studies find facilitation. During comprehension, names demonstrate competition by causing a Moses illusion: People erroneously answer invalid questions such as “How many animals did Moses take on the ark?”, failing to detect that Noah is the correct name for the question. Errors in both name retrieval and comprehension are more likely when the correct name and distractor name sound similar, share biographical characteristics, or have some visual resemblance. However, shared visual information has played a competitive role more consistently in name comprehension than retrieval, an asymmetry that remains to be investigated.

Interference resolution in face perception and name retrieval

Selective retrieval is a rather difficult task, and especially so when one attempts to retrieve personal representations such as faces or names. Retrieval of memories under strong competition conditions is pervasive in human memory and some have suggested that inhibitory control is used to overcome interference between competing stimuli. In the present study, we used the retrieval practice paradigm to investigate if competition among personal representations (such as facial features and names) is also resolved by inhibitory mechanisms. This question is theoretically relevant, since personal representations have been said to have a special status on cognition. Moreover, some models of face recognition assume that interference can arise between different representations, but that this interference would be automatically and rapidly solved, with no need for a controlled inhibitory mechanism to act. In two experiments we showed RIF for facial features and familiar names, but only when participants had to actively retrieve some information. This suggests that personal information is subject to mechanisms of inhibitory control, which could help explain everyday life difficulties in processes such as face feature recognition or name retrieval.