Neural correlates of explicit and implicit memory at encoding and retrieval: A unified framework and meta-analysis of functional neuroimaging studies

Differential effects of bilateral hippocampal CA3 damage on the implicit learning and recognition of complex event sequences

Learning regularities in the environment is a fundament of human cognition, which is supported by a network of brain regions that include the hippocampus. In two experiments, we assessed the effects of selective bilateral damage to human hippocampal subregion CA3, which was associated with autobiographical episodic amnesia extending ~50 years prior to the damage, on the ability to recognize complex, deterministic event sequences presented either in a spatial or a non-spatial configuration. In contrast to findings from related paradigms, modalities, and homologue species, hippocampal damage did not preclude recognition memory for an event sequence studied and tested at four spatial locations, whereas recognition memory for an event sequence presented at a single location was at chance. In two additional experiments, recognition memory for novel single-items was intact, whereas the ability to recognize novel single-items in a different location from that presented at study was at chance. The results are at variance with a general role of the hippocampus in the learning and recognition of complex event sequences based on non-adjacent spatial and temporal dependencies. We discuss the impact of the results on established theoretical accounts of the hippocampal contributions to implicit sequence learning and episodic memory.

Neural correlates of musical familiarity: a functional magnetic resonance study

Existing neuroimaging studies on neural correlates of musical familiarity often employ a familiar vs. unfamiliar contrast analysis. This singular analytical approach reveals associations between explicit musical memory and musical familiarity. However, is the neural activity associated with musical familiarity solely related to explicit musical memory, or could it also be related to implicit musical memory? To address this, we presented 130 song excerpts of varying familiarity to 21 participants. While acquiring their brain activity using functional magnetic resonance imaging (fMRI), we asked the participants to rate the familiarity of each song on a five-point scale. To comprehensively analyze the neural correlates of musical familiarity, we examined it from four perspectives: the intensity of local neural activity, patterns of local neural activity, global neural activity patterns, and functional connectivity. The results from these four approaches were consistent and revealed that musical familiarity is related to the activity of both explicit and implicit musical memory networks. Our findings suggest that: (1) musical familiarity is also associated with implicit musical memory, and (2) there is a cooperative and competitive interaction between the two types of musical memory in the perception of music.

Neurofunctional underpinnings of individual differences in visual episodic memory performance

Episodic memory, the ability to consciously recollect information and its context, varies substantially among individuals. While prior fMRI studies have identified certain brain regions linked to successful memory encoding at a group level, their role in explaining individual memory differences remains largely unexplored. Here, we analyze fMRI data of 1,498 adults participating in a picture encoding task in a single MRI scanner. We find that individual differences in responsivity of the hippocampus, orbitofrontal cortex, and posterior cingulate cortex account for individual variability in episodic memory performance. While these regions also emerge in our group-level analysis, other regions, predominantly within the lateral occipital cortex, are related to successful memory encoding but not to individual memory variation. Furthermore, our network-based approach reveals a link between the responsivity of nine functional connectivity networks and individual memory variability. Our work provides insights into the neurofunctional correlates of individual differences in visual episodic memory performance.

Conscious awareness and memory systems in the brain

The term "memory" typically refers to conscious retrieval of events and experiences from our past, but experience can also change our behaviour without corresponding awareness of the learning process or the associated outcome. Based primarily on early neuropsychological work, theoretical perspectives have distinguished between conscious memory, said to depend critically on structures in the medial temporal lobe (MTL), and a collection of performance-based memories that do not. The most influential of these memory systems perspectives, the declarative memory theory, continues to be a mainstay of scientific work today despite mounting evidence suggesting that contributions of MTL structures go beyond the kinds or types of memory that can be explicitly reported. Consistent with these reports, more recent perspectives have focused increasingly on the processing operations supported by particular brain regions and the qualities or characteristics of resulting representations whether memory is expressed with or without awareness. These alternatives to the standard model generally converge on two key points. First, the hippocampus is critical for relational memory binding and representation even without awareness and, second, there may be little difference between some types of priming and explicit, familiarity-based recognition. Here, we examine the evolution of memory systems perspectives and critically evaluate scientific evidence that has challenged the status quo. Along the way, we highlight some of the challenges that researchers encounter in the context of this work, which can be contentious, and describe innovative methods that have been used to examine unconscious memory in the lab. This article is categorized under: Psychology > Memory Psychology > Theory and Methods Philosophy > Consciousness.

Spurious Autobiographical Memory of Psychosis: A Mechanistic Hypothesis for the Resolution, Persistence, and Recurrence of Positive Symptoms in Psychotic Disorders

Psychotic disorders are complex disorders with multiple etiologies. While increased dopamine synthesis capacity has been proposed to underlie psychotic episodes, dopamine-independent processes are also involved (less responsive to dopamine receptor-blocking medications). The underlying mechanism(s) of the reduction in antipsychotic responsiveness over time, especially after repeated relapses, remain unclear. Despite the consistent evidence of dopamine overactivity and hippocampal volume loss in schizophrenia, few accounts have been provided based on the interactive effect of dopamine on hippocampal synapse plasticity mediating autobiographical memory processes. The present hypothesis builds upon previous works showing the potential effects of dopamine overactivity on hippocampal-mediated neuroplasticity underlying autobiographical memory, alongside known patterns of autobiographical memory dysfunction in psychosis. We propose that spurious autobiographical memory of psychosis (SAMP) produced during active psychosis may be a key mechanism mediating relapses and treatment non-responsiveness. In a hyperdopaminergic state, SAMP is expected to be generated at an increased rate during active psychosis. Similar to other memories, it will undergo assimilation, accommodation, and extinction processes. However, if SAMP fails to integrate with existing memory, a discontinuity in autobiographical memory may result. Inadequate exposure to normalizing experiences and hyposalience due to overmedication or negative symptoms may also impede the resolution of SAMP. Residual SAMP is hypothesized to increase the propensity for relapse and treatment non-responsiveness. Based on recent findings on the role of dopamine in facilitating hippocampal synapse plasticity and autobiographical memory formation, the SAMP hypothesis is consistent with clinical observations of DUP effects, including the repetition of contents in psychotic relapses as well as the emergence of treatment non-responsiveness after repeated relapses. Clinical implications of the hypothesis highlight the importance of minimizing active psychosis, integrating psychosis memory, avoiding over-medication, and fostering normalizing experiences.

Neurocognitive effects of binge drinking on verbal episodic memory. An ERP study in university students

Background: Verbal memory may be affected by engagement in alcohol binge drinking during youth, according to the findings of neuropsychological studies. However, little is known about the dynamics of the neural activity underlying this cognitive process in young, heavy drinkers. Aims: To investigate brain event-related potentials associated with cued recall from episodic memory in binge drinkers and controls. Methods: Seventy first-year university students were classified as binge drinkers (32: 17 female) or controls (38: 18 female). The participants completed a verbal paired associates learning task during electroencephalogram (EEG) recording. ERPs elicited by old and new word pairs were extracted from the cued-recall phase of the task by using Principal Component Analysis. Subjects also performed a standardized neuropsychological verbal learning test. Results: Two of the three event-related potentials components indicating old/new memory effects provided evidence for anomalies associated with binge drinking. The old/new effects were absent in the binge drinkers in the two subsequent posterior components, identified with the late parietal component and the late posterior negativity The late frontal component revealed similar old/new effects in both groups. Binge drinkers showed similar behavioural performance to controls in the verbal paired associates task, but performed poorly in the more demanding short-term cued-recall trial of a neuropsychological standardized test. Conclusion: Event-related potentials elicited during a verbal cued-recall task revealed differences in brain functioning between young binge drinkers and controls that may underlie emergent deficits in episodic memory linked to alcohol abuse. The brain activity of binge drinkers suggests alterations in the hippocampal - posterior parietal cortex circuitry subserving recognition and recollection of the cue context and generation of the solution, in relation to verbal information shallowly memorised.

Greater interruption of visual processing and memory encoding by visceral than somatic pain in healthy volunteers - An fMRI study

Visceral pain is regarded as more salient than somatic pain. It has greater affective and emotional components, i.e., it elicits higher levels of pain-related fear and is perceived as more unpleasant than somatic pain. In this fMRI study, we examined the neural effects of painful visceral as compared to painful somatic stimulation on visual processing and memory encoding in a visual categorization and surprise recognition task in healthy volunteers. During the categorization task, participants received either rectal distensions or heat stimuli applied to the forearm, with stimuli being individually matched for unpleasantness. Behaviorally, visceral pain reduced memory encoding as compared to somatic pain (Kleine-Borgmann et al., 2021). Imaging analyses now revealed that visceral pain was associated with reduced activity (i.e., greater pain-related interruption) in neural areas typically involved in visual processing and memory encoding. These include the parahippocampal gyrus, fusiform gyrus, striatum, occipital cortex, insula, and the amygdala. Moreover, reduced engagement of the lateral occipital complex during visual categorization under visceral pain was associated with higher visceral pain-related fear. These findings obtained in healthy volunteers shed light on the neural circuitry underlying the interruptive effect of visceral pain and pave the way for future studies in patient samples.

Attention- versus significance-driven memory formation: Taxonomy, neural substrates, and meta-analyses

Functional neuroimaging data on episodic memory formation have expanded rapidly over the last 30 years, which raises the need for an integrative framework. This study proposes a taxonomy of episodic memory formation to address this need. At the broadest level, the taxonomy distinguishes between attention-driven vs. significance-driven memory formation. The three subtypes of attention-driven memory formation are selection-, fluctuation-, and level-related. The three subtypes of significance-driven memory formation are novelty-, emotion-, and reward-related. Meta-analytic data indicated that attention-driven memory formation affects the functioning of the extra-medial temporal lobe more strongly than the medial temporal lobe (MTL) regions. In contrast, significance-driven memory formation affects the functioning of the MTL more strongly than the extra-MTL regions. This study proposed a model in which attention has a stronger impact on the formation of neocortical traces than hippocampus/MTL traces, whereas significance has a stronger impact on the formation of hippocampus/MTL traces than neocortical traces. Overall, the taxonomy and model provide an integrative framework in which to place diverse encoding-related findings into a proper perspective.

Neural reinstatement reveals divided organization of fear and extinction memories in the human brain

Neurobiological research in rodents has revealed that competing experiences of fear and extinction are stored as distinct memory traces in the brain. This divided organization is adaptive for mitigating overgeneralization of fear to related stimuli that are learned to be safe while also maintaining threat associations for unsafe stimuli. The mechanisms involved in organizing these competing memories in the human brain remain unclear. Here, we used a hybrid form of Pavlovian conditioning with an episodic memory component to identify overlapping multivariate patterns of fMRI activity associated with the formation and retrieval of fear versus extinction. In healthy adults, distinct regions of the medial prefrontal cortex (PFC) and hippocampus showed selective reactivation of fear versus extinction memories based on the temporal context in which these memories were encoded. This dissociation was absent in participants with posttraumatic stress disorder (PTSD) symptoms. The divided neural organization of fear and extinction may support flexible retrieval of context-appropriate emotional memories, while their disorganization may promote overgeneralization and increased fear relapse in affective disorders.

Food-Related Brain Activation Measured by fMRI in Adults with Prader-Willi Syndrome

(1) Background: Prader–Willi syndrome (PWS) is characterized by hyperphagia, resulting in morbid obesity if not controlled. The primary aim of this study was to investigate whether PWS patients show altered activation of brain areas involved in hunger. As a secondary objective, we assessed whether there is an association between these brain areas and several endocrine and metabolic factors in the fasting state. (2) Methods: 12 PWS adults and 14 healthy controls (siblings) performed a food-related experimental task after an overnight fast while brain activation in regions of interest was measured by functional MRI. (3) Results: In controls, significantly more activation was found in the left insula (p = 0.004) and the bilateral fusiform gyrus (p = 0.003 and 0.013) when the individuals were watching food as compared to non-food pictures, which was absent in PWS patients. Moreover, in PWS adults watching food versus non-food pictures a significant negative correlation for glucose and right amygdala activation (p_fwe = 0.007) as well as a positive correlation for leptin and right anterior hippocampus/amygdala activation (p_fwe = 0.028) was demonstrated. No significant associations for the other hormonal and metabolic factors were found. (4) Conclusions: PWS individuals show aberrant food-related brain activation in the fasting state. Leptin is associated with activation within the neural motivation/reward circuitry, while the opposite is true for glucose.

Modulating Long Term Memory at Late-Encoding Phase: An rTMS Study

Despite a huge effort of the scientific community, the functioning of Long-Term Memory (LTM) processes is still debated and far from being elucidated. Functional and neurophysiological data point to an involvement of Dorsolateral Prefrontal Cortex (DLPFC) in both encoding and retrieval phases. However, the recently proposed Explicit/Implicit Memory Encoding and Retrieval (EIMER) model proposes that LTM at the encoding phase consists of anatomically and chronologically different sub-phases. On this basis, we aimed to investigate the role of right DLPFC during a late-encoding phase by means of low-frequency rTMS. Thirty right-handed healthy subjects were divided into three experimental groups. Inhibitory rTMS was applied over right-DLPFC immediately after the encoding phase (Late-Encoding Group) or before recognition phase (Pre-Recognition Group), 24 h after, of an LTM task. Both groups also received sham stimulation during the non-target phase, while the third group (Sham Group) received only sham stimulation in both phases. The Late-Encoding Group collected a lower number of correct responses compared with Sham Group (p = 0.00), while Pre-Retrieval Group increased accuracy as compared to the Sham Group (p = 0.0). rTMS-inhibition of the right DLPFC seems able to interfere with LTM memory performances when delivered at a late stage of the encoding phase, with opposite effects at the pre-retrieval phase.

Higher functional connectivity between prefrontal regions and the dorsal attention network predicts absence of renewal

Renewal describes the recovery of an extinguished response when extinction and recall contexts differ, demonstrating the context-dependency of extinction. The unexpected outcome change during extinction presumably directs attention to the context and promotes renewal. Accordingly, studies show that context processing for renewal is modulated by salience of and attention to context. Besides context-processing hippocampus, renewal involves ventromedial prefrontal cortex, orbitofrontal cortex and inferior frontal gyrus, which mediate response processing. Since showing renewal is a trait-like processing tendency, individuals with and without renewal may differ in resting-state functional connectivity of prefrontal regions with networks mediating attentional and salience processing. We analyzed resting-state functional MRI data from healthy participants (n = 70) of a non-fear-related contextual extinction task particularly suited for investigation of renewal. Participants without renewal exhibited significantly higher functional connectivity between prefrontal regions and bilateral intraparietal sulcus of the dorsal attention network. Functional connectivity between these regions correlated negatively with renewal level. Only in participants with renewal, the renewal level correlated positively with connectivity between left frontal eye field and several prefrontal regions. In contrast, functional connectivity of prefrontal regions with the salience network did not differ between groups. The results deliver first-time evidence for differences in resting-state functional connectivity between participants with and without renewal in non-fear-related extinction. Intraparietal-sulcus-guided top-down attentional control appears more strongly related to prefrontal activity in participants without renewal, and thus may have a role in their default processing mode of focusing on the stimulus and disregarding the context.

Imaging recollection, familiarity, and novelty in the frontoparietal control and default mode networks and the anterior-posterior medial temporal lobe: An integrated view and meta-analysis

A network-level model of recollection-based recognition (R), familiarity-based recognition (F), and novelty recognition (N) was constructed, and its validity was evaluated through meta-analyses to produce an integrated view of neuroimaging data. The model predicted the following: (a) the overall magnitude of the frontoparietal control network (FPCN) activity (which supports retrieval and decision effort) is in the order of F > R > N; (b) that of the posterior medial temporal network (MTL) activity (which plays a direct role in retrieval) is in the order of R > N > F; (c) that of the anterior MTL activity (which supports novelty-encoding) is in the order of N > R > F; (d) that of the default mode network (DMN) activity (which supports the subjective experience of remembering) is in the order of R > N > F. The meta-analyses results were consistent with these predictions. Subsystem analysis indicated a functional dissociation between the cingulo-opercular vs. frontoparietal components of the FPCN and between the core vs. medial temporal components of the DMN.

Recallable but not recognizable: The influence of semantic priming in recall paradigms

When people can successfully recall a studied word, they should be able to recognize it as having been studied. In cued-recall paradigms, however, participants sometimes correctly recall words in the presence of strong semantic cues but then fail to recognize those words as actually having been studied. Although the conditions necessary to produce this unusual effect are known, the underlying neural correlates have not been investigated. Across five experiments, involving both behavioral and electrophysiological methods (EEG), we investigated the cognitive and neural processes that underlie recognition failures. Experiments 1 and 2 showed behaviorally that assuming that recalled items can be recognized in cued-recall paradigms is a flawed assumption, because recognition failures occur in the presence of cues, regardless of whether those failures are measured. With event-related potentials (ERPs), Experiments 3 and 4 revealed that successfully recalled words that are recognized are driven by recollection at recall and then by a combination of recollection and familiarity at ensuing recognition. In contrast, recognition failures did not show that memory signature and may instead be driven by semantic priming at recall and followed at recognition stages by negative-going ERP effects consistent with implicit processes, such as repetition fluency. These results demonstrate that recall - long-characterized as predominantly reflecting recollection-based processing in episodic memory - may at times also be served by a confluence of implicit cognitive processes.

The contributions of the left fusiform subregions to successful encoding of novel words

The left fusiform cortex has been identified as a crucial structure in visual word learning and memory. Nevertheless, the specific roles of the fusiform subregions in word memory and their consistency across different writings have not been elaborated. To address these questions, the present study performed two experiments, in which study-test paradigm was used. Participants' brain activity was measured with fMRI while memorizing novel logographic words in Experiment 1 and novel alphabetic words in Experiment 2. A post-scan recognition memory test was then administered to acquire the memory performance. Results showed that, neural responses in the left anterior and middle fusiform subregions during encoding were positively correlated with recognition memory of novel words. Moreover, the positive brain-behavior correlations in the left anterior and middle fusiform cortex were evident for both logographic and alphabetic writings. The present findings clarify the relationship between the left fusiform subregions and novel word memory.

Behavioral and Neural Effects of Familiarization on Object-Background Associations

Associative memory is the ability to link together components of stimuli. Previous evidence suggests that prior familiarization with study items affects the nature of the association between stimuli. More specifically, novel stimuli are learned in a more context-dependent fashion than stimuli that have been encountered previously without the current context. In the current study, we first acquired behavioral data from 62 human participants to conceptually replicate this effect. Participants were instructed to memorize multiple object-scene pairs (study phase) and were then tested on their recognition memory for the objects (test phase). Importantly, 1 day prior, participants had been familiarized with half of the object stimuli. During the test phase, the objects were either matched to the same scene as during study (intact pair) or swapped with a different object’s scene (rearranged pair). Our results conceptually replicated the context-dependency effect by showing that breaking up a studied object-context pairing is more detrimental to object recognition performance for non-familiarized objects than for familiarized objects. Second, we used functional magnetic resonance imaging (fMRI) to determine whether medial temporal lobe encoding-related activity patterns are reflective of this familiarity-related context effect. Data acquired from 25 human participants indicated a larger effect of familiarization on encoding-related hippocampal activity for objects presented within a scene context compared to objects presented alone. Our results showed that both retrieval-related accuracy patterns and hippocampal activation patterns were in line with a familiarization-mediated context-dependency effect.

Effective Connectivity for Default Mode Network Analysis of Alcoholism

Introduction: With the recent technical advances in brain imaging modalities such as magnetic resonance imaging, positron emission tomography, and functional magnetic resonance imaging (fMRI), researchers' interests have inclined over the years to study brain functions through the analysis of the variations in the statistical dependence among various brain regions. Through its wide use in studying brain connectivity, the low temporal resolution of the fMRI represented by the limited number of samples per second, in addition to its dependence on brain slow hemodynamic changes, makes it of limited capability in studying the fast underlying neural processes during information exchange between brain regions. Materials and Methods: In this article, the high temporal resolution of the electroencephalography (EEG) is utilized to estimate the effective connectivity within the default mode network (DMN). The EEG data are collected from 20 subjects with alcoholism and 25 healthy subjects (controls), and used to obtain the effective connectivity diagram of the DMN using the Partial Directed Coherence algorithm. Results: The resulting effective connectivity diagram within the DMN shows the unidirectional causal effect of each region on the other. The variations in the causal effects within the DMN between controls and alcoholics show clear correlation with the symptoms that are usually associated with alcoholism, such as cognitive and memory impairments, executive control, and attention deficiency. The correlation between the exchanged causal effects within the DMN and symptoms related to alcoholism is discussed and properly analyzed. Conclusion: The establishment of the causal differences between control and alcoholic subjects within the DMN regions provides valuable insight into the mechanism by which alcohol modulates our cognitive and executive functions and creates better possibility for effective treatment of alcohol use disorder.

Neural Correlates of Repetition Priming: A Coordinate-Based Meta-Analysis of fMRI Studies

Repetition priming is a form of implicit memory, whereby classification or identification of a stimulus is improved by prior presentation of the same stimulus. Repetition priming is accompanied with a deceased fMRI signal for primed vs. unprimed stimuli in various brain regions, often called "repetition suppression," or RS. Previous studies proposed that RS in posterior regions is associated with priming of perceptual processes, whereas RS in more anterior (prefrontal) regions is associated with priming of conceptual processes. To clarify which regions exhibit reliable RS associated with perceptual and conceptual priming, we conducted a quantitative meta-analysis using coordinate-based activation likelihood estimation. This analysis included 65 fMRI studies that (i) employed visual repetition priming during either perceptual or conceptual tasks, (ii) demonstrated behavioral priming, and (iii) reported the results from whole-brain analyses. Our results showed that repetition priming was mainly associated with RS in left inferior frontal gyrus and fusiform gyrus. Importantly, RS in these regions was found for both perceptual and conceptual tasks, and no regions show RS that was selective to one of these tasks. These results question the simple distinction between conceptual and perceptual priming, and suggest consideration of other factors such as stimulus-response bindings.

Forgotten but not gone: FMRI evidence of implicit memory for negative stimuli 24 hours after the initial study episode

Endel Tulving conducted pioneering work on the explicit and implicit memory systems and demonstrated that priming effects can be long-lasting. It is also well-established that emotion can amplify explicit and implicit memory. Prior work has utilized repetition suppression (RS) of the fMRI-BOLD signal-a reduction in the magnitude of activity over repeated presentations of stimuli-to index implicit memory. Using an explicit recognition memory paradigm, we examined emotional modulation of long-term implicit memory effects as revealed by repetition suppression (i.e., comparing second-exposure forgotten items to first-exposure correct rejections). Forty-seven participants incidentally encoded line-drawings of negative, positive, and neutral scenes followed by the full color image. Twenty-four hours later, participants underwent fMRI during a recognition memory test in which old and new line-drawings were presented. Implicit and explicit memory effects were defined by the contrasts of New-Correct Rejections > Old-Misses and Old-Hits > New-Correct Rejections, respectively. Wide-spread Negative RS was found in frontal and occipito-temporal cortex that was greater than Neutral RS in the right orbito-frontal cortex and inferior frontal gyri. Valence-specific Negative RS, compared to Positive RS, was observed in the left inferior occipital gyrus. There was no strong evidence for emotional modulation of amygdala RS, but functional connectivity analyses revealed valence-specificity: Negative and positive valence were associated with repetition suppression and repetition enhancement of amygdala-occipital connectivity, respectively. Negative implicit memory patterns in most frontal regions-but not occipital areas-overlapped with explicit memory effects. Thus, implicit memory effects for a single visual stimulus presentation are modulated by emotional valence, can be observed 24hours after initial exposure, and show some overlap with explicit memory.

Neural activity during working memory encoding, maintenance, and retrieval: A network-based model and meta-analysis

It remains unclear whether and to what extent working memory (WM) temporal subprocesses (i.e., encoding, maintenance, and retrieval) involve shared or distinct intrinsic networks. To address this issue, I constructed a model of intrinsic network contributions to different WM phases and then evaluated the validity of the model by performing a quantitative meta-analysis of relevant functional neuroimaging data. The model suggests that the transition from the encoding to maintenance and to retrieval stages involves progressively decreasing involvement of the dorsal attention network (DAN), but progressively increasing involvement of the frontoparietal control network (FPCN). Separate meta-analysis of each phase effect and direct comparisons between them yielded results that were largely consistent with the model. This evidence included between-phase double dissociations that were consistent with the model, such as encoding > maintenance contrast showing some DAN, but no FPCN, regions, and maintenance > encoding contrast showing the reverse, that is, some FPCN, but no DAN, regions. Two closely juxtaposed regions that are members of the DAN and FPCN, such as inferior frontal junction versus caudal prefrontal cortex and superior versus inferior intraparietal sulcus, showed a high degree of functional differentiation. Although all regions identified in the present study were already identified in previous WM studies, this study uniquely enhances our understating of their roles by clarifying their network membership and specific associations with different WM phases.