Material-common and material-specific neural activity during encoding of words and scenes: A neuroimaging meta-analysis

This study examined the extent to which neural activity during memory encoding demonstrates material-commonness or material-specificity. A meta-analysis of functional magnetic resonance imaging studies was conducted to compare the brain regions associated with subsequent memory effects for word and scene stimuli. The main results were as follows. First, significant subsequent memory effects for both words and scenes were primarily observed within the dorsal attention network. This finding aligns with the perspective that temporal fluctuations in attention modulate the intensity of encoding activity, influencing the success and failure of encoding. Second, multiple prefrontal cortex regions, particularly the left inferior frontal cortex, exhibited stronger subsequent memory effects for words compared to scenes. Conversely, multiple visual processing regions revealed an opposite pattern, with heightened subsequent memory effects for scenes relative to words. These findings suggest that words are more strongly encoded through semantic processing, whereas scenes are primarily encoded through visuo-perceptual processing. In conclusion, these results clarify the material specificity and commonness of encoding-related neural activity, emphasizing the significant role of attention and the distinctions between verbal and pictorial information.

From vision to memory: How scene-sensitive regions support episodic memory formation during child development

Previous brain imaging studies have identified three brain regions that selectively respond to visual scenes, the parahippocampal place area (PPA), the occipital place area (OPA), and the retrosplenial cortex (RSC). There is growing evidence that these visual scene-sensitive regions process different types of scene information and may have different developmental timelines in supporting scene perception. How these scene-sensitive regions support memory functions during child development is largely unknown. We investigated PPA, OPA and RSC activations associated with episodic memory formation in childhood (5-7 years of age) and young adulthood, using a subsequent scene memory paradigm and a functional localizer for scenes. PPA, OPA, and RSC subsequent memory activation and functional connectivity differed between children and adults. Subsequent memory effects were found in activations of all three scene regions in adults. In children, however, robust subsequent memory effects were only found in the PPA. Functional connectivity during successful encoding was significant among the three regions in adults, but not in children. PPA subsequently memory activations and PPA-RSC subsequent memory functional connectivity correlated with accuracy in adults, but not children. These age-related differences add new evidence linking protracted development of the scene-sensitive regions to the protracted development of episodic memory.

Representing the Good and Bad: fMRI signatures during the encoding of multisensory positive, negative, and neutral events

Few studies have examined how multisensory emotional experiences are processed and encoded into memory. Here, we aimed to determine whether, at encoding, activity within functionally-defined visual- and auditory-processing brain regions discriminated the emotional category (i.e., positive, negative, or neutral) of the multisensory (audio-visual) events. Participants incidentally encoded positive, negative, and neutral multisensory stimuli during event-related functional magnetic resonance imaging (fMRI). Following a 3-h post-encoding delay, their memory for studied stimuli was tested, allowing us to identify emotion-category-specific subsequent-memory effects focusing on medial temporal lobe regions (i.e., amygdala, hippocampus) and visual- and auditory-processing regions. We used a combination of univariate and multivoxel pattern fMRI analyses (MVPA) to examine emotion-category-specificity in mean activity levels and neural patterning, respectively. Univariate analyses revealed many more visual regions that showed negative-category-specificity relative to positive-category-specificity, and auditory regions only showed negative-category-specificity. These results suggest that negative emotion is more closely tied to information contained within sensory regions, a conclusion that was supported by the MVPA analyses. Functional connectivity analyses further revealed that the visual amplification of category-selective processing is driven, in part, by mean signal from the amygdala. Interestingly, while stronger representations in visuo-auditory regions were related to subsequent-memory for neutral multisensory stimuli, they were related to subsequent-forgetting of positive and negative stimuli. Neural patterning in the hippocampus and amygdala were related to memory for negative multisensory stimuli. These results provide new evidence that negative emotional stimuli are processed with increased engagement of visuosensory regions, but that this sensory engagement-that generalizes across the entire emotion category-is not the type of sensory encoding that is most beneficial for later retrieval.

Functional activation features of memory in successful agers across the adult lifespan

Much neuroimaging research has explored the neural mechanisms underlying successful cognitive aging. Two different patterns of functional activation, maintenance of youth-like activity and compensatory novel recruitment, have been proposed to represent different brain functional features underlying individual differences in cognitive aging. In this study, we investigated the functional features in individuals across the adult lifespan who appeared to resist age-related cognitive decline, in comparison to those with typical age-related declines, over the course of four years. We first implemented latent mixture modeling, a data-driven approach, to classify participants as successful and average agers in middle-aged, young-old, and very old groups, based on their baseline and longitudinal cognitive performance. Then, using fMRI with a subsequent memory paradigm at the follow-up visit, brain activation specifically related to successful encoding (i.e., subsequent memory effect: subsequently remembered with high confidence > subsequently forgotten) was compared between people who established successful cognitive aging versus average aging in the three age groups. Several differences in the subsequent memory effect were revealed. First, across core task-related regions commonly used during successful encoding, successful agers exhibited high subsequent memory effect, at a level comparable to the young control group, until very old age; in contrast, average agers showed reduced subsequent memory effect, compared to successful agers, beginning in young-old age when memory performance also reduced in average agers, compared to successful agers. Second, additional recruitment in prefrontal clusters, distant from the core task-related regions, were identified in the left superior frontal and right orbitofrontal cortices in successful agers of young-old age, possibly reflecting functional compensation in successful aging. In summary, successful agers demonstrate a pattern of youth-like activation spanning from middle age to young-old age, as well as novel frontal recruitment in young-old age. Overall, our study demonstrated evidence of two neural patterns related to successful cognitive aging, offering an integrated view of functional features underlying successful aging, and suggests the importance of studying individuals across the lifespan to understand brain changes occurring in mid and early-late life.

Reliability of subsequent memory effects in children and adults: The good, the bad, and the hopeful

Functional MRI (fMRI) is a key tool for investigating neural underpinnings of cognitive development. Yet, in recent years, the reliability of fMRI effects has come into question and with it, the feasibility of using task-based fMRI to identify developmental changes related to cognition. Here, we investigated the reliability of task-based fMRI activations with a widely used subsequent memory paradigm using two developmental samples: a cross-sectional sample (n = 85, age 8–25 years) and a test-retest sample (n = 24, one-month follow up, age 8–20 years). In the large cross-sectional sample, we found good to excellent group-level reliability when assessing activation patterns related to the encoding task and subsequent memory effects. In the test-retest sample, while group-level reliability was excellent, the consistency of activation patterns within individuals was low, particularly for subsequent memory effects. We observed consistent activation patterns in frontal, parietal, and occipital cortices, but comparatively lower test-retest reliability in subcortical regions and the hippocampus. Together, these findings highlight the limitations of interpreting task-based fMRI effects and the importance of incorporating reliability analyses in developmental studies. Leveraging larger and densely collected longitudinal data may help contribute to increased reproducibility and the accumulation of knowledge in developmental sciences.

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Reliability of fMRI memory encoding was assessed in cross-sectional and test-retest developmental samples.

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Subsequent memory effects were less consistent than activations across all encoding trials, regardless of memory outcome.

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Group-level effects showed excellent consistency in both cross-sectional and test-retest reliability analyses.

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Individual test-retest reliability was good in cortical regions but lower in subcortical regions and the hippocampus.

Age moderation of the association between negative subsequent memory effects and episodic memory performance

Negative subsequent memory effects in functional MRI studies of memory formation have been linked to individual differences in memory performance, yet the effect of age on this association is currently unclear. To provide insight into the brain systems related to memory across the lifespan, we examined functional neuroimaging data acquired during episodic memory formation and behavioral performance from a memory recognition task in a sample of 109 participants, including three developmental age groups (8-12, 13-17, 18-25 year-olds) and one additional group of older adults (55-85 year-olds). Young adults showed the highest memory performance and strongest negative subsequent memory effects, while older adults showed reduced negative subsequent memory effects relative to young adults. Across the sample, negative subsequent memory effects were associated with better memory performance, and there was a significant interaction between negative subsequent memory effects and memory performance by age group. Posthoc analyses revealed that this moderation effect was driven by a stronger association between negative subsequent memory effects and memory performance in young adults than children, and that neither children nor older adults showed a significant association. These findings suggest that negative subsequent memory effects may differentially support memory performance across a lifespan trajectory characterized by developmental maturation and support further investigation of this effect in aging.

Whole-brain functional correlates of memory formation in mesial temporal lobe epilepsy

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Large study of encoding and subsequent memory for words, faces, and scenes.

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Less ipsilateral mesial temporal activity in mesial temporal lobe epilepsy (mTLE).

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Extra-mTL activity in mTLE only partly relevant for memory formation.

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Across materials contralateral mTL decisive to maintain intact memory in mTLE.

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Left frontal activation correlates with better verbal memory only in left mTLE.

The mesial temporal lobe is a key region for episodic memory. Accordingly, memory impairment is frequent in patients with mesial temporal lobe epilepsy. However, the functional relevance of potentially epilepsy-induced reorganisation for memory formation is still not entirely clear. Therefore, we investigated whole-brain functional correlates of verbal and non-verbal memory encoding and subsequent memory formation in 56 (25 right sided) mesial temporal lobe epilepsy patients and 21 controls. We applied an fMRI task of learning scenes, faces, and words followed by an out-of-scanner recognition test. During encoding of faces and scenes left and right mesial temporal lobe epilepsy patients had consistently reduced activation in the epileptogenic mesial temporal lobe compared with controls. Activation increases in patients were apparent in extra-temporal regions, partly associated with subsequent memory formation (left frontal regions and basal ganglia), and patients had less deactivation in regions often linked to the default mode and auditory networks. The more specific subsequent memory contrast indicated only marginal group differences. Correlating patients’ encoding activation with memory performance both within the paradigm and with independent clinical measures demonstrated predominantly increased contralateral mesio-temporal activation supporting intact memory performance. In left temporal lobe epilepsy patients, left frontal activation was also correlated with better verbal memory performance. Taken together, our findings hint towards minor extra-temporal plasticity in mesial temporal lobe epilepsy patients, which is in line with pre-surgical impairment and post-surgical memory decline in many patients. Further, data underscore the importance of particularly the contralateral mesial temporal lobe itself, to maintain intact memory performance.

Attention control processes that prioritise task execution may come at the expense of incidental memory encoding

Attention underpins episodic memory encoding by gating information processing. However, it is unclear how different forms of attention affect encoding. Using fMRI, we implemented a novel task that separates top-down and bottom-up attention (TDA; BUA) to test how these forms of attention influence encoding. Twenty-seven subjects carried out a scanned incidental encoding task that required semantic categorisation of stimuli. Trials either required visual search (TDA) to locate a target, or the target blinked and captured attention (BUA). After a retention period, subjects performed a surprise recognition test. Univariate analyses showed that ventral visual regions and right hippocampus indexed encoding success. Psychophysiological interaction analyses showed that, during TDA, there was increased coupling between dorsal parietal cortex and fusiform gyrus with encoding failure, and between lateral occipital cortex and fusiform gyrus with encoding success. No significant connectivity modulations were observed during BUA. We propose that increased TDA to objects in space is mediated by parietal cortex and negatively impacts encoding. Also, increases in connectivity within ventral visual cortex index the integration of stimulus features, promoting encoding. Finally, the influences of attention on encoding likely depend on task demands: as cognitive control increases, task execution is emphasised at the expense of memory encoding.

Effects of distinctiveness and memory on lateralized and unlateralized brain-electrical activity during visual word encoding

The present study investigated a recently introduced left-lateralized component in the event-related potential (ERP), the posterior semantic asymmetry (PSA), in the context of an isolation paradigm. The PSA is a relative negativity that is most pronounced at temporoparietal electrodes, peaks around 300 ms, and is assumed to reflect early semantic processing of visual words. A free-recall, word-list-learning paradigm was conducted. The learning list comprised two stimuli which were physically isolated from the other stimuli (by different font size or different typeface). The typical behavioral isolation effect with higher recall for isolated stimuli was observed. Furthermore, ERP effects of stimulus type and subsequent memory were analyzed. A left-lateralized negativity that matched the topography of the PSA but occurred somewhat later showed an effect of stimulus distinctiveness, with increased amplitudes for isolates, thus suggesting their deeper semantic processing. However, PSA amplitude did not predict subsequent recall. Unlateralized ERPs replicated previous findings of a greater late frontal positivity during elaborated encoding of both isolated stimuli and subsequently recalled stimuli. This recall effect was greater for isolated than standard stimuli. We argue that physical distinctiveness during encoding facilitates recall to the extent that it promotes the frontally-mediated processes that predict better recall in general.

Mind-wandering and task stimuli: Stimulus-dependent thoughts influence performance on memory tasks and are more often past- versus future-oriented

Although many studies have indicated that participants frequently mind-wander during experimental tasks, relatively little research has examined the extent to which such thoughts are triggered by task stimuli (stimulus-dependent thoughts; SDTs) versus internally triggered (stimulus-independent thoughts; SITs). In the current experiment, we assessed differences in the frequency and characteristics of SDTs and SITs, as well as their associations with subsequent memory in young adults. Whereas frequency of SDTs (but not SITs) increased in a task with more meaningful stimuli, frequency of SITs (but not SDTs) increased in an easier task. Furthermore, only SDTs were more likely to be past- versus future-oriented. Finally, frequency and vividness of SDTs during a shallow, but not a deep, incidental encoding task both correlated with later memory performance for word stimuli. These results suggest that SDTs differ from SITs in several important ways.

Neural activation patterns of successful episodic encoding: Reorganization during childhood, maintenance in old age

The two-component framework of episodic memory (EM) development posits that the contributions of medial temporal lobe (MTL) and prefrontal cortex (PFC) to successful encoding differ across the lifespan. To test the framework’s hypotheses, we compared subsequent memory effects (SME) of 10–12 year-old children, younger adults, and older adults using functional magnetic resonance imaging (fMRI). Memory was probed by cued recall, and SME were defined as regional activation differences during encoding between subsequently correctly recalled versus omitted items. In MTL areas, children’s SME did not differ in magnitude from those of younger and older adults. In contrast, children’s SME in PFC were weaker than the corresponding SME in younger and older adults, in line with the hypothesis that PFC contributes less to successful encoding in childhood. Differences in SME between younger and older adults were negligible. The present results suggest that, among individuals with high memory functioning, the neural circuitry contributing to successful episodic encoding is reorganized from middle childhood to adulthood. Successful episodic encoding in later adulthood, however, is characterized by the ability to maintain the activation patterns that emerged in young adulthood.

Neural substrates of successful working memory and long-term memory formation in a relational spatial memory task

Working memory (WM) tasks may involve brain activation actually implicated in long-term memory (LTM). In order to disentangle these two memory systems, we employed a combined WM/LTM task, using a spatial relational (object-location) memory paradigm and analyzed which brain areas were associated with successful performance for either task using fMRI. Critically, we corrected for the performance on the respective memory task when analyzing subsequent memory effects. The WM task consisted of a delayed-match-to-sample task assessed in an MRI scanner. Each trial consisted of an indoor or outdoor scene in which the exact configuration of four objects had to be remembered. After a short delay (7–13 s), the scene was presented from a different angle and spatial recognition for two objects was tested. After scanning, participants received an unexpected subsequent recognition memory (LTM) task, where the two previously unprobed objects were tested. Brain activity during encoding, delay phase and probe phase was analyzed based on WM and LTM performance. Results showed that successful WM performance, when corrected for LTM performance, was associated with greater activation in the inferior frontal gyrus and left fusiform gyrus during the early stage of the maintenance phase. A correct decision during the WM probe was accompanied by greater activation in a wide network, including bilateral hippocampus, right superior parietal gyrus and bilateral insula. No voxels exhibited supra-threshold activity during the encoding phase, and we did not find any differential activity for correct versus incorrect trials in the WM task when comparing LTM correct versus LTM incorrect trials.

Decoupling of large-scale brain networks supports the consolidation of durable episodic memories

At a large scale, the human brain is organized into modules of interconnected regions, some of which play opposing roles in supporting cognition. In particular, the Default-Mode Network (DMN) has been linked to operations on internal representations, while task-positive networks are recruited during interactions with the external world. Here, we test the hypothesis that the generation of durable long-term memories depends on optimal recruitment of such antagonistic large-scale networks. As long-term memory consolidation is a process ongoing for days and weeks after an experience, we propose that individuals characterized by strong decoupling of the DMN and task-positive networks at rest operate in a mode beneficial for the long-term stabilization of episodic memories. To capture network connectivity unaffected by transient task demands and representative of brain behavior outside an experimental setting, 87 participants were scanned during rest before performing an associative encoding task. To link individual resting-state functional connectivity patterns to time-dependent memory consolidation processes, participants were given an unannounced memory test, either after a brief interval or after a retention period of ~6 weeks. We found that participants with a resting state characterized by high synchronicity in a DMN-centered network system and low synchronicity between task-positive networks showed superior recollection weeks after encoding. These relationships were not observed for information probed only hours after encoding. Furthermore, the two network systems were found to be anticorrelated. Our results suggest that this memory-relevant antagonism between DMN and task-positive networks is maintained through complex regulatory interactions between the systems.

Neural mechanisms of the spacing effect in episodic memory: A parallel EEG and fMRI study

Although behavioral studies have consistently reported the spacing effect in learning, its cognitive and neural mechanisms are still not clearly elucidated. According to the storage/retrieval strength framework proposed by Bjork (1999; Bjork & Bjork, 1992), which was built on the study-phase retrieval hypothesis and the deficient processing hypothesis, the spacing effect is achieved by reducing memomtary retrieval strength during subsequent repetitions and thus enhancing storage strength. The present study tested this hypothesis with parallel fMRI and EEG. Participants were asked to study 180 words that were repeated three times, with half of the words having an inter-repetition-lag of 1-3 words (i.e., massed learning) and the other half an inter-repetition-lag of 25-35 words (i.e., spaced learning). An unexpected recognition test was administered 24 h after learning. Consistent with Bjork's hypothesis, the EEG data suggested that spaced learning was associated with weaker retrieval strength, as indicated by a reduced familiarity effect in frontal N400. Meanwhile, spaced learning effectively enhanced the encoding process and thus led to stronger storage strength, as indicated by greater fMRI responses during learning in brain regions whose activities were associated with subsequent memory. Interestingly, no direct association was found between repetition priming and episodic memory. These results furthered our understanding of the neural mechanisms underlying the spacing effect.

"I remember thinking …": Neural activity associated with subsequent memory for stimulus-evoked internal mentations

Conscious thought involves an interpretive inner monologue pertaining to our waking experiences. Previous studies focused on the mechanisms that allow us to remember externally presented stimuli, but the neurobiological basis of the ability to remember one's internal mentations remains unknown. In order to investigate this question, we presented participants with sentences and scanned their neural activity using functional magnetic resonance imaging (fMRI) as they incidentally produced spontaneous internal mentations. After the scan, we presented the sentences again and asked participants to describe the specific thoughts they had during the initial presentation of each sentence. We categorized experimental trials for each participant according to whether they resulted in subsequently reported internal mentations or not. The results show that activation within classic language processing areas was associated with participants' ability to recollect their thoughts. Activation within mostly right lateralized and medial "default-mode network" regions was associated with not reporting such thoughts.

Human intracranial high-frequency activity maps episodic memory formation in space and time

Noninvasive neuroimaging studies have revealed a network of brain regions that activate during human memory encoding; however, the relative timing of such activations remains unknown. Here we used intracranially recorded high-frequency activity (HFA) to first identify regions that activate during successful encoding. Then, we leveraged the high-temporal precision of HFA to investigate the timing of such activations. We found that memory encoding invokes two spatiotemporally distinct activations: early increases in HFA that involve the ventral visual pathway as well as the medial temporal lobe and late increases in HFA that involve the left inferior frontal gyrus, left posterior parietal cortex, and left ventrolateral temporal cortex. We speculate that these activations reflect higher-order visual processing and top-down modulation of attention/semantic information, respectively.

Complementary attentional components of successful memory encoding

Attention during encoding improves later memory, but how this happens is poorly understood. To investigate the role of attention in memory formation, we combined a variant of a spatial attention cuing task with a subsequent memory fMRI design. Scene stimuli were presented in the periphery to either the left or right of fixation, preceded by a central face cue whose gaze oriented attention to the probable location of the scene. We contrasted activity for scenes appearing in cued versus uncued locations to identify: (1) regions where cuing facilitated processing, and (2) regions involved in reorienting. We then tested how activity in these facilitation and reorienting regions of interest predicted subsequent long-term memory for individual scenes. In facilitation regions such as parahippocampal cortex, greater activity during encoding predicted memory success. In reorienting regions such as right temporoparietal junction, greater activity during encoding predicted memory failure. We interpret these results as evidence that memory formation benefits from attentional facilitation of perceptual processing combined with suppression of the ventral attention network to prevent reorienting to distractors.