Reactivation in Working Memory: An Attractor Network Model of Free Recall

The dynamic nature of human working memory, the general-purpose system for processing continuous input, while keeping no longer externally available information active in the background, is well captured in immediate free recall of supraspan word-lists. Free recall tasks produce several benchmark memory phenomena, like the U-shaped serial position curve, reflecting enhanced memory for early and late list items. To account for empirical data, including primacy and recency as well as contiguity effects, we propose here a neurobiologically based neural network model that unifies short- and long-term forms of memory and challenges both the standard view of working memory as persistent activity and dual-store accounts of free recall. Rapidly expressed and volatile synaptic plasticity, modulated intrinsic excitability, and spike-frequency adaptation are suggested as key cellular mechanisms underlying working memory encoding, reactivation and recall. Recent findings on the synaptic and molecular mechanisms behind early LTP and on spiking activity during delayed-match-to-sample tasks support this view.     

Does Recall after Sleep-Dependent Memory Consolidation Reinstate Sensitivity to Retroactive Interference?

Previous studies have shown that newly encoded memories are more resistant to retroactive interference when participants are allowed to sleep after learning the original material, suggesting a sleep-related strengthening of memories. In the present study, we investigated delayed, long-term effects of sleep vs. sleep deprivation (SD) on the first post-training night on memory consolidation and resistance to interference. On day 1, participants learned a list of unrelated word pairs (AB), either in the morning or in the evening, then spent the post-training night in a sleep or sleep deprivation condition, in a within-subject paradigm. On day 4, at the same time of day, they learned a novel list of word pairs (AC) in which 50% of the word pairs stemmed with the same word than in the AB list, resulting in retroactive interference. Participants had then to recall items from the AB list upon presentation of the “A” stem. Recall was marginally improved in the evening, as compared to the morning learning group. Most importantly, retroactive interference effects were found in the sleep evening group only, contrary to the hypothesis that sleep exerts a protective role against intrusion by novel but similar learning. We tentatively suggest that these results can be explained in the framework of the memory reconsolidation theory, stating that exposure to similar information sets back consolidated items in a labile form again sensitive to retroactive interference. In this context, sleep might not protect against interference but would promote an update of existing episodic memories while preventing saturation of the memory network due to the accumulation of dual traces.     

Autobiographical memory in mild cognitive impairment and Alzheimer's disease: A comparison between the Levine and Kopelman interview methodologies

Previous studies have produced inconsistent results concerning the two components of autobiographical memory-personal semantic memory and episodic memory. Results in subjects with mild cognitive impairment (MCI) and dementia of Alzheimer's type (DAT) have varied concerning the existence of a temporal gradient in retrograde amnesia. These results have important theoretical implications regarding multiple trace theory versus standard consolidation models of long-term memory (LTM). We investigated whether this variability arises from differences in the methods used in assessing autobiographical memory. We examined patterns of memory impairment in 20 healthy elderly controls, 20 MCI subjects, and 10 DAT subjects using the Autobiographical Memory Interview (AMI) of Kopelman and the Autobiographical Interview (AI) of Levine. Both the AMI and AI were modified to allow for the test scores to be derived from a single interview without fatiguing the subjects. On the AMI, DAT subjects were significantly impaired on both components of autobiographical memory-episodic memory and personal semantics-with episodic memory showing a significant though gentle temporal gradient sparing childhood memories. Using the AI test, subjects with DAT showed impaired recall of episodic details (but not personal semantics), again with a gentle temporal gradient. Differences between the two interview methods (fewer epochs in the AMI; fewer memories per epoch in the AI) were found to have a significant impact on the pattern of findings; fewer epochs in the AMI brought out the temporal gradient, and fewer memories per epoch (in the AI) diminished it. These data show the importance of technical details of the different tests in favouring one versus another LTM theory. The data are not purely compatible with either theory. ? 2012 Wiley Periodicals, Inc.     

Genetic modulation of Rpd3 expression impairs long-term courtship memory in Drosophila

There is increasing evidence that regulation of local chromatin structure is a critical mechanism underlying the consolidation of long-term memory (LTM), however considerably less is understood about the specific mechanisms by which these epigenetic effects are mediated. Furthermore, the importance of histone acetylation in Drosophila memory has not been reported. The histone deacetylase (HDAC) Rpd3 is abundant in the adult fly brain, suggesting a post-mitotic function. Here, we investigated the role of Rpd3 in long-term courtship memory in Drosophila. We found that while modulation of Rpd3 levels predominantly in the adult mushroom body had no observed impact on immediate recall or one-hour memory, 24-hour LTM was severely impaired. Surprisingly, both overexpression as well as RNAi-mediated knockdown of Rpd3 resulted in impairment of long-term courtship memory, suggesting that the dose of Rpd3 is critical for normal LTM.     

On recency and echoic memory

In short-term memory, the tendency for the last few (recency) items from a verbal sequence to be increasingly well recalled is more pronounced if the items are spoken rather than written. This auditory recency advantage has been quite generally attributed to echoic memory, on the grounds that in the auditory, but not the visual, mode, sensory memory persists just long enough to supplement recall of the most recent items. This view no longer seems tenable. There are now several studies showing that an auditory recency advantage occurs not only in long-term memory, but under conditions in which it cannot possibly be attributed to echoic memory. Also, similar recency phenomena have been discovered in short-term memory when the items are lip-read, or presented in sign-language, rather than heard. This article provides a partial review of these studies, taking a broad theoretical position from which these particular recency phenomena are approached as possible exceptions, to a general theory according to which recency is due to temporal distinctiveness. Much of the fresh evidence reviewed is of a somewhat preliminary nature and it is as yet unexplained by any theory of memory. The need for additional, converging experimental tests is obvious; so too is the need for further theoretical development. Several alternative theoretical resolutions are mentioned, including the possibility that enhanced recency may reflect movement, from sequentially occurring stimulus features, and the suggestion that it may be associated with the primary linguistic mode of the individuals concerned. But special weight is attached to the conjecture that all these recency phenomena might be accounted for in terms of distinctiveness or discriminability. On this view, the enhanced recency effects observed with certain modes, including the auditory mode, are attributed to items possessing greater temporal discriminability in those modes.     

Episodic memory: insights from semantic dementia

Semantic dementia, also known as the temporal lobe variant of fronto-temporal dementia, results in a progressive yet relatively pure loss of semantic knowledge about words, objects and people, and is associated with asymmetric, focal atrophy of the antero-lateral temporal lobes. Semantic dementia provides a unique opportunity to study the organization of long-term memory particularly since initial observations suggested sparing of episodic memory. Recent studies reveal, however, a more complex but theoretically revealing pattern. On tests of autobiographical memory, patients with semantic dementia show a 'reverse step function' with sparing of recall of events from the most recent 2 to 5 years but impairment on more distant life periods. Anterograde recognition memory for visual materials is extremely well preserved, except in the most deteriorated cases, although performance is heavily reliant upon perceptual information about the studied stimuli, particularly for items that are no longer known by the subjects. On tests of verbal anterograde memory such as word learning, performance is typically poor even for words which are 'known' to the patients. A source discrimination experiment, designed to evaluate familiarity and recollection-based anterograde memory processes, found that patients with semantic dementia showed good item detection, although recollection of source was sometimes impaired. Semantic knowledge about studied items and measures of item detection and source discrimination were largely independent. The implications of these findings for models of long-term memory are discussed. The results support the concept that episodic memory, or at least the recall of temporally specific autobiographical experiences, draws upon a number of separable memory processes, some of which can function independently of semantic knowledge.     

轻度认知损伤患者的联系记忆损伤特征

遗忘型轻度认知损伤患者(aMCI)在项目记忆和联系记忆上都有损伤．本文通过临床记忆量表中的项目记忆和联系记忆测验,研究aMCI的联系记忆是否比项目记忆有更显著的损伤．另外,通过分析配对联想学习测验,进一步研究aMCI联系记忆损伤的特点．25名aMCI和28名健康老人参与了两个联系记忆测验(配对联想学习测验和联想回忆测验)和两个项目记忆测验(图像自由回忆和无意义图形再认),aMCI患者在联系记忆测验上表现出了更显著的损伤,即使控制了项目记忆的损伤,aMCI的联系记忆仍然比健康老人显著降低．另外,ROC分析表明联系记忆测验比项目记忆测验对aMCI病人有更高的区分度．对配对联想学习测验的分析表明,相对于健康老人,aMCI患者在记忆有语言联系的词对要比记忆无语义联系的词对更为困难．本研究进一步表明aMCI患者的联系记忆比项目记忆有更大的损伤．相对于健康老人,aMCI患者不仅难以在两个无关项目间创建记忆连接,而且在有效利用项目间本身的语义联系方面存在更大的损伤．联系记忆测验比项目记忆测验对aMCI患者有更高的区分度．在神经心理评估中增加联系记忆测验,能更加有效地识别aMCI患者．     

Ventromedial prefrontal cortex activation is associated with memory formation for predictable rewards

During reinforcement learning, dopamine release shifts from the moment of reward consumption to the time point when the reward can be predicted. Previous studies provide consistent evidence that reward-predicting cues enhance long-term memory (LTM) formation of these items via dopaminergic projections to the ventral striatum. However, it is less clear whether memory for items that do not precede a reward but are directly associated with reward consumption is also facilitated. Here, we investigated this question in an fMRI paradigm in which LTM for reward-predicting and neutral cues was compared to LTM for items presented during consumption of reliably predictable as compared to less predictable rewards. We observed activation of the ventral striatum and enhanced memory formation during reward anticipation. During processing of less predictable as compared to reliably predictable rewards, the ventral striatum was activated as well, but items associated with less predictable outcomes were remembered worse than items associated with reliably predictable outcomes. Processing of reliably predictable rewards activated the ventromedial prefrontal cortex (vmPFC), and vmPFC BOLD responses were associated with successful memory formation of these items. Taken together, these findings show that consumption of reliably predictable rewards facilitates LTM formation and is associated with activation of the vmPFC.     

Active maintenance of associative mnemonic signal in monkey inferior temporal cortex

We investigated the contribution of the inferior temporal (IT) cortical neurons to the active maintenance of internal representations. The activity of single neurons in the IT cortex was recorded while the monkeys performed a sequential-type associative memory task in which distractor stimuli interrupted the delay epoch between the cue and target (paired-associate) stimuli. For each neuron, information about each stimulus conveyed by the delay activity was estimated as a coefficient of multiple regression analysis. We found that target information derived from long-term memory (LTM) persisted despite the distractors. By contrast, cue information derived from the visual system was attenuated and frequently replaced by distractor information. These results suggest that LTM-derived information required for upcoming behavior is actively maintained in the IT neurons, whereas visually derived information tends to be updated irrespective of behavioral relevance.     

Semantic memory and the human hippocampus

It has been unclear whether the hippocampus is uniquely important for episodic memory (memory for events that are specific to time and place) or whether the hippocampus is also important for learning and remembering facts (semantic memory). In two studies, we assessed the capacity for semantic memory in patients with bilateral damage thought to be restricted primarily to the hippocampal region who developed memory impairment at a known time. Since the onset of their memory impairment, the patients have acquired less factual knowledge than controls. The patients also exhibit temporally limited retrograde amnesia for factual information from the several years preceding the onset of memory impairment. Remote memory for factual knowledge (from 11-30 years before amnesia) is intact. The results show that the hippocampal region supports semantic memory as well as episodic memory and that its role in the acquisition and storage of semantic knowledge is time limited.     

Effects of DGAVP on verbal memory

Effects of DGAVP (desglycinamide-arginine-vasopressin, a synthetic vasopressin analog) on verbal memory were investigated in 13 healthy male volunteers. Ten word lists, each consisting of 15 words, were presented to the subjects who had to recall them according to a free recall paradigm. The total number of recalled words was not different between DGAVP and placebo treatment; but DGAVP had an effect on memory performance depending on the serial position of the words. It attenuated the primacy effect and enhanced the recency effect of memory performance. The pattern of changes after DGAVP may be consistent with an effect of the peptide on general arousal. Since the experiment was not designed to test influences of DGAVP on arousal, these considerations remain tentative.     

Attention and available long-term memory in an activation-based model

The influence of attention on memorizing related items and on available long-term memory (ALTM) was explored, showing that N400 of no-memory items was more negative than that of the memory item. The results of the category comparison task indicated that information processing under attention-driven in WM determined the availability of related long-term memory, i.e., specific content, which was formerly concerned or ignored, yielding different indirect semantic priming effects. These indicate that the orientation of conceptual attention leads the related representations of LTM to diverse activation patterns, supporting the activation-based model.     

Prolonged activation of cAMP-dependent protein kinase during conditioning induces long-term memory in honeybees

To investigate the function cAMP-dependent protein kinase (PKA) exerts in the induction of long-term memory, changes in PKA activity induced by associative learning in vivo were measured in the antennal lobes (ALs) of honeybees. The temporal dynamics of PKA activation depend on both the sequence of conditioned and unconditioned stimuli and the number of conditioning trials. Only multiple-trial conditioning, which induces long-term memory (LTM), leads to a profound prolongation of PKA activation mediated by the NO/cGMP system. Imitation of this prolonged PKA activation in the ALs in combination with single-trial conditioning is sufficient to induce LTM. These findings not only demonstrate the close connection between conditioning procedure and temporal dynamics in PKA activation but also reveal that already during conditioning a distinct temporal pattern of PKA activation is critical for LTM induction in intact animals.     

Information Retrieval during Free Listing Is Biased by Memory: Evidence from Medicinal Plants

Free listing is a methodological tool that is widely used in various scientific disciplines. A typical assumption of this approach is that individual lists reflect a subset of total knowledge and that the first items listed are the most culturally important. However, little is known about how cognitive processes influence free lists. In this study, we assess how recent memory of use, autonoetic and anoetic memory, and long-term associative memory can affect the composition and order of items in free lists and evaluate whether free lists indicate the most important items. Based on a model of local knowledge about medicinal plants and their therapeutic targets, which was collected via individual semi-structured interviews, we classify each item recorded in free lists according to the last time that the item was used by the informant (recently or long ago), the type of relevant memory (autonoetic or anoetic memory) and the existing associations between therapeutic targets (similar or random). We find that individuals have a tendency to recall information about medicinal plants used during the preceding year and that the recalled plants were also the most important plants during this period. However, we find no trend in the recall of plants from long-term associative memory, although this phenomenon is well established in studies on cognitive psychology. We suggest that such evidence should be considered in studies that use lists of medicinal plants because this temporal cognitive limit on the retrieval of knowledge affects data interpretation.     

Memory for self-performed actions in individuals with Asperger syndrome

Memory for action is enhanced if individuals are allowed to perform the corresponding movements, compared to when they simply listen to them (enactment effect). Previous studies have shown that individuals with Autism Spectrum Disorders (ASD) have difficulties with processes involving the self, such as autobiographical memories and self performed actions. The present study aimed at assessing memory for action in Asperger Syndrome (AS). We investigated whether adults with AS would benefit from the enactment effect when recalling a list of previously performed items vs. items that were only visually and verbally experienced through three experimental tasks (Free Recall, Old/New Recognition and Source Memory). The results showed that while performance on Recognition and Source Memory tasks was preserved in individuals with AS, the enactment effect for self-performed actions was not consistently present, as revealed by the lower number of performed actions being recalled on the Free Recall test, as compared to adults with typical development. Subtle difficulties in encoding specific motor and proprioceptive signals during action execution in individuals with AS might affect retrieval of relevant personal episodic information. These disturbances might be associated to an impaired action monitoring system.     

Evidence for human fronto-central gamma activity during long-term memory encoding of word sequences

Although human gamma activity (30-80 Hz) associated with visual processing is often reported, it is not clear to what extend gamma activity can be reliably detected non-invasively from frontal areas during complex cognitive tasks such as long term memory (LTM) formation. We conducted a memory experiment composed of 35 blocks each having three parts: LTM encoding, working memory (WM) maintenance and LTM retrieval. In the LTM encoding and WM maintenance parts, participants had to respectively encode or maintain the order of three sequentially presented words. During LTM retrieval subjects had to reproduce these sequences. Using magnetoencephalography (MEG) we identified significant differences in the gamma and beta activity. Robust gamma activity (55-65 Hz) in left BA6 (supplementary motor area (SMA)/pre-SMA) was stronger during LTM rehearsal than during WM maintenance. The gamma activity was sustained throughout the 3.4 s rehearsal period during which a fixation cross was presented. Importantly, the difference in gamma band activity correlated with memory performance over subjects. Further we observed a weak gamma power difference in left BA6 during the first half of the LTM rehearsal interval larger for successfully than unsuccessfully reproduced word triplets. In the beta band, we found a power decrease in left anterior regions during LTM rehearsal compared to WM maintenance. Also this suppression of beta power correlated with memory performance over subjects. Our findings show that an extended network of brain areas, characterized by oscillatory activity in different frequency bands, supports the encoding of word sequences in LTM. Gamma band activity in BA6 possibly reflects memory processes associated with language and timing, and suppression of beta activity at left frontal sensors is likely to reflect the release of inhibition directly associated with the engagement of language functions.     

Molecular pharmacological dissection of short- and long-term memory

1. It has been discussed for over 100 years whether short-term memory (STM) is separate from, or just an early phase of, long-term memory (LTM). The only way to solve this dilemma is to find out at least one treatment that blocks STM while keeping LTM intact for the same task in the same animal.2. The effect of a large number of treatments infused into the hippocampus, amygdala, and entorhinal, posterior parietal or prefrontal cortex on STM and LTM of a one-trial step-down inhibitory avoidance task was studied. The animals were tested at 1.5 h for STM, and again at 24 h for LTM. The treatments were given after training.3. Eleven different treatments blocked STM without affecting LTM. Eighteen treatments affected the two memory types differentially, either blocking or enhancing LTM alone. Thus, STM is separate from, and parallel to the first hours of processing of, LTM of that task.4. The mechanisms of STM are different from those of LTM. The former do not include gene expression or protein synthesis; the latter include a double peak of cAMP-dependent protein kinase activity, accompanied by the phosphorylation of CREB, and both gene expression and protein synthesis.5. Possible cellular and molecular events that do not require mRNA or protein synthesis should account for STM. These might include a hyperactivation of glutamate AMPA receptors, ribosome changes, or the exocytosis of glycoproteins that participate in cell addition.     

Primacy and recency effects in extinction and latent inhibition: a selective review with implications for models of learning

In the framework of animal conditioning and human associative learning, primacy and recency effects on acquired stimulus control of behavior refer to the superior influence of first-learned and last-learned associations, respectively. Most contemporary associative models of learning anticipate unwavering recency effects and claim support from numerous published studies. But, for pragmatic reasons, almost all of these studies were conducted under select conditions that favored recency effects. When these conditions are not met, recency effects are far from ubiquitous. We review the literature on primacy and recency effects regarding extinction and latent inhibition (i.e., interference between outcomes), with special emphasis on the impact of certain post-training manipulations and test conditions on conditioned responding. Evidence for recency-to-primacy shifts and for memory integration is examined in light of contemporary models of learning.     

Astrocyte-neuron lactate transport is required for long-term memory formation

We report that, in the rat hippocampus, learning leads to a significant increase in extracellular lactate levels that derive from glycogen, an energy reserve selectively localized in astrocytes. Astrocytic glycogen breakdown and lactate release are essential for long-term but not short-term memory formation, and for the maintenance of long-term potentiation (LTP) of synaptic strength elicited in vivo. Disrupting the expression of the astrocytic lactate transporters monocarboxylate transporter 4 (MCT4) or MCT1 causes amnesia, which, like LTP impairment, is rescued by L-lactate but not equicaloric glucose. Disrupting the expression of the neuronal lactate transporter MCT2 also leads to amnesia that is unaffected by either L-lactate or glucose, suggesting that lactate import into neurons is necessary for long-term memory. Glycogenolysis and astrocytic lactate transporters are also critical for the induction of molecular changes required for memory formation, including the induction of phospho-CREB, Arc, and phospho-cofilin. We conclude that astrocyte-neuron lactate transport is required for long-term memory formation.     

What Determines Auditory Distraction? On the Roles of Local Auditory Changes and Expectation Violations

Both the acoustic variability of a distractor sequence and the degree to which it violates expectations are important determinants of auditory distraction. In four experiments we examined the relative contribution of local auditory changes on the one hand and expectation violations on the other hand in the disruption of serial recall by irrelevant sound. We present evidence for a greater disruption by auditory sequences ending in unexpected steady state distractor repetitions compared to auditory sequences with expected changing state endings even though the former contained fewer local changes. This effect was demonstrated with piano melodies (Experiment 1) and speech distractors (Experiment 2). Furthermore, it was replicated when the expectation violation occurred after the encoding of the target items (Experiment 3), indicating that the items'' maintenance in short-term memory was disrupted by attentional capture and not their encoding. This seems to be primarily due to the violation of a model of the specific auditory distractor sequences because the effect vanishes and even reverses when the experiment provides no opportunity to build up a specific neural model about the distractor sequence (Experiment 4). Nevertheless, the violation of abstract long-term knowledge about auditory regularities seems to cause a small and transient capture effect: Disruption decreased markedly over the course of the experiments indicating that participants habituated to the unexpected distractor repetitions across trials. The overall pattern of results adds to the growing literature that the degree to which auditory distractors violate situation-specific expectations is a more important determinant of auditory distraction than the degree to which a distractor sequence contains local auditory changes.