Competitive Semantic Memory Retrieval: Temporal Dynamics Revealed by Event-Related Potentials

Memories compete for retrieval when they are related to a common retrieval cue. Previous research has shown that retrieval of a target memory may lead to subsequent retrieval-induced forgetting (RIF) of currently irrelevant competing memories. In the present study, we investigated the time course of competitive semantic retrieval and examined the neurocognitive mechanisms underlying RIF. We contrasted two theoretical accounts of RIF by examining a critical aspect of this memory phenomenon, namely the extent to which it depends on successful retrieval of the target memory. Participants first studied category-exemplar word-pairs (e.g. Fruit—Apple). Next, we recorded electrophysiological measures of brain activity while the participants performed a competitive semantic cued-recall task. In this task, the participants were provided with the studied categories but they were instructed to retrieve other unstudied exemplars (e.g. Fruit—Ma__?). We investigated the event-related potential (ERP) correlates of retrieval success by comparing ERPs from successful and failed retrieval trials. To isolate the ERP correlates of continuous retrieval attempts from the ERP correlates of retrieval success, we included an impossible retrieval condition, with incompletable word-stem cues (Drinks—Wy__) and compared it with a non-retrieval presentation baseline condition (Occupation—Dentist). The participants’ memory for all the studied exemplars was tested in the final phase of the experiment. Taken together, the behavioural results suggest that RIF is independent of target retrieval. Beyond investigating the mechanisms underlying RIF, the present study also elucidates the temporal dynamics of semantic cued-recall by isolating the ERP correlates of retrieval attempt and retrieval success. The ERP results revealed that retrieval attempt is reflected in a late posterior negativity, possibly indicating construction of candidates for completing the word-stem cue and retrieval monitoring whereas retrieval success was reflected in an anterior positive slow wave.     

Mechanisms of Cache Retrieval in the Group Nesting Southern Flying Squirrel (Glaucomys volans)

We examined the proximate mechanisms of cache retrieval in the group‐living, southern flying squirrel, Glaucomys volans, through a series of behavioral experiments conducted in a large indoor arena. The effectiveness of several retrieval mechanisms was determined including spatial memory, olfaction, random searching and a heuristic under different environmental conditions. Our goal was to elucidate the hoarding strategy of individuals in a nest group and to address whether food storing individuals possess a retrieval advantage over pilfering nestmates. A storer's retrieval advantage is necessary for scatter hoarding to be an evolutionarily stable strategy (ESS) within aggregations of unrelated individuals. Our previous work has shown that, G. volans lives in such groups, and consequently it was important to address the storer's retrieval advantage under a range of environmental conditions. Initially in our baseline experiment, we developed methods to eliminate olfactory‐based retrieval and to control for random searching. Subsequently, we experimentally determined the effectiveness of cache retrieval via spatial memory, a heuristic and olfaction. We examined the mechanisms of cache retrieval in three additional experiments using two independent subject populations and found that under dry, odorless conditions spatial memory was the most effective retrieval mechanism in support of a storer's retrieval advantage. In a fifth experiment we examined cache retrieval under wet environmental conditions and showed that olfactory‐based retrieval was effective and the storer's advantage was reduced. We interpret these laboratory results based on considerations of natural environmental conditions and game theory. We propose a conditional ESS strategy where animals store and retrieve their own caches as the primary food hoarding tactic and opportunistically pilfer caches as a secondary tactic.     

Patterns of feeding and food pellet retrieval by Norway rats during food deprivation

This study examined whether, for Norway rats suffering a body weight deficit of sufficient magnitude, food hoarding is prepotent over feeding. Rats food-deprived to 85% ad lib weight were videotaped during hoarding sessions that lasted until they ceased to hoard or eat to obtain complete records of retrieval and feeding. During the early minutes of a trial, feeding was the predominant activity, and food pellet retrieval occurred singly or in short bouts. This gave way over time to diminished eating and extended bouts of retrieval (hoarding). Closer analysis suggested that retrieval and feeding were aroused together and at the beginning of such arousal retrieval was momentarily prepotent over feeding. But once retrieval had occurred, there was a shift in favor of feeding. After the rat had consumed a sufficient amount of food, feeding was no longer aroused to the same degree, so bouts of retrieval occurred without feeding. Electronic Publication     

On the tip of the tongue: an event-related fMRI study of semantic retrieval failure and cognitive conflict.

The tip of the tongue (TOT) state refers to a temporary inaccessibility of information that one is sure exists in long-term memory and is on the verge of recovering. Using event-related fMRI, we assessed the neural correlates of this semantic retrieval failure to determine whether the anterior cingulate-lateral prefrontal neural circuit posited to mediate conflict resolution is engaged during metacognitive conflicts that arise during the TOT. Results revealed that, relative to successful retrieval or unsuccessful retrieval not accompanied by a TOT, retrieval failures accompanied by TOTs elicited a selective response in anterior cingulate-prefrontal cortices. During a TOT, cognitive control mechanisms may be recruited in attempts to resolve the conflict and retrieval failure that characterize this state.     

Recovering meaning: left prefrontal cortex guides controlled semantic retrieval

Prefrontal cortex plays a central role in mnemonic control, with left inferior prefrontal cortex (LIPC) mediating control of semantic knowledge. One prominent theory posits that LIPC does not mediate semantic retrieval per se, but rather subserves the selection of task-relevant knowledge from amidst competing knowledge. The present event-related fMRI study provides evidence for an alternative hypothesis: LIPC guides controlled semantic retrieval irrespective of whether retrieval requires selection against competing representations. With selection demands held constant, LIPC activation increased with semantic retrieval demands and with the level of control required during retrieval. LIPC mediates a top-down bias signal that is recruited to the extent that the recovery of meaning demands controlled retrieval. Selection may reflect a specific instantiation of this mechanism.     

Molecular mechanisms of memory retrieval

Memory retrieval is a fundamental component or stage of memory processing. In fact, retrieval is the only possible measure of memory. The ability to recall past events is a major determinant of survival strategies in all species and is of paramount importance in determining our uniqueness as individuals. Most biological studies of memory using brain lesion and/or gene manipulation techniques cannot distinguish between effects on the molecular mechanisms of the encoding or consolidation of memories and those responsible for their retrieval from storage. Here we examine recent findings indicating the major molecular steps involved in memory retrieval in selected brain regions of the mammalian brain. Together the findings strongly suggest that memory formation and retrieval may share some molecular mechanisms in the hippocampus and that retrieval initiates extinction requiring activation of several signaling cascades and protein synthesis.     

The dynamics of memory retrieval in hierarchical networks

Memory retrieval is of central importance to a wide variety of brain functions. To understand the dynamic nature of memory retrieval and its underlying neurophysiological mechanisms, we develop a biologically plausible spiking neural circuit model, and demonstrate that free memory retrieval of sequences of events naturally arises from the model under the condition of excitation-inhibition (E/I) balance. Using the mean-field model of the spiking circuit, we gain further theoretical insights into how such memory retrieval emerges. We show that the spiking neural circuit model quantitatively reproduces several salient features of free memory retrieval, including its semantic proximity effect and log-normal distributions of inter-retrieval intervals. In addition, we demonstrate that our model can serve as a platform to examine memory retrieval deficits observed in neuropsychiatric diseases such as Parkinson’s and Alzheimer’s diseases. Furthermore, our model allows us to make novel and experimentally testable predictions, such as the prediction that there are long-range correlations in the sequences of retrieved items.     

Rer1, a putative transmembrane domain receptor responsible for targeting proteins to the endoplasmic reticulum

Summary Localization of resident proteins provides identity to subcellular compartments. Most proteins depend on a combination of both retention and retrieval to maintain their steady-state distribution. Rerl is a putative receptor protein mediating retrieval of membrane proteins of the endoplasmic reticulum. This retrieval relies on an unusual hydrophobic target sequence, the transmembrane domain. Apart from Rerl, coatomer is also required to retrieve escaped membrane proteins from the early Golgi region back to the endoplasmic reticulum. Current evidence suggests that the Rerl-mediated retrieval of membrane proteins is a general sorting pathway in eukaryotic cells contributing to the maintenance of compartmental identity in the early secretory pathway.     

DNA extraction from archival formalin-fixed,paraffin-embedded tissues: heat-induced retrieval in alkaline solution

Based on the antigen retrieval principle, our previous study has demonstrated that heating archival formalin-fixed, paraffin-embedded (FFPE) tissues at a higher temperature and at higher pH value of the retrieval solution may achieve higher efficiency of extracted DNA, when compared to the traditional enzyme digestion method. Along this line of heat-induced retrieval, this further study is focused on development of a simpler and more effective heat-induced DNA retrieval technique by testing various retrieval solutions. Three major experiments using a high temperature heating method to extract DNA from FFPE human lymphoid and other tissue sections were performed to compare: (1) different concentrations of alkaline solution (NaOH or KOH, pH 11.5–12) versus Britton and Robinson type of buffer solution (BR buffer) of pH 12 that was the only retrieval solution tested in our previous study; (2) several chemical solutions (SDS, Tween 20, and GITC of various concentrations) versus BR buffer or alkaline solution; and (3) alkaline solution mixed with chemicals versus BR buffer or single alkaline solution. Efficiency of DNA extraction was evaluated by measuring yields using spectrophotometry, electrophoretic pattern, semiquantitation of tissue dissolution, PCR amplification, and kinetic thermocycling-PCR methods. Results showed that boiling tissue sections in 0.1 M NaOH or KOH or its complex retrieval solutions produced higher yields and better quality of DNA compared to BR buffer or chemical solutions alone. The conclusion was that boiling FFPE tissue sections in 0.1 M alkaline solution is a simpler and more effective heat-induced retrieval protocol for DNA extraction. Combination with some chemicals (detergents) may further significantly improve efficiency of the heat-induced retrieval technique.     

Neural coding for the retrieval of multiple memory patterns

We investigate the retrieval dynamics in a feature-based semantic memory model, in which the features are coded by neurons of the Hindmarsh–Rose type in the chaotic regime. We consider the retrieval process as consisting of the synchronized firing activity of the neurons coding for the same memory pattern. The retrieval dynamics is investigated for multiple patterns, with particular attention to the case of overlapping memories. In this case, we hypothesize a dynamical nontransitive mechanism based on synchronization, that allows for a shared feature to participate in multiple memory representations. The problem of the choice of a cognitive plausible time-scale for the retrieval analysis is investigated by analyzing the information that can be inferred from finite-time analyses. Different types of indicators are proposed in order to evaluate the temporal dynamics of the neurons engaged in the retrieval process. We interpret the simulation results as suggestive of a role for chaotic dynamics in allowing for flexible composition of elementary meaningful units in memory representations.     

Chaotic EEG patterns during recall of stressful memory related to panic attack

Chaotic transitions likely emerge in a wide variety of cognitive phenomena and may be linked to specific changes during the development of mental disorders. They represent relatively short periods in the behavior of a system, which are extremely sensitive to very small changes. This increased sensitivity has been suggested to occur also during retrieval of stressful emotional experiences because of their fragmentary, temporally and spatially disorganized character. To test this hypothesis we recorded EEG during retrieval of fearful memories related to panic attack in 7 patients and retrieval of anxiety-related memories in 11 healthy controls. Nonlinear data analysis of EEG records showed a statistically significant increase in degree of chaotic dynamics after retrieval of stressful memories in majority of patients as well as in control subjects. This change correlated with subjective intensity of anxiety induced during the memory retrieval. The data suggest a role of nonlinear changes of neural dynamics in the processing of stressful anxiety-related memories, which may play an important role in the pathophysiology of panic disorder.     

Neural coding for the retrieval of multiple memory patterns

We investigate the retrieval dynamics in a feature-based semantic memory model, in which the features are coded by neurons of the Hindmarsh-Rose type in the chaotic regime. We consider the retrieval process as consisting of the synchronized firing activity of the neurons coding for the same memory pattern. The retrieval dynamics is investigated for multiple patterns, with particular attention to the case of overlapping memories. In this case, we hypothesize a dynamical nontransitive mechanism based on synchronization, that allows for a shared feature to participate in multiple memory representations. The problem of the choice of a cognitive plausible time-scale for the retrieval analysis is investigated by analyzing the information that can be inferred from finite-time analyses. Different types of indicators are proposed in order to evaluate the temporal dynamics of the neurons engaged in the retrieval process. We interpret the simulation results as suggestive of a role for chaotic dynamics in allowing for flexible composition of elementary meaningful units in memory representations.     

Generic Information Can Retrieve Known Biological Associations: Implications for Biomedical Knowledge Discovery

Motivation

Weighted semantic networks built from text-mined literature can be used to retrieve known protein-protein or gene-disease associations, and have been shown to anticipate associations years before they are explicitly stated in the literature. Our text-mining system recognizes over 640,000 biomedical concepts: some are specific (i.e., names of genes or proteins) others generic (e.g., ‘Homo sapiens’). Generic concepts may play important roles in automated information retrieval, extraction, and inference but may also result in concept overload and confound retrieval and reasoning with low-relevance or even spurious links. Here, we attempted to optimize the retrieval performance for protein-protein interactions (PPI) by filtering generic concepts (node filtering) or links to generic concepts (edge filtering) from a weighted semantic network. First, we defined metrics based on network properties that quantify the specificity of concepts. Then using these metrics, we systematically filtered generic information from the network while monitoring retrieval performance of known protein-protein interactions. We also systematically filtered specific information from the network (inverse filtering), and assessed the retrieval performance of networks composed of generic information alone.

Results

Filtering generic or specific information induced a two-phase response in retrieval performance: initially the effects of filtering were minimal but beyond a critical threshold network performance suddenly drops. Contrary to expectations, networks composed exclusively of generic information demonstrated retrieval performance comparable to unfiltered networks that also contain specific concepts. Furthermore, an analysis using individual generic concepts demonstrated that they can effectively support the retrieval of known protein-protein interactions. For instance the concept “binding” is indicative for PPI retrieval and the concept “mutation abnormality” is indicative for gene-disease associations.

Conclusion

Generic concepts are important for information retrieval and cannot be removed from semantic networks without negative impact on retrieval performance.     

Extremely Dilute Modular Neuronal Networks: Neocortical Memory Retrieval Dynamics

A model of columnar networks of neocortical association areas is studied. The neuronal network is composed of many Hebbian autoassociators, or modules, each of which interacts with a relatively small number of the others, randomly chosen. Any module encodes and stores a number of elementary percepts, or features. Memory items, or patterns, are peculiar combinations of features sparsely distributed over the multi-modular network. Any feature stored in any module can be involved in several of the stored patterns; feature-sharing is in fact source of local ambiguities and, consequently, a potential cause of erroneous memory retrieval spreading through the model network in pattern completion tasks.The memory retrieval dynamics of the large modular autoassociator is investigated by combining mathematical analysis and numerical simulations. An oscillatory retrieval process is proposed that is very efficient in overcoming feature-sharing drawbacks; it requires a mechanism that modulates the robustness of local attractors to noise, and neuronal activity sparseness such that quiescent and active modules are about equally noisy to any post-synaptic module.Moreover, it is shown that statistical correlation between 'kinds' of features across the set of memory patterns can be exploited to obtain a more efficient achievement of memory retrieval capabilities.It is also shown that some spots of the network cannot be reached by retrieval activity spread if they are not directly cued by the stimulus. The locations of these activity isles depend on the pattern to retrieve, while their extension only depends (in large networks) on statistics of inter-modular connections and stored patterns. The existence of activity isles determines an upper-bound to retrieval quality that does not depend on the specific retrieval dynamics adopted, nor on whether feature-sharing is permitted. The oscillatory retrieval process nearly saturates this bound.     

Retrieval of stored seeds by Marsh Tits Parus palustris in the field

The retrieval of stored food by Marsh Tits was documented using Hall-plate detectors to register visits to storage sites by marked individuals. In each experimental session a marked bird was allowed to store up to 50 radioactively labelled half-peanuts. By placing detectors next to each stored item that we located it was possible to show that birds attempt to retrieve about 25% of their stored food. Most of the retrieval is within one or two days (confirming Cowie et al. 1981) and the proportion of recorded retrieval attempts that are successful declines to zero after about three days. By this time storage sites have already been emptied by cache robbers. There may be a tendency for retrieval to occur in the afternoon.     

Research on Similarity Measurement for Texture Image Retrieval

A complete texture image retrieval system includes two techniques: texture feature extraction and similarity measurement. Specifically, similarity measurement is a key problem for texture image retrieval study. In this paper, we present an effective similarity measurement formula. The MIT vision texture database, the Brodatz texture database, and the Outex texture database were used to verify the retrieval performance of the proposed similarity measurement method. Dual-tree complex wavelet transform and nonsubsampled contourlet transform were used to extract texture features. Experimental results show that the proposed similarity measurement method achieves better retrieval performance than some existing similarity measurement methods.     

Age-Related Changes in the Functional Network Underlying Specific and General Autobiographical Memory Retrieval: A Pivotal Role for the Anterior Cingulate Cortex

Age-related changes in autobiographical memory (AM) recall are characterized by a decline in episodic details, while semantic aspects are spared. This deleterious effect is supposed to be mediated by an inefficient recruitment of executive processes during AM retrieval. To date, contrasting evidence has been reported on the neural underpinning of this decline, and none of the previous studies has directly compared the episodic and semantic aspects of AM in elderly. We asked 20 young and 17 older participants to recall specific and general autobiographical events (i.e., episodic and semantic AM) elicited by personalized cues while recording their brain activity by means of fMRI. At the behavioral level, we confirmed that the richness of episodic AM retrieval is specifically impoverished in aging and that this decline is related to the reduction of executive functions. At the neural level, in both age groups, we showed the recruitment of a large network during episodic AM retrieval encompassing prefrontal, cortical midline and posterior regions, and medial temporal structures, including the hippocampus. This network was very similar, but less extended, during semantic AM retrieval. Nevertheless, a greater activity was evidenced in the dorsal anterior cingulate cortex (dACC) during episodic, compared to semantic AM retrieval in young participants, and a reversed pattern in the elderly. Moreover, activity in dACC during episodic AM retrieval was correlated with inhibition and richness of memories in both groups. Our findings shed light on the direct link between episodic AM retrieval, executive control, and their decline in aging, proposing a possible neuronal signature. They also suggest that increased activity in dACC during semantic AM retrieval in the elderly could be seen as a compensatory mechanism underpinning successful AM performance observed in aging. These results are discussed in the framework of recently proposed models of neural reorganization in aging.     

Frontal brain potentials during recognition are modulated by requirements to retrieve perceptual detail

To assess the role of prefrontal cortex in retrieval and address the controversy about whether prefrontal retrieval operations are engaged only following successful retrieval, we recorded event-related brain potentials during two recognition tests with differing demands on retrieval effort. Both tests included object drawings that were (1) identical to those studied, (2) the same but with altered aspect ratios, and (3) previously unseen. Instructions were to respond "old" only if drawings were not modified (specific test) or regardless of modifications (general test). Frontal potentials were enhanced during the specific relative to the general test for all three types of drawings. We conclude that these potentials reflected differential engagement of strategic retrieval, that this function relied on left prefrontal cortex, and that it was not contingent on successful retrieval.     

The hippocampus is coupled with the default network during memory retrieval but not during memory encoding

The brain's default mode network (DMN) is activated during internally-oriented tasks and shows strong coherence in spontaneous rest activity. Despite a surge of recent interest, the functional role of the DMN remains poorly understood. Interestingly, the DMN activates during retrieval of past events but deactivates during encoding of novel events into memory. One hypothesis is that these opposing effects reflect a difference between attentional orienting towards internal events, such as retrieved memories, vs. external events, such as to-be-encoded stimuli. Another hypothesis is that hippocampal regions are coupled with the DMN during retrieval but decoupled from the DMN during encoding. The present fMRI study investigated these two hypotheses by combining a resting-state coherence analysis with a task that measured the encoding and retrieval of both internally-generated and externally-presented events. Results revealed that the main DMN regions were activated during retrieval but deactivated during encoding. Counter to the internal orienting hypothesis, this pattern was not modulated by whether memory events were internal or external. Consistent with the hippocampal coupling hypothesis, the hippocampus behaved like other DMN regions during retrieval but not during encoding. Taken together, our findings clarify the relationship between the DMN and the neural correlates of memory retrieval and encoding.