Consolidation theory and retrograde amnesia in humans

Human retrograde amnesia and memory consolidation

This paper reports a reassessment of published literature on the question of whether retrograde amnesia data from patients with severe trauma supports the idea that there is ongoing consolidation of long-lasting memories. That is, memory consolidation continues for decades with older memories being increasingly consolidated, and, thus, more protected from forgetting. Our analysis was limited to patients with specific traumas rather than neurodegenerative conditions that can be complicated by the additional presence of significant anterograde amnesia. These constraints were used because trauma patients have a definitive start to their amnesia allowing comparison of their memories before this event, unlike when there is an undefined amnesia onset. Our results revealed that the standard account of retrograde amnesia only fits part of the data, with more than half not conforming to this account. Specifically, damage to different brain areas was associated with different patterns of retrograde amnesia. Those cases where the standard retrograde amnesia account was held tended to involve damage to the hippocampus and temporal lobes, as expected. Future directions to better understand the influence of retrograde amnesia and memory consolidation are suggested.

Stochastic consolidation of lifelong memory

Humans have the remarkable ability to continually store new memories, while maintaining old memories for a lifetime. How the brain avoids catastrophic forgetting of memories due to interference between encoded memories is an open problem in computational neuroscience. Here we present a model for continual learning in a recurrent neural network combining Hebbian learning, synaptic decay and a novel memory consolidation mechanism: memories undergo stochastic rehearsals with rates proportional to the memory's basin of attraction, causing self-amplified consolidation. This mechanism gives rise to memory lifetimes that extend much longer than the synaptic decay time, and retrieval probability of memories that gracefully decays with their age. The number of retrievable memories is proportional to a power of the number of neurons. Perturbations to the circuit model cause temporally-graded retrograde and anterograde deficits, mimicking observed memory impairments following neurological trauma.

Current Developments on the Role of α1-Adrenergic Receptors in Cognition, Cardioprotection, and Metabolism

The α₁-adrenergic receptors (ARs) are G-protein coupled receptors that bind the endogenous catecholamines, norepinephrine, and epinephrine. They play a key role in the regulation of the sympathetic nervous system along with β and α₂-AR family members. While all of the adrenergic receptors bind with similar affinity to the catecholamines, they can regulate different physiologies and pathophysiologies in the body because they couple to different G-proteins and signal transduction pathways, commonly in opposition to one another. While α₁-AR subtypes (α_1A, α_1B, α_1C) have long been known to be primary regulators of vascular smooth muscle contraction, blood pressure, and cardiac hypertrophy, their role in neurotransmission, improving cognition, protecting the heart during ischemia and failure, and regulating whole body and organ metabolism are not well known and are more recent developments. These advancements have been made possible through the development of transgenic and knockout mouse models and more selective ligands to advance their research. Here, we will review the recent literature to provide new insights into these physiological functions and possible use as a therapeutic target.

Forgetting in chimpanzees (Pan troglodytes): What is the role of interference?

Humans are constantly acquiring new information and skills. However, forgetting is also a common phenomenon in our lives. Understanding the lability of memories is critical to appreciate how they are formed as well as forgotten. Here we investigate the lability of chimpanzees’ short-term memories and assess what factors cause forgetting in our closest relatives. In two experiments, chimpanzees were presented with a target task, which involved remembering a reward location, followed by the presentation of an interference task—requiring the recollection of a different reward location. The interference task could take place soon after the presentation of the target task or soon before the retrieval of the food locations. The results show that chimpanzees’ memories for the location of a reward in a target task were compromised by the presentation of a different food location in an interference task. Critically, the temporal location of the interference task did not significantly affect chimpanzees’ performance. These pattern of results were found for both Experiment 1—when the retention interval between the encoding and retrieval of the target task was 60 seconds- and Experiment 2—when the retention interval between the encoding and retrieval of the target task was 30 seconds. We argue that the temporal proximity of the to-be-remembered information and the interference item during encoding is the factor driving chimpanzees’ performance in the present studies.

Retrograde Autobiographical Memory From PTA Emergence to Six-Month Follow-Up in Moderate to Severe Traumatic Brain Injury

OBJECTIVE

The overwhelming focus of research on memory following traumatic brain injury (TBI) has been on anterograde amnesia, and very little attention has been paid to retrograde amnesia. There is evidence to suggest that retrograde autobiographical memory deficits exist after severe TBI, although there have been no prospective studies of autobiographical memory in a representative sample of moderate to severe cases recruited from hospital admissions.

METHODS

The purpose of the present study was to report changes in autobiographical memory performance among a group of patients soon after emergence from posttraumatic amnesia (PTA) and at the 6-month follow-up compared with a healthy control (HC) group. The authors also examined associations with anterograde memory function and community integration to assist in understanding the functional impact of autobiographical memory deficits and potential underlying mechanisms. The Autobiographical Memory Interview and the Rey Auditory Verbal Learning Test were used as measures of retrograde and anterograde memory, respectively, and the Community Integration Questionnaire was used as a measure of functional outcome in the TBI group.

RESULTS

The results demonstrated that both personal semantic and episodic autobiographical memory scores were impaired following emergence from PTA and at the 6-month follow-up. Only subtle differences emerged in change over time in different injury severity groups. Recent retrograde memory function was associated with anterograde memory performance, which supports some degree of overlap in underlying mechanisms.

CONCLUSIONS

The findings suggest that autobiographical memory deficits are prevalent following moderate to severe TBI and warrant consideration in rehabilitation.

Correlation Between Hippocampal Volume and Autobiographical Memory Depending on Retrieval Frequency in Healthy Individuals and Patients with Alzheimer's Disease

The hippocampus plays an indispensable role in episodic memory, particularly during the consolidation process. However, its precise role in retrieval of episodic memory is still ambiguous. In this study, we investigated the correlation of hippocampal morphometry and the performance in an autobiographical memory task in 27 healthy controls and 24 patients suffering from Alzheimer's disease (AD). Most importantly, correlations were defined separately and comparatively for memory contents with different retrieval frequency in the past. In healthy subjects, memory performance for seldom retrieved autobiographical events was significantly associated with gray matter density in the bilateral hippocampus, whereas this correlation was not present for events with high retrieval frequency. This pattern of findings confirms that retrieval frequency plays a critical role in the consolidation of episodic autobiographical memories, thereby making them more independent of the hippocampal system. In AD patients, on the other hand, successful memory retrieval appeared to be related to hippocampal morphometry irrespective of the contents' retrieval frequency, comprising events with high retrieval frequency, too. The observed differences between patients and control subjects suggest that AD-related neurodegeneration not only impairs the function, but also decreases the functional specialization of the hippocampal memory system, which, thus, may be considered as marker for AD.

Electrophysiological evidence for strategically orienting retrieval toward the specific age of a memory

For over a century, memory researchers have extensively studied the differences between retrieving memories that were encoded in the remote past as opposed to recently. Although this work has largely focused on the changes that these memory traces undergo over time, an unexplored issue is whether retrieval attempts and other strategic processes might be differentially oriented in order to effectively access memories of different ages. The current study addressed this issue by instructing participants to retrieve words that were encoded either one week (remote) or about 30 minutes earlier (recent). To maximize the possibility that participants adopted distinct retrieval orientations, separate blocks of the memory test employed exclusion task procedures in which the words from only one encoding period were targeted at a given time, in the face of distractors from the other period. Event-related potentials (ERPs) elicited by correctly-rejected new items were contrasted to minimize confounding effects of retrieval success. The new-item ERPs revealed differences according to the targeted week, such that the ERPs over posterior scalp were more positive-going for the recent compared to remote blocks. Furthermore, using multiple methods, these ERP effects were dissociated from differences in difficulty across the two conditions. The findings provide novel evidence that knowledge about when a memory was initially encoded leads to differences in the adoption of retrieval processing strategies.

Functional (dissociative) retrograde amnesia

Retrograde amnesia is described as condition which can occur after direct brain damage, but which occurs more frequently as a result of a psychiatric illness. In order to understand the amnesic condition, content-based divisions of memory are defined. The measurement of retrograde memory is discussed and the dichotomy between "organic" and "psychogenic" retrograde amnesia is questioned. Briefly, brain damage-related etiologies of retrograde amnesia are mentioned. The major portion of the review is devoted to dissociative amnesia (also named psychogenic or functional amnesia) and to the discussion of an overlap between psychogenic and "brain organic" forms of amnesia. The "inability of access hypothesis" is proposed to account for most of both the organic and psychogenic (dissociative) patients with primarily retrograde amnesia. Questions such as why recovery from retrograde amnesia can occur in retrograde (dissociative) amnesia, and why long-term new learning of episodic-autobiographic episodes is possible, are addressed. It is concluded that research on retrograde amnesia research is still in its infancy, as the neural correlates of memory storage are still unknown. It is argued that the recollection of episodic-autobiographic episodes most likely involves frontotemporal regions of the right hemisphere, a region which appears to be hypometabolic in patients with dissociative amnesia.

Generalization from episodic memories across time: a route for semantic knowledge acquisition

The storage of input regularities, at all levels of processing complexity, is a fundamental property of the nervous system. At high levels of complexity, this may involve the extraction of associative regularities between higher order entities such as objects, concepts and environments across events that are separated in space and time. We propose that such a mechanism provides an important route towards the formation of higher order semantic knowledge. The present study assessed whether subjects were able to extract complex regularities from multiple associative memories and whether they could generalize this regularity knowledge to new items. We used a memory task in which subjects were required to learn face-location associations, but in which certain facial features were predictive of locations. We assessed generalization, as well as memory for arbitrary stimulus components, over a 4-h post-encoding consolidation period containing wakefulness or sleep. We also assessed the stability of regularity knowledge across a period of several weeks thereafter. We found that subjects were able to detect the regularity structure and use it in a generalization task. Interestingly, the performance on this task increased across the 4hr post-learning period. However, no differential effects of cerebral sleep and wake states during this interval were observed. Furthermore, it was found that regularity extraction hampered the storage of arbitrary facial features, resulting in an impoverished memory trace. Finally, across a period of several weeks, memory for the regularity structure appeared very robust whereas memory for arbitrary associations showed steep forgetting. The current findings improve our understanding of how regularities across memories impact memory (trans)formation.

A mathematical model of forgetting and amnesia

We describe a mathematical model of learning and memory and apply it to the dynamics of forgetting and amnesia. The model is based on the hypothesis that the neural systems involved in memory at different time scales share two fundamental properties: (1) representations in a store decline in strength (2) while trying to induce new representations in higher-level more permanent stores. This paper addresses several types of experimental and clinical phenomena: (i) the temporal gradient of retrograde amnesia (Ribot’s Law), (ii) forgetting curves with and without anterograde amnesia, and (iii) learning and forgetting curves with impaired cortical plasticity. Results are in the form of closed-form expressions that are applied to studies with mice, rats, and monkeys. In order to analyze human data in a quantitative manner, we also derive a relative measure of retrograde amnesia that removes the effects of non-equal item difficulty for different time periods commonly found with clinical retrograde amnesia tests. Using these analytical tools, we review studies of temporal gradients in the memory of patients with Korsakoff’s Disease, Alzheimer’s Dementia, Huntington’s Disease, and other disorders.

Tracelink: A model of consolidation and amnesia

A connectionist model is presented, the TraceLink model, that implements an autonomous "off-line" consolidation process. The model consists of three subsystems: (1) a trace system (neocortex), (2) a link system (hippocampus and adjacent regions), and (3) a modulatory system (basal forebrain and other areas). The model is able to account for many of the characteristics of anterograde and retrograde amnesia, including Ribot gradients, transient global amnesia, patterns of shrinkage of retrograde amnesia, and correlations between anterograde and retrograde amnesia or the absence thereof (e.g., in isolated retrograde amnesia). In addition, it produces normal forgetting curves and can exhibit permastore. It also offers an explanation for the advantages of learning under high arousal for long-term retention.

Aging memories: differential decay of episodic memory components

Some memories about events can persist for decades, even a lifetime. However, recent memories incorporate rich sensory information, including knowledge on the spatial and temporal ordering of event features, while old memories typically lack this "filmic" quality. We suggest that this apparent change in the nature of memories may reflect a preferential loss of hippocampus-dependent, configurational information over more cortically based memory components, including memory for individual objects. The current study systematically tests this hypothesis, using a new paradigm that allows the contemporaneous assessment of memory for objects, object pairings, and object-position conjunctions. Retention of each memory component was tested, at multiple intervals, up to 3 mo following encoding. The three memory subtasks adopted the same retrieval paradigm and were matched for initial difficulty. Results show differential decay of the tested episodic memory components, whereby memory for configurational aspects of a scene (objects' co-occurrence and object position) decays faster than memory for featured objects. Interestingly, memory requiring a visually detailed object representation decays at a similar rate as global object recognition, arguing against interpretations based on task difficulty and against the notion that (visual) detail is forgotten preferentially. These findings show that memories undergo qualitative changes as they age. More specifically, event memories become less configurational over time, preferentially losing some of the higher order associations that are dependent on the hippocampus for initial fast encoding. Implications for theories of long-term memory are discussed.

Equal Learning Does Not Result in Equal Remembering: The Importance of Post-Encoding Processes

Explanations of variability in long-term recall typically appeal to encoding and/or retrieval processes. However, for well over a century, it has been apparent that for memory traces to be stored successfully, they must undergo a post-encoding process of stabilization and integration. Variability in post-encoding processes is thus a potential source of age-related and individual variance in long-term recall. We examined post-encoding variability in each of two experiments. In each experiment, 20-month-old infants were exposed to novel three-step sequences in each of three encoding conditions: watch only, imitate, and learn to criterion. They were tested for recall after 15 min (as a measure of the success of encoding) and either weeks (1, 2, or 3: Experiment 1) or days (1, 2, or 4: Experiment 2) later. In each experiment, differential relative levels of performance among the conditions were observed at the two tests. The results implicate post-encoding processes are a source of variance in long-term recall.

Effect of an acute d-amphetamine administration on context information memory in healthy volunteers: evidence from a source memory task

RATIONALE

Previous research demonstrated a positive effect of d-amphetamine on long-term verbal memory. An improvement in memory for contextual information is proposed as a possible mechanism underlying the d-amphetamine facilitation effect.

OBJECTIVES

A double blind, placebo controlled experiment was used to examine the processes involved in episodic memory affected by an acute administration of d-amphetamine. We investigated whether positive effects of d-amphetamine on item memory could be extended to context information by using a source memory paradigm.

METHODS

In a within-subjects design with two sessions, two study lists were presented in each session and participants were required to make an old/new recognition decision (item memory) and a list discrimination judgement (source memory) after delays of 1 h, 1 day and 1 week.

RESULTS

Enhancement of item memory after d-amphetamine intake was observed on delayed tests only, confirming that amphetamine does not affect short-term memory or memory acquisition, but rather a process operating after initial encoding. Importantly, we found an enhancement in remembering the source of recognized items after d-amphetamine administration.

CONCLUSION

The present study suggests that an acute administration of d-amphetamine helps to bind different features of an item in memory, in turn leading to an increased ability to recollect both the item and its context.

The role of the hippocampus in long-term memory: is it memory store or comparator?

Several attempts have been made to reconcile a number of rival theories on the role of the hippocampus in long-term memory. Those attempts fail to explain the basic effects of the theories from the same point of view. We are reviewing the four major theories, and shall demonstrate, with the use of mathematical models of attention and memory, that only one theory is capable of reconciling all of them by explaining the basic effects of each theory in a unified fashion, without altogether sacrificing their individual contributions. The key issue here is whether or not a memory trace is ever stored in the hippocampus itself, and there is no reconciliation unless the answer to that question is that there is not. As a result of the reconciliation that we are proposing, there is a simple solution to several outstanding problems concerning the neurobiology of memory such as: consolidation and reconsolidation, persistency of long term memory, novelty detection, habituation, long-term potentiation, and the multifrequency oscillatory self-organization of the brain.

Persistence of long-term memory storage requires a late protein synthesis- and BDNF- dependent phase in the hippocampus

Persistence is the most characteristic attribute of long-term memory (LTM). To understand LTM, we must understand how memory traces persist over time despite the short-lived nature and rapid turnover of their molecular substrates. It is widely accepted that LTM formation is dependent upon hippocampal de novo protein synthesis and Brain-Derived Neurotrophic Factor (BDNF) signaling during or early after acquisition. Here we show that 12 hr after acquisition of a one-trial associative learning task, there is a novel protein synthesis and BDNF-dependent phase in the rat hippocampus that is critical for the persistence of LTM storage. Our findings indicate that a delayed stabilization phase is specifically required for maintenance, but not formation, of the memory trace. We propose that memory formation and memory persistence share some of the same molecular mechanisms and that recurrent rounds of consolidation-like events take place in the hippocampus for maintenance of the memory trace.

Retrograde Amnesia for Autobiographical Memories and Public Events in Mild and Moderate Alzheimer's Disease

Patients with mild to moderate Alzheimer's disease and normal controls were tested on two retrograde memory tests, one based on public events, and the other querying autobiographical memory. On both tests, patients showed strong decrements as compared to normal controls, pointing to retrograde amnesia. Evidence for a gradient in retrograde amnesia was conflicted, with analyses of variance revealing no gradient beyond the most recent period, and more sensitive analyses pointing to shallow Ribot gradients on both tests. A literature review shows that this is the case in most published studies. In autobiographical remote memory patients generated many incorrect answers, a tendency correlated with the number of false alarms on an anterograde memory test administered several months earlier. This suggests a stable, possibly executive, factor underlying memory errors.

Mechanisms of memory stabilization: are consolidation and reconsolidation similar or distinct processes?

Consolidation of new memories depends on a crucial phase of protein synthesis. It is widely held that, once consolidated, memories are stable and resilient to disruption. However, established memories become labile when recalled and require another phase of protein synthesis to be maintained. Therefore, it has been proposed that when a memory is reactivated it must undergo additional consolidation (reconsolidation) to persist. To determine whether reconsolidation recapitulates consolidation, in the past few years several groups have investigated whether the same molecules and pathways mediate the formation of a memory and its maintenance after reactivation. At first glance, the results appear conflicting: although both processes appear to engage the same molecules and mechanisms, brain areas involved in consolidation after initial training are not required for reconsolidation. In addition, the formation of a memory and its maintenance after reactivation seem to have distinctive temporal molecular requirements. This review concludes with a working model that could explain the apparent controversy of memory vulnerability after reactivation.

Retrograde amnesia in dementia: comparison of HIV-associated dementia, Alzheimer's disease, and Huntington's disease

Remote memory was assessed in persons with HIV-associated dementia (HIV-D), probable Alzheimer's disease (AD), and Huntington's disease (HD) and in healthy controls. The clinical groups were similar in overall dementia severity. Each clinical group exhibited impairments on remote memory tests relative to controls; however, temporally graded memory loss with selective preservation of older information was observed in the AD group but not the HD or HIV-D group. Analysis of cued retrieval indicated a preferential cuing benefit for the HIV-D and HD groups relative to the AD group. The similar pattern of remote memory performance demonstrated by the HIV-D and HD groups is a novel finding and suggests a subcortically mediated retrograde amnesia in HIV-D. The temporally graded pattern and the abnormal cued retrieval performance in the AD group are consistent with a consolidation deficit associated with extrahippocampal (cortical) and hippocampal damage.

Retrograde amnesia for semantic information in Alzheimer's disease

Patients with mild to moderate Alzheimer's disease and normal controls were tested on a retrograde amnesia test with semantic content (Neologism and Vocabulary Test, or NVT), consisting of neologisms to be defined. Patients showed a decrement as compared to normal controls, pointing to retrograde amnesia within semantic memory. No evidence for a gradient within this amnesia was found, although one was present on an autobiographic test of retrograde amnesia that had a wider time scale. Several explanations for these results are presented, including one that suggests that extended retrograde amnesia and semantic memory deficits are in fact one and the same deficit.

The Psychology and Neuroscience of Forgetting

Traditional theories of forgetting are wedded to the notion that cue-overload interference procedures (often involving the A-B, A-C list-learning paradigm) capture the most important elements of forgetting in everyday life. However, findings from a century of work in psychology, psychopharmacology, and neuroscience converge on the notion that such procedures may pertain mainly to forgetting in the laboratory and that everyday forgetting is attributable to an altogether different form of interference. According to this idea, recently formed memories that have not yet had a chance to consolidate are vulnerable to the interfering force of mental activity and memory formation (even if the interfering activity is not similar to the previously learned material). This account helps to explain why sleep, alcohol, and benzodiazepines all improve memory for a recently learned list, and it is consistent with recent work on the variables that affect the induction and maintenance of long-term potentiation in the hippocampus.

Consolidation of Long-Term Memory: Evidence and Alternatives

Memory loss in retrograde amnesia has long been held to be larger for recent periods than for remote periods, a pattern usually referred to as the Ribot gradient. One explanation for this gradient is consolidation of long-term memories. Several computational models of such a process have shown how consolidation can explain characteristics of amnesia, but they have not elucidated how consolidation must be envisaged. Here findings are reviewed that shed light on how consolidation may be implemented in the brain. Moreover, consolidation is contrasted with alternative theories of the Ribot gradient. Consolidation theory, multiple trace theory, and semantization can all handle some findings well but not others. Conclusive evidence for or against consolidation thus remains to be found.