Behavioural and Magnetoencephalographic Evidence for the Interaction Between Semantic and Episodic Memory in Healthy Elderly Subjects
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Retrospective memory
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Animal models of episodic memory—the ability to recall events from the past along with relevant spatial, temporal, and other contextual detail—have been criticized on the grounds that it is difficult to determine if animals engage in the “mental time travel” that accompanies episodic recall in humans. It is possible that animals rely on mechanisms different from those operative in humans. We have added a new dimension to these concerns by showing that a test widely considered to measure episodic memory in humans can be constructed such that it measures nonconscious (implicit) memory under certain test parameters. The same parameters are common in tests frequently used to study recognition in nonhuman animals. In particular, recognition was driven by implicit memory when (1) recognition decisions could not be guided by semantic information, and (2) decisions were made with relatively high automaticity. These findings suggest that nonhuman animals may rely on implicit memory processing in many episodic-memory testing circumstances, as semantic content is minimal and automaticity is likely after the extended practice usually required. Identifying factors that promote and discourage recognition based on implicit memory is therefore necessary for creating accurate cognitive and neurobiological models of memory processing in humans and other animals.
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Amnesic patients can acquire new semantic knowledge despite a profound deficit of episodic memory. Although retrospective studies have been carried out on adults and children, prospective studies have been restricted to adults. The aim of the present work was to assess the acquisition of new semantic knowledge in amnesic children. The semantic protocol, composed of short, illustrated texts, was based on an original methodology which included an assessment of episodic memory on two occasions. Two amnesic children acquired new concepts despite major episodic disturbance, illustrated notably by a lack of recollection in episodic tasks. Our findings lend weight to the assumption that forming new semantic knowledge does not necessarily involve episodic memory, and provide methodological pointers for children's neuropsychological rehabilitation.
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Episodic memory and semantic memory are two types of declarative memory. There have been two principal views about how this distinction might be reflected in the organization of memory functions in the brain. One view, that episodic memory and semantic memory are both dependent on the integrity of medial temporal lobe and midline diencephalic structures, predicts that amnesic patients with medial temporal lobe/diencephalic damage should be proportionately impaired in both episodic and semantic memory. An alternative view is that the capacity for semantic memory is spared, or partially spared, in amnesia relative to episodic memory ability. This article reviews two kinds of relevant data: 1) case studies where amnesia has occurred early in childhood, before much of an individual's semantic knowledge has been acquired, and 2) experimental studies with amnesic patients of fact and event learning, remembering and knowing, and remote memory. The data provide no compelling support for the view that episodic and semantic memory are affected differently in medial temporal lobe/diencephalic amnesia. However, episodic and semantic memory may be dissociable in those amnesic patients who additionally have severe frontal lobe damage. Hippocampus 1998;8:205–211. Published 1998 Wiley-Liss, Inc.1 This article is a US Government work and, as such, is in the public domain in the United States of America.
Retrospective memory
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The ability to envisage personally relevant events at a future time point represents an incredibly sophisticated cognitive endeavor and one that appears to be intimately linked to episodic memory integrity. Far less is known regarding the neurocognitive mechanisms underpinning the capacity to envisage non-personal future occurrences, known as semantic future thinking. Moreover the degree of overlap between the neural substrates supporting episodic and semantic forms of prospection remains unclear. To this end, we sought to investigate the capacity for episodic and semantic future thinking in Alzheimer's disease (n = 15) and disease-matched behavioral-variant frontotemporal dementia (n = 15), neurodegenerative disorders characterized by significant medial temporal lobe and frontal pathology. Participants completed an assessment of past and future thinking across personal (episodic) and non-personal (semantic) domains, as part of a larger neuropsychological battery investigating episodic and semantic processing, and their performance was contrasted with 20 age- and education-matched healthy older Controls. Participants underwent whole-brain T1 weighted structural imaging and voxel-based morphometry analysis was conducted to determine the relationship between grey matter integrity and episodic and semantic future thinking. Relative to Controls, both patient groups displayed marked future thinking impairments, extending across episodic and semantic domains. Analyses of covariance revealed that while episodic future thinking deficits could be explained solely in terms of episodic memory proficiency, semantic prospection deficits reflected the interplay between episodic and semantic processing. Distinct neural correlates emerged for each form of future simulation with differential involvement of prefrontal, lateral temporal and medial temporal regions. Notably, the hippocampus was implicated irrespective of future thinking domain, with the suggestion of lateralization effects depending on the type of information being simulated. Whereas episodic future thinking related to right hippocampal integrity, semantic future thinking was found to relate to left hippocampal integrity. Our findings support previous observations of significant medial temporal lobe involvement for semantic forms of prospection and point to distinct neurocognitive mechanisms which must be functional to support future-oriented forms of thought across personal and non-personal contexts.
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Episodic memory and semantic memory are two types of declarative memory. There have been two principal views about how this distinction might be reflected in the organization of memory functions in the brain. One view, that episodic memory and semantic memory are both dependent on the integrity of medial temporal lobe and midline diencephalic structures, predicts that amnesic patients with medial temporal lobe/diencephalic damage should be proportionately impaired in both episodic and semantic memory. An alternative view is that the capacity for semantic memory is spared, or partially spared, in amnesia relative to episodic memory ability. This article reviews two kinds of relevant data: 1) case studies where amnesia has occurred early in childhood, before much of an individual's semantic knowledge has been acquired, and 2) experimental studies with amnesic patients of fact and event learning, remembering and knowing, and remote memory. The data provide no compelling support for the view that episodic and semantic memory are affected differently in medial temporal lobe/diencephalic amnesia. However, episodic and semantic memory may be dissociable in those amnesic patients who additionally have severe frontal lobe damage. Hippocampus 1998;8:205–211. Published 1998 Wiley-Liss, Inc.1 This article is a US Government work and, as such, is in the public domain in the United States of America.
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Retrograde amnesia
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Abstract It has been proposed that declarative memories can be dependent on both an episodic and a semantic memory system. While the semantic system deals with factual information devoid of reference to its acquisition, the episodic system, characterized by mental time travel, deals with the unique past experience in which an event took place. Episodic memory is characteristically hippocampus‐dependent. Place cells are recorded from the hippocampus of rodents and their firing reflects many of the key characteristics of episodic memory. For example, they encode information about “what” happens “where,” as well as temporal information. However, when these features are expressed during an animal's behavior, the neuronal activity could merely be categorizing the present situation and could therefore reflect semantic memory rather than episodic memory. We propose that mental time travel is the key feature of episodic memory and that it should take a form, in the awake animal, similar to the replay of behavioral patterns of activity that has been observed in hippocampus during sleep. Using tasks designed to evoke episodic memory, one should be able to see memory reactivation of behaviorally relevant sequences of activity in the awake animal while recording from hippocampus and other cortical structures. © 2006 Wiley‐Liss, Inc.
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