Long-term Episodic Memory Research Articles

Spatial deficits in an amnesic patient with hippocampal damage: Questioning the multiple trace theory

Mediotemporal lobe structures are involved in both spatial processing and long-term memory. Patient M.R. suffers from amnesia, due to bilateral hippocampal lesion and temporoparietal atrophy following carbon monoxide poisoning. We compared his performance in immediate spatial memory tasks with the performance of ten healthy matched participants. Using an immediate reproduction of path, we observed a dissociation between his performance in three allocentric tasks and in five egocentric-updating tasks. His performance was always impaired on tasks requiring the use of an egocentric-updating representation but remained preserved on allocentric tasks. These results fit with the hypothesis that the hippocampus plays a role in spatial memory, but they also suggest that allocentric deficits previously observed in amnesia might actually reflect deficits in egocentric-updating processes. Furthermore, the co-occurrence of deficits in episodic long-term memory and short-term egocentric-updating representation without any short-term allocentric deficit suggests a new link between the mnemonic and navigational roles of the hippocampus. The Cognitive Map theory, the Multiple Trace theory, as well as further models linking spatial and nonspatial functions of the hippocampus are discussed.

Episodic long-term memory as a predictor of change from retrieval to storage memory profile in amnestic mild cognitive impairment

Episodic long-term memory as a predictor of change from retrieval to storage memory profile in amnestic mild cognitive impairment

Cooperation between the Hippocampus and the Striatum during Episodic Encoding

The hippocampus and the striatum are thought to play distinct roles in learning and memory, each supporting an independent memory system. A fundamental question is whether, and how, these systems interact to jointly contribute to learning and memory. In particular, it remains unknown whether the striatum contributes selectively to implicit, habitual learning, or whether the striatum may also contribute to long-term episodic memory. Here, we show with functional magnetic resonance imaging (fMRI) that the hippocampus and the striatum interact cooperatively to support episodic memory formation. Participants were scanned during a memory encoding paradigm and, subsequently, were tested for memory of encoded items. fMRI data revealed that successful memory was associated with greater activity in both the hippocampus and the striatum (putamen) during encoding. Furthermore, activity in the hippocampus and the striatum was correlated within subjects for items that were later remembered, but not for items that were forgotten. Finally, across subjects, the strength of the correlation between the hippocampus and the striatum predicted memory success. These findings provide novel evidence for contributions of both the striatum and the hippocampus to successful episodic encoding and for a cooperative interaction between them.

Increased inhibition and enhancement of memory retrieval are associated with reduced hippocampal volume.

Putative control of encoding and retrieval processes have been linked to communication between the lateral prefrontal cortex (LPFC) and the hippocampus. Moreover, correlations between the LPFC (e.g., MFG) and hippocampus have predicted individuals' ability to inhibit memory retrieval. Anatomically, differences in volume of the hippocampus have been related to changes in long-term episodic memories. Although the relationship between these ideas is clear, few studies have examined the association of how anatomy may affect the role of control over brain regions involved in distint memory processes. The current study sought to examine hippocampal volume and its relationship to LPFC control over the hippocampus. Using an automated cortical/subcortical segmentation technique (FIRST) on brain imaging gata from the Think/No-Think task, we show that hippocampal volume is associated to changes in both enhancement and inhibitory processes of memory retrival.

Cognitive Hearing Science

Stuart Gatehouse was one of the pioneers of cognitive hearing science. The ease of language understanding (ELU) model (Rönnberg) is one example of a cognitive hearing science model where the interplay between memory systems and signal processing is emphasized. The mismatch notion is central to ELU and concerns how phonological information derived from the signal, matches/mismatches phonological representations in lexical and semantic long-term memory (LTM). When signals match, processing is rapid, automatic and implicit, and lexical activation proceeds smoothly. Given a mismatch, lexical activation fails, and working or short-term memory (WM/STM) is assumed to be invoked to engage in explicit repair strategies to disambiguate what was said in the conversation. In a recent study, negative long-term consequences of mismatch were found by means of relating hearing loss to episodic LTM in a sample of old hearing-aid wearers. STM was intact (Rönnberg et al.). Beneficial short-term consequences of a binary masking noise reduction scheme on STM was obtained in 4-talker babble for individuals with high WM capacity, but not in stationary noise backgrounds (Ng et al.). This suggests that individuals high on WM capacity inhibit semantic auditory distraction in 4-talker babble while exploiting the phonological benefits in terms of speech quality provided by binary masking (Wang). Both long-term and short-term mismatch effects, apparent in data sets including behavioral as well as subjective (Rudner et al.) data, need to be taken into account in the design of future hearing instruments.

Behavioral specifications of reward-associated long-term memory enhancement in humans

Recent functional imaging studies link reward-related activation of the midbrain substantia nigra-ventral tegmental area (SN/VTA), the site of origin of ascending dopaminergic projections, with improved long-term episodic memory. Here, we investigated in two behavioral experiments how (1) the contingency between item properties and reward, (2) the magnitude of reward, (3) the uncertainty of outcomes, and (4) the contextual availability of reward affect long-term memory. We show that episodic memory is enhanced only when rewards are specifically predicted by the semantic identity of the stimuli and changes nonlinearly with increasing reward magnitude. These effects are specific to reward and do not occur in relation to outcome uncertainty alone. These behavioral specifications are relevant for the functional interpretation of how reward-related activation of the SN/VTA, and more generally dopaminergic neuromodulation, contribute to long-term memory.

Higher intraindividual variability is associated with more forgetting and dedifferentiated memory functions in old age

Intraindividual trial-to-trial reaction time (RT) variability is commonly found to be higher in clinical populations or life periods that are associated with impaired cognition. In the present study, higher within-person trial-to-trial RT variability in a perceptual speed task is related to more forgetting and dedifferentiation of memory functions in older adults (aged 60–71 years). More specifically, our study showed that individuals in a high-variability group (n=175) forgot more memory scenes over a 1-week retention interval than individuals in the low-variability group (n=174). In contrast, slower RT speed was associated with poorer episodic memory in general, but unrelated to the amount of forgetting. Moreover, results from multiple group latent factor analyses showed that episodic memory and working memory functions were more highly correlated in the high-variability (r=.63) than in the low-variability (r=.25) group. Given that deficits in dopamine (DA) modulation may underlie increases in RT variability, the present findings are in line with (i) recent animal studies implicating DA in long-term episodic memory consolidation and (ii) neurocomputational work linking DA modulation of performance variability to dedifferentiation of cognitive functions in old age.

Neuropsychological and neurobiological markers of the preclinical stage of Alzheimer’s disease.

Dementia, especially Alzheimer's disease, has a high prevalence in the elderly population. Therefore, identifying individuals who are at a high risk for early diagnosis is crucial to allow both pharmacological and behavioral therapeutic interventions, which in some cases can delay the progression of dementia. This paper describes neuropsychological and neurobiological markers for the early diagnosis of Alzheimer's disease and presents the main risk factors, including neuropathological, neuroanatomical, neurofunctional, genetic, and neuropsychological. The literature shows that the combination of these markers is the best method for predicting Alzheimer's disease, years before its clinical manifestation. The most prevalent neurobiological and neuropsychological risk factors include (1) senile plaques and neurofibrillary tangles in the medial temporal lobe and cortical regions, (2) low concentrations of Aβ1-42 peptide and high concentrations of total tau protein and phosphorylated tau protein in cerebrospinal fluid, (3) reduced global cerebral volume, increased ventricular volume, and atrophy in the hippocampal formation and entorhinal cortex, (4) global reductions in cerebral metabolism and perfusion in the temporoparietal junction, temporal, parietal, and frontal lobes, hippocampal formation, and posterior cingulate cortex, (5) the presence of the apolipoprotein E e4 allele, and (6) verbal anterograde episodic long-term memory impairment and executive dysfunction. The present review discusses the evidence for markers that identify individuals who are at a high risk of developing Alzheimer's disease and the importance of longitudinal studies in this context.

Prefrontal and Medial Temporal Lobe Activity at Encoding Predicts Temporal Context Memory

One of the defining features of episodic long-term memory is that it includes information about the temporal context in which an event occurred. Little is known about the regions that support the encoding of temporal information in the human brain, although previous work has suggested a role for the lateral prefrontal cortex (PFC) and medial temporal lobes (MTL). Here we used event-related fMRI to examine the relationship between activity at encoding and subsequent memory for temporal context. Participants were scanned while performing a serial order working memory task with pictures of common objects and were later tested for temporal memory at two different timescales. In the coarse temporal memory test, participants viewed one object from each trial and indicated approximately when during the course of the experiment it had appeared. In the fine temporal memory test, participants were shown the remaining objects from each trial and asked to recall the order in which they had been originally presented. Activity in the parahippocampal cortex predicted subsequent fine temporal accuracy, whereas coarse temporal accuracy was predicted by activity in several regions of the PFC, as well as in the hippocampus. Additional multivoxel pattern analyses revealed evidence implicating the rostrolateral PFC in the representation of time-varying contextual states in a manner similar to that proposed by computational theories of temporal context memory. These results highlight MTL and PFC contributions to temporal memory at the time of encoding and suggest a particular role for the rostrolateral PFC in encoding coarse temporal context.

Obstructive sleep apnea and cognitive impairment in the elderly

Obstructive Sleep Apnea Syndrome (OSAS) is characterized by repeated episodes of upper airway obstruction during sleep that result in intermittent hypoxemia and arousal. The prevalence of OSAS increases with aging, occurring in up to 25% of older adults and up to 48% in patients with Alzheimer's disease. OSAS causes hypoxia, fragmented sleep, daytime sleepiness, cognitive dysfunction, functional decline, and brain damage resulting from reduced cerebral blood flow, ischemic brain lesions, microvascular reactivity, white matter lesions, and grey matter loss. OSAS is considered as an independent risk factor for hypertension, stroke and mortality. The treatment of choice for OSAS is continuous positive airway pressure (CPAP). OSAS-related cognitive dysfunction has been shown in a variety of domains including attention, executive functioning, motor efficiency, working memory, and long-term episodic memory. Proposed mechanisms include hypoxemia, sleep fragmentation and inflammatory process, but it remains unclear which mechanisms underlie the relationship between OSAS and disturbances in the different cognitive domains. Recent studies suggest that OSAS may exacerbate cognitive functioning in dementia and that CPAP therapy can be applied to these patients and improve cognitive functioning.

With time on our side? Task-dependent compensatory processes in graceful aging

Graceful aging has been associated with frontal hyperactivations in working- and episodic long-term memory tasks, a compensatory process, according to some, that allows the best normal elders to perform these tasks at a juvenile level, in spite of natural cortical impoverishment. In this study, 24 young and 24 healthy elderly participants were compared. Graceful aging was explored by investigating domains where most healthy elders perform like youngers (e.g. lexical-semantic knowledge) and tasks that are typically more challenging, like episodic long-term recognition memory tasks. With voxel-based morphometry, we also studied to what extent changes of fMRI activation were consistent with the pattern of brain atrophy. We found that hyperactivations and hypoactivations of the elders were not restricted to the frontal lobes, rather they presented with task-dependent patterns. Only hypoactivations and normal levels of activation systematically overlapped with regional atrophy. We conclude that compensatory processes associated with graceful aging may not necessarily be a sign of early saturation of executive resources, if this was to be represented by a systematic frontal hyperactivation, but rather they may represent the ability of recruiting new cognitive strategies. We discuss two possible approaches to further test this hypothesis.

Developmental dyslexia and explicit long‐term memory

The reduced verbal long-term memory capacities often reported in dyslexics are generally interpreted as a consequence of their deficit in phonological coding.The present study was aimed at evaluating whether the learning deficit exhibited by dyslexics was restricted only to the verbal component of the long-term memory abilities or also involved visual-object and visual-spatial domain. A further goal of the present study was to investigate the predictive value of non-verbal long-term memory abilities with respect to word and non-word reading in dyslexic children.In accordance with these aims, performances of 60 dyslexic children were compared with that of 65 age-matched normal readers on verbal, visual-spatial and visual-object task.Results documented a generalized impairment of episodic long-term memory capacities in dyslexic children and the results did not vary as a function of children's age.Furthermore, in addition to verbal measures, also individual differences in non-verbal long-term memory tasks turn out to be good predictors of reading difficulties in dyslexics.Our findings indicate that the long-term memory deficit in dyslexia is not limited to the dysfunction of phonological components but also involves visual-object and visual-spatial aspect, thus suggesting that dyslexia is associated to multiple cognitive deficits.

Cognitive function is related to fronto-striatal serotonin transporter levels – a brain PET study in young healthy subjects

Pharmacological manipulation of serotonergic neurotransmission in healthy volunteers impacts on cognitive test performance. Specifically, markers of serotonin function are associated with attention and executive functioning, long-term memory, and general cognitive ability. The serotonin transporter (SERT) protein is a key regulator in the serotonin system. We hypothesized that higher performance on tests sensitive to serotonin would be associated with higher SERT levels in specific fronto-striatal brain regions. Thirty-two healthy subjects (25 males, mean age 26.0 years, range 19-37) underwent positron emission tomography using the SERT ligand [(11)C]DASB. Subjects underwent the following tests: Stroop Color Word Test, Trail Making Test B, Rey's Auditory Verbal Learning Test and Complex Figure Test, logical reasoning subtest from Intelligenz-Struktur-Test 2000 R, and a Danish version of National Adult Reading Test. We found positive associations between performance on the Stroop Color Word Test and right-sided dorsolateral prefrontal SERT binding (R(2) = 0.12, p = 0.048). Furthermore, scores of logical reasoning (correlating with IQ) and educational level associated positively with SERT binding in the caudate, most prominent on the left side (logical reasoning: R(2) = 0.34, p = 0.0026 (left), R(2) = 0.2, p = 0.022 (right), educational level: R(2) = 0.19, p = 0.012 (left), R(2) = 0.15, p = 0.027 (right)). Scores of logical reasoning also associated with left-sided ventrolateral prefrontal cortex (R(2) = 0.24, p = 0.014). There were no significant associations between SERT binding and tests of long-term episodic memory. The results imply that in healthy subjects, high SERT binding in fronto-striatal regions is associated with better performance on tasks involving executive function and logical reasoning.

Estrogen treatment impairs cognitive performance after psychosocial stress and monoamine depletion in postmenopausal women

Recent studies have shown that women experience an acceleration of cognitive problems after menopause and that estrogen treatment can improve or at least maintain current levels of cognitive functioning in postmenopausal women. However, we have previously shown that the negative emotional effects of psychosocial stress are magnified in normal postmenopausal women after estrogen treatment. This study examined whether estradiol (E2) administration can modify cognitive performance after exposure to psychological stress and monoamine depletion. Participants consisted of 22 postmenopausal women placed on either oral placebo or 17beta-E2 (1 mg/d for 1 mo, then 2 mg/d for 2 mo). At the end of the 3-month treatment phase, participants underwent three depletion challenges in which they ingested one of three amino acid mixtures: deficient in tryptophan, deficient in phenylalanine/tyrosine, or balanced. Five hours later, participants performed the Trier Social Stress Test (TSST), followed by mood and anxiety ratings and cognitive testing. Cognitive measures included tests of attention, psychomotor function, and verbal episodic memory. E2-treated compared with placebo-treated participants exhibited significant worsening of cognitive performance on tasks measuring attentional performance and psychomotor speed. Similar trends for impairment were seen in measures of long-term episodic memory compared with placebo-treated postmenopausal women. E2-treated participants also showed a significant increase in negative mood and anxiety compared with placebo-treated women after, but not before, the TSST, although the worsening of both cognitive and behavioral functioning was not correlated. These effects were independent of tryptophan or tyrosine/phenylalanine depletion and were not manifested before the TSST or at baseline. These data suggest that the relationship between estrogen administration and cognitive/behavioral performance in postmenopausal women may be more complex than initially appreciated and that the effects of psychosocial stress may influence whether hormone effects are beneficial.

Evidence for similar principles in episodic and semantic memory: The presidential serial position function

When people recall a list of items that they have just experienced (an episodic memory task), the resulting serial position function looks strikingly similar to that observed when people are asked to recall the presidents of the United States (a semantic memory task). Despite the similarity in appearance, there is disagreement about whether the two functions arise from the same processes. A local distinctiveness model of memory, SIMPLE, successfully fit the presidential data using two underlying dimensions: one corresponding to item (or presidential) distinctiveness and the other to order (or positional) distinctiveness. According to the model, presidential primacy and recency are due to the same mechanisms that give rise to primacy and recency effects in both short- and long-term episodic memory. All of these primacy and recency effects reflect the relative distinctiveness principle (Surprenant & Neath, 2009): Items will be well remembered to the extent that they are more distinct than competing items at the time of retrieval.

Role of the hippocampus in goal-oriented tasks requiring retrieval of spatial versus non-spatial information

The role of the hippocampus in non-spatial memory has been issue of some controversy. To investigate the nature of dorsal hippocampus engagement in spatial and non-spatial memory we performed discrete excitotoxic lesions of this region before mice (C57/BL6) were trained in one of two tasks that required the animals to retrieve a hidden food reward. In the visuospatial task animals had to remember a particular spatial location, independent of odor cues. In contrast, in a non-spatial olfactory task animals had to remember a particular odor, independent of spatial location. The mice were trained in one of these tasks over a period of three days. We found that lesions restricted to the dorsal hippocampus affected performance only in the spatial task. In contrast, lesions that also encompassed a larger portion of the ventral hippocampus caused a moderate deficit in the olfactory task. These results are consistent with the role of the dorsal hippocampus in long-term spatial episodic memory, and support the involvement of larger portions of the hippocampus on the encoding of non-spatial olfactory representations.

La révolution des neurosciences

About 50 years ago, three discoveries marked the beginning of revolutions in biology and neurosciences; namely, the tri-dimensional structure of DNA, the action of temporal hippocampus in long-term episodic memory, and the proof of an innate neurological organization. Since then, subsequent work in these three fields has always made scientific improvements on the front page. It is time to evaluate consequences, not only in the field of rehabilitation, but also in renewed humanistic culture. DNA analysis drove to minimizing gene contents and to emphasizing the true construction process from the embryo in an activity-dependent development, with the result that there is no aspect of psychism at birth. Temporal hippocampal activity by the way of synaptic modifications gives a brand new light to what is long-term episodic memory. Innate cerebral organization specifies radical constructivism.

Signatures of memory: brain coactivations during retrieval distinguish correct from incorrect recollection

Are specific distributed coactivations in the brain during memory retrieval a signature of retrieval outcome? Here we show that this is indeed the case. Widespread brain networks were reported to be involved in the retrieval of long-term episodic memories. Although functional coactivation among particular regions occurs during episodic memory retrieval, it is unknown to what extent it contributes to the accuracy and confidence of recollection. In this study we set out to explore this question. Participants saw a narrative documentary movie. A week later they underwent an fMRI scan during which they either accepted or rejected factual or fictitious verbal statements concerning the movie. Correct vs. incorrect responses to factual statements were more common and were provided with higher confidence than those made to fictitious statements. Whereas activity in the retrieval network correlated mostly with confidence, coactivations primarily correlated with memory accuracy. Specifically, coactivations of left medial temporal lobe regions with temporal and parietal cortices were greater during correct responses to factual statements, but did not differ between responses to fictitious statements. We propose that network coactivations play a role in recovering memory traces that are relevant to online retrieval cues, culminating in distinct retrieval outcomes.

The temporal attributes of episodic memory

From a temporal dynamic processing point of view, episodic memory can be divided into three critical time periods: short-term episodic memory with a duration of seconds, intermediate-term episodic memory with a duration from minutes to hours, and long-term or remote episodic memory with a duration from days to years. We propose that short-term episodic memory is mediated by the CA3 subregion of the hippocampus, intermediate-term episodic memory is mediated by the CA1 subregion of the hippocampus (in certain situations aided by the CA3 subregion), and that long-term or remote episodic memory may be mediated by the CA1 subregion. In support of the above mentioned proposal data are presented to support the short-term and intermediate episodic memory functions of CA3 and CA1 based on single item object, spatial location, and object–place association tasks. Additional data are presented for a role for CA3 in short-term episodic memory based on multiple sequential spatial locations, visual objects, and odors tasks. The same episodic memory model based on duration mentioned above cannot easily be applied to the functions of the CA3 (short-term episodic) and CA1 (intermediate-term episodic) for a multiple sequentially presented item, such as a places, objects or odors. The reason for this is that the CA1 region supports, in addition to intermediate episodic memory, temporal pattern separation processes which would reduce interference among sequentially experienced items. The consequence is that this temporal pattern separation process can result in CA1 involvement in short-term episodic tasks based on duration. Also, data are presented based on tasks that involved multiple-trials tested within a day and between days short-term and intermediate-term episodic memory. Furthermore, the mechanisms for understanding the interactions and dissociations between CA3 and CA1 are discussed. The DG appears to have a modulatory influence on the CA3 and CA1 mediation of short-term and intermediate-term episodic memory. The role of CA1 in supporting remote episodic memory requires more experimentation.

Is the hippocampus necessary for visual and verbal binding in working memory?

A series of experiments test the recent claim that the hippocampus is necessary for the binding of features in working memory. Some potential limitations of studies underlying this claim are discussed, and an attempt is made to further test the hypothesis by studying a case of developmental amnesia whose extensively investigated pathology appears to be principally limited to the hippocampus, and who shows the expected deficit in episodic long-term memory. One series of experiments studied the short-term visual binding of color and shape under conditions ranging from simple presentation of colored objects through the more demanding task of combining the features when separated in space, or in time. A second set of experiments studied the capacity to use sentence structure to bind words into chunks in short-term verbal memory. Hippocampal pathology did not lead to a decrement on any of these tasks, suggesting that the hippocampus is not essential for short-term binding in working memory.