Directed forgetting of source memory in normal aging and Alzheimer's disease

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Repeated recall on source misattribution in Alzheimer's disease

We assessed the effect of repeated recall on item memory and source monitoring in Alzheimer's disease (AD). AD patients and controls were instructed to either look at or imagine items. They then had to either retrieve the items without indicating their source in three consecutive free recall tests, or to remember the source of the retrieved items in three consecutive source tests. Results showed a significant increase in item memory across all three free recall tests and all three source monitoring tests in control participants, but not in AD patients. More source misattributions were observed in AD patients and controls in the third source test than in the first and second tests. The absence of beneficial effect of repeated recall on item memory in AD patients may be attributed to compromise in memory consolidation processes, while the increase in source misattributions due to repetitions may be attributed to the inflation effect.

The picture of the past: Pictures to cue autobiographical memory in Alzheimer's disease

INTRODUCTION

This study investigated the effect of pictorial cues on autobiographical memory in Alzheimer's disease (AD). We assessed autobiographical memory of patients with AD and cognitively normal older adults in two conditions.

METHODS

In one condition, the participants were provided with verbal instructions to retrieve three autobiographical memories. In the second condition, the same verbal instructions were provided; however, the participants were simultaneously presented with three pictures. We analyzed autobiographical memory regarding specificity, that is, the ability to remember unique events situated in time and space.

RESULTS

Analysis demonstrated higher autobiographical memory after verbal-and-visual cuing than after the no cue condition in both patients with AD and cognitively normal older adults.

DISCUSSION

Pictorial cues seem to be an effective method to alleviate autobiographical compromise in AD.

The subjective experience of mind wandering in Alzheimer's disease

Introduction: Little is known about mind wandering in Alzheimer's disease (AD). In this study, we evaluated the subjective experience of mind wandering in AD.Methods: We invited AD patients and control participants to rate the occurrence, intentionality, emotionality, visual imagery, specificity, self-relatedness and temporal orientation of mind wandering.Results: Analysis showed that AD patients rated their mind wandering as more frequent, negative, and more oriented toward the past, but less vivid and specific than that of control participants. No significant differences were observed between AD patients and control participants regarding the intentionality or self-relatedness of mind wandering.Conclusions: These findings demonstrate the negative content in AD. Regarding the reduction of visual imagery and specificity during mind wandering, this reduction may mirror a diminished subjective experience of mind wandering in AD. Regarding temporality, our results may reflect a tendency of AD patients to reminisce over past experiences. Finally, mind wandering in AD seems to trigger significant self-related content.

Memory of decisions: Relationship between decline of autobiographical memory and temporal discounting in Alzheimer's disease

Temporal discounting refers to the tendency to attribute higher value to a reward received early than to one received later. We evaluated this tendency in patients with Alzheimer's Disease. We also evaluated whether temporal discounting is associated with decline in autobiographical memory (i.e., the ability to remember past personal experiences), with executive dysfunction, and/or with general cognitive decline. We invited patients with AD and control participants to answer binary questions involving the choice between receiving a smaller amount of money earlier or a larger one later (e.g., "Which do you prefer, 10 euros in cash right now or 50 euros in a month?"). Results demonstrated higher temporal discounting in patients with AD than in control participants. Temporal discounting was significantly correlated with decline in AM and general cognitive decline but not with executive dysfunction in patients with AD. The tendency to decide based on immediate rewards (i.e., temporal discounting) in AD is related with difficulty in remembering information about experiences of previous decisions, and/or their consequences (i.e., decline in autobiographical memory).

False Memory in Alzheimer's Disease

Patients with Alzheimer's Disease (AD) not only are suffering from amnesia but also are prone to memory distortions, such as experiencing detailed and vivid recollections of episodic events that have never been encountered (i.e., false memories). To describe and explain these distortions, we offer a review to synthesize current knowledge on false memory in AD into a framework allowing for better understanding of the taxonomy and phenomenology of false memories and of the cognitive mechanisms that may underlie false memory formation in AD. According to this review, certain phenomenological characteristics of memories (e.g., high emotional load, high vividness, or high familiarity) result in misattributions in AD. More specifically, this review proposes that generalized decline in cognitive control and inhibition in AD may result in difficulties in suppressing irrelevant information during memory monitoring, especially when irrelevant (i.e., false) information is characterized by high emotion, vividness, or familiarity. This review also proposes that binding deficits in AD decrease the ability to retrieve relevant contextual details, leading to source monitoring errors and false memories. In short, this review depicts how phenomenological characteristics of memories and failures of monitoring during retrieval contribute to the occurrence of false memory in AD.

Intentional inhibition but not source memory is related to hallucination-proneness and intrusive thoughts in a university sample

Proneness to unusual perceptual states - such as auditory or visual hallucinations - has been proposed to exist on a continuum in the general population, but whether there is a cognitive basis for such a continuum remains unclear. Intentional cognitive inhibition (the ability to wilfully control thoughts and memories) is one mechanism that has been linked to auditory hallucination susceptibility, but most evidence to date has been drawn from clinical samples only. Moreover, such a link has yet to be demonstrated over and above relations to other cognitive skills (source monitoring) and cognitive states (intrusive thoughts) that often correlate with both inhibition and hallucinations. The present study deployed two tests of intentional inhibition ability - the Inhibition of Currently Irrelevant Memories (ICIM) task and Directed Forgetting (DF) task - and one test of source monitoring (a source memory task) to examine how cognitive task performance relates to self-reported i) auditory hallucination-proneness and ii) susceptibility to intrusive thoughts in a non-clinical student sample (N = 76). Hierarchical regression analyses were used to assess the independent and combined contributions of task performance to proneness scores. ICIM performance but not DF or source memory scores were significantly related to both hallucination-proneness and intrusive thoughts. Further analysis suggested that intrusive thoughts may mediate the link between intentional inhibition skills and auditory hallucination-proneness, suggesting a potential pathway from inhibition to perception via intrusions in cognition. The implications for studying cognitive mechanisms of hallucination and their role in "continuum" views of psychosis-like experiences are discussed.

Self-defining Memories in Normal Aging

BACKGROUND

Self-defining memories refer to events that are vivid, effectively intense, and include enduring concerns about oneself.

OBJECTIVE

We investigated the relationship between the production of these memories in normal aging and the ability to integrate new information into existing knowledge in memory (i.e., updating).

METHOD

Older participants were asked to perform an updating task as well as to retrieve autobiographical memories that were later analyzed for their self-defining relevance.

RESULTS

Analyses showed significant positive correlations between updating and the production of self-defining memories.

CONCLUSION

Updating our life story is an important psychological process,which enables us to refine and enrich our life story with new experiences, roles and/or challenges, and this ability seems to be related to the capacity to produce memories that draw on the pursuit of long-term goals, meaning making, emotional regulation, and/or activation of self-images (i.e., self-defining memories). These findings suggest that updating one's identity throughout life, at least in normal aging, may be related to the shaping and retrieval of self-defining memories, memories that lead to the creation of narrative scripts, which themselves serve as the ingredients for "chapters" across the lifespan.

Hallucinations in Alzheimer's disease: failure to suppress irrelevant memories

INTRODUCTION

Research with patients with schizophrenia suggests that inhibitory dysfunction leads to the emergence of redundant or irrelevant information from long-term memory into awareness, and that this process may be involved in generating hallucinations. We investigated whether inhibitory dysfunction in Alzheimer's disease (AD) leads to hallucinations.

METHOD

AD participants and healthy matched controls were assessed with a hallucinations scale and a directed forgetting task. On the directed forgetting task, they were asked to retain a list of 10 words (i.e., List 1). Thereafter, half of the participants were asked to forget this list whereas the other half were asked to retain the list in memory. After the List 1 presentation, all participants were asked to retain another list of 10 words and, successively, were asked to remember all of the words from both lists, regardless of the previous forget or remember instruction.

RESULTS

Relative to healthy matched controls, AD participants showed difficulties in suppressing the words from List 1. AD participants also showed more hallucinatory experiences than healthy matched controls. Interestingly, a significant correlation was observed between the score on the hallucinations measure and difficulties in suppressing List 1 in AD participants.

DISCUSSION

Hallucinations in AD may, at least in part, be related to difficulties in suppressing memory representations, such that unwanted or repetitive thoughts intrude into consciousness.

Beneficial Effect of Minimal Interference on Item Memory But Not on Source Memory in Alzheimer's Disease

OBJECTIVES

Research suggests beneficial effects of minimal interference on item memory in mild Alzheimer's disease (AD). We conducted a study to test whether these beneficial effects extend to source memory, i.e., the ability to remember the episodic context in which an information was previously acquired.

METHOD

Twenty-six mild AD participants and 28 controls manipulated six objects or watched the experimenter manipulating six other objects. After immediate item recall ("what were the items?") and source recall ("by whom the items were previously manipulated?"), an interference or a minimal interference condition was administrated. In the interference condition, participants were assessed with neuropsychological tests for 10 min. In the minimal interference condition, they rested alone in a dark and quiet room for 10 minutes. Both interference and minimal interference conditions were followed by the delayed recall, on which participants were asked to remember the previously-presented objects and their source.

RESULTS

Higher item memory was observed following minimal interference than following interference in AD participants (p < .01) and controls (p < .01). Also, AD participants demonstrated higher item memory on immediate recall than following interference (p < .001) or minimal interference (p < .001); controls also demonstrated higher item memory on immediate recall than following interference (p < .001) or minimal interference (p < .001). Considering source memory, similar source memory was observed following interference and minimal interference in AD participants (p > .1) and controls (p > .1). Also, AD participants demonstrated higher source memory on immediate recall than following interference (p < .001) or minimal interference (p < .001); controls also demonstrated higher source memory on immediate recall than following interference (p < .001) or minimal interference (p < .001).

CONCLUSION

Failures of hippocampus-dependent associative or consolidation processes in AD may preclude benefits of minimal interference for source memory. Nevertheless, AD patients may show some capacity to retain simple material, should the material presentation be followed by short delays that are free of further stimuli.

Negative Prospective Memory in Alzheimer's Disease: "Do Not Perform That Action"

Relatively to "standard" prospective memory, i.e., remembering to perform a future action, little is known about negative prospective memory, i.e., remembering not to perform a future action. This study investigated the latter ability in Alzheimer's disease (AD). AD participants and healthy older adults were asked to click on the keyboard or not to click on it when a cue word was encountered. Results showed more omissions (i.e., forgetting to click the keyboard when the instruction was to do so) in AD participants than in healthy older adults, suggesting a prospective memory deficit. Interestingly, more commissions (i.e., clicking the keyboard when the instruction was not to do so) were also observed in AD participants than in healthy older adults. Similar levels of commissions and omissions were observed in AD participants and in healthy older adults. Also, commissions and omissions were correlated with performance on an inhibition assessment task. Our findings reveal that AD is characterized by not only difficulty in the retrieval of recent information, but also difficulty to inhibit no-longer appropriate stimulus-response associations previously learned, suggesting a specific deficit of negative prospective memory in AD.

The use of metacognitive strategies to decrease false memories in source monitoring in patients with mild cognitive impairment

Patients with amnestic mild cognitive impairment (MCI) and Alzheimer's disease (AD) often demonstrate high rates of false memories, leading to stressful and frustrating situations for both patients and caregivers in everyday life. Sometimes these false memories are due to failures in monitoring the source of the information. In the current study, we examined interventions aimed to enhance the use of the metacognitive "recall-to-reject" memory strategy. Such interventions could improve source memory and decrease false memory in patients with MCI. Because the picture superiority effect (better memory for pictures compared to words) has been shown to be present in both patients with MCI and healthy older controls, we investigated whether pictures could help patients with MCI use a recall-to-reject strategy in a simulation of real-world source memory task. In this experiment, patients with MCI and healthy older adults were asked to simulate preparing for and then taking a trip to the market. Subjects first studied 30 pictures of items in their "cupboard," followed by a list of 30 words of items on their "shopping list." At test, participants saw 90 pictures (30 cupboard, 30 list, 30 new) organized as they would be if walking down the market aisles, and are provided with either standard or metacognitive instructions. With standard instructions, they were asked if they needed to buy the item. With the metacognitive instructions, they were asked a series of questions to help guide them through a recall-to-reject strategy to highlight the different sources of memories. Results showed that the metacognitive instructions did significantly reduce the false memory rates for patients with MCI. Further studies need to investigate how to best implement these practical strategies into the everyday lives of patients.

Apolipoprotein E (APOE) ε4 and episodic memory decline in Alzheimer's disease: A review

A growing body of research has examined the relationship between episodic memory decline, the cognitive hallmark of Alzheimer's disease (AD), and the presence of Apolipoprotein E ε4 (APOE ε4) allele, a major genetic risk factor for the disease. Our review attempts to summarize and critically evaluate this literature. We performed a systematic search for studies assessing episodic memory in AD patients who were genotyped for APOE ε4 and identified fourteen papers. Although most of these papers reported significant relationships between APOE ε4 and episodic memory decline in AD, some papers did not confirm this relationship. Our review links this controversy to the conflicting literature about the effects of APOE ε4 on general cognitive functioning in AD. We identify several shortcoming and limitations of the research on the relationship between APOE ε4 and episodic memory in AD, such as small sample sizes, non-representative populations, lack of comparison of early-onset vs. late-onset disease, and lack of comparison among different genotypes that include APOE ε4 (i.e., zero, one, or two ε4 alleles). Another major shortcoming of the reviewed literature was the lack of comprehensive evaluation of episodic memory decline, since episodic memory was solely evaluated with regard to encoding and retrieval, omitting evaluation of core episodic features that decline in AD, such as context recall (e.g., how, where, and when an episodic event has occurred) and subjective experience of remembering (e.g., reliving, emotion and feeling during episodic recollection). Future research taking these limitations into consideration could illuminate the nature of the relationship between APOE ε4 and episodic memory decline in AD.

Inhibitory deterioration may contribute to hallucinations in Alzheimer's disease

INTRODUCTION

Although delusions and hallucinations are relatively common symptoms in individuals with Alzheimer's disease (AD), very little is known regarding underlying mechanisms. We examined whether these manifestations could be underpinned by psychological distress and executive impairments.

METHODS

Thirty-one participants with probable mild AD and 33 healthy older adults were administered a neuropsychological and clinical battery assessing delusions, hallucinations, anxiety, depression, episodic memory and executive functions (shifting, updating and inhibition).

RESULTS

Prevalence of delusions and hallucinations were significantly higher in AD participants compared to control participants. Further, hallucinations in AD participants were significantly correlated with poor inhibition, with the latter uniquely predicting the former, as compared to other variables. In addition, hallucinations in AD participants were associated with depression, a relationship that was further mediated by inhibition.

CONCLUSION

Hallucinations in individuals with AD seem to be related to difficulties suppressing irrelevant thoughts, resulting in these irrelevant thoughts becoming confused with ongoing reality.

Memory suppression in Alzheimer's disease

An important challenge for memory is the competition between appropriate and inappropriate information during retrieval. This competition is normally reduced thanks to controlled inhibitory processes that suppress irrelevant memories. In Alzheimer's disease (AD), compromise of suppression ability may result in strong competition between relevant and irrelevant memories during retrieval. The present review highlights this issue by examining studies using the directed forgetting method in AD. This method in which participants are typically instructed to forget no longer relevant information is argued to reflect suppression in memory. Studies using the directed forgetting method suggest that AD participants experience difficulties when they are asked to suppress no longer relevant information in working, autobiographical, source and destination memory. Difficulties in suppressing irrelevant information, as may be observed in AD, may hamper memory retrieval by activating irrelevant memories at the expense of relevant ones.

Autobiographical memory decline in Alzheimer's disease, a theoretical and clinical overview

Autobiographical memory, or memory for personal experiences, allows individuals to define themselves and construct a meaningful life story. Decline of this ability, as observed in Alzheimer's disease (AD), results in an impaired sense of self and identity. In our model (AMAD: Autobiographical Memory in Alzheimer's Disease), we present a critical review of theories and findings regarding cognitive and neuroanatomical underpinnings of autobiographical memory and its decline in AD and highlight studies on its clinical rehabilitation. We propose that autobiographical recall in AD is mainly characterized by loss of associated episodic information, which leads to de-contextualization of autobiographical memories and a shift from reliving past events to a general sense of familiarity. This decline refers to retrograde, but also anterograde amnesia that affects newly acquired memories besides remote ones. One consequence of autobiographical memory decline in AD is decreased access to memories that shape self-consciousness, self-knowledge, and self-images, leading to a diminished sense of self and identity. The link between autobiographical decline and compromised sense of self in AD can also manifest itself as low correspondence and coherence between past memories and current goals and beliefs. By linking cognitive, neuroanatomical, and clinical aspects of autobiographical decline in AD, our review provides a theoretical foundation, which may lead to better rehabilitation strategies.

"Forget to whom you have told this proverb": directed forgetting of destination memory in Alzheimer's disease

Destination memory is the ability to remember the receiver of transmitted information. By means of a destination memory directed forgetting task, we investigated whether participants with Alzheimer's Disease (AD) were able to suppress irrelevant information in destination memory. Twenty-six AD participants and 30 healthy elderly subjects were asked to tell 10 different proverbs to 10 different celebrities (List 1). Afterwards, half of the participants were instructed to forget the destinations (i.e., the celebrities) whereas the other half were asked to keep them in mind. After telling 10 other proverbs to 10 other celebrities (List 2), participants were asked to read numbers aloud. Subsequently, all the participants were asked to remember the destinations of List 1 and List 2, regardless of the forget or remember instructions. The results show similar destination memory in AD participants who were asked to forget the destinations of List 1 and those who were asked to retain them. These findings are attributed to inhibitory deficits, by which AD participants have difficulties to suppress irrelevant information in destination memory.