Autobiographical memory in amnestic Mild Cognitive Impairment

Episodic and Semantic Autobiographical Memory in Mild Cognitive Impairment (MCI): A Systematic Review

INTRODUCTION

Mild cognitive impairment (MCI) is a syndrome defined as a decline in cognitive performance greater than expected for an individual according to age and education level, not interfering notably with daily life activities. Many studies have focused on the memory domain in the analysis of MCI and more severe cases of dementia. One specific memory system is represented by autobiographical memory (AM), which has been largely studied in Alzheimer's disease and its effect on AM; however, the impairment of AM in moderate forms of decline, such as MCI, is still controversial.

OBJECTIVE

The main aim of this systematic review is to analyze the functioning of autobiographical memory in patients with MCI, considering both the semantic and the episodic components.

MATERIALS

The review process was conducted according to the PRISMA statement. The search was conducted until 20 February 2023 in the following bibliographical databases: PubMed, Web of Science, Scopus, and PsycInfo, and twenty-one articles were included.

RESULTS

The results highlight controversial findings concerning the semantic component of AM since only seven studies have found a worse semantic AM performance in patients with MCI compared to the HC group. The results of impaired episodic AM in individuals with MCI are more consistent than those concerning semantic AM.

CONCLUSIONS

Starting from the evidence of this systematic review, further studies should detect and investigate the cognitive and emotional mechanisms that undermine AM performance, allowing the development of specific interventions targeting these mechanisms.

Memory for public events in amnestic mild cognitive impairment: The role of hippocampus and ventro-medial prefrontal cortex

BACKGROUND

Current theories assume that retrograde memory deficits for semantic information in amnestic mild cognitive impairment (aMCI) are temporally graded and partially sparing most remote memories. Moreover, these models assume a prevalent role of the hippocampus in early phases of memory consolidation and of the prefrontal mesial neocortical areas in permanent consolidation of traces.

PURPOSE

To explore the relationship between hippocampus and memory accuracy for the most recent public events and between the ventro-medial prefrontal cortex (vmPFC) and memory accuracy irrespective of the memory age, we investigated in aMCI patients the retrograde memory for public events and its relationship with grey matter volume reductions in the hippocampus and vmPFC.

METHODS

18 aMCI patients and 13 healthy subjects (HS) underwent a modified version of the Famous Events questionnaire (FEq) to assess their memory performance for public events. Patients underwent 3T-MRI scanning to assess correlations between FEq's scores and grey matter volumes.

RESULTS

aMCI showed significantly reduced performances on FEq compared to HS in the recollection of most recent events, while no significant difference was observed for more remote memories, thus demonstrating a temporal gradient. Moreover, hippocampal volumes predicted accuracy scores for most recent, but not older, public events. Finally, an area in the subcallosal portion of the vmPFC, corresponding to BA32, predicted accuracy scores on FEq irrespective of the period examined.

CONCLUSIONS

Pathological changes in a neural circuit linking hippocampal to medial prefrontal cortical regions are responsible for impaired recollection of retrograde memories in aMCI.

Episodic memory decline is associated with deficits in coherence of discourse

This study investigates coherence of discourse in the production of autobiographical narratives by individuals with aMCI. Autobiographical interviews were analyzed to determine whether reduced episodic recall was related to deficits in discourse coherence. A coherence rating scale was used to evaluate relatedness of the autobiographical details produced by participants to the topic of discourse. Interviews were transcribed, segmented into details, and divided into sets of episodic, semantic, or supplementary information, which were subsequently analysed with the coherence rating scale. We predicted that the known episodic deficits observed in aMCI could also affect the retrieval of coherent episodic information. The results revealed deficits in coherence could be found in both episodic and semantic information in the aMCI group. These results suggest that the cognitive deficits experienced by individuals with aMCI may go beyond their known difficulty in recalling episodic details, as they also affect the controlled retrieval of both episodic and semantic information.

The role of hippocampus in the retrieval of autobiographical memories in patients with amnestic Mild Cognitive Impairment due to Alzheimer's disease

The role of the hippocampus and neocortical areas in the retrieval of past memories in pre-dementia Alzheimer's disease (AD) patients was investigated. The aim was to assess whether the hippocampus has a temporary role in memory trace formation, according to the Cortical Reallocation Theory (CRT), or whether it continuously updates and enriches memories, according to the Multiple Trace Theory. According to the former theory, hippocampal damage should affect more recent memories, whereas the association cortex is expected to affect memories of the entire lifespan. In the second case, damage to either the hippocampus or the association cortices should affect memories of the entire lifespan. Seventeen patients with amnestic Mild Cognitive Impairment due to AD were submitted to autobiographical (i.e., episodic and semantic personal) memory assessment. Patients underwent MRI for the acquisition of T1-weighted brain volumes. Voxel-based morphometry was used to assess correlations between grey matter (GM) volumes and autobiographical memory. Correlation analyses revealed a strict association between GM volumes in the hippocampus and patients' ability to retrieve the most recent but not the oldest autobiographical memories in both aspects, episodic and semantic. Moreover, patients' GM volumes in the pre-frontal and temporal polar areas were associated with recollection of episodic and semantic events, respectively. Finally, GM volumes in the precuneus and occipital cortex were associated with retrieval of the most recent episodic events. These findings indicate that the hippocampus has a specific time-dependent role; thus, they support the CRT.

Retrograde memory for public events in mild cognitive impairment and its relationship to anterograde memory and neuroanatomy

OBJECTIVE

The study characterized the status of retrograde amnesia (RA) in amnestic mild cognitive impairment (MCI).

METHOD

We measured RA, anterograde amnesia (AA), brain measures, apolipoprotein-E status (ApoE), and conversion to probable Alzheimer's disease (AD) across 3 years in 15 individuals with MCI. We compared the severity of amnesia and brain atrophy in MCI to a group of patients with limited damage to the hippocampus (H) or more extensive damage to the medial temporal lobe (MTL).

RESULTS

The MCI group exhibited modest AA, together with severe RA, covering nearly 4 decades before their diagnosis. Compared with H-MTL patients, the temporal extent of RA was disproportionate to the severity of AA. The MCI group exhibited more modest AA and MTL atrophy than H-MTL patients, together with more severe RA and neocortical atrophy than H-MTL patients. The severity of AA corresponded to the integrity of MTL structures, whereas the severity of RA corresponded to the integrity of both MTL and neocortical structures. RA (but not AA, nor measures of cognitive status) was related to ApoE status and subsequent diagnosis of probable AD. RA was predicted by heritable risk for AD, in addition to the integrity of MTL and neocortical structures.

CONCLUSIONS

Compared with H-MTL patients, the MCI group exhibited RA that was disproportionate to their AA and that was more severe than would be expected if their atrophy were limited primarily to the MTL. Heritable risk for AD, as well as the integrity of brain regions within and beyond the MTL, are important for understanding RA in MCI.