Neural correlates of autobiographical memory in amnestic Mild Cognitive Impairment

Neural Correlates of Autobiographical Memory: Evidence From a Positron Emission Tomography Study in Patients With Mild Cognitive Impairment and Alzheimer's Disease

Background: Autobiographical memory (AM) changes are the hallmark of Alzheimer's disease (AD) and mild cognitive impairment (MCI). In recent neuroimaging studies, AM changes have been associated with numerous cerebral sites, such as the frontal cortices, the mesial temporal lobe, or the posterior cingulum. Regional glucose uptake in these sites was investigated for underlying subdimensions using factor analysis. Subsequently, the factors were examined with respect to AM performance in a subgroup of patients. Methods: Data from 109 memory clinic referrals, who presented with MCI (n = 60), mild AD (n = 49), or were cognitively intact, were analyzed. The glucose metabolic rates determined by positron emission tomography (PET) with 18F-fluorodeoxyglucose (FDG) in 34 cerebral sites important for AM were investigated for underlying subdimensions by calculating factor analysis with varimax rotation. Subsequently, the respective factor scores were correlated with the episodic and semantic AM performance of 22 patients, which was measured with a semi-structured interview assessing episodic memories (characterized by event-related emotional, sensory, contextual, and spatial-temporal details) and personal semantic knowledge from three periods of life (primary school, early adulthood, and recent years). Results: Factor analysis identified seven factors explaining 69% of the variance. While patients with MCI and AD showed lower values than controls on the factors frontal cortex, mesial temporal substructures, and occipital cortex, patients with MCI presented with increased values on the factors posterior cingulum and left temporo-prefrontal areas. The factors anterior cingulum and right temporal cortex showed only minor, non-significant group differences. Solely, the factor mesial temporal substructures was significantly correlated with both episodic memories (r = 0.424, p < 0.05) and personal semantic knowledge (r = 0.547, p < 0.01) in patients with MCI/AD. Conclusions: The factor structure identified corresponds by large to the morphological and functional interrelations of the respective sites. While reduced glucose uptake on the factors frontal cortex, mesial temporal substructures, and occipital cortex in the patient group may correspond to neurodegenerative changes, increased values on the factors posterior cingulum and left temporo-prefrontal areas in MCI may result from compensatory efforts. Interestingly, changes of the mesial temporal substructures were correlated with both semantic and episodic AM. Our findings suggest that AM deficits do not only reflect neurodegenerative changes but also refer to compensatory mechanisms as they involve both quantitative losses of specific memories and qualitative changes with a semantization of memories.

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.

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.

Henry, where have you lost your Self?

The Self is a complex construct encompassing distinct components, including episodic and semantic autobiographical memory, the Self-concept, and the subjective sense of Self, which highest level consists of Self-awareness. The neuro-anatomical correlates are complex, and it is debated as to whether a common region could support these different components of the Self, with a particular interest for the cortical midline structures and the medial prefrontal cortex (MPFC). Alzheimer's disease (AD) constitutes an interesting model for the study of Self as autobiographical memory typically deteriorates as the disease progresses. Here, we report the unexpected case of Henry, a patient with MCI due to AD who was unable to produce any personal autobiographical memories, nor describe his Self-concept, had a poor personal semantic memory, and disclosed unusual anosognosia for this stage of the disease. His cognitive performance was compared to a group of matched AD patients and a group of healthy controls confirming that the main components of his Self were degraded. We hypothesized that it was due to a marked atrophy within the cortical midline, as visually assessed on his MRI. We further elucidated these findings through Voxel-based morphometry analysis, which confirmed a significant atrophy of the MPFC that was specific to this patient. Moreover, this revealed significant atrophy within the bilateral insular cortex. Given the stage of the disease, the degradation of the Self is unlikely to be accounted for by deficient mnemonic processes, especially as the presence of discrete temporal atrophy was noted. We suggest that this specific pattern of MPFC and insular atrophy is responsible for the systematic collapse of the patient's Self, through the breakdown of the subjective sense of Self, which is proposed as a prerequisite to all other components, according to the model proposed by Prebble, Addis, and Tippett (2013).

Social cognition and the cerebellum: A meta-analytic connectivity analysis

This meta-analytic connectivity modeling (MACM) study explores the functional connectivity of the cerebellum with the cerebrum in social cognitive processes. In a recent meta-analysis, Van Overwalle, Baetens, Mariën, and Vandekerckhove (2014) documented that the cerebellum is implicated in social processes of "body" reading (mirroring; e.g., understanding other persons' intentions from observing their movements) and "mind" reading (mentalizing, e.g., inferring other persons' beliefs, intentions or personality traits, reconstructing persons' past, future, or hypothetical events). In a recent functional connectivity study, Buckner et al. (2011) offered a novel parcellation of cerebellar topography that substantially overlaps with the cerebellar meta-analytic findings of Van Overwalle et al. (2014). This overlap suggests that the involvement of the cerebellum in social reasoning depends on its functional connectivity with the cerebrum. To test this hypothesis, we explored the meta-analytic co-activations as indices of functional connectivity between the cerebellum and the cerebrum during social cognition. The MACM results confirm substantial and distinct connectivity with respect to the functions of (a) action understanding ("body" reading) and (b) mentalizing ("mind" reading). The consistent and strong connectivity findings of this analysis suggest that cerebellar activity during social judgments reflects distinct mirroring and mentalizing functionality, and that these cerebellar functions are connected with corresponding functional networks in the cerebrum.

Social cognition and the cerebellum: a meta-analysis of over 350 fMRI studies

This meta-analysis explores the role of the cerebellum in social cognition. Recent meta-analyses of neuroimaging studies since 2008 demonstrate that the cerebellum is only marginally involved in social cognition and emotionality, with a few meta-analyses pointing to an involvement of at most 54% of the individual studies. In this study, novel meta-analyses of over 350 fMRI studies, dividing up the domain of social cognition in homogeneous subdomains, confirmed this low involvement of the cerebellum in conditions that trigger the mirror network (e.g., when familiar movements of body parts are observed) and the mentalizing network (when no moving body parts or unfamiliar movements are present). There is, however, one set of mentalizing conditions that strongly involve the cerebellum in 50-100% of the individual studies. In particular, when the level of abstraction is high, such as when behaviors are described in terms of traits or permanent characteristics, in terms of groups rather than individuals, in terms of the past (episodic autobiographic memory) or the future rather than the present, or in terms of hypothetical events that may happen. An activation likelihood estimation (ALE) meta-analysis conducted in this study reveals that the cerebellum is critically implicated in social cognition and that the areas of the cerebellum which are consistently involved in social cognitive processes show extensive overlap with the areas involved in sensorimotor (during mirror and self-judgments tasks) as well as in executive functioning (across all tasks). We discuss the role of the cerebellum in social cognition in general and in higher abstraction mentalizing in particular. We also point out a number of methodological limitations of some available studies on the social brain that hamper the detection of cerebellar activity.