Autobiographical memory and autonoetic consciousness: triple dissociation in neurodegenerative diseases

Abstract

Few studies have investigated autobiographical amnesia in neurodegenerative diseases and yet these pathologies are particularly relevant when addressing the issue of theories of long-term memory consolidation. According to the standard model, the medial temporal lobe (MTL) is involved in the storage and retrieval of episodic and semantic memories during a limited period of years. An alternative model, the multiple trace theory (MTT), suggests that the capacity of the MTL to recollect episodic memories is of a more permanent nature. In order to test these models, we studied three groups of patients with a neurodegenerative disease predominantly affecting different cerebral structures namely the MTL (13 patients in the early stages of Alzheimer's disease) and the neocortex involving either the anterior temporal lobe (10 patients with semantic dementia) or the frontal lobe (15 patients with the frontal variant of frontotemporal dementia, fv-FTD). We compared these groups of patients with control subjects using an original and reliable autobiographical memory task designed specially to assess strictly episodic memory over the entire lifespan. This task, developed on the basis of the most up-to-date definition of episodic memory, takes into account the ability to mentally travel back in time and re-experience the source of acquisition (remembering, i.e. autonoetic consciousness) via the remember/know paradigm. All three groups of patients produced strongly contrasting profiles of autobiographical amnesia (regardless of the nature of the memories), which also differed markedly from that of the control group: temporally graded memory loss in Alzheimer's disease, showing that remote memories are better preserved than recent ones; memory loss with a reversed gradient in semantic dementia; and memory loss without any clear gradient in fv-FTD. Most strictly episodic memories (i.e. unique, specific in time and space, and detailed) were impaired, whatever the time interval considered in the three groups, though the memory loss was ungraded in Alzheimer's disease and fv-FTD, and temporally graded in semantic dementia, sparing the most recent period. A deficit of autonoetic consciousness emerged in Alzheimer's disease and fv-FTD, but not in semantic dementia, though beyond the most recent 12-month period, the latter group could not justify their subjective sense of remembering to the same extent as the controls, in terms of the actual contextual information retrieved-phenomenological, spatial or temporal details. Our results demonstrate that autobiographical amnesia varies according to the nature of the memories under consideration and the locus of cerebral dysfunction. They are discussed in the light of the two competing models of long-term memory consolidation and recent conceptions of autobiographical recollection: new insights based on current concepts of episodic memories challenge the standard model and tend to support the MTT instead.

Anatomical and functional alterations in semantic dementia: A voxel-based MRI and PET study

“…While episodic memory deficits could be partly due to semantic memory impairments, the use of a visual episodic task in our study suggests genuine episodic memory impairment, although definitely less serious than in Alzheimer Disease patients [52,58]. Finally, all the patients who underwent the behavioural assessment presented various changes, in line with growing evidence that many patients with SD have behavioural changes, sometimes identical to those suffering from the frontal variant of frontotemporal dementia [5,18,39,54,65,67].…”

“…Left-sided predominant atrophy is more frequent than right predominant or symmetrical involvement (e.g., [7,26,29]). Visual inspection of MRI brain scans has suggested that the hippocampal complex is preserved in SD, which might fit with normal day-to-day memory, or near normal performance on episodic memory tests in some patients [27,28,57,58]. Some authors [29,9] did not find significant atrophy of the hippocampus and adjacent structures using the SPM software which allows a voxel-by-voxel analysis (Voxel Based Morphometry, VBM) of the entire brain.…”

Sense of identity in advanced Alzheimer’s dementia: A cognitive dissociation between sameness and selfhood?

“…These results may reflect two sides of sense of identity, one being relatively general and quite stable, and therefore preserved even in the presence of extensive cognitive and neural damage, the other varying over time and with experience, and therefore more fragile, especially in the case of severe impairment of both episodic and semantic retrograde memory (Haslam et al, 2011;Piolino et al, 2003).…”

“…When memories of past experiences are impaired, as they are in AD, the sense of existing in the past must be dramatically compromised. Given the temporal gradient of memory impairment in AD (Piolino et al, 2003), patients may travel back to the time in their lives for which they have the clearest memories, the resulting gap being resolved by their continuity of sense of identity. The selfrepresentation of age, a component of sense of identity, is thus dependent upon subjective time and becomes tied to the best preserved episodic memories.…”

Item familiarity and controlled associative retrieval in Alzheimer's disease: An fMRI study

“…More concretely, participants are asked to report whether they recognise items through recollection of episodic 3 details of the encoding context (Remember) or through a feeling of familiarity without any recollective experience (Know). Most of the studies that assessed the subjective aspects of recollection and familiarity in AD patients by using the Remember-Know procedure found that the experience of remembering, but not familiarity, was significantly impaired in AD patients (Dalla Barba, 1997;Piolino et al, 2003;Rauchs et al, 2007). However, a more recent study has found that both Remember and Know responses were reduced in AD patients (Hudon et al, 2009).…”