In any event, a conclusion is necessary now: even at an early stage of the disease, Alzheimer’s patients are deficient in terms of learning and consolidation of memories. 

Decontextualized memories

A distinction is generally made between explicit (conscious) and implicit (unconscious) memories. Numerous authors postulate that the recovery of a piece of information in explicit memory can operate in accordance with two processes. Sometimes the information will be recalled with its encoded context (moment, place, circumstances, emotions felt, and personal thoughts) – this is referred to as “recollection”, an inherent concept in the context of episodic memory. Sometimes the information with which the subject is confronted will cause him to feel a “simple” sentiment of familiarity denuded of every contextual detail (I have a feeling I know that person but I don’t remember where I met her). “In one of our recent studies (2), we showed that the capacity for recovery of a piece of information by means of a feeling of familiarity is similar in patients suffering from the early onset of Alzheimer’s and in normal subjects”,  says Eric Salmon. The fMRI scan shows that, in the two samples, this capacity depends on the parietal areas involved in attentional processes.

The capacity for recollection of information is drastically reduced in Alzheimer’s, even at an early stage. The researchers from Liege discovered that, in both patients and controlled subjects, it is linked to the activity of the posterior cingulate cortex and that this region “works” in the patient suffering from dementia. So where exactly is the problem? It is to be found in the loss of connectivity between this cortex and the medial temporal areas, particularly the hippocampus. Consequently the traces of contextualized memories (that can be recollected) cannot be created. “The most atrophied structure in the Alzheimer’s patient is the hippocampus and not the posterior cingulate cortex. According to several authors, this cortex is a communication node. Because it is receiving less information from the hippocampus, it shows a reduced carbohydrate metabolism and is incapable of redistributing the tasks necessary for encoding information”, explains Professor Salmon.

Familiarity

The autobiographical memory gathers all the information (episodic and semantic) in direct relationship with our personal history. It is an intimate part of us and is eminently subjective as it is charged with emotions. The group Ageing and Memory explored it in Alzheimer’s patients and in normal elderly subjects by focusing on autobiographical memories some of which were related to their distant past (their young adulthood) and the others to the previous year (3).

The first assessment revealed that even in patients at very early stages of Alzheimer’s, autobiographical memories are a great deal less rich in contextual details. A second assessment by means of FDG-PET scan images revealed that the loss of episodic characteristics in memories of the period when the subjects were young adults stems from a metabolic impairment of the occipital and occipitoparietal regions involved in the reactivation of sensory and perceptual details. For recent memories, the dysfunction is located elsewhere: it is in the lateral temporal regions that are solicited for recovering semantic clues about personal experiences and in the temporoparietal regions which are involved in the capacity to establish a personal perspective (relate information to oneself) which can favor episodic recollection.

(2) Genon S., Recollection and familiarity in Alzheimer's disease: An fMRI study, Cortex (in press).
(3) Bastin C. et al., Episodic autobiographical memory in amnesic cognitive impairment: What are the neural correlates? Human Brain Map (in press).