The role of the hippocampus in declarative memory consolidation is a matter of intense debate. We investigated the neural substrates of memory retrieval for recent and remote information using functional magnetic resonance imaging (fMRI). 18 young, healthy participants learned a series of pictures. Then, during two fMRI recognition sessions, 3 days and 3 months later, they had to determine whether they recognized or not each picture using the “Remember/Know” procedure. Presentation of the same learned images at both delays allowed us to track the evolution of memories and distinguish consistently episodic memories from those that were initially episodic and then became familiar or semantic over time and were retrieved without any contextual detail. Hippocampal activation decreased over time for initially episodic, later semantic memories, but remained stable for consistently episodic ones, at least in its posterior part. For both types of memories, neocortical activations were observed at both delays, notably in the ventromedial prefrontal and anterior cingulate cortices. These activations may reflect a gradual reorganization of memory traces within neural networks. Our data indicate maintenance and strengthening of hippocampal and cortico-cortical connections in the consolidation and retrieval of episodic memories over time, in line with the Multiple Trace theory (Nadel and Moscovitch, 1997). At variance, memories becoming semantic over time consolidate through strengthening of cortico-cortical connections and progressive disengagement of the hippocampus.

Memories are not stored as they were initially encoded but rather undergo a gradual reorganization process, termed memory consolidation. Numerous data indicate that sleep plays a major role in this process, notably due to the specific neurochemical environment and the electrophysiological activity observed during the night. Two putative, probably not exclusive, models (“hippocampo-neocortical dialogue” and “synaptic homeostasis hypothesis”) have been proposed to explain the beneficial effect of sleep on memory processes. However, all data gathered until now emerged from studies conducted in young subjects. The investigation of the relationships between sleep and memory in older adults has sparked off little interest until recently. Though, aging is characterized by memory impairment, changes in sleep architecture, as well as brain and neurochemical alterations. All these elements suggest that sleep-dependent memory consolidation may be impaired or occurs differently in older adults. This review outlines the mechanisms governing sleep-dependent memory consolidation, and the crucial points of this complex process that may dysfunction and result in impaired memory consolidation in aging.

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BackgroundCognitive and mood disorders negatively impact daily life in patients with multiple sclerosis (MS). Pharmacological treatments did not demonstrate any effect on cognition compared with cognitive rehabilitation (CR). However, if CR programs offer promising results on cognition, they are less consistent concerning mood and quality of life (QoL). In this context, we designed a randomized controlled trial to evaluate the efficacy of an innovative computerized CR program, conducted at home, on QoL. Secondary objectives will estimate the improvement, or the stabilization over time, of patients’ cognitive performances and their emotional affects.MethodsForty MS patients (relapsing-remitting or secondary progressive forms) who have cognitive impairment will be recruited for the trial (called SEPIA-NCT03471338) and randomly assigned to either the experimental group or the control group. Patients randomly assigned in the experimental group will perform a home-based CR program with psychological support during eight consecutive weeks. CR will be based on computerized cognitive exercises from the PRESCO® software developed by HAPPYneuron©. Training sessions (three sessions of 45 min per week) will consist of short exercises evaluating a broad range of cognitive domains and will be personalized for each patient (tracking tool and supervised guidance). The control group, designed to control for non-specific elements of the intervention, will receive only psychological support consisting of various issues related to MS, such as everyday cognitive-related difficulties or management of emotions. QoL, assessed by the MUSIQOL (Multiple Sclerosis International Quality Of Life) questionnaire, will be evaluated three times (at baseline and after 1 week and 25 weeks after home-based intervention) as well as secondary outcomes measuring self-esteem, cognition, depression, anxiety, metacognition, fatigue, and sleep quality. Given the expected MUSIQOL variation, the inclusion of 20 patients per group (alpha risk 5% and power 80%) will be required.DiscussionEvidence suggests that computerized programs may be a practice option for CR for people with MS, but there is a paucity of studies evaluating QoL. We hope that this innovative program will highlight such benefits over time in patients’ daily life. In the future, such programs will allow a wider range of available therapeutic options for MS patients with cognitive impairment and for practitioners in charge of their care.Trial registrationClinicalTrials.gov identifier: NCT03471338. Retrospectively registered on 25 April 2018. https://clinicaltrials.gov/ct2/show/NCT03471338?term=NCT03471338&cond=Multiple+Sclerosis&draw=2&rank=1.

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