Franck Doidy

How aging affects sleep-dependent memory consolidation?

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.

Does the heart forget? Modulation of cardiac activity induced by inhibitory control over emotional memories

The subjective construction surrounding the perception of negative experiences is partly built upon bodily afferent information, comprising heart, gut or respiratory signals. While this bottom-up influence has been extensively described, the opposite pathway, the putative influence of cognitive processes over autonomic response, is still debatable. Emotion regulation and the ability to control maladjusted physiological response associated with thoughts and memories is a recurrent concern for most psychiatric disorders and mental health as a whole. Memory suppression (i.e. exerting inhibitory control toward unwanted memories) has been proposed as a possible solution to perform such regulation. This method, however, also holds debates as it could putatively worsen distressing reactions when unsuccessful, or simply be in-effective on the physiological roots of emotions. Here, we tested the hypothesis that memory suppression can influence the physiological marker of emotions in two studies using the “Think/No-Think” paradigm. We measured in healthy participants the aftereffect of suppressing negative memories on their cardiac reaction toward the excluded memories. Results of Study 1 revealed that an efficient control of memories was associated with a long-term inhibition of the cardiac deceleration normally induced by disgusting stimuli. Attempts to suppressing sad memories, on the opposite, aggravated cardiac response, an effect that was largely related to the inability to forget this specific material. Study 2 recorded concurrent electroencephalographic (EEG) response during the TNT task to track the oscillatory dynamics supporting the inhibitory control of memories and estimate their role in the inhibition of the later cardiac response. We found that the decrease of the 5-9 Hz frequency band, a prominent electrophysiological marker of memory suppression, was indeed related to the subsequent inhibition of the cardiac response toward disgusting stimuli. These results show that the modulation of cognitive representations can have long-term effects on the cardiac system. They further support the notion that cognitive control over unwanted emotional memories, when successful, can reduce the autonomic aversive processes to achieve emotional regulation. Our findings open new avenues for possible interventions focused on training this suppression mechanism to reduce the impact of distressing intrusive memories on mental health.

Alexithymia in Amyotrophic Lateral Sclerosis and Its Neural Correlates

Introduction: Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disease that causes progressive and extensive motor deficits. Patients may also have cognitive impairments or alteration of emotional processing. Very few studies, however, have looked at deficits in how they experience their own feelings (alexithymia). Methods: We assessed alexithymia in 28 patients with ALS using the 20-item Toronto Alexithymia Scale (TAS-20), comparing them with a control group matched for sex, age, and education level. We took into account both the total score of the TAS-20 and its three subscores corresponding to the three dimensions of alexithymia: Difficulty Identifying Feelings (DIF), Difficulty Describing Feelings (DDF), and Externally Oriented Thinking (EOT). Patients also underwent a neuropsychological assessment and anatomical magnetic resonance imaging (MRI) in order to correlate cognitive performances and gray matter volume and level of alexithymia. Results: On average, ALS subjects had a significantly higher total score and DIF sub-score of the TAS-20 than controls indicating an increased alexithymia in patients. Total and DIF Scores correlated significantly and negatively to gray matter volume of the prefrontal cortex, right superior temporal pole and parahippocampal gyri. No correlations were found between scores on executive functions and those on the TAS-20. Conclusion: The first stage of ones own emotional processing seems to be affected in ALS independently of executive dysfunction. This trouble seems to be underpinned by cerebral regions that are well known to be both implicated in alexithymia in healthy subjects and altered in ALS.

Impact of Semantic Relatedness on Associative Memory: An ERP Study

Encoding and retrieval processes in memory for pairs of pictures are thought to be influenced by inter-item similarity and by features of individual items. Using Event-Related Potentials (ERP), we aimed to identify how these processes impact on both the early mid-frontal FN400 and the Late Positive Component (LPC) potentials during associative retrieval of pictures. Twenty young adults undertook a sham task, using an incidental encoding of semantically related and unrelated pairs of drawings. At test, we conducted a recognition task in which participants were asked to identify target identical pairs of pictures, which could be semantically related or unrelated, among new and rearranged pairs. We observed semantic (related and unrelated pairs) and condition effects (old, rearranged and new pairs) on the early mid-frontal potential. First, a lower amplitude was shown for identical and rearranged semantically related pairs, which might reflect a retrieval process driven by semantic cues. Second, among semantically unrelated pairs, we found a larger negativity for identical pairs, compared to rearranged and new ones, suggesting additional retrieval processing that focuses on associative information. We also observed an LPC old/new effect with a mid-parietal and a right occipito-parietal topography for semantically related and unrelated old pairs, demonstrating a recollection phenomenon irrespective of the degree of association. These findings suggest that associative recognition using visual stimuli begins at early stages of retrieval, and differs according to the degree of semantic relatedness among items. However, either strategy may ultimately lead to recollection processes.

Rôle du sommeil dans la consolidation des souvenirs

De nombreuses etudes indiquent que le sommeil favorise la consolidation en memoire a long terme des informations recemment acquises. Dans cet article, nous proposons une synthese des travaux menes sur les liens entre sommeil et memoire, au moyen de differentes approches experimentales (privation de sommeil, imagerie cerebrale…). Ces etudes ont contribue a preciser les substrats neurobiologiques sous-tendant l’effet benefique du sommeil sur la memoire et ont abouti a la proposition de deux modeles : l’hypothese du dialogue hippocampo-neocortical et la theorie de l’homeostasie synaptique (ou recalibrage synaptique). Ces deux modeles sont decrits et nous exposons quelques arguments experimentaux en faveur de chacun d’eux.