Grâce à l’imagerie cérébrale fonctionnelle, nos connaissances concernant le réseau par défaut (resting state ou default mode network) ne cessent de s’étendre.

Le réseau par défaut est composé de plusieurs régions cérébrales à présent bien identifiées qui s’activent spontanément lorsque le sujet n’est pas engagé dans une activité cognitive orientée vers un but précis et dont le rôle cognitif fait l’objet de nombreux travaux.

Cette revue dresse le tableau des connaissances anatomiques, physiologiques et phénoménologiques actuelles concernant ce réseau chez le sujet jeune, ainsi que ses modifications au cours du vieillissement normal et dans la pathologie (maladie d’Alzheimer et sclérose en plaques).

Le réseau par défaut pourrait refléter diverses activités introspectives (planification et simulations mentales du futur basées sur la recollection des expériences autobiographiques passées, théorie de l’esprit) et/ou une activité attentionnelle diffuse destinée à surveiller l’environnement. Des atteintes spécifiques de ce réseau sont rapportées au cours du vieillissement normal, dans la maladie d’Alzheimer et dans la sclérose en plaques.

A “resting state” or “default mode network” has been highlighted in functional neuroimaging studies as a set of brain regions showing synchronized activity at rest or in task-independent cognitive state.

A considerable and increasing number of studies have been conducted over the last few years so as to unravel the cognitive function(s) of this brain network.

This review gives an overview of anatomical, physiological and phenomenological data regarding the default mode network. Different hypotheses have been proposed regarding the role of this network. Several studies have highlighted its involvement in autobiographical memory, prospection, self, attention, and theory of mind. The influence of the attention level and consciousness onto resting state brain network activity has also been discussed. Specific changes have been described in normal aging, Alzheimer’s disease (AD) and multiple sclerosis (MS).

These studies altogether contribute to a better definition of the default mode network, in terms of implicated brain structures, subtending mechanisms, and potential cognitive roles. For instance, similarities and relationships were found between self-related brain activity and resting-state activity in regions belonging to this network, namely posterior cingulate and prefrontal areas that may reflect introspective activity experienced, more or less consciously, when the brain is not specifically engaged in a cognitive task. As a whole, the default mode network appears as a non human-specific intrinsic functional network, active all over the life from birth until aging where it is progressively modified, and sensitive to different pathologies including AD and MS. On the other hand, many points remain to be clarified concerning this network, such as the exact part of its activity dedicated to self-related cognitive processes (introspection, imaginary mental scenario based on past autobiographical experiences) and that involved in a sentinel-like attentional process designated to react to possible environmental events. Indeed, it seems that this network is functional even in case of low level of consciousness, i.e., during light sleep. Conversely, a loss of self and environment perception as in coma, deep sleep or anesthesia might modulate its connectivity along the anteroposterior axis, i.e., frontal activity disappearance associated with a parietal reinforcement of connectivity. Since studies aiming at highlighting these points are still uncommon to date, exhaustive and objective explorations are needed to better understand all these resting state processes.