Combining Sensory Experiences with Internal Milieu in the Brain

Understanding the brain is not only intrinsically fascinating to apprehend life’s complexity or to further progress in fundamental biosciences, but it is also highly relevant to increase our well-being since the brain exhibits a power over the body that makes it capable both of triggering illness or facilitating the healing process. Considering the dual role at play by the brain, using ascending and descending pathways to combine information issued from the external world and the internal environment, this review challenges the cerebro-centered vision of the brain. In our daily life, we construct a representation of the outside world by transforming chemical, pressure changes, and light waves into tastes, smells, touches, sounds and sights. In doing so, we create our experience of the external world by interpreting our senses through a process called exteroception. But to be compelling, this Descartes’ vision of the brain has to be completed by integration of events from inside our body. The way the brain constructs our inner sensations called interoception, is now starting to be unrevealed. As such, brain sciences have undergone, and will undergo, an important revolution, redefining its boundaries beyond the skull to prefer a more holistic vision carried out by the notion of an embodied brain acting as a coincidence detector to combine sensory experiences with corporeal homeostasis. The goal of this review is to highlight some mechanisms by which the brain activity is controlled by internal cues for better prediction. The gut-brain axis is here taken as a canonical example to discuss about the communication between the milieu intérieur and brain functions that shapes how we feel, and how we think.