Most research in cognitive neuroscience explores how external stimuli are processed by the brain. However, the brain also receives input from the internal body. We discuss here how the heart and gastrointestinal (GI) tract intrinsically generate their own electrical activity, thereby continuously sending information to the brain. These ongoing ascending signals actively shape brain dynamics at rest, complementing canonical resting-state networks (RSNs). Cardiac signals also influence the processing of external sensory information and the production of spontaneous, internal cognition. These findings are discussed in relation to interpretative frameworks regarding the functional role of visceral inputs. This active field of research offers a unique opportunity to draw new theories blurring the border between cognition, emotion, and consciousness, as well as between mind and body. A Paradigm Shift in Cognitive Neuroscience 'The brain is bombarded by information from the environment.' This sentence, in one form or another, acts as an introduction to many textbooks and articles about perception and cognition (e.g., [1]). Although the internal environment is sometimes mentioned, the vast majority of the experimental work pertains to the processing of external stimuli. This reflects the dominant paradigm in cognitive neuroscience, where an agent collects information from the external environment via the senses and then reacts to this environment by producing actions (Figure 1A). By studying the brain, cognitive neuroscience has been highly successful at providing mechanistic explanations of behavior and at revealing the existence of hidden rules and variables, such as for instance the reward prediction error [2]. In the following we review extensions of this paradigm that include the interplay between the brain and the internal bodily environment (Figure 1B) to account for the neural implementation of cognition. More specifically, we focus on the heart and the gastrointestinal (GI) tract (see Glossary), and refer the reader to [3] for a comprehensive review of other types of somatic influences, including notably humoral and immune factors.

Resting-state networks offer a unique window into the brain’s functional architecture, but their characterization remains limited to instantaneous connectivity thus far. Here, we describe a novel resting-state network based on the delayed connectivity between the brain and the slow electrical rhythm (0.05 Hz) generated in the stomach. The gastric network cuts across classical resting-state networks with partial overlap with autonomic regulation areas. This network is composed of regions with convergent functional properties involved in mapping bodily space through touch, action or vision, as well as mapping external space in bodily coordinates. The network is characterized by a precise temporal sequence of activations within a gastric cycle, beginning with somato-motor cortices and ending with the extrastriate body area and dorsal precuneus. Our results demonstrate that canonical resting-state networks based on instantaneous connectivity represent only one of the possible partitions of the brain into coherent networks based on temporal dynamics.