Interoception network in the rat brain.

The brain is never truly at rest. Even in the absence of external stimuli, the central nervous system continuously monitors and regulates visceral organs. However, it remains unclear to what extent this interoceptive signaling shapes the brain's resting state networks. Here, we tested the hypothesis that resting-state functional connectivity is actively sustained by visceral inputs. Using functional magnetic resonance imaging in 34 anesthetized rats, we identified a cohesive "interoception network" characterized by dense, reciprocal functional connectivity among the dorsal vagal complex, thalamus, hypothalamus, amygdala, insula, and cingulate cortex. The topology of this network was not static but dynamically coupled to the physiological state. Functional integration was robust in the digestive (fed) phase but significantly attenuated in the inter-digestive (fasted) phase. The network relied on the integrity of the vagus nerve. When vagal signaling was eliminated by bilateral cervical vagotomy, the network effectively disintegrated. These findings demonstrate that resting-state functional connectivity is not an intrinsic property of the isolated brain but an embodied phenomenon that relies on continuous neural signaling between the brain and viscera.