The physiology of interoception and its adaptive role in consciousness.

The interoceptive nervous system uses signalling mechanisms distinct from those of exteroceptive, voluntary motor, cognitive and linguistic processes. While interoception relies mainly on analogue-like processing, the other systems favour digital-like signalling. This physiological feature allows interoception to map the state of the organism's internal milieu in real time, in stark contrast to the rapid, precise and time-sensitive processing of exteroceptive, proprioceptive, cognitive and linguistic signals. This distinction arises from the unique features of the interoceptive process, namely the unmyelinated and lightly myelinated nature of afferent interoceptive neurons, which facilitates non-synaptic signalling throughout interoceptive pathways, notably in those of the vagus nerve and of neural structures devoid of a blood-brain barrier. We propose that by continuously representing the body's state, spontaneously conscious homoeostatic feelings constitute the foundational substrate of subjectivity and the grounding for consciousness. This idea builds on our prior work, which grounds consciousness in core biology rather than high-level cognition. Consciousness enables adaptive and protective responses that maintain homeostasis and secure life.This article is part of the theme issue 'Evolutionary functions of consciousness'.