Brain Interoception: The Missing Organ on the Interoceptive Map

Interoception — the sensing of the body's internal state — is catalogued across cardiac,pulmonary, gastrointestinal, and other organ systems. The brain is absent from thiscatalogue. No published paper proposes "brain interoception" as a defined concept, despitesubstantial evidence that the brain senses its own tissue state. This review argues thatbrain interoception is a coherent category forming a single continuum. At the pathologicalend, patients detect brain-state changes via meningeal nociceptors (headache), corticalspreading depression (migraine aura), focal neural discharge (epileptic aura), andintracranial hemorrhage (thunderclap headache) — phenomena accepted in clinicalneurology but never unified as interoception. In the middle range, universal experiences ofsleepiness, brain fog, mental fatigue, and caffeine withdrawal reflect the detection ofmetabolic brain-state changes through established mechanisms: central chemoreceptors(explicitly called interoception in respiratory physiology), astrocyte-derived adenosinesignaling, and microglial surveillance. At the high-precision end, some individuals reportdiscriminating subtle variations in cognitive readiness with sufficient resolution to guide taskselection. Drawing an analogy to skeletal muscle — which has both proprioception(spindles, Golgi tendon organs) and interoception (Group III/IV metabolic afferents) — wepropose that the brain similarly has both metacognition (monitoring cognitive processes)and interoception (monitoring tissue state), and that these are phenomenologicallydistinguishable. The classification gap reflects an unexamined Cartesian inheritance inwhich the brain is treated as the interpreter of interoceptive signals rather than as an organwhose state is itself interocepted. Six testable predictions are offered.