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 this
catalogue. No published paper proposes "brain interoception" as a defined concept, despite
substantial evidence that the brain senses its own tissue state. This review argues that
brain interoception is a coherent category forming a single continuum. At the pathological
end, patients detect brain-state changes via meningeal nociceptors (headache), cortical
spreading depression (migraine aura), focal neural discharge (epileptic aura), and
intracranial hemorrhage (thunderclap headache) — phenomena accepted in clinical
neurology but never unified as interoception. In the middle range, universal experiences of
sleepiness, brain fog, mental fatigue, and caffeine withdrawal reflect the detection of
metabolic brain-state changes through established mechanisms: central chemoreceptors
(explicitly called interoception in respiratory physiology), astrocyte-derived adenosine
signaling, and microglial surveillance. At the high-precision end, some individuals report
discriminating subtle variations in cognitive readiness with sufficient resolution to guide task
selection. Drawing an analogy to skeletal muscle — which has both proprioception
(spindles, Golgi tendon organs) and interoception (Group III/IV metabolic afferents) — we
propose that the brain similarly has both metacognition (monitoring cognitive processes)
and interoception (monitoring tissue state), and that these are phenomenologically
distinguishable. The classification gap reflects an unexamined Cartesian inheritance in
which the brain is treated as the interpreter of interoceptive signals rather than as an organ
whose state is itself interocepted. Six testable predictions are offered.