Interoception as Ground State Reading: The Insular Cortex as Displacement Sensor, Interoceptive Accuracy as Return Capacity, and Predictive Error in Chronic Illness

We apply the Displacement Framework to interoception, formalizing it as the nervous system's mechanism for reading D(xi_body) — how far the body currently is from its ground state. The insular cortex is identified as the displacement sensor: the posterior insula transduces ascending visceral signals, the anterior insula computes the estimated displacement D_hat(t). Nine formal propositions are derived covering interoceptive accuracy as sensor fidelity, predictive coding as dual displacement (actual vs. predicted), fibromyalgia as prediction-system wrong attractor (predicted >> actual displacement), PTSD as recalibrated sensor zero point, alexithymia as emotion-displacement blindness, panic disorder as catastrophic precision-weight amplification, the somatic marker hypothesis as displacement-based emotion theory, and vagal tone as signal-to-noise ratio of the interoceptive channel.

Body scan, yoga, and HRV biofeedback are formalized as sensor recalibration interventions.