Stable insular spectral patterns underlie dynamic pain encoding in chronic neuropathic pain

Chronic neuropathic pain persists for years yet fluctuates markedly from day to day, raising a fundamental question: how do stable neural patterns relate to dynamic pain experience? The insula is a central hub for pain processing, but it remains unclear whether ongoing pain is associated with enduring baseline neural patterns within the insula or day-to-day variation in neural activity. We analysed multiday bilateral intracranial electroencephalography recordings from six individuals with chronic unilateral neuropathic pain, comprising 34 days of resting-state data. Spectral power, peak-derived spectral metrics and intra-insular functional connectivity (coherence and phase-locking value) were quantified across anterior and posterior insular subregions. Linear mixed-effects models were used to distinguish stable within-cohort neural patterns from within-subject, day-to-day pain-related fluctuations in pain intensity and unpleasantness, with explicit control for longitudinal temporal drift. The insula exhibited stable within-cohort spectral organization characterized by an anterior-posterior gradient with lower posterior fast-band power and spectral slowing, together with greater posterior spectral power in the hemisphere contralateral to the chronic pain side. Superimposed on this stable pattern, daily pain intensity was most strongly associated with increased contralateral posterior alpha power, accounting for approximately 33% of within-subject variance. Pain unpleasantness showed its strongest association with increased contralateral posterior beta power, accounting for approximately 19% of within-subject variance. Peak-derived spectral metrics and intra-insular connectivity largely reflected stable baseline patterns, with comparatively modest day-to-day pain-related variation. These exploratory findings suggest that chronic neuropathic pain is associated with stable insular spectral patterns upon which frequency-specific oscillatory variation relates to daily pain experience. Posterior insular alpha and beta oscillations showed partially distinct associations with pain intensity and unpleasantness, respectively, whereas intra-insular connectivity primarily reflected stable network structure. Posterior insular spectral power may therefore provide a candidate physiological signal for tracking pain states.