Asymmetry of vestibular pathways predicts upper-brainstem auditory imbalance in subjective somatosensory tinnitus: A tract-specific chronological reorganization model.

OBJECTIVE: This study evaluated subcortical auditory and peripheral vestibular pathway alterations in normal-hearing individuals with unilateral subjective somatic tinnitus, isolating cross-modal electrophysiological relationships using an intra-subject, internally controlled paradigm. METHODS: Twenty-four participants (aged 17-61 years) with unilateral subjective somatic tinnitus and symmetric normal hearing thresholds (≤15 dB HL across 0.125-8 kHz) underwent auditory brainstem response (ABR), cervical vestibular evoked myogenic potential (cVEMP), and ocular VEMP (oVEMP) testing. Mathematically normalized Interaural Asymmetry Indices (AI) isolated side-specific neural imbalances. RESULTS: Conventional audiological profiles, middle ear compliance, and absolute ABR parameters (waves I-V) displayed interaural symmetry (p > .05). Conversely, absolute oVEMP evaluations demonstrated a statistically significant reduction in P1-N1 peak-to-peak amplitudes within the tinnitus-affected ears (M = 4.23 µV, SD = 2.98) relative to the healthy contralateral ears (M = 5.76 µV, SD = 4.72, p = .014), while absolute cVEMP parameters remained balanced (p = .396). Correlation analysis of normalized indices revealed a strong positive relationship between cVEMP AI and ABR wave V AI (r(22) = .555, p = .005). Multiple linear regression modeling showed that combined vestibulocollic (cVEMP) and vestibulo-ocular (oVEMP) neural asymmetries significantly predicted upper-brainstem auditory asymmetry at the level of the inferior colliculus, F(2, 21) = 8.66, p = .002, accounting for 45.2% of the variance in ABR wave V asymmetry (R2 = .452). Tinnitus chronicity showed an exploratory positive correlation with cVEMP AI (r(22) = .456, unadjusted p = .025; did not survive FDR correction). CONCLUSION: Unilateral subjective somatic tinnitus appears to involve tract-specific subcortical electrophysiological asymmetries. We discuss these side-specific differences within a chronological reorganization framework. While acute ascending vestibulo-ocular suppression marks the initial gating failure, a progressively deepening descending vestibulocollic imbalance is strongly associated with upper auditory brainstem asymmetry. These outcomes validate combined VEMP indices as objective biomarkers and provide a neurophysiological rationale for future interventional trials to test whether early targeted bimodal neuromodulation can effectively reset subcortical hyperactivity before persistent neuroplastic consolidation.