Vestibulo-sympathetic Reflex and Pupillary Responses Elicited by Vertical Semicircular Canal Stimulation.

The vestibulo-sympathetic reflex has been studied mainly using horizontal semicircular canal and otolith stimulation, whereas the autonomic role of the vertical canals remains unclear. We quantified cardiovascular and pupillary responses to vertical canal stimulation and compared stimulus-frequency-locked pupil oscillations between vertical and horizontal rotations in 18 healthy adults. Vertical stimulation used a 70°-tilted rotary chair aligning the vertical canal planes with the earth-horizontal axis. Participants underwent sinusoidal harmonic acceleration at 0.08, 0.16, and 0.64 Hz and the vertical velocity step test at 100°/s. Systolic blood pressure rose 5.8-7.8 mmHg with medium-to-large effect sizes but did not survive multiple-comparison correction. Pupil area increased significantly at all frequencies, and vertical stimulation produced larger stimulus-frequency-locked oscillation amplitude compared to horizontal stimulation (β = 0.568 mm², p = 0.018). Additionally, the pupillary oscillation was phase-locked to the stimulus per canal plane, reaching significance at 0.64 Hz (sign-aligned cross-plane phase-locking value 0.27-0.37; Rayleigh p = 0.015). A two-stage gain model (cupula velocity transduction filtered by iris smooth-muscle dynamics; gain ≈ 0.09 mm²/(deg/s); iris corner ≈ 0.71 Hz) was more consistent with a velocity-coded than an acceleration-coded drive, although the sample size precluded resolving the absolute phase. These findings suggest that vertical canal inputs may engage human vestibulo-autonomic pathways and that spectral pupillometry may provide a noninvasive index of vestibulo-sympathetic reflex function.