Chapter 2

Anatomy & Physiology of the Angular VOR

Three sensors, three relays, three muscle groups. Everything that appears on a chair report — gain, phase, symmetry, time constant — falls out of this small circuit.

The horizontal canal lies at 30°

earth horizontallateral canal30°Head tilted 30° forward → lateral canal in plane of rotation
Tilting the head 30° forward brings the lateral semicircular canal into the earth-horizontal plane that the chair rotates in, maximising the stimulus.

In upright posture the lateral (horizontal) semicircular canal is tilted ~30° above earth-horizontal. To stimulate it maximally during SHA, the head is tilted forward by 30° so that the canal lies flat in the rotation plane Wang Y 2022. This is a non-negotiable starting position; without it, gain estimates drift toward those of the vertical canals.

The membranous labyrinth

ampullalateral canalanterior canalposterior canalvestibulecochleaCN VIIIRCT primarily probes the lateral (horizontal) semicircular canalvia the superior division of CN VIII.
Schematic of the membranous labyrinth. The lateral canal — the primary RCT target — is angled ~30° above horizontal, which is why the head is tilted 30° forward during testing.

Each semicircular canal contains a fluid-filled duct of endolymph and, at one end, an ampulla housing the cupula. Hair cells embedded in the cupula transduce mechanical deflection into receptor potentials. The three canals are arranged near-orthogonally; rotation in any plane excites at least two of them.

Cranial nerve VIII: superior and inferior divisions

DivisionInnervatesRCT impact
Superior vestibular nerveLateral & anterior canals, utricleMajor contributor to SHA gain & phase
Inferior vestibular nervePosterior canal, sacculeHorizontal SHA largely preserved; cVEMP loss

Acute viral neuritis classically targets the superior division, which is why a typical right-sided neuritis produces a reduced right horizontal vHIT gain, a caloric paresis on the right, and an SHA pattern of reduced gain with phase lead and right-direction asymmetryLe TN 2021. The posterior canal vHIT is spared.

The three-neuron arc

Hair cellVestibularnucleusOculomotor /abducensEOMThree-neuron arc · latency ≈ 7–15 ms
The simplest VOR pathway: hair cell → vestibular nucleus → extra-ocular motoneuron.

Hair cell → vestibular afferent → vestibular nucleus → ocular motoneuron → extra-ocular muscle. The whole loop takes 7–15 ms. The beauty of the design is its symmetry: the right and left horizontal canals are wired antagonistically into the vestibular nuclei, so opposite-direction nystagmus is what every step in the chair must produce.

Velocity storage

The cupula returns to its neutral position with a time constant of roughly 4–6 s after a step in head velocity Raphan T 2002. Yet the eye-velocity response decays with a Tc of 15–25 s in healthy adults. The extra duration is produced by a brainstem integrator — the velocity-storage mechanism — that combines canal, otolith and optokinetic inputs. Vestibular-only (VO) and vestibular-pause-saccade (VPS) neurons in the medial and superior vestibular nuclei carry the prolonged signal.