Recording Technique

A clean RCT depends on three habits: a properly positioned head, an alert subject, and complete denial of vision. Get those wrong and the gain numbers tell you nothing.

Sinusoidal harmonic acceleration (SHA)

SHA delivers low- and mid-frequency stimulation by oscillating the chair in a sine pattern. The standard protocol tests an octave-spaced sequence — usually 0.01, 0.02, 0.04, 0.08, 0.16, 0.32 and 0.64 Hz — at a peak velocity of 50–60°/s Wang Y 2022. Many laboratories start at a mid frequency (0.08 Hz) and work outward, because the lowest frequencies take more than a minute to record one cycle.

Velocity step test

The chair accelerates to a constant velocity (commonly 60–100°/s) and holds it; the per-rotational nystagmus decays exponentially. The chair then stops, and a post-rotational nystagmus of opposite direction appears and decays in the same way. Two velocities and two directions give four time-constant measurements, one per canal per phase.

Visual suppression

Asking the subject to fixate an earth-fixed light during chair rotation normally suppresses > 60 % of the nystagmus. Failure of suppression — the “fixation index” staying near 1 — localises a lesion to the cerebellum, particularly the flocculus and the nodulus/uvula Raphan T 2002.

The alerting task

Drowsiness collapses VOR gain. The tester therefore engages the subject in continuous arithmetic, animal-naming or country-listing through every recording. If a frequency returns a spectral purity below 60 %, the response is too noisy to be useful and the frequency should be re-tested after re-alerting Interacoustics A/S (technical reference). 2023.

Vision denied

Any visible target drives optokinetic eye movements or lets the subject suppress the VOR — both contaminate the recording. Modern systems use video goggles with the cover firmly seated; older systems use a light-tight booth.

RCT vs caloric — when to use each