We measured the gain of the human rotational vestibulo-ocular reflex (VOR) in a total of 75 human subjects, about 10 times as many as most other studies, under both active and passive motion at 0.45 Hz in open-loop conditions. Examining a large number of subjects allowed an assessment of VOR gain resistant to individual differences; many prior studies used small numbers of subjects, and this is a likely cause of the variation of gains reported in the literature. While undergoing sinusoidal oscillation, active VOR gain averaged 0.99 and passive gain averaged 1.0, very close to complete VOR compensation. These gain measurements, while differing from older estimates, agree closely with some more recent estimates, and support a reinterpretation of the role of VOR in eye stabilisation during head movement. The human rotational VOR appears sufficient for complete rotary compensation; visual (i.e. optokinetic) feedback compensation functions only to correct vestibular error, and in many subjects it operates in the same direction as head acceleration.

We made a number of decisions regarding parameters:

1. We employed sinusoidal oscillation at 0.45 Hz, 30 deg amplitude since this imitates a natural rate of head motion.
2. Subjects were asked to imagine an earth-fixed target, as this seemed the best way to examine the normal compensatory properties of the VOR.
3. We employed both active and passive rotation to assess VOR in both these situations.
4. Since subjects were in the apparatus for less than 2 min, it is unlikely they experienced decreased arousal, and so they were asked merely to fixate targets when present, and perform saccade-free fixations when absent. Recording intervals were limited to 5 s, so that fatigue was not an issue and the eyes did not drift significantly from their original laser-oriented position.
5. Computation was performed by taking the ratio of root-mean-square eye velocity to root-mean-square head velocity, since this is the most common approach in the literature.

Subjects were trained to not make saccades during dark phases.

A spot of red laser light was beamed via a mirror onto a hemicylindrical screen (viewing distance = 90 cm). Head position was measured with a Polhemus Fastrak magnetic field emitter unit in conjunction with a receiver mounted on a helmet that was tightly strapped to the subject's head. Eye position was monitored by paired infrared sensitive photocells attached to the helmet and positioned below the right eye. A head-mounted infrared LED provided constant illumination of the eye. Bandpass of the photocell system exceeded 0 - 400 Hz, and spatial resolution is 15 minarc. The eye monitor was calibrated by having subjects fixate 3 fixation marks 15 deg apart in succession, with background lights illuminating the marks. When fixation was stable at a mark, the experimenter pressed a key to enter the corresponding a/d value into the computer. Recorded eye positions were interpolated from these values.