Is Ocular Counter-Roll Otolith-Ocular Reflex?

Purpose Ocular counter-roll has been thought to be an otolith-ocular reflex. However, there is a hypothesis that it is a semicircular canal (SCC) ocular reflex. This study aimed to validate whether ocular counter-roll can be explained by the SCC theory. Methods Experiment 1: Two springs with plastic balls (different turning radius) were fixed to a swivel chair and were rotated suddenly. Experiment 2: Subject was seated and the entire upper body was bent forward 60 degrees with the neck fixed. Experiment 3: Subject was seated, the head was twisted 45 degrees to the right and the entire upper body was bent forward 60 degrees with the neck fixed. Experiment 4 (roll rotation): Subject was asked to tilt his upper body 60 degrees to the left. Eye movements in the dark were observed and recorded using an infrared camera. In Experiments 2, 3 and 4, subjects were four healthy humans. Results Experiment 1: Immediately after the movement, the two balls moved tangentially. The ball on the outside spring moved more compared to the ball attached to the inside spring. Experiment 2: In all subjects, nystagmus was vertical. The direction was downward. Experiment 3: In all subjects, nystagmus was vertical/torsional. The direction was downward (torsional component was leftward). Experiment 4: In all subjects, nystagmus was torsional, and the direction was leftward. Conclusion Since the role of SCCs is to sense angular acceleration in all directions of 360 degrees in a coordinated manner, bilateral vertical SCCs are stimulated in roll rotation. Therefore, SCC theory can explain the generation of ocular counter-roll. We proposed the “difference of inertial forces” theory to explain the mechanism of vestibulo-ocular reflex.