Objectives: This study aimed to quantify the nystagmus induced by four kinds of angular accelerations exerted on the head, and to clarify whether the semicircular canal theory can explain the mechanism of ocular counter-roll. Methods: Subjects were 12 healthy humans. Experiment 1: Head was passively moved back and forth for three cycles (pitch rotation). Experiment 2: Head was rotated 45？ to the left, moved back and forth for three cycles. Experiment 3: Head was rotated 60？ to the left, moved back and forth for three cycles. Experiment 4: Head was tilted laterally to the left and right for three cycles without axial rotation (roll rotation). In all experiments, the amplitude was approximately 30？, and the frequency was 0.33 Hz. Eye movements in the dark were recorded and analyzed. Results: In Experiment 1, nystagmus was purely vertical. In Experiments 2 to 4, nystagmus was vertical/torsional. The intensity of the vertical component was greatest in Experiment 1, followed by Experiment 2, then Experiment 3, and was smallest in Experiment 4. The intensity of the torsional component was in reverse pattern. Conclusion: Considering the nystagmus direction and intensity, ocular counter-roll can be explained by the semicircular canal theory.
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