Three-dimensional analysis of linear vestibulo-ocular reflex in humans during eccentric rotation while facing downwards.

When participants undergo eccentric rotation (ER), i.e., they are rotated while displaced from the axis of rotation, they undergo both rotational stimulation and linear acceleration, which induces both the angular vestibulo-ocular reflex (aVOR) and linear VOR (lVOR). During ER, the lVOR induced by tangential linear acceleration enhances the eye movement induced by aVOR. In this study, we attempted to measure aVOR and lVOR separately, while participants underwent ER while facing the ground in a dark room. We analyzed three-dimensional eye movements using a video-oculography system. The participants sat on the ER chair either directly above the center of rotation, or with their head out, head in, right ear out, or left ear out against the center of rotation. Under these conditions, the rotational axis of the eye was perpendicular to the ground for rotational stimulation (aVOR), and the axis was parallel to the ground for linear stimulation (lVOR). Thus, measured eye movements could be separated into these two components. At 0.1 and 0.3 Hz rotation, we observed aVOR but not lVOR. However, when the stimulation frequency was above 0.5 Hz, we observed both aVOR and lVOR. These data indicate that lVOR is activated when the stimulation frequency is above 0.5 Hz. We conclude that it is possible to separately analyze aVOR and lVOR, and to simultaneously assess the function of aVOR and lVOR by analyzing eye movements induced when participants undergo ER above 0.5 Hz while facing the ground.