Vestibulo-ocular responses to vertical translation in normal human subjects

Abstract

Prior studies of the human translational vestibulo-ocular reflex (tVOR) report that eye rotations amount to less than 60% of those required to keep the eyes pointed at a stationary visual target, unlike the angular VOR (aVOR) which is optimized to maintain stable gaze. Our first goal was to determine if the performance of the tVOR improves when head translations are combined with head rotations in ambient lighting. A second goal was to measure tVOR during vertical head translations (bob), which has not received systematic study. We measured tVOR alone and in combination with the aVOR in 20 normal human subjects, aged 25-72 years, as they sat on a moving platform that bobbed at 2.0 Hz while rotating horizontally (yaw) at 1.0 Hz. When subjects viewed a visual target at 2 m, median "compensation gain" (eye rotational velocity/required eye rotational velocity to maintain foveal target fixation) was 0.52 during pure bob and 0.59 during combined bob-yaw; during viewing of a near target at approximately 17 cm, compensation gain was 0.58 for pure bob and 0.60 for combined bob-yaw. Mean phase lag of eye-in-head velocity for the tVOR was approximately 19 degrees with respect to the ideal compensatory response, irrespective of whether translation was accompanied by rotation. Thus, the tVOR changed only slightly during translation-rotation versus pure translation, and our subjects' ocular rotations remained at about 60% of those required to point the eyes at the target. Comparison of response during binocular or monocular viewing, and ambient or reduced illumination, indicated that relative image motion between the target and background was an important determinant of tVOR behavior. We postulate that tVOR evolved not to stabilize the image of the target on the fovea, but rather to minimize retinal image motion between objects lying in different planes, in order to optimize motion parallax information.

Recurrence quantification analysis of gait in normal and hypovestibular subjects

“…[3][4][5] in the UVH patients is consistent with the important role of the vestibular system in controlling dynamic stability during walking. The findings suggest that the RQA may prove useful as a quantitative tool to assess walking performance (Fig.…”

“…During walking, the vestibular system contributes to head and whole body orientation and stabilization through vestibulocollic reflexes (VCR) and other vestibulo-spinal mechanisms [1][2][3]. At the same time, balance control requires head stability so that the maximum head velocity does not exceeds the range over which the vestibulo-ocular reflexes (VOR) can enable optimal gaze control [4][5][6][7]. Indeed, upper body accelerations are gradually attenuated from the bottom to the upper portions of the body, a process that may be associated with the top-down control and recruitment of paraspinal muscles during walking [7][8][9].…”

Why do patients with PSP fall?

“…9 On the other hand, tVOR does not need to generate eye rotations during viewing of distant objects, but during near viewing, the magnitude of required eye movements is inversely related to target distance. [10][11][12] Taking this information together, it is possible to formulate laboratory stimuli that will approximate the head movements occurring during natural locomotion under a range of viewing conditions and could be used to test aVOR and tVOR in patients with PSP. For our experiments, we applied combined yaw rotations at 1.0 Hz and bob translations at 2.0 Hz, and compared responses of aVOR and tVOR during viewing of targets located at far and near.…”

“…Subjects sat in a chair on a Moog 6DOF2000E electric motion platform (East Aurora, NY) that could move with six degrees of rotational and translational freedom through a range of Ϯ20 degrees and Ϯ20 cm, with peak rotational acceleration of 400 deg/second 2 and peak linear acceleration of 0.5 g. 12 Belts were used to secure the subject's torso and a snugly fitting skateboard helmet, inlaid with foam, was used to stabilize the subject's head. Any head movements that were decoupled from chair or platform motion were measured, as described below.…”

“…Threedimensional eye rotations were measured using the magnetic search coil technique, 16 and head movements were monitored by an infrared reflection system (Vicon Motion Systems, Los Angeles, CA). 12 Coil signals were digitized at 500 Hz with 16-bit precision after Butterworth filtering (0 to 150 Hz) to avoid aliasing and positions of the eyes were calculated in rotation vectors and converted to degrees. Head position signals from the infrared reflection system were digitized at 120 Hz.…”