Ocular responses to head rotations during mirror viewing

The vergence-mediated gain increase: Physiology and clinical relevance

BACKGROUND

During near-viewing, the vestibulo-ocular reflex (VOR) response/gain increases to compensate for the relatively larger translation of the eyes with respect to the target.

OBJECTIVE

To review vergence-mediated gain increase (VMGI) testing methods stimuli and responses (latency and amplitude), peripheral/central pathways and clinical relevance.

METHODS

The authors discuss publications listed in PUBMED since 1980 in the light of their own studies.

RESULTS

The VMGI can be measured during rotational, linear and combined head accelerations. It has short-latency, non-compensatory amplitude, and relies on irregularly discharging peripheral afferents and their pathways. It is driven by a combination of perception, visual-context and internal modelling.

CONCLUSIONS

Currently, there are technical barriers that hinder VMGI measurement in the clinic. However, the VMGI may have diagnostic value, especially with regards to measuring otolith function. The VMGI also may have potential value in rehabilitation by providing insight about a patient's lesion and how to best tailor a rehabilitation program for them, that potentially includes VOR adaptation training during near-viewing.

Vestibulo-ocular responses to vertical translation in normal human subjects

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.

Vestibular and non-vestibular contributions to eye movements that compensate for head rotations during viewing of near targets

Geometry dictates that when subjects view a near target during head rotation the eyes must rotate more than the head. The relative contribution to this compensatory response by adjustment of the vestibulo-ocular reflex gain (Gvor), visual tracking mechanisms including prediction, and convergence is debated. We studied horizontal eye movements induced by sinusoidal 0.2-2.8 Hz, en-bloc yaw rotation as ten normal humans viewed a near target that was either earth-fixed (EFT) or head-fixed (HFT). For EFT, group median gain was 1.49 at 0.2 Hz declining to 1.08 at 2.8 Hz. For HFT, group median gain was 0.03 at 0.2 Hz increasing to 0.71 at 2.8 Hz. By applying transient head perturbations (peak acceleration >1,000 degrees s(-2)) during sinusoidal rotation, we determined that Gvor was similar during either EFT or HFT conditions, and contributed only approximately 75% to the compensatory response. We confirmed that retinal image slip contributed to the compensatory response by demonstrating reduced gain during EFT viewing under strobe illumination. Gain also declined during sum-of-sines head rotations, confirming the contribution of predictive mechanisms. The gain of compensatory eye movements was similar during monocular or binocular viewing, although vergence angle was greater during binocular viewing. Comparison with previous studies indicates that mechanisms for generation of eye rotations during near viewing depend on head stimulus type (rotation or translation), waveform (transient or sinusoidal), and the species being tested.

Effects of retinal image slip on modulation of visual vestibulo-ocular reflex during near viewing

During viewing of a near target, the gain of the visually modulated vestibulo-ocular reflex (vVOR) exceeds 1.0. We investigated whether retinal image slip contributes to this gain increase by measuring responses during strobe illumination. Subjects showed greater gain values during normal versus strobe illumination of the near target, even during rotation at frequencies above 1.0 Hz, indicating that retinal image velocity information is used to optimize vVOR.