The Under-compensatory Roll aVOR Does Not Affect Dynamic Visual Acuity

Abstract

Rotations of the head evoke compensatory reflexive eye rotations in the orbit to stabilize images onto the fovea. In normal humans, the angular vestibulo-ocular reflex (aVOR) gain (eye/head velocity) changes depending on the head rotation plane. For pitch and yaw head rotations, the gain is near unity, but during roll head rotations, the aVOR gain is ∼0.7. The purpose of this study was to determine whether this physiological discrepancy affects dynamic visual acuity (DVA)-a functional measure of the aVOR that requires subjects to identify letters of varying acuities during head rotation. We used the scleral search coil technique to measure eye and head velocity during passive DVA testing in yaw, roll, and pitch head impulses in healthy controls and patients with unilateral vestibular hypofunction (UVH). For control subjects, the mean aVOR gain during roll impulses was significantly lower than the mean aVOR gain during yaw and pitch impulses; however, there was no difference in DVA between yaw, roll, or pitch. For subjects with UVH, only aVOR gain during head rotations toward the affected side (yaw) were asymmetric (ipsilesional, 0.32±0.17, vs. contralesional, 0.95±0.05), with no asymmetry during roll or pitch. Similarly, there was a large asymmetry for DVA only during yaw head rotations, with no asymmetry in roll or pitch. Interestingly, DVA during roll toward the affected ear was better than DVA during yaw toward the affected ear-even though the ipsilesional roll aVOR gain was 60 % lower. During roll, the axis of eye rotation remains nearly perpendicular to the fovea, resulting in minimal displacement between the fovea and fixation target image projected onto the back of the eye. For subjects with UVH, the DVA score during passive horizontal impulses is a better indicator of poor gaze stability than during passive roll or pitch.

Stability of the aVOR to Repeat Head Impulse Testing

“…This is functionally relevant as the roll aVOR in frontal eyed species has a naturally smaller adaptive drive to bring the gain closer to unity because during pure head roll the target image is not displaced off the fovea (32). This too has been validated in gaze stability testing where dynamic visual acuity is normal in ipsilesional roll but not ipsilesional yaw for patients with vestibular hypofunction (33). Another possible explanation for the lower roll VOR gain is coil slippage (34,35).…”

Angular vestibuloocular reflex responses in Otop1 mice. II. Otolith sensor input improves compensation after unilateral labyrinthectomy

“…A possible behavioral explanation for the flatter midfrequency vertical VOR is that for a lateral-eyed animal, such as a mouse where the eyes at rest point along an axis~30°with respect to the interaural axis, a pitch head movement results in a mostly torsional eye rotation. Torsional retinal image slip might be more tolerated by the mouse VOR, as it is in the human VOR (Schubert et al 2012), because unlike horizontal and vertical image slip the image remains on the central retinal area where photoreceptor density is highest. Consequently, there is less need for a highly compensatory vertical VOR gain during midfrequency head rotations, whereas at higher frequencies torsional retinal slip might be less tolerated.…”

Consensus Paper: Neurophysiological Assessments of Ataxias in Daily Practice

“…One notes the number of lines on the acuity chart lost with head shaking; more than two is abnormal. More objective and quantitative testing can be obtained by recording head movements and only displaying the acuity optotypes when the head has reached a specific speed [195,196].…”