Optokinetic stimulation induces vertical vergence, possibly through a non-visual pathway

Tobias Wibble,Tony Pansell

doi:10.1038/s41598-020-72646-8

Abstract

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion. However, vertical vergence can also be induced by seemingly unrelated visual conditions, such as optokinetic rotations. This study aims to investigate the effect of vision on this latter form of vertical vergence. Eight subjects (4m/4f) viewed a visual scene in head erect position in two different viewing conditions (monocular and binocular). The scene, containing white lines angled at 45° against a black background, was projected at an eye-screen distance of 2 m, and rotated 28° at an acceleration of 56°/s2. Eye movements were recorded using a Chronos Eye-Tracker, and eye occlusions were carried out by placing an infrared-translucent cover in front of the left eye during monocular viewing. Results revealed vergence amplitudes during binocular viewing to be significantly lower than those seen for monocular conditions (p = 0.003), while torsion remained unaffected. This indicates that vertical vergence to optokinetic stimulation, though visually induced, is visually suppressed during binocular viewing. Considering that vertical vergence is generally viewed as a vestibular signal, the findings may reflect a visually induced activation of a vestibular pathway.

Highlights

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion
In addition to the vestibular response seen during head tilts, vertical visual disparities will induce a combination of torsion and vertical vergence, which has been associated with less reliable depth-perception[9,10]
Paired T-tests revealed no significant differences between CCW and CW stimulus rotation for neither vertical vergence (t(7) = 0.079; p = 0.93) nor ocular torsion (t(7) = 1.176; p = 0.278), allowing for further analysis to be indiscriminative of rotation direction

Summary

Introduction

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion. We have previously shown that a rotating visual image, simulating the optokinetic field-of-view as seen during a head rotation, will lead to a vertical v ergence[3,4] This eye movement is accompanied by ocular torsion in the same direction as the visual rotation[5]. Despite the intrinsic physiological link between torsion and vertical vergence, evidence generally points to different neural pathways of induction during optokinetic roll stimulations, with torsion showing a greater dependency on visual information density[3,11] This is true from a vestibular perspective, with the otolith organs sensing static head position and inducing vertical vergence, while dynamic head movements are relayed through the semi-circular canals, which lead to ocular counter-rolling[12,13].

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