Short-Latency Covert Saccades - The Explanation for Good Dynamic Visual Performance After Unilateral Vestibular Loss?

Julia Sjögren,Craig Hickson,Måns Magnusson,Fredrik Tjernström,Mikael Karlberg,Per-Anders Fransson

doi:10.3389/fneur.2021.695064

Julia Sjögren, Craig Hickson + Show 4 more

Open Access

https://doi.org/10.3389/fneur.2021.695064

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Abstract

Background: Functional head impulse test (fHIT) tests the ability of the vestibulo-ocular reflex (VOR) to allow visual perception during head movements. Our previous study showed that active head movements to the side with a vestibular lesion generated a dynamic visual performance that were as good as during movements to the intact side.Objective: To examine the differences in eye position during the head impulse test when performed with active and passive head movements, in order to better understand the role of the different saccade properties in improving visual performance.Method: We recruited 8 subjects with complete unilateral vestibular loss (4 men and 4 women, mean age 47 years) and tested them with video Head Impulse Test (vHIT) and Functional Head Impulse Test (fHIT) during passive and active movements while looking at a target. We assessed the mean absolute position error of the eye during different time frames of the head movement, the peak latency and the peak velocity of the first saccade, as well as the visual performance during the head movement.Results: Active head impulses to the lesioned side generated dynamic visual performances that were as good as when testing the intact side. Active head impulses resulted in smaller position errors during the visual perception task (p = 0.006) compared to passive head-impulses and the position error during the visual perception time frame correlated with shorter latencies of the first saccade (p < 0.001).Conclusion: Actively generated head impulses toward the side with a complete vestibular loss resulted in a position error within or close to the margin necessary to obtain visual perception for a brief period of time in patients with chronic unilateral vestibular loss. This seems to be attributed to the appearance of short-latency covert saccades, which position the eyes in a more favorable position during head movements.

Highlights

The vestibulo-ocular reflex (VOR) stabilizes gaze during head movements, enabling the eyes to maintain focus on visual targets
The saccades during the active head movement were performed so early that the position error of the eyes was small during the time frame when the optotype was displayed (Figures 2E,F), and enabled the subjects to determine the orientation of the optotype
Our material was too small to evaluate subjective parameters such as DHI score but we found strong relationships between active head movements, short latency covert saccades and moderate position errors during the optotype presentation, which might explain an almost normal dynamic visual performance despite a complete vestibular loss [9]

Summary

Introduction

The vestibulo-ocular reflex (VOR) stabilizes gaze during head movements, enabling the eyes to maintain focus on visual targets. During the acute stage (days) of unilateral vestibular loss, catch-up eye saccades are induced after the head stops moving. Detecting such saccades is the fundament of the clinical head impulse test [1]. At a later stage of vestibular loss recovery, the catch-up saccades may occur as early as during the head movement. This makes them impossible to perceive without recording equipment and they are referred to as ‘covert saccades. Our previous study showed that active head movements to the side with a vestibular lesion generated a dynamic visual performance that were as good as during movements to the intact side

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Results

Conclusion