Effects of age, target characteristics, and viewing distance on ocular counterroll in healthy humans

Abstract

The ocular counter-roll (OCR) reflex generates compensatory torsional eye movements during static head tilt. OCR gain is defined as the ratio of change in eye position to change in head position. Previous reports of OCR gain have varied considerably among normal subjects, perhaps due to the use of different targets and viewing distances. This study compared OCR gains during viewing of well-controlled targets (simple vs complex, near vs far viewing) in healthy subjects (n=60; age 10–62). Two visual stimuli were used: a simple target consisting of a central fixation cross against a uniform gray background, and a complex target consisting of the same fixation cross against a high-contrast grid pattern. Each stimulus was presented at near (0.33m) and far (1.0m) viewing distance. Eye movements were recorded using a video-based eye tracker. Change in torsional eye position was plotted as a function of head position step (0±40° in 5° steps), and a sigmoidal fit was performed as part of a non-linear regression. Mean OCR gain was calculated from the slope of the regression line. Factorial and repeated measures ANOVA were performed. Mean OCR gains were higher in younger subjects ([[lt]]30 years), during viewing of far targets (p[[lt]]0.001), and also during viewing of complex targets (p[[lt]]0.05). There was significant interaction between target distance and complexity, with a larger reduction in gains for simple targets during near viewing (p[[lt]]0.03). OCR gain is highest when using complex targets, which provide additional visual spatial orientation cues. Decreased OCR gain during near viewing may minimize the vertical disparity induced by the specific geometry of eye rotations during torsional movements, thereby optimizing stereopsis during convergence. The normal database created by this study can now be used to evaluate patients with different disorders, including skew deviation and peripheral vestibulopathy.