Abstract
PurposeFixation stability for binocular anomalies with a phoria cannot be detected by direct observations. This study aimed to quantitatively evaluate fixation stability using an eye tracker rather than direct directions in binocular vision with abnormal and normal phorias.MethodsThirty-five and 25 participants with abnormal and normal phoria, respectively, were included in the study. The horizontal and vertical gaze points and convergence were recorded for 10 s using a remote eye tracker while binocularly viewing a target on a display screen 550 mm away. Fixation stability was quantified using bivariate contour ellipse areas (BCEA).ResultsThe fixation stability for all participants-based evaluations as a single cluster in the abnormal phoria group was lower than that in the normal phoria group (p = 0.005). There was no difference between the two groups in the evaluation based on the BCEA for each participant-based evaluation (p = 0.66). Fixation stability was also more related to convergence for the abnormal phoria group than for the normal phoria group (r = 0.769, p < 0.001; r = 0.417, p = 0.038, respectively).ConclusionAs the first study to evaluate fixation stability using an eye-tracker to differentiate between abnormal and normal phoria for non-strabismus, these findings may provide evidence for improving the evaluation of binocular vision not detected with clinical diagnostic tests.
Highlights
Fixation stability is the ability of the eyes to hold the image of an object on the fovea by keeping a constant steady gaze on the fixation target
Fixation stability can be measured by eye trackers clinically other than direct observation under the binocular vision, most of them were evaluated under manifest strabismus
Significant differences were found between the abnormal (n = 35) and normal phoria groups (n = 25) in the distance and near phoria, and in the calculated accommodative convergence to accommodation ratio
Summary
Fixation stability is the ability of the eyes to hold the image of an object on the fovea by keeping a constant steady gaze on the fixation target. The ability to maintain steady fixation plays important roles in eye movements, including saccadic and pursuit functions (Subramanian et al, 2013; Hessels et al, 2018). These eye movements are widely researched in ophthalmology, neurology, psychology, cognitive science, and human factors (Holmqvist et al, 2011; Laretzaki et al, 2011; Metsing and Ferreira, 2016; Krauzlis et al, 2017; Thielen et al, 2019). For evaluating fixation stability in binocular function, an objective assessment with observable eye movements is carried out to determine whether visual fixations are normal or abnormal. An objective assessment or diagnostic test for fixation stability by observation with the naked eye is not detected in non-strabismus; it is evaluated as normal. Evaluation of fixation stability in clinical practice is limited to visual observable manifests of strabismus with or without amblyopia
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