Abstract
Background/aimsThe ability to extract depth from disparity may be hindered under fusional stress, as alignment of the eyes may be more difficult to maintain consistently. Therefore we aim to determine the effect of fusional demand on stereoacuity in individuals with no known binocular vision impairments.MethodsA novel static and dynamic binocular depth detection task, capable of assessing many discrete levels of stereoacuity, was presented on digital displays attached to each tube of the Synoptophore. Stereoacuity was measured with any latent deviation fully corrected and compared to that measured at the ‘recovery’ angle. This recovery angle is where single vision is restored after decompensation to diplopia, during vergence range assessment.ResultsSeventy-two subjects (50 Female, 22 Male; mean (SD) age 22 (6) years) were assessed. The amount of fusional demand was between 1 and 26 prism dioptres (PD), with a mean (SD) of 8(6)PD. Under zero fusion demand the mean (SD) static and dynamic depth detection thresholds were 322(53)” and 69(23)”. Under fusional stress these were 224(40)” and 77(21)”. There was no significant difference between thresholds in stressed and zero demand fusion (p = 0.08). Dynamic depth detection thresholds were significantly lower than static (P < 0.01).ConclusionFusional stress does not appear to impact on stereoacuity. The numerical value of the recovery point varied amongst individuals, but this represents a common point, where single vision is easily restored and binocularity well established. Due to individual differences in the ability to control a certain amount of fusional stress (e.g. vergences stress of 10PD, when recovery is 8PD, will perturb binocularity more than a person with a recovery of 20PD), previous reports may not accurately represent the effect of fusional stress. Whilst our findings are contrary to previous reports, we did not stress fusion beyond the recovery point and used a more accurate/repeatable method to measure stereoacuity.
Highlights
High grade stereo acuity requires the precise alignment of the visual axis, and the sensory ability to determine the presence of binocular disparity between the left and right visual fields, and use this information to extract depth information
This study aims to evaluate the effect of the fusional stress on stereoacuity in both static and dynamic presentations controlled with a computerised staircase procedure with the ability to present numerous levels of disparity, in subjects undergoing similar stress on fusional control
To determine if the size of the fusional reserves available had any bearing on stereoacuity thresholds, a Pearson’s correlation was run between the size of the recovery angle and the static and dynamic stereoacuity thresholds with zero fusional stress (Fig. 4)
Summary
High grade stereo acuity requires the precise alignment of the visual axis, and the sensory ability to determine the presence of binocular disparity between the left and right visual fields, and use this information to extract depth information. In subjects with good binocular control, the motor fusion system responds to any diplopia perceived, ensuring the visual axes are positioned on the point of fixation, resulting in zero retinal disparity at fixation. Many individuals experience difficulty with ocular motor control, with varying impact on levels of stereoacuity, for example, the deterioration of fusional control in intermittent exotropia, can lead to in an increase in threshold [3,4,5,6]. While the consequences of a breakdown in ocular motor control are seen clinically with (2019) 19:6 patients reporting a variety of symptoms resulting from the effort of maintaining binocular vision, the impact of exerting motor fusion on stereoacuity is not clear, and experiments designed to determine the importance of motor control have resulted in conflicting conclusions. The choice of stereoacuity test may have contributed to the variability in response, as any change may be encompassed by test/re-test variability (TRV)
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