Abstract
Purpose :
3D displays, like clinical prism vergences, allow limited ranges of clear and single binocular vision because of accommodation/convergence mismatch. Hogan and McCormack (ARVO 2012) showed that these ranges differ between 3D displays and clinical vergences, and Kulowski and McCormack (ARVO 2013) demonstrated that 3D fixation target size is a cause of this difference. We now present additional data from the latter research, testing two hypotheses for the target size effect: (1) fusional vergence gain increases with target size (restraining retinal image defocus) and (2) the threshold of blur increases with target size (masking retinal image defocus).
Methods :
20 visually normal adults aged 18 to 28 years were dilated with 2.5% phenylephrine. 3D images were viewed at 40cm in an electronic mirror stereoscopic. The fixation target, a Maltese cross, moved in depth at 2 prism diopters/sec by way of changing crossed and uncrossed disparity until blur and diplopia ensued. We used four target sizes: (1) “small” (0.21°w x 0.63°h), (2) “medium” (1.43°w x 4.3°h), (3) “large” (3.6°w x 10.8°h), and (4) “3D” - size changing congruently with disparity (as in naturalistic viewing). Subjects reported blur and diplopia by finger taps on a touch surface. A first generation PowerRefractor (TM) quantified retinal image defocus at the moment of the blur tap. The effect of target size on vergence blur and break limits, and on the threshold of blur, were tested by mixed ANOVAs with target size as the repeated measures variable.
Results :
Vergence blurs and breaks increased with target size by 31% and 36% respectively (F=41.9, p<.0001). Blur changes paralleled break changes across target size. For subjects who reported blur, increasing target size raised the threshold of blur (F=5.9, p=.0027), from 1.2D to 2D in convergence and from .99D to 1.5D in divergence.
Conclusions :
The similarity of the target size effect on blur and break responses argues that fusional vergence gain change plays a role in the target size effect because depth of focus would not impact breaks. We suggest that the fusional vergence gain and blur threshold effects on clarity are additive. Those concerned with optimizing visual clarity in 3D displays should consider the influence of fixation target size on fusional vergence gain and blur thresholds.
This is a 2020 ARVO Annual Meeting abstract.