Scotopic and Photopic Conventional Visual Acuity and Hyperacuity - Binocular Summation

Abstract

Purpose: The purpose of this study was to determine and compare binocular summation (BiS) of conventional visual acuity (cVA) versus hyperacuity (hVA) for photopic and scotopic luminance conditions as a potential biomarker to assess the outcome of interventions on binocular function.

Methods: Sixteen young adults (age range [years] = 21-31; 8 women; cVA logMAR < 0.0) participated in this study. The Freiburg Visual Acuity Test (FrACT) was used for VA testing and retested on another day. Both cVA and hVA were determined for dark grey optotypes on light grey background. Participants underwent 40 minutes of dark adaptation prior to scotopic VA testing. Binocular and monocular VA testing was performed. The eye with better VA over the 2 days of testing was selected, the BiS was quantified (binocular VA - better monocular VA) and repeated measures ANOVAs were performed.

Results: Binocular VA exceeded monocular VA for all luminance conditions, VA-types, and sessions. We report BiS estimates for photopic and scotopic cVA and hVA, (logMAR BiS ± SEM [decimal BiS]): photopic = -0.01 ± 0.01 [1.03] and -0.06 ± 0.03 [1.15]; and scotopic = -0.05 ± 0.01 [1.12] and -0.11 ± 0.04 [1.28], respectively). Improvement for binocular vision estimates ranged from 0.01 to 0.11 logMAR. A repeated-measures ANOVA (RM ANOVA) did not reveal significant effects of LUMINANCE or VA TYPE on BiS, albeit a trend for strongest BiS for scotopic hVA (15% vs. 28%, photopic versus scotopic, respectively) and weakest for photopic cVA (3% vs. 12%, photopic versus scotopic conditions, respectively).

Conclusions: Our results indicate that BiS of VA is relevant to scotopic and photopic hVA and cVA. It appears therefore a plausible candidate biomarker to assess the outcome of retinal therapies restoring rod or cone function on binocular vision.

Translational relevance: Binocular summation of visual acuity might serve as a clinical biomarker to monitor therapy outcome on binocular rod and cone-mediated vision.

Figure 1.

Acuity comparison between sessions. For each of the four visual acuity conditions (scotopic versus photopic), individual cVA and hVA values (small symbols) for OD, OS, and OU, are shown along with group averages (large symbols; n = 16; SEMs are smaller than symbol size). No significant effects on visual acuity were observed across sessions. Note the inverted axes for logMAR values (this measure describes visual loss), such that poor visual acuity is in the bottom left quadrant and good visual acuity is in the top right quadrant; the identity line is dashed.

Figure 2.

Mean binocular summation across different luminance and VA conditions determined with three different approaches. Mean values and SEM of mean BiS for both luminance conditions (photopic and scotopic) and both VA types (cVA and hVA) [logMAR] are given for each group of calculations. BiS are calculated as the difference of binocular LogMAR and monocular LogMAR (A) For the better eye of each day; (B) for the better eye of both days; (C) for the mean across both days); approach B is less prone to misestimations (see text). Binocular improvement is given in logMAR, left, and as the corresponding binocular summation ratio (BSR), right. Note the inverted axis for the logMAR values so that binocular inhibition is shown at the bottom and good summation is at the top. There were no significant main effects of VA type and of luminance on BiS. The horizontal line indicates the established binocular improvement in literature, 40%.

Figure 3.

BiS comparison between sessions. For each of the four VA conditions, the average BiS ± SEM indicate the binocular VA improvement (cVA = squares; hVA = circles; photopic = light fill; and scotopic = dark fill; colors indicate BiS calculation method, see Methods). Although there is a trend for hVA improvement in session 2, no significant session effect was evident as detailed in Results and Figure 1. Note the inverted axes for logMAR values resulting in weaker BiS in the bottom left and stronger BiS in the top right. The identity line is dashed, the two solid zero lines indicate the transition from BiS to binocular inhibition for sessions 1 and 2.