Advancing Hyperacuity for Vision Screening

Abstract

Introduction: The human visual system is extremely sensitive to certain spatial visual tasks. It can detect the subtle misalignment of closer objects to a degree of 2-5 arcseconds, which is smaller than the foveal cone diameter or spacing. This ability is referred to as hyperacuity, and one such visual task is the Vernier task, which involves misalignment detection of Vernier lines or dots. It is also called Vernier acuity and has a significant diagnostic value for screening various eye abnormalities. However, due to methodological and technical limitations, its utility was restricted to laboratory applications due to concerns over test reliability and testing time. I hypothesized that applying advanced psychophysical procedures, techniques, and modern technological interventions might improve the Vernier acuity testing standards for clinical consideration. Therefore, I attempted to address the challenges noticed in the literature by advancing the methodology and technicality to improve the Vernier acuity test efficiency for clinical application.

Aims: ⸰ Experiment 1 (Chapter-2): To develop a software application and assess the Vernier acuity program performance, measurements, and stimuli characteristics. ⸰ Experiment 2 (Chapter-3): To enhance the Vernier acuity program efficiency, assess program performance and reliability for technical validation. ⸰ Experiment 3 (Chapter-4): To modify the Vernier acuity program for the visual field quantification (Hyperacuity perimetry) and assess reliability for technical validation. ⸰ Experiment 4 (Chapter-5): To develop a software application for the visual distortions quantification (Metamorphopsia) using Vernier acuity-based bisectional program and assess reliability for technical validation.

Methods: This study was performed through two pilot studies: the first pilot study had five adult volunteers with the best corrected visual acuity of 20/20 vision in the right eye (tested right eye only) and included all the experiments from Chapter 2, whereas the second pilot study was carried out on 21 adult emmetrope (unaided 20/20 vision) volunteers (tested both eyes individually) and included all the experiments from Chapters-3, 4, and 5. I used PsychoPy3 to develop each software program. However, I employed two methods to provide the test results efficiently and reliably for clinical testing. I developed three software applications and could only perform technical validation because of the pandemic. ⸰ Experiment 1 (Chapter-2): I programmed a software program and employed a 3-Down, 1-Up adaptive staircase method and three alternative forced choices technique to quantify the Vernier acuity. The Vernier acuity was measured at seven vertical separations (gaps) to assess test performance. The initial testing was focused on determining test performance using stimuli shapes, followed by technical validation of the software application program and assessment of stimuli contrast for standardization. ⸰ Experiment 2 (Chapter-3): I adjusted the measurement parameters to improve test efficiency. I assessed the program performance from response accuracy, reaction time, and testing time, along with repeatability of measurements for technical validation of the Vernier acuity program. ⸰ Experiment 3 (Chapter-4): I modified the Vernier acuity program to quantify the hyperacuity perimetry in superior, inferior, nasal, and temporal visual fields. I assessed the repeatability of measurements for technical validation of this modified Vernier acuity program. ⸰ Experiment 4 (Chapter-5): I programmed a Vernier bisection program to quantify the metamorphopsia using a method of adjustment. The testing involved Vernier stimuli in two different orientations and referred to them as patterns (A and B), Pattern-A had a presentation of two vertical and horizontal line stimuli that are equally away from the center of the screen in either direction, whereas pattern B involved presentations of pattern-A at oblique angles to the screen center. Using both patterns, I measured the metamorphopsia in central 5 degrees and assessed the repeatability of the measurements and testing time for technical validation of the Vernier bisection program.

Results: ⸰ Experiment 1 (Chapter-2): In the initial experiment, the line stimuli achieved comparable measurements to dot stimuli at most gap sizes except at 32 arcminutes of gap size. The test detected the lowest misalignment of 2 arcseconds at 2 arcminutes of gap size. The mean lowest acuity was below 8 arcseconds at 2 arcminutes, and the highest acuity was within an arcminute at 128 arcminutes. The negative contrast line stimuli were comparably precise to positive line stimuli at most gap sizes except at 16 arcminutes. ⸰ Experiment 2 (Chapter-3): The right eyes were repeatable at most of the gap sizes except for the 32 and 64 gap sizes, whereas the left eyes were repeatable at all gap sizes except 128. The right and left eye measurements were statistically the same at both visits. Since no difference was observed between the eyes, results from both eyes were compiled to assess the test performance and response accuracy. The Vernier acuity measured at 16, 8, 4, and 2 gap sizes were statistically repeatable, and the correlation was positive but weak. The response accuracy was estimated to be above 90% for mean correct responses, below 3% for mean incorrect responses, and about 5% for mean aligned responses through the gap sizes at both visits. The estimated reaction time was just below a second for mean correct responses, below 0.75 seconds for the mean incorrect responses, and about 2.5 seconds for the mean aligned responses. The test time was below 2 minutes at each gap size at both visits. ⸰ Experiment 3 (Chapter-4): The right eye hyperacuity perimetry results were repeatable in all four quadrants of 15 gap size, and the correlation was positive but weak in all quadrants except the superior visual field, where the correlation was negative and weak. Whereas for gap size 30, the results were repeatable in all quadrants except the inferior visual field, where the results were not repeatable. However, results from all the quadrants had a positive but weak correlation. The left eye hyperacuity perimetry results were repeatable in all quadrants of 15 gap size, except the nasal visual field, where the results were not repeatable. However, results from all the quadrants had a positive but weak correlation. For gap size 30, the results were repeatable in all four quadrants, and all quadrants had a positive but weak correlation. The results from both eyes showed no significant difference for a gap of 15 arcminutes. However, there was a substantial difference between the eyes only at the inferior field for a gap of 30 arcminutes. ⸰ Experiment 4 (Chapter-5): The right eye metamorphopsia results were repeatable in the central 5 degrees using a pattern A with a positive but weak correlation at all degrees except 5 and 1 degrees, where the correlation was negative and weak. Similarly, the results were repeatable in the central 5 degrees using a pattern B except for 5 degrees, and the correlation was positive but weak at all degrees except 3 and 2 degrees, where there was a negative and weak correlation. On the other hand, the left eye results were repeatable in the central 5 degrees using a pattern A with positive and moderate to strong correlations at all degrees. Similarly, the results were repeatable in the central 5 degrees using a pattern B except for 4 degrees, and the correlation was positive and moderate to strong at all degrees. There was no difference between the eyes for individual patterns at both visits.

Conclusions: ⸰ Experiment 1 (Chapter-2): The developed software application program measured the Vernier acuity precisely using 3-down, 1-up, an adaptive staircase method, and the 3AFC technique. In addition, I standardized stimuli for shape and contrast to measure the Vernier acuity. The calculated results are precise and consistent with the previously reported data. Therefore, motivating to advance the test further for Vernier acuity testing. While performing the test, some areas for improvement were identified. By adjusting the necessary parameters, the test's efficiency can be enhanced. I plan to make those adjustments in the following pilot testing and perform technical validation. ⸰ Experiment 2 (Chapter-3): I adjusted the necessary parameters to improve the efficiency of the Vernier acuity testing. The results showed that the program is efficient, robust, and repeatable. The mean Vernier acuity measured at seven different gap sizes was consistent with previously reported data and comparable with pilot study 1 results. This cohort’s Vernier acuity measured at smaller gap sizes was highly dependable. The measurements were significantly repeatable at most gap sizes but were poorly dependable. Therefore, this program may need further modifications to achieve better reliable results for clinical testing. ⸰ Experiment 3 (Chapter-4): I modified the Vernier acuity program to quantify the Vernier acuity in eccentric 5 degrees of the macula (para-foveal area). The right eye results were repeatable for 15 arc minutes of gap size, whereas the left eye results were repeatable for the 30 arc minutes of gap size. However, both eyes had poor reliability at most of the gap sizes except the superior field of gap 30, where the reliability was moderate. This inconsistency between the eyes could be due to distinct reasons and therefore needs further investigations to address the underlying cause. ⸰ Experiment 4 (Chapter-5): I programmed a software application to quantify the metamorphopsia using a method of adjustment. The right eye results were repeatable at most gap sizes for both patterns. However, the measurements were poorly dependable at most gap sizes of both patterns. Similarly, the left eye results were repeatable at most gap sizes for both patterns. However, the measurements were poorly dependable at most gap sizes of both patterns, except at 3 degrees of pattern A and 5 and 2 degrees of pattern B, where the reliability was moderate. Further modifications in the program may provide better reliability for clinical testing.