Humza J. Tahir

Purpose Many eye diseases require on-going assessment for optimal management, creating an ever-increasing burden on patients and hospitals that could potentially be reduced through home vision monitoring. However, there is limited evidence for the utility of current applications and devices for this. To address this, we present a new automated, computer tablet-based method for self-testing near visual acuity (VA) for both high and low contrast targets. We report on its reliability and agreement with gold standard measures. Methods The Mobile Assessment of Vision by intERactIve Computer (MAVERIC) system consists of a calibrated computer tablet housed in a bespoke viewing chamber. Purpose-built software automatically elicits touch-screen responses from subjects to measure their near VA for either low or high contrast acuity. Near high contrast acuity was measured using both the MAVERIC system and a near Landolt C chart in one eye for 81 patients and low contrast acuity using the MAVERIC system and a 25 % contrast near EDTRS chart in one eye of a separate 95 patients. The MAVERIC near acuity was also retested after 20 min to evaluate repeatability. Results Repeatability of both high and low contrast MAVERIC acuity measures, and their agreement with the chart tests, was assessed using the Bland-Altman comparison method. One hundred and seventy-three patients (96 %) completed the self- testing MAVERIC system without formal assistance. The resulting MAVERIC vision demonstrated good repeatability and good agreement with the gold-standard near chart measures. Conclusions This study demonstrates the potential utility of the MAVERIC system for patients with ophthalmic disease to self-test their high and low contrast VA. The technique has a high degree of reliability and agreement with gold standard chart based measurements.

It has been proposed that the accommodation system could perform contrast discrimination between the two dioptric extremes of accommodative microfluctuations to extract directional signals for reflex accommodation. Higher-order aberrations (HOAs) may have a significant influence on the strength of these contrast signals. Our goal was to compute the effect HOAs may have on contrast signals for stimuli within the upper defocus limit by comparing computed microcontrast fluctuations with psychophysical contrast increment thresholds (Bradley & Ohzawa, 1986). Wavefront aberrations were measured while subjects viewed a Maltese spoke stimulus monocularly. Computations were performed for accommodation or disaccommodation stimuli from a 3 Diopter (D) baseline. Microfluctuations were estimated from the standard deviation of the wavefronts over time at baseline. Through-focus Modulation Transfer, optical contrast increments (ΔC), and Weber fractions (ΔC/C) were derived from point spread functions computed from the wavefronts at baseline for 2 and 4 cycles per degree (cpd) components, with and without HOAs. The ΔCs thus computed from the wavefronts were compared with psychophysical contrast increment threshold data. Microfluctuations are potentially useful for extracting directional information for defocus values within 3 D, where contrast increments for the 2 or 4 cpd components exceed psychophysical thresholds. HOAs largely reduce contrast signals produced by microfluctuations, depending on the mean focus error, and their magnitude in individual subjects, and they may shrink the effective stimulus range for reflex accommodation. The upper defocus limit could therefore be constrained by discrimination of microcontrast fluctuations.

Technological advances have led to the development of powerful yet portable tablet computers whose touch-screen resolutions now permit the presentation of targets small enough to test the limits of normal visual acuity. Such devices have become ubiquitous in daily life and are moving into the clinical space. However, in order to produce clinically valid tests, it is important to identify the limits imposed by the screen characteristics, such as resolution, brightness uniformity, contrast linearity and the effect of viewing angle. Previously we have conducted such tests on the iPad 3. Here we extend our investigations to 2 other devices and outline a protocol for calibrating such screens, using standardised methods to measure the gamma function, warm up time, screen uniformity and the effects of viewing angle and screen reflections. We demonstrate that all three devices manifest typical gamma functions for voltage and luminance with warm up times of approximately 15 minutes. However, there were differences in homogeneity and reflectance among the displays. We suggest practical means to optimise quality of display for vision testing including screen calibration.

Most of the high-order aberrations of the eye are not circularly symmetric. Hence, while it is well known that human vision is subject to cortically based orientation preference in cell tuning, the optics of the eye might also introduce some orientational anisotropy. We tested this idea by measuring contrast sensitivity at different orientations of sine-wave gratings when viewing through a closed-loop adaptive optics phoropter. Under aberration-corrected conditions, mean contrast sensitivity improved for all observers by a factor of 1.8× to 5×. The detectability of some orientations improved more than others. As expected, this orientation-specific effect varied between individuals. The sensitivity benefits were accurately predicted from MTF model simulations, demonstrating that the observed effects reflected the individual's pattern of high-order aberrations. In one observer, the orientation-specific effects were substantial: an improvement of 8× at one orientation and 2× in another orientation. The experiments confirm that, for conditions that are not diffraction limited, the optics of the eye introduce rotational asymmetry to the luminance distribution on the retina and that this impacts vision, inducing orientational anisotropy. These results suggest that the traditional view of meridional anisotropy having an entirely neural origin may be true for diffraction-limited pupils but that viewing through larger pupils introduces an additional orientation-specific optical component to this phenomenon.

Higher‐order aberrations degrade visual performance when the pupil is large, particularly in eyes that have abnormally high amounts of aberrations, such as those that have undergone refractive surgery. Geometrical optics predicts that asymmetrical aberrations such as coma can produce orientation selective effects, much like uncorrected astigmatism. Coma is also one of the main aberrations to increase following refractive surgery. Orientation may therefore be an important parameter when testing grating based contrast sensitivity, particularly in refractive surgery patients. Contrast sensitivity to four orientations of a 12 cycles deg ⁻¹ sine wave grating was measured in normals ( n = 34) and refractive surgery patients ( n = 12). In over a third of normal subjects the higher‐order aberrations produced a significant orientation‐specific change in contrast sensitivity (13 out of 34 eyes, p < 0.05). No significant differences existed in the aberrations between those that displayed orientation‐selective changes and those that did not. In subjects who underwent refractive surgery those that displayed orientation selective changes had significantly higher amounts of overall aberrations and also higher amounts of primary coma than those that did not. These results indicate that grating orientation is an important factor when assessing the effects of higher‐order aberrations on contrast sensitivity.

Local minima or notches in the defocused contrast sensitivity function (CSF) have been linked to the aberrations of the eye. We use theoretical modeling of the effects of the aberrations to show these notches can be orientation-selective due to the effects of aberration terms such as coma and trefoil. Notches that changed with orientation were observed in the defocused CSF of four subjects. The measured CSFs were found to match well with theoretical predictions produced using the individual aberrations. Theoretical modeling highlighted orientation-specific differences in notches for both positive and negative blur. The results indicate that orientation is an important variable when testing for the functional effects of higher-order aberrations.

It is well known that refractive surgery leads to an increase in higher‐order aberrations. These can cause problems such as glare, starburst and halos in vision, particularly in low light levels when the pupil is large. The overall wavefront aberration of the eye is typically non‐circularly symmetric due to the presence of aberrations such as coma and trefoil. These asymmetries have been shown to induce orientation‐selective effects in grating‐based contrast sensitivity tests. Classically, contrast sensitivity is assumed to be independent of orientation and is measured at only one orientation. Examining more orientations may give a better indicator of visual performance when abnormal amounts of aberration are present. To examine this, contrast sensitivity through a dilated pupil was determined for four orientations of a 12 cycles deg ⁻¹ sine wave grating in 12 subjects both before and after refractive surgery. Surgery produced orientation‐selective changes in contrast sensitivity in most patients (9 out of 12, p < 0.05). The correlation between contrast sensitivity and higher‐order aberrations was found to improve when several orientations were tested compared with data from only one orientation (combined orientations, r ²  = 0.46 , p =  0.007; single orientation r ² = 0.55, p <  0 .05 to r ² = 0.09, p >  0.05). These results indicate that grating orientation is an important factor when investigating the link between visual performance and higher order aberrations.