Humza J. Tahir

An assessment of a modern touch-screen tablet computer with reference to core physical characteristics necessary for clinical vision testing.

There are a multitude of applications using modern tablet computers for vision testing that are accessible to ophthalmology patients. While these may be of potential future benefit, they are often unsupported by scientific assessment. This report investigates the pertinent physical characteristics behind one of the most common highest specification tablet computers with regard to its capacity for vision testing. We demonstrate through plotting of a gamma curve that it is feasible to produce a precise programmable range of central luminance levels on the device, even with varying background luminance levels. It may not be possible to display very low levels of contrast, but carefully using the gamma curve information allows a reasonable range of contrast sensitivity to be tested. When the screen is first powered on, it may require up to 15 min for the luminance values to stabilize. Finally, luminance of objects varies towards the edge of the screen and when viewed at an angle. However, the resulting effective contrast of objects is less variable. Details of our assessments are important to developers, users and prescribers of tablet clinical vision tests. Without awareness of such findings, these tests may never reach satisfactory levels of clinical validity and reliability.

Optimisation and assessment of three modern touch screen tablet computers for clinical vision testing.

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.

Higher-order aberrations produce orientation-specific notches in the defocused contrast sensitivity function

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.

The oblique effect has an optical component: Orientation-specific contrast thresholds after correction of high-order aberrations

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 individuals 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.

Effects of Accommodation Training on Accommodation and Depth of Focus in an Eye Implanted with a Crystalens Intraocular Lens

PURPOSE To investigate objective measures of the effects of accommodative training of a pseudophakic eye implanted with a Crystalens AT-52SE (eyeonics Inc) intraocular lens (IOL) on reading performance, accommodation, and depth of focus. METHODS Objective dynamic measures of accommodation, pupil size, and depth of focus were quantified from wavefront measures before and after 1 week of accommodative training that began 29 months after implantation of an accommodating IOL in one patient. Depth of focus was estimated from 50% cut-off of peak performance levels for defocus curves that were computed from the image quality metric VSOTF based on ocular wavefront aberrations. RESULTS The patient reported improved near vision reading performance after completing the training procedure. After training, there was a shift in conjugate focus in the hyperopic direction, yet the depth of focus increased significantly for near objects. Simulated retinal images and the calculated modulation transfer function of the eye both demonstrated improved quality for near vision after training. CONCLUSIONS The subjective report of improved near vision after training was correlated with improvement of objective measures. Depth of focus increased for near objects with attempts to accommodate after training. This change was linked to increases in aberrations and pupil size and occurred despite the conjugate focus shifting in the hyperopic direction. These results demonstrate that accommodative training may be useful in improving near vision in patients with accommodating IOLs.

Orientation selectivity for sinusoidal gratings; evidence for an optical component

The human visual system exhibits lower contrast thresholds for detecting vertical and horizontal gratings compared with those orientated obliquely. This is known as the oblique effect and has been thought to be due wholly to neural factors. In this study, sensitivity to gratings of different orientation is compared when viewing with small and large pupils. Optical aberrations increase with pupil size. In some subjects, we find a statistically significant change in sensitivity to some orientations for the large pupil condition. This suggests that there is an optical component to orientation selectivity induced by the higher-order aberrations of the eye.

Potential role for microfluctuations as a temporal directional cue to accommodation.

The goal was to revisit an important, yet unproven notion that accommodative microfluctuations facilitate the determination of direction (sign) of abrupt focus changes in the stimulus to accommodation. We contaminated the potential temporal cues from natural accommodative microfluctuations by presenting uncorrelated external (screen) temporal defocus noise that combined with the retinal image effects of natural microfluctuations. A polychromatic Maltese spoke pattern thus either modulated defocus at a combination of two temporal frequencies (on-screen noise condition) or was static (control condition). The on-screen conditions were combined with step changes in optical vergence that were randomized in direction and magnitude. Five subjects monocularly viewed stimuli through a Badal optical system in a Maxwellian view. An artificial 4-mm aperture was imaged at the entrance pupil of the eye. Wavefront aberrations were measured dynamically at 50 Hz using a custom Shack–Hartmann aberrometer. Dynamic changes in the Zernike defocus term with step changes in optical vergence were analyzed. We calculated the percentage of correct directional responses for 1, 2, and 3 D accommodative and disaccommodative step stimuli using preset criteria for latency, velocity, and persistence of the response. The on-screen noise condition reduced the percent-correct responses compared to the static stimulus, suggesting that this manipulation affected the detectability of the sign of the accommodative stimulus. Several possible reasons and implications of this result are discussed.

Contrast sensitivity based orientation changes in refractive surgery patients

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, r2 = 0.46, p = 0.007; single orientation r2 = 0.55, p < 0 .05 to r2 = 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.

Slowed dark adaptation in older eyes; effect of location

Purpose The rate of rod sensitivity recovery following a photobleach is a basic measure of the integrity of the outer retina. Rods are selectively impaired in aging and many disorders of the retina, notably Age‐Related Macular Degeneration (AMD). It is not known for certain whether the age‐related deficit is a pan‐retinal effect or if there are localised regions of impaired rod function. To address this important issue a dual arc stimulus was developed that samples sensitivity recovery in two retinal locations. Methods Arc‐shaped stimuli were presented on a black CRT screen at two locations, in the inferior visual field. Following a bleach, which was localised to the stimuli, recovery of sensitivity was measured using a modified method of adjustment technique. Neutral density filters were used to extend the luminance range of the CRT. Sensitivity recovery functions were fitted by non‐linear regression to a seven‐parameter model. Results Pairs of sensitivity recovery functions were generated from the stimuli. The cone phases of these functions were identical. The slopes of the S2 sections of the curves were steeper for the outer stimuli for both young (p < 0.001) and older (p = 0.003) observers. The difference between the two was the same for the two groups. The &agr; point was reached slightly earlier for the young observers and with the outer stimulus but neither of these effects reached statistical significance. The &bgr; point occurred earlier for the outer stimuli and this effect was statistically significant only for the older group. Conclusions The method places minimal demands on observers. The fact that rod sensitivity recovery is slowed in the older normal eye to the same extent in the two locations suggests that this deficit may be uniform across the retina. As there are localised losses in scotopic function in AMD, the technique is ideally suited to distinguishing impaired recovery dynamics due to normal ageing from those caused by disease. HighlightsRate of rod sensitivity recovery following a photobleach is a basic measure of retinal integrity.Rods can be selectively impaired in aging and diseases of the retina but it is unknown if the deficit is pan‐retinal.A dual arc stimulus was developed that samples sensitivity recovery in two retinal locations.Data suggest that although rod sensitivity recovery is slowed in the older normal eye, it is uniform across the retina.The technique could be used to distinguish between impaired sensitivity due to ageing or disease.

The importance of grating orientation in contrast sensitivity following refractive surgery

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.