Sotiris Plainis

The effect of ocular aberrations on steady-state errors of accommodative response

It is well accepted that the accommodation system is characterized by steady-state errors in focus. The purpose of this study was to correlate these errors with changes in ocular wavefront aberration and corresponding image quality when accommodating. A wavefront analyzing system, the Complete Ophthalmic Analysis System (COAS), was used in conjunction with a Badal optometer to allow continuous recording of the aberration structure of the eye for a range of accommodative demands (up to 8 D). Fifty consecutive recordings from seven subjects were taken. Monocular accommodative response was calculated as (i) the equivalent refraction minimizing wavefront error and (ii) the defocus needed to optimize the modulation transfer function at high spatial frequencies. Previously reported changes in ocular aberrations with accommodation (e.g., the shift of spherical aberration to negative values) were confirmed. Increased accommodation errors for near targets (lags) were evident for all subjects, although their magnitude showed a significant intersubject variability. It is concluded that the one-to-one stimulus/response slope in accommodation function should not always be considered as ideal, because higher order aberrations, especially changes of spherical aberration, may influence the actual accommodative demand. Fluctuations may serve to preserve image quality when errors of accommodation are moderate, by temporarily searching for the best focus.

Neurophysiological interpretation of human visual reaction times: effect of contrast, spatial frequency and luminance

Neurophysiological studies have demonstrated that in primates Magno and Parvo neurons have distinct contrast gain properties. Reaction Times (RTs) can be used to study supra-threshold contrast coding in humans over the same range of stimulus parameters. RTs to achromatic sinusoidal gratings were measured for a range of spatial frequencies (0.49-17.7 c/degree), stimulus luminances (0.005-20 cd/m2) and contrasts (from threshold to 0.5). The stimuli subtended an angle of 7.2 degrees at a viewing distance of 114 cm. RTs exhibit a linear relationship when plotted against the reciprocal of suprathreshold contrast. The slope of these functions reveals how contrast is linked to RT and can, therefore, be referred to as the RT-contrast factor with units of msec x contrast. A general equation is derived which accounts for all stimulus combinations. RT-based contrast functions resemble closely those obtained neurophysiologically for Magno (M) and Parvo (P) cells. Furthermore, the RT equivalent of contrast gain exhibits qualitatively similar gain characteristics to these neurons for a wide range of luminances and spatial frequencies. Our data support the notion that the sensory component of RTs is limited by the properties of pre-cortical neurons.

Variability of wavefront aberration measurements in small pupil sizes using a clinical Shack-Hartmann aberrometer

BackgroundRecently, instruments for the measurement of wavefront aberration in the living human eye have been widely available for clinical applications. Despite the extensive background experience on wavefront sensing for research purposes, the information derived from such instrumentation in a clinical setting should not be considered a priori precise. We report on the variability of such an instrument at two different pupil sizes.MethodsA clinical aberrometer (COAS Wavefront Scienses, Ltd) based on the Shack-Hartmann principle was employed in this study. Fifty consecutive measurements were perfomed on each right eye of four subjects. We compared the variance of individual Zernike expansion coefficients as determined by the aberrometer with the variance of coefficients calculated using a mathematical method for scaling the expansion coefficients to reconstruct wavefront aberration for a reduced-size pupil.ResultsWavefront aberration exhibits a marked variance of the order of 0.45 microns near the edge of the pupil whereas the central part appears to be measured more consistently. Dispersion of Zernike expansion coefficients was lower when calculated by the scaling method for a pupil diameter of 3 mm as compared to the one introduced when only the central 3 mm of the Shack – Hartmann image was evaluated. Signal-to-noise ratio was lower for higher order aberrations than for low order coefficients corresponding to the sphero-cylindrical error. For each subject a number of Zernike expansion coefficients was below noise level and should not be considered trustworthy.ConclusionWavefront aberration data used in clinical care should not be extracted from a single measurement, which represents only a static snapshot of a dynamically changing aberration pattern. This observation must be taken into account in order to prevent ambiguous conclusions in clinical practice and especially in refractive surgery.

Contrast coding and magno/parvo segregation revealed in reaction time studies

Reaction times (RTs) are obtained for a wide range of contrasts of vertical sinusoidal gratings. The data are plotted as a function of the reciprocal of contrast. In some conditions, a single linear function accounts for the data. In others a clear bi-linear function is obtained. The low and high contrast regions of the function are interpreted as representing magno and parvo activity, respectively. RT-based supra-threshold sensitivity functions are obtained for different luminances, stimulus durations and eccentricities and these are compared with conventional threshold-based sensitivities to establish the extent to which RTs and contrast sensitivity are constrained by the same sensory processes.

Power Profiles of Multifocal Contact Lenses and Their Interpretation

Purpose Many contact lens (CL) manufacturers produce simultaneous-image lenses in which power varies either smoothly or discontinuously with zonal radius. We present in vitro measurements of some recent CLs and discuss how power profiles might be approximated in terms of nominal distance corrections, near additions, and on-eye visual performance. Methods Fully hydrated soft, simultaneous-image CLs from four manufacturers (Air Optix AQUA, Alcon; PureVision multifocal, Bausch & Lomb; Acuvue OASYS for Presbyopia, Vistakon; Biofinity multifocal- “D” design, Cooper Vision) were measured with a Phase focus Lens Profiler (Phase Focus Ltd., Sheffield, UK) in a wet cell and powers were corrected to powers in air. All lenses had zero labeled power for distance. Results Sagittal power profiles revealed that the “low” add PureVision and Air Optix lenses exhibit smooth (parabolic) profiles, corresponding to negative spherical aberration. The “mid” and “high” add PureVision and Air Optix lenses have bi-aspheric designs, leading to different rates of power change for the central and peripheral portions. All OASYS lenses display a series of concentric zones, separated by abrupt discontinuities; individual profiles can be constrained between two parabolically decreasing curves, each giving a valid description of the power changes over alternate annular zones. Biofinity lenses have constant power over the central circular region of radius 1.5 mm, followed by an annular zone where the power increases approximately linearly, the gradient increasing with the add power, and finally an outer zone showing a slow, linear increase in power with a gradient being almost independent of the add power. Conclusions The variation in power across the simultaneous-image lenses produces enhanced depth of focus. The through-focus nature of the image, which influences the “best focus” (distance correction) and the reading addition, will vary with several factors, including lens centration, the wearer’s pupil diameter, and ocular aberrations, particularly spherical aberration; visual performance with some designs may show greater sensitivity to these factors.

Choice of Analytic Approach for Eye-Specific Outcomes: One Eye or Two?

PURPOSE To investigate the use of analytic approaches for eye-specific outcomes in ophthalmology publications. DESIGN A review of analytic approaches used in original research articles published in ophthalmology journals. METHODS All 161 research articles published in 5 ophthalmology journals in the first 2 months of 2008 were considered. Publications were categorized according to analytic approach: 1 eye selected, both eyes contribute, or per-individual outcome. Studies were considered suboptimal when criteria for eye selection were not provided or when measurements from both eyes were included without interocular correlation being considered. Visual impairment prevalence data were used to illustrate analytic approach choices. RESULTS Measurements from both eyes were included in 38% of the 112 studies that used statistical inferential techniques. In 31 (74%), there was no mention of possible correlation. Only 7% used statistical methods appropriate for correlated outcomes. In 35 studies (31%), measurements from 1 eye were selected; 31% of these did not provide selection criteria. In 67%, only univariate tests were used. A review of 47 articles published in 2011 produced similar findings. Characteristics of studies were not found to differ according whether the studies were suboptimal. Using a test appropriate for correlated outcomes resulted in a P value 3.5 times that obtained ignoring the correlation. CONCLUSIONS Between-eye correlation seems not to be assessed commonly in ophthalmology publications, although its knowledge aids the choice of analytic approach when eye-specific variables are of interest. Statistical methods appropriate for correlated ocular outcome data are not being applied widely.

Through-focus performance with multifocal contact lenses: effect of binocularity, pupil diameter and inherent ocular aberrations

To evaluate the effects of the wearers pupil size and spherical aberration on visual performance with centre‐near, aspheric multifocal contact lenses (MFCLs). The advantage of binocular over monocular vision was also investigated.

The Ocular Stress Monitor; a New Device for Measuring Discomfort Glare.

Discomfort glare is always accompanied by a strong contraction or spasm in the muscles surrounding the eye. A portable device for measuring the electrical activity generated by this muscle spasm is described. The device samples the signal from electrodes placed around the eye. It is composed of a narrow-band amplifier/transmitter, a receiver/frequency converter and a tone generator. The signal amplitude is proportional to the vertical illuminance at the eye and can therefore be used as an objective index of the discomfort induced. The results compare favourably with subjective assessment.

Raised visual detection thresholds depend on the level of complexity of cognitive foveal loading

The objective of the study was to measure the interactions between visual thresholds for a simple light (the secondary task) presented peripherally and a simultaneously performed cognitive task (the primary task) presented foveally. The primary task was highly visible but varied according to its cognitive complexity. Interactions between the tasks were determined by measuring detection thresholds for the peripheral task and accuracy of performance of the foveal task. Effects were measured for 5, 10, 20, and 30 deg eccentricity of the peripherally presented light and for three levels of cognitive complexity. Mesopic conditions (0.5 lx) were used. As expected, the concurrent presentation of the foveal cognitive task reduced peripheral sensitivity. Moreover, performance of the foveal task was adversely affected when conducting the peripheral task. Performance on both tasks was reduced as the level of complexity of the cognitive task increased. There were qualitative differences in task interactions between the central 10 deg and at greater eccentricities. Within 10 deg there was a disproportionate effect of eccentricity, previously interpreted as the ‘tunnel-vision’ model of visual field narrowing. Interactions outside 10 deg were less affected by eccentricity. These results are discussed in terms of the known neurophysiological characteristics of the primary visual pathway.

Efficacy of 2 types of silicone hydrogel bandage contact lenses after photorefractive keratectomy

PURPOSE: To compare the efficacy of 2 types of silicone hydrogel bandage contact lenses with high oxygen transmissibility after photorefractive keratectomy (PRK). SETTING: Institute of Vision and Optics, Faculty of Medicine, University of Crete, Heraklion, Crete, Greece. METHODS: In this prospective study, 1 eye of patients having bilateral PRK was randomly fitted with a bandage contact lens of lotrafilcon A (Night & Day) and the fellow eye, with a bandage contact lens of lotrafilcon B (O2Optix). The patients and the examiner were masked to which bandage contact lens type was in which eye. Patients were examined on the day of surgery and 1, 3, and 5 days postoperatively. Postoperative examinations included uncorrected distance visual acuity and slitlamp biomicroscopy to assess epithelial defect size. Subjective evaluation of pain and vision was recorded 1, 2, 3, and 4 days postoperatively. RESULTS: The study enrolled 44 patients (88 eyes). The mean epithelial defect size immediately after surgery was 47.0 mm2 with both types of bandage contact lenses. There was no statistically significant difference in epithelial defect size between the 2 lenses at any postoperative visit. Three days postoperatively, reepithelialization was complete in 75.0% of eyes in the lotrafilcon A group and 72.7% of the eyes in the lotrafilcon B group. CONCLUSION: There were no differences in corneal reepithelialization or subjective measurements after PRK between the 2 types of silicone hydrogel bandage contact lenses.