Temporal multiplexing with adaptive optics for simultaneous vision

Temporal fluctuations in defocus may reverse the acuity loss encountered with induced refractive errors

A recent template-matching model hypothesized that simulated visual acuity loss with uncorrected refractive errors may be recovered by adding temporal defocus fluctuations up to the magnitude of the refractive error. Acuity recovery saturates or gets attenuated beyond this magnitude. These predictions were confirmed for monocular high-contrast visual acuity of 10 young, cyclopleged adults with 0.5-2.0D of induced myopia combined with the same range of temporal defocus fluctuations at 4.0 Hz frequency. The outcomes reinforce that spatial resolution may be optimized by averaging time-varying defocus over the entire stimulus presentation epoch or around the point of least defocus within this epoch.

Catastrophe optics theory unveils the localised wave aberration features that generate ghost images

Monocular polyplopia (ghost or multiple images) is a serious visual impediment for some people who report seeing two (diplopia), three (triplopia) or even more images. Polyplopia is expected to appear if the point spread function (PSF) has multiple intensity cores (a dense concentration of a large portion of the radiant flux contained in the PSF) relatively separated from each other, each of which contributes to a distinct image. We present a theory that assigns these multiple PSF cores to specific features of aberrated wavefronts, thereby accounting optically for the perceptual phenomenon of monocular polyplopia. The theory provides two major conclusions. First, the most likely event giving rise to multiple PSF cores is the presence of hyperbolic, or less probably elliptical, umbilic caustics (using the terminology of catastrophe optics). Second, those umbilic caustics formed on the retinal surface are associated with certain points of the wave aberration function, called cusps of Gauss, where the gradient of a curvature function vanishes. However, not all cusps of Gauss generate those umbilic caustics. We also provide necessary conditions for those cusps of Gauss to be fertile. To show the potential of this theoretical framework for understanding the nature and origin of polyplopia, we provide specific examples of ocular wave aberration functions that induce diplopia and triplopia. The polyplopia effects in these examples are illustrated by depicting the multi-core PSFs and the convolved retinal images for clinical letter charts, both through computer simulations and through experimental recording using an adaptive optics set-up. The number and location of cores in the PSF is thus a potentially useful metric for the existence and severity of polyplopia in spatial vision. These examples also help explain why physiological pupil constriction might reduce the incidence of ghosting and multiple images of daily objects that affect vision with dilated pupils. This mechanistic explanation suggests a possible role for optical phase-masking as a clinical treatment for polyplopia and ghosting.

Predictability of sinusoidally moving stimuli does not improve the accuracy of the accommodative response

Previous research work suggests that predictable target motion such as sinusoidal movement can be anticipated by the visual system, thereby improving the accommodative response. The validity of predictable motion for studying human dynamic accommodation is sometimes put into question. The aim of this work was to assess the effect of anticipation along with learning (and motivation, etc.) and fatigue (and boredom, loss of attention, etc.) on dynamic accommodation experiments from a practical perspective. Specifically, changes in amplitude and temporal phase lag were estimated within and between trials as 9 adult observers were instructed to focus on a stimulus that oscillated sinusoidally towards and away from the eye at specific temporal frequencies. On average, amplitude decreased whereas phase increased within trials. No evidence of anticipation or learning was observed either within or between trials. Fatigue consistently dominated anticipation and learning within the course of each trial. Even if the eye is equipped by a prediction operator as it is often assumed, fatigue confounds the results from dynamic accommodation experiments more than anticipation or learning.

Multifocal and Extended Depth-of-Focus Intraocular Lenses in 2020

Ophthalmic surgeons have been overwhelmed by the influx of multifocal intraocular lens (IOL) options in recent years, with close to 100 IOLs on the market in 2020. This practical and technical update on a representative group of established as well as newly launched multifocal IOLs on the market focuses on multifocal IOLs, including extended depth-of-focus lenses. We also describe the optical basis of lens platforms used and thorough preoperative planning to aid decision making. This allows the surgeon the knowledge base to deliver the required relative customized spectacle independence with the least photic phenomenon and loss of contrast possible while achieving high individual patient satisfaction. Data of reviewed IOLs displayed in tabular format include mean monocular uncorrected distance, intermediate, and near visual acuities (logarithm of the minimum angle of resolution), with standard deviations and ranges where available. The range of vision targeted, pupil dependence, toric availability, as well as type of optical platform, are provided as a practical guide to demystify existing terminology on the market that may create interest around a seemingly new design that is actually not novel at all. Halos and glare experienced, levels of patient satisfaction, and spectacle independence achieved also are summarized. A wide range of multifocal IOLs options are available on the market to surgeons. Comprehensive patient selection and examination, combined with knowledge of the most recent options and adequate patient counseling, including neuroadaptation, can avoid dissatisfaction. Many recently available IOLs are awaiting formal results, but the methods by which we label and compare these types of IOLs must also be standardized.

Autofocals: Evaluating gaze-contingent eyeglasses for presbyopes

As humans age, they gradually lose the ability to accommodate, or refocus, to near distances because of the stiffening of the crystalline lens. This condition, known as presbyopia, affects nearly 20% of people worldwide. We design and build a new presbyopia correction, autofocals, to externally mimic the natural accommodation response, combining eye tracker and depth sensor data to automatically drive focus-tunable lenses. We evaluated 19 users on visual acuity, contrast sensitivity, and a refocusing task. Autofocals exhibit better visual acuity when compared to monovision and progressive lenses while maintaining similar contrast sensitivity. On the refocusing task, autofocals are faster and, compared to progressives, also significantly more accurate. In a separate study, a majority of 23 of 37 users ranked autofocals as the best correction in terms of ease of refocusing. Our work demonstrates the superiority of autofocals over current forms of presbyopia correction and could affect the lives of millions.

Defocus vibrations in optical systems-considerations in reference to the human eye

Experimental visual acuity (VA) of eight subjects was measured using the Freiburg vision test in a custom-made adaptive optics system. Measurements were conducted under one control and five defocus-induced conditions. In the defocus-induced conditions, 1 diopter of myopic defocus was added to the system using the Badal stage, and defocus vibrations with five different levels of amplitude were generated by a deformable mirror at 50 Hz. Computational simulations of the visual Strehl ratio (VSOTF) were performed using average aberrations of each subject recorded in the control condition. For the first time, to the best of our knowledge, it has been shown experimentally that both the simulated VSOTF and experimentally measured VA improve when defocus vibrations are added to a defocused eye.

Tunable lenses: dynamic characterization and fine-tuned control for high-speed applications

Tunable lenses are becoming ubiquitous, in applications including microscopy, optical coherence tomography, computer vision, quality control, and presbyopic corrections. Many applications require an accurate control of the optical power of the lens in response to a time-dependent input waveform. We present a fast focimeter (3.8 KHz) to characterize the dynamic response of tunable lenses, which was demonstrated on different lens models. We found that the temporal response is repetitive and linear, which allowed the development of a robust compensation strategy based on the optimization of the input wave, using a linear time-invariant model. To our knowledge, this work presents the first procedure for a direct characterization of the transient response of tunable lenses and for compensation of their temporal distortions, and broadens the potential of tunable lenses also in high-speed applications.

Presbyopia: Effectiveness of correction strategies

Presbyopia is a global problem affecting over a billion people worldwide. The prevalence of unmanaged presbyopia is as high as 50% of those over 50 years of age in developing world populations, due to a lack of awareness and accessibility to affordable treatment, and is even as high as 34% in developed countries. Definitions of presbyopia are inconsistent and varied, so we propose a redefinition that states "presbyopia occurs when the physiologically normal age-related reduction in the eye's focusing range reaches a point, when optimally corrected for distance vision, that the clarity of vision at near is insufficient to satisfy an individual's requirements". Strategies for correcting presbyopia include separate optical devices located in front of the visual system (reading glasses) or a change in the direction of gaze to view through optical zones of different optical powers (bifocal, trifocal or progressive addition spectacle lenses), monovision (with contact lenses, intraocular lenses, laser refractive surgery and corneal collagen shrinkage), simultaneous images (with contact lenses, intraocular lenses and corneal inlays), pinhole depth of focus expansion (with intraocular lenses, corneal inlays and pharmaceuticals), crystalline lens softening (with lasers or pharmaceuticals) or restored dynamics (with 'accommodating' intraocular lenses, scleral expansion techniques and ciliary muscle electrostimulation); these strategies may be applied differently to the two eyes to optimise the range of clear focus for an individual's task requirements and minimise adverse visual effects. However, none fully overcome presbyopia in all patients. While the restoration of natural accommodation or an equivalent remains elusive, guidance is given on presbyopic correction evaluation techniques.

Optical quality of rotationally symmetrical contact lenses derived from their power profiles

PURPOSE

To present a methodology for evaluating the optical quality of rotationally symmetrical contact lenses (CLs) from a single power profile.

METHODS

Simulated rotationally symmetrical power profiles corresponding to different CLs designs (monofocal, two-zone center-near bifocal, and four-zone center-distance bifocal) were used to calculate the wavefront error profile by means of numerical integration. Then, each lens wavefront error profile was spun around the center to obtain the lens wavefront error surface. From the surface, monochromatic optical transfer functions (OTF), simulated images and the visual Strehl ratio based on the OTF (VSOTF) were obtained for different distances and pupil sizes (3 and 5.5mm) after performing a through-focus.

RESULTS

VSOTF variations, taking into account both vergence and pupil size, were presented for the three CLs designs. The monofocal design showed excellent optical quality only for far vision, whereas the bifocal designs exhibited good optical quality for far and near vision. Modulation transfer function (MTF) from each lens design, pupil size, and work distance agreed with the previous results.

CONCLUSIONS

The methodology presented here allows for a rapid and thorough assessment of the optical quality of rotationally symmetrical CLs by means of optical quality metrics, with a special interest in simultaneous image contact lenses. This methodology may be useful for choosing the most suitable lens for each subject's visual demands.