In this research we present a basis for a solution for Age Related Macular Degeneration (AMD) patients. The proposed solution is a binocular passive optical device composed of a contact lens and spectacles, both coated by light-reflecting material in order to generate a Fabry-Perot-like resonator. This bounces the light rays several times between the two surfaces, achieving optical simultaneous magnifications for near and far distances as needed for AMD patients in early stages of the disease. Our work has two parts: numerical simulation of the magnification achieved by the device and a clinical experiment, with non-AMD patients, in which we examined visual skills with simultaneous magnifications. The numerical simulations proved mathematically that the device can produce several different magnifications simultaneously, Zemax simulations confirmed this. In the clinical study, simultaneous vision was found to have little effect on visual acuity, but slightly increased reaction time to stimuli. Thus, the proposed device may improve visual capabilities of AMD patients, allow patients in stages where the peripheral retina still functions to use these areas to maximize their remaining visual potential and thus function better in everyday life.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
AMD is an irreversible eye condition affecting older people and involves the loss of the person's central field of vision, caused by the macula developing degenerative lesions. It is the most common cause of untreatable blindness in industrialized countries among the elderly population, and the third most frequent cause of blindness globally. Its prevalence before the age of 50 years is 0.05%, rising to 11.8% after 80 years of age. This prevalence is expected to increase dramatically in the near future with the increasing ageing of the population. AMD is typically bilateral, but vision loss may be asymmetrical in the two eyes [1–5]. Currently there is no available cure for AMD. Scientific studies have shown that vitamins and supplements may lower the risk of macular degeneration progressing toward the advanced stages of the disease. Yet the beneficial effects are limited, and the treatment has side effects [3,4,6,7].
The macula is the centrally located portion of the retina, responsible for high resolution and best acuity vision and thus the ability to discern fine details. In the early stages of AMD, patients suffer from degeneration of the central retina, impairing their ability to perform simple daily activities that require precise vision, such as driving, recognizing faces and reading. However, in the early stages of the disease peripheral retinal areas still function normally. As the disease develops, the peripheral retina may also degenerate leading to serious injury and eventually to blindness [8–11]. In the earlier stages of the disease, while the peripheral retina still functions, magnifiers can be adjusted to allow focusing of central stimuli on the peripheral retina, a solution that enables optimal realization of the patient's remaining visual potential. Usually, AMD patients need different magnifications required for different activities, for near and for far distances [1,2,12–14].
“Simultaneous vision” relates to a situation in which different images are simultaneously projected onto the retina, with the final image formed resulting from the combination of the various images. Simultaneous vision is used in some optical solutions for various vision-related situations, such as bifocal or multifocal contact lenses for presbyopia and myopia control and IOLs implanted during cataract surgery. Implantation of a high-add IOL after cataract removal enables an approximately ×2.5 - ×3 magnification for near. These simultaneous vision lenses have multiple powers, creating different images reaching the retina at the same time [15–25]. “Regular” IOL (positioned in the natural lens capsule) do not cause significant unwanted magnification.
These different magnifications and different contrasts of the images usually result in the simultaneous formation of focused (sharp) and defocused (blurred) images on the retina. Superimposing these images make them compete in the subject's visual system and may cause optical degradation produced by the image overlap . If the patient is unable to suppress the defocused image, the large amount of blur generated may reduce the contrast sensitivity . Thus, to be successful, a neural adaptation process is needed for the patient to learn how to process the image, by selectively ignoring or suppressing the defocused image components that are not desirable for a given task, while preserving the focused ones. An improvement over time in visual performance after adaptation to simultaneous vision has been reported [21–23,26,27].
The treatment approach for AMD patients proposed here aims to create optical magnifications of the image seen by the eye, so as to allow the patient, whose central retina is damaged or nonfunctional, to use the peripheral retina. The treatment approach is based on a binocular passive optical device composed of a contact lens and spectacles (ophthalmic lens) worn by the patient. The external surface of the contact lens and the internal surface of the ophthalmic lens are coated in order to generate a Fabry-Perot-like resonator that bounces the optical rays several times between these two surfaces, creating different simultaneous magnifications of the image. Different magnifications are needed for the near and far distance vision of AMD patients, and simultaneous magnification meets this need .
This paper, presenting the device, is comprised of two parts: the numerical simulation of the magnifications achieved by the device and a clinical experiment in which we examined visual acuity and reaction time in healthy subjects when viewing simultaneous magnification.
2. Materials and methods
2.1 Numerical simulation of the magnification achieved by the two coated lenses
To create the required magnification for focusing the image on functional peripheral retina in AMD patients, the patient wears a contact lens coated with reflective coating on its external surface and an ophthalmic lens coated by reflective coating on its internal surface as demonstrated in Fig. 1.
The realizable magnification factor can be computed as follows:
Due to geometric relations, we have:
2.1 Clinical Experiment
A clinical experiment was conducted with ten subjects, healthy and without AMD or other ocular pathologies (20 eyes), in order to test visual system skills when multiple magnifications of the visual stimulus are presented, that is, in simultaneous viewing conditions. Informed consent was obtained from all subjects after the study was explained to them, and the approval of the ethics committee of Bar-Ilan University was obtained. Research procedures used in this study followed the tenets of the Declaration of Helsinki.
A psychophysical task was conducted monocularly while the contralateral eye was covered. Visual stimuli were presented via a personal computer on a BENQ XL 2411 color monitor. Screen resolution was 1920 X 1080 pixels at a 120 Hz refresh rate, and gamma correction was applied. Viewing distance was 60 cm from the screen. The experiments were conducted in a dark environment in which the only ambient light came from the monitor. The “Tumbling-E pattern (TeVA) test” paradigm (Fig. 2) was used [29–31]. Figure 3 shows a subject participating in the experiment.
The stimuli consisted of 5 X 5 E-patterns, which correspond to a subset of the LogMAR chart, with a baseline pattern size (TeVA = 0) corresponding to the baseline 6/6 visual acuity. They were presented for 100 milliseconds, a very common duration in psychophysical experiments . Subjects were asked to detect the direction of the open part of the E-letter, one of four directions as presented in Fig. 2, using the arrows on the keyboard.
A staircase with the pattern size modified by 0.1 log unit in each step was used to determine the size for 50% correct (chance was 25%) using a four-alternative forced-choice (4AFC) method. That is, subjects had to choose a direction and answer. The next stimulus was presented only after a response to the previous one. No feedback was given for correct or incorrect responses.
The experiment consisted of two stages with different stimulus colors, each performed twice. First, the visual acuity threshold and reaction time were measured for a single E-letter presented in one of the four directions shown in Fig. 2. The figure was shown in different sizes, and subjects were asked to detect its direction. In the second stage, the same E-letter was presented with a magnified constant size E-letter in the background for testing visual acuity and reaction time in the presence of a simultaneous magnified stimulus. The staircase for pattern size was performed only for the central E-letter, as tested in the first stage of the experiment.
In the first (“gray”) experiment the stimulus consisted of a single light gray E-pattern on a gray background (Fig. 4(a)). After measuring visual acuity threshold and reaction time, using different sizes and directions, the same gray E-letter was presented in the presence of a magnified darker gray E-letter in the background, again using different sizes and directions (Fig. 4(b)). Visual acuity and reaction time were measured again. This allowed comparing results for the single stimulus and the simultaneous vision condition.
The same procedures as in the gray experiment were performed again but this time the stimulus consisted of a single red E-pattern on a black background (Fig. 4(c)). After measuring visual acuity threshold and reaction time, using different sizes and directions, the same red E-letter was presented in the presence of a magnified green E-letter in the background, again using different sizes and directions of the central E-letter (Fig. 4(d)). We chose to use red and green colors which are commonly used in studies [33,34]. Visual acuity and reaction time were measured.
Results of the experiments with a single stimulus were compared to those in the presence of the magnified ones in the background to show the differences, if any, in the visual acuity and reaction time between the two situations.
3.1. Numerical simulation
The first simulation, plotted by the Zemax software as a ray diagram, is shown in Fig. 5 and validated the basic concept that light passing through different parts of the lens may be magnified differently. In this simulation the rays passing through the central part of the lenses were not magnified, while those passing peripherally through the lens were. A magnification of about X4 was realized for light passing through the peripheral part of the lenses. The effective focal length (EFL) for the simulated system was 65 mm and 17 mm for the unaided eye. Please note that the numerical Zemax simulations according to which we have concluded about the magnification factor do show a full numerical validation of the proposed concept since the magnification factors were extracted by computing the change in the EFL of the imaging module. One may for instance see in Fig. 5 that a sharp focus is obtained on the retinal plane while the optical rays traveled much longer optical path due to the back and forth reflections. The overall optical path was of 65 mm which is 4 times larger than what would have been obtained without the forth and back traveling and yet a sharp focus was generated as seen in our simulation.
Figure 6 is the simulation of a design that can produce two different magnifications for central and peripheral rays of light, as desired in the case of AMD. In this simulation a smaller magnification for near objects and a largest magnification for far objects was achieved. In the simulations presented on the left of Fig. 6, the refractive central magnification was X2.8. The catadioptric peripheral magnification was X12. This is shown in the simulation on the right of Fig. 6.
In the simulations, the external surface of the contact lens and the internal surface of the ophthalmic lens were coated in order to generate a Fabry-Perot-like resonator that would bounce the optical rays between the two surfaces to achieve optical magnification of the imaged point source. Figure 7 shows a zoomed simulation of the optical elements with these coatings.
The central field of view was narrowed to about 20 degrees. The typical energetic efficiency ratio of the proposed configuration was proportional to the ration between the area of the peripheral part of the lens and its central area. In the simulations of Fig. 7, this equals:
We suggest that the magnification achieved by this configuration will allow people with AMD, whose macula is deteriorated, to read via the peripheral areas of the retina, which are still functional. Note that in the proposed solution large magnification is realized without any need for an invasive and implantable solution, as required by the common solution available today for AMD patients.
In Fig. 8 we present some preliminary experimental results taken from an optical bench. In the optical bench two partial reflection surfaces (50% reflectivity) were placed on both sides of the imaging lens. Instead of the retinal imaging plane a camera was placed. The optical system can be seen in Fig. 8(a). The right image of Fig. 8(b) shows a single passage through the optical system. One of the reflecting surfaces was intentionally slightly tilted to have the zoomed and the non-zoomed images side by side for comparison reasons. The left image shows 3 passages (forth, back and forth transmission). One can clearly see the magnification of the resolution chart. The measured magnification factor was of X1.3. The designed increase in the EFL was aimed for X1.35.
3.2. Clinical Experiment
The clinical experiment was comprised of two experiments performed under the same conditions with different stimulus colors, in order to make the test closer to realistic conditions. Visual acuity threshold and reaction time were first measured for a single grey E-letter, and then when this was presented with a magnified darker gray letter in the background (“gray experiment”). The same measurements were then made with a single red E-letter (“color experiment”), after which simultaneous vision was tested with a magnified green letter in the background. Each stimulus was presented for 100 milliseconds, and subjects were asked to detect the direction of the E-letter, using a staircase with the pattern size modified by a 0.1 log unit in each step. Visual acuities were measured monocularly and given in LogMAR units, as in the logarithmic ETDRS visual acuity chart. Each 0.1 LogMAR is equal to one acuity line in the ETDRS chart.
Visual acuity differences (TeVA elevation/reduction) and reaction time differences were computed for each eye as the difference between the visual acuities and reaction times with the E-letter in the presence of the magnified E-letter (simultaneous vision) and those with the single E-letter. These results are summarized in Figs. 9 and 10.
The average difference in visual acuity between the two types of presentation was −0.0615 log units, thus the simultaneous vision under these conditions resulted in improved visual acuity.
Reaction time was longer with simultaneous vision than with the single stimulus, with an average increase of 33.45 milliseconds.
The average difference in visual acuity in the color experiment was 0.0605 log units. That is, when using colors, visual acuity was reduced on average in simultaneous vision, contrasting with the improvement of acuity seen with gray stimuli.
As in the gray experiment, reaction time was longer in simultaneous vision conditions than with a single stimulus. The average increase was 50.3 milliseconds.
4. Discussion and conclusions
This paper explores the possibility of a device for simultaneously creating different magnifications that would alleviate some of the problems of AMD patients. In this disease the center of the retina is damaged. To achieve better resolution for the different parts of the visual field and for different functions, the patient may require different degrees of magnification. Simultaneous magnifications may therefore allow the patient to utilize different parts of the visual field without the need for replacing magnifiers, which require different distances for different functions. We hypothesized that a binocular passive optical device based on the combination of a contact lens and spectacles (ophthalmic lenses) can achieve such magnifications and allow the patient to use undamaged peripheral retina. If the external surface of the contact lens and the internal surface of the ophthalmic lens are coated, we suggested that a Fabry-Perot-like resonator would be formed that creates the different magnifications needed. We suggest that simultaneous magnifications or simultaneous vision achieved by this device may lead to reasonable visual potential and visual skills that will improve the quality of life of AMD patients.
This paper did not examine binocular skills. However, it can be estimated that AMD patients’ binocular skills is primarily affected by the functional state of the two retinas. If the functional state of the two retinas is very different, i.e. when one eye is much more damaged than the contralateral one, binocular vision may be almost non-existent. In cases where the visual system is able to fuse figures from the peripheral retinas, we anticipate that when identical magnifications are given to the two eyes, by using the device on both eyes, binocular visual processing of the magnified figures projected on the peripheral retinas will be performed. Panum’s area, that is narrowest at the fixation point and becomes broader in the periphery, allows this retinal images fusion into a single object.
The numerical simulation proved that the coated lenses do create a Fabry-Perot-like resonator that functions like an external telescope to create different magnifications for peripheral and central parts of the visual field, as needed for AMD patients. Although actual coating was not attempted here, it can be achieved by surface roughening using nano-pillars that are generated inside the original grooves along the plane of the lens. These are sub-wavelength structures, which, due to surface tension and viscosity, prevent the tears from penetrating the surface grooves. As the nano-structures are sub-wavelength in size, they preserve the ophthalmic functionality of the contact lens without generating undesired diffraction and scattering effects.
The clinical experiment compared visual acuity and reaction time between simultaneous vision and a single stimulus in healthy subjects. Stimuli were presented in gray and in color to test different conditions and make the test more realistic. We expected visual acuity in both the gray and the color experiments to be poorer than acuities obtained in an optometric examination room, since stimuli were only presented for 100 milliseconds and were not static. Such “temporal visual acuity” is known to be reduced in comparison to non-temporal visual acuity tests with static stimuli [35,36].
In addition, the experiments were carried out in a dark environment in which the only ambient light came from the monitor (mesopic condition, without dark-adaptation). Mesopic vision is more complicated than photopic (day) vision or scotopic (night) vision. In scotopic vision, visual perception is rod-mediated, and perceptions are principally achromatic. The rod system has poorer spatial resolution, giving lower visual acuity and poorer temporal resolution. In photopic vision, visual perception is primarily cone-mediated and characterized by good visual acuity and color vision. Mesopic vision is an in-between condition in which rods and cones are active simultaneously, and visual perception depends on the relative activity of these two photoreceptor types and systems. Contrast sensitivity declines rapidly as the eye moves to mesopic operation and reaction time becomes longer [37,38]. In the experiments here, the reduced acuity due to mesopic conditions had no adverse effect, since mesopic limitations similarly influenced visual acuity for both single letter and simultaneous vision.
In the gray experiment, visual acuity of some of the subjects was improved in the simultaneous visual condition, while others’ visual acuity decreased. The average difference in visual acuity of the central E-letter between the two stimuli was an improvement of −0.0615 log units. A possible explanation is the better contrast between the central E-letter and the darker background E-letter, in comparison to the gray background for the single-letter presentation. The most widely used definition for physical contrast is the Michelson formula, which relates contrast sensitivity to the magnitude of the difference in light intensity between the light and dark areas in relation to the overall luminance of the stimulus . In simultaneous vision the larger E-letter created increased contrast by increasing the gap between the stimulus and background light intensities.
Clinically this visual acuity improvement is almost negligible because it constitutes less than one visual acuity line on the ETDRS chart. Also, there was variability between subjects, with some showing better acuity with the background letter and others worse acuity. These results are reasonable due to individual differences in contrast sensitivity skills under these examination conditions of mesopic illumination that are not optimal for our visual system .
In the color experiment, the average difference in visual acuity between simultaneous stand single stimuli was 0.0605 log units. That is, the simultaneous vision under these conditions on average reduced visual acuity of the central E-letter, contrary to the results of the gray experiment. As mentioned above, under dim illumination, there is no obvious color vision due to reduced color perception of the rod system. Visual perceptions become graded variations of light and dark, which reduces visual acuity to colored stimuli. Like the improvement in the gray experiment, this reduction in visual acuity in the color experiment is also negligible, constituting less than one visual acuity line on the ETDRS chart. This experiment also showed individual variability, with some subjects having greater acuity with the magnified letter in the background, while in others visual acuity decreased. Again, these results are reasonable due to individual variation in contrast sensitivity skills under mesopic conditions.
Reaction time in the gray experiment was longer for simultaneous vision than for the single stimulus, with an average lengthening of 33.45 milliseconds. In the color experiment, reaction time lengthened by an average of 50.3 milliseconds in simultaneous vision conditions compared to single stimulus and was longer than the average reaction time in the gray experiment. The cause may have been the poorer temporal resolution of colored stimuli by the mesopic system.
Thus, this experiment showed that visual acuity was hardly affected by simultaneous vision. It was slightly improved with stimulus and background in different gray levels and was slightly reduced with colored stimuli. Both improvements and reductions in visual acuity are clinically negligible, representing about half a visual acuity line on a visual acuity examination chart. For AMD patients with central scotoma, the ability to use peripheral areas of the retina for different functions is much more important and effective than this difference in visual acuity in healthy subjects.
When healthy people look at an object in space, they unconsciously focus it on their fovea, the central part of the retina that allows high resolution vision, to achieve sharp and clear vision. In AMD, the center of the retina is damaged, and these patients have no means for achieving high resolution vision. However, in the early stages of the disease, peripheral retinal areas still function normally. Thus, one proposed solution for these patients is to use magnifiers that allow them to use the functional peripheral retina. The quality of vision in this case is not optimal, since the peripheral retina cannot provide high resolution vision. However, low resolution vision is still preferable for these patients and may improve their vision and quality of life.
Different functions and situations require different magnifications. The device proposed here creates magnifications which produce simultaneous vision and different simultaneous magnifications of the object the subject is looking at with the same eye (monocular simultaneous vision). The clinical experiment showed that the existence of another simultaneous figure does not significantly impair visual acuity. AMD patients would require adaptation to simultaneous vision, but if the patients eventually managed to deal with this form of vision and learned to focus on the desired figure while ignoring the other magnifications created, they would be able to maximally utilize their peripheral retina to achieve the best vision possible for them.
The authors declare that they do not have any conflict of interest.
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