Aiswaryah Radhakrishnan

Short-term neural adaptation to simultaneous bifocal images

Simultaneous vision is an increasingly used solution for the correction of presbyopia (the age-related loss of ability to focus near images). Simultaneous Vision corrections, normally delivered in the form of contact or intraocular lenses, project on the patients retina a focused image for near vision superimposed with a degraded image for far vision, or a focused image for far vision superimposed with the defocused image of the near scene. It is expected that patients with these corrections are able to adapt to the complex Simultaneous Vision retinal images, although the mechanisms or the extent to which this happens is not known. We studied the neural adaptation to simultaneous vision by studying changes in the Natural Perceived Focus and in the Perceptual Score of image quality in subjects after exposure to Simultaneous Vision. We show that Natural Perceived Focus shifts after a brief period of adaptation to a Simultaneous Vision blur, similar to adaptation to Pure Defocus. This shift strongly correlates with the magnitude and proportion of defocus in the adapting image. The magnitude of defocus affects perceived quality of Simultaneous Vision images, with 0.5 D defocus scored lowest and beyond 1.5 D scored “sharp”. Adaptation to Simultaneous Vision shifts the Perceptual Score of these images towards higher rankings. Larger improvements occurred when testing simultaneous images with the same magnitude of defocus as the adapting images, indicating that wearing a particular bifocal correction improves the perception of images provided by that correction.

Testing vision with angular and radial multifocal designs using Adaptive Optics

ABSTRACT Multifocal vision corrections are increasingly used solutions for presbyopia. In the current study we have evaluated, optically and psychophysically, the quality provided by multizone radial and angular segmented phase designs. Optical and relative visual quality were evaluated using 8 subjects, testing 6 phase designs. Optical quality was evaluated by means of Visual Strehl‐based‐metrics (VS). The relative visual quality across designs was obtained through a psychophysical paradigm in which images viewed through 210 pairs of phase patterns were perceptually judged. A custom‐developed Adaptive Optics (AO) system, including a Hartmann‐Shack sensor and an electromagnetic deformable mirror, to measure and correct the eyes aberrations, and a phase‐only reflective Spatial Light Modulator, to simulate the phase designs, was developed for this study. The multizone segmented phase designs had 2–4 zones of progressive power (0 to +3D) in either radial or angular distributions. The response of an “ideal observer” purely responding on optical grounds to the same psychophysical test performed on subjects was calculated from the VS curves, and compared with the relative visual quality results. Optical and psychophysical pattern‐comparison tests showed that while 2‐zone segmented designs (angular & radial) provided better performance for far and near vision, 3‐ and 4‐zone segmented angular designs performed better for intermediate vision. AO‐correction of natural aberrations of the subjects modified the response for the different subjects but general trends remained. The differences in perceived quality across the different multifocal patterns are, in a large extent, explained by optical factors. AO is an excellent tool to simulate multifocal refractions before they are manufactured or delivered to the patient, and to assess the effects of the native optics to their performance.

Differences in visual quality with orientation of a rotationally asymmetric bifocal intraocular lens design

Purpose To evaluate visual and perceptual performance for different orientations of a rotationally asymmetric bifocal intraocular lens (IOL) (M‐Plus) simulated optically using a simultaneous vision simulator. Setting Instituto de Optica, Madrid, Spain. Design Prospective observational study. Methods Perceptual quality and decimal high‐contrast visual acuity (HCVA) was measured under cycloplegia for 8 orientations of the asymmetric bifocal IOL phase pattern at far, intermediate, and near distances simulated with a simultaneous vision simulator using face images and tumbling E targets. The preferred orientation at each distance was calculated as the centroid of the data for 8 orientations. The visual Strehl value was calculated using the subjects’ ocular aberrations and multifocal pattern at each orientation. Optical predictions were obtained by implementing a differential visual Strehl values–based ideal observer model. Results The study comprised 20 subjects (aged 21 to 62 years). Horizontal orientation (near segment at 0 or 180 degrees ± 45 [SD]) was preferred by 14 subjects and by 13 subjects at far and near distances, respectively; 8 subjects showed strong orientation preferences. The mean difference in preferred orientation between far and near was 27 ± 22 degrees. No significant differences in HCVA were observed. Optical predictions correlated strongly and significantly with measurements (far r = 0.71, near r = 0.62; P < .0001). The mean difference between measurement and simulation in the preferred orientation was 28 ± 29 degrees at far and 36 ± 28 degrees at near. Conclusions The perception varied for different orientations of an asymmetric bifocal IOL design tested using a simultaneous vision simulator. Optimum orientation was driven by interactions of the design with the eye’s optical aberrations. Financial Disclosure None of the authors has a financial or proprietary interest in any material or method mentioned.

Perceived image quality with simulated segmented bifocal corrections

Bifocal contact or intraocular lenses use the principle of simultaneous vision to correct for presbyopia. A modified two-channel simultaneous vision simulator provided with an amplitude transmission spatial light modulator was used to optically simulate 14 segmented bifocal patterns (+ 3 diopters addition) with different far/near pupillary distributions of equal energy. Five subjects with paralyzed accommodation evaluated image quality and subjective preference through the segmented bifocal corrections. There are strong and systematic perceptual differences across the patterns, subjects and observation distances: 48% of the conditions evaluated were significantly preferred or rejected. Optical simulations (in terms of through-focus Strehl ratio from Hartmann-Shack aberrometry) accurately predicted the pattern producing the highest perceived quality in 4 out of 5 patients, both for far and near vision. These perceptual differences found arise primarily from optical grounds, but have an important neural component.

Portable simultaneous vision device to simulate multifocal corrections

Multifocal lenses are increasingly used solutions for presbyopia, the age-related loss of crystalline lens focus ability. These lenses work by the principle of simultaneous vision, superimposing focused and defocused images on the retina. Providing the experience of simultaneous vision to a patient before permanent implantation of a multifocal lens is a recognized unmet need to increase the patient’s confidence and optimize the lens selection. We developed a hand-held, see-through multifocal vision simulator based on temporal multiplexing of a tunable lens. The device was calibrated and validated using focimetry and Hartmann–Shack aberrometry revealing high reproducibility of the through-focus multifocal energy distribution and high optical quality. We measured visual acuity and perceptual quality on nine cyclopeged patients with three monofocal, two bifocal, and two trifocal corrections with different far/intermediate/near energy distributions simulated using the device. Visual performance and perceptual quality with multifocal corrections varied across patients, although they were more uniform across distances than monofocal corrections. Among the bifocal and trifocal designs, a trifocal with more energy at far was the most frequently identified as providing better quality. The simultaneous vision simulator proved a promising compact tool to study visual performance with multifocal corrections and to select the lens design best suited for each patient, alternative to costly and bulky adaptive optics based devices.

Single neural code for blur in subjects with different interocular optical blur orientation

The ability of the visual system to compensate for differences in blur orientation between eyes is not well understood. We measured the orientation of the internal blur code in both eyes of the same subject monocularly by presenting pairs of images blurred with real ocular point spread functions (PSFs) of similar blur magnitude but varying in orientations. Subjects assigned a level of confidence to their selection of the best perceived image in each pair. Using a classification-images-inspired paradigm and applying a reverse correlation technique, a classification map was obtained from the weighted averages of the PSFs, representing the internal blur code. Positive and negative neural PSFs were obtained from the classification map, representing the neural blur for best and worse perceived blur, respectively. The neural PSF was found to be highly correlated in both eyes, even for eyes with different ocular PSF orientations (rPos = 0.95; rNeg = 0.99; p < 0.001). We found that in subjects with similar and with different ocular PSF orientations between eyes, the orientation of the positive neural PSF was closer to the orientation of the ocular PSF of the eye with the better optical quality (average difference was ∼10°), while the orientation of the positive and negative neural PSFs tended to be orthogonal. These results suggest a single internal code for blur with orientation driven by the orientation of the optical blur of the eye with better optical quality.

Comparison of vision through surface modulated and spatial light modulated multifocal optics

Spatial-light-modulators (SLM) are increasingly used as active elements in adaptive optics (AO) systems to simulate optical corrections, in particular multifocal presbyopic corrections. In this study, we compared vision with lathe-manufactured multi-zone (2-4) multifocal, angularly and radially, segmented surfaces and through the same corrections simulated with a SLM in a custom-developed two-active-element AO visual simulator. We found that perceived visual quality measured through real manufactured surfaces and SLM-simulated phase maps corresponded highly. Optical simulations predicted differences in perceived visual quality across different designs at Far distance, but showed some discrepancies at intermediate and near.

Role of parafovea in blur perception

ABSTRACT The blur experienced by our visual system is not uniform across the visual field. Additionally, lens designs with variable power profile such as contact lenses used in presbyopia correction and to control myopia progression create variable blur from the fovea to the periphery. The perceptual changes associated with varying blur profile across the visual field are unclear. We therefore measured the perceived neutral focus with images of different angular subtense (from 4° to 20°) and found that the amount of blur, for which focus is perceived as neutral, increases when the stimulus was extended to cover the parafovea. We also studied the changes in central perceived neutral focus after adaptation to images with similar magnitude of optical blur across the image or varying blur from center to the periphery. Altering the blur in the periphery had little or no effect on the shift of perceived neutral focus following adaptation to normal/blurred central images. These perceptual outcomes should be considered while designing bifocal optical solutions for myopia or presbyopia.

Perceived image quality with simulated segmented bifocal corrections: Publisher’s note

Bifocal contact or intraocular lenses use the principle of simultaneous vision to correct for presbyopia. A modified two-channel simultaneous vision simulator provided with an amplitude transmission spatial light modulator was used to optically simulate 14 segmented bifocal patterns (+ 3 diopters addition) with different far/near pupillary distributions of equal energy. Five subjects with paralyzed accommodation evaluated image quality and subjective preference through the segmented bifocal corrections. There are strong and systematic perceptual differences across the patterns, subjects and observation distances: 48% of the conditions evaluated were significantly preferred or rejected. Optical simulations (in terms of through-focus Strehl ratio from Hartmann-Shack aberrometry) accurately predicted the pattern producing the highest perceived quality in 4 out of 5 patients, both for far and near vision. These perceptual differences found arise primarily from optical grounds, but have an important neural component.