Effect of phenylephrine on static and dynamic accommodation / Efecto de la fenilefrina sobre la acomodación estática y dinámica

Purpose: We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics. Methods: We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2 Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size. Results: When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51 ± 0.53 D, and 0.12 ± 0.15, for static and dynamic accommodation, respectively, and were statistically significant (p < 0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were -0.20 ± 0.51 D, and -0.05 ± 0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p > 0.313). Conclusions: The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation

Objetivo: Probamos la hipótesis de que los cambios de la acomodación tras la instilación de Hidrocloruro de Fenilefrina (PHCl) observados en algunos estudios podrían estar originados por los cambios en la óptica. Métodos: Realizamos dos experimentos para probar los efectos de PHCl sobre la acomodación estática y dinámica en 8 y 6 sujetos, respectivamente. Se registraron las mediciones objetivas de frente de onda de la respuesta acomodativa estática a un estímulo a diferentes distancias, o la respuesta acomodativa dinámica a un estímulo con movimiento sinusoidal (entre 1 y 3 D de demanda acomodativa a 0,2Hz). Las respuestas se caracterizaron utilizando dos métodos: uno que tiene en cuenta los efectos ópticos midriáticos sobre la acomodación producida por aberraciones de alto orden, y otro que considera únicamente los cambios de potencia paraxialmente, debido a la acción del músculo ciliar, independientemente del tamaño de la pupila. Resultados: Al tenerse en cuenta los efectos ópticos midriáticos, las diferencias de las respuestas antes y después de la instilación de PHCl fueron de 0,51 ± 0,53 D, y 0,12 ± 0,15, para la acomodación estática y dinámica, respectivamente, siendo estadísticamente significativas (p < 0,039). Al no considerarse los efectos ópticos midriáticos, las diferencias en cuanto a las respuestas antes y después de la instilación de PHCl fueron de -0,2 ± 0,51 D, y -0,05 ± 0,14, para la acomodación estática y dinámica, respectivamente, no siendo estadísticamente significativas (p > 0,313). Conclusiones: El efecto midriático de PHCl origina cambios ópticos en el ojo que pueden reducir la medición objetiva y subjetiva de la acomodación

Influence of contrast polarity on the accommodative response / Influencia de la polaridad de contraste en la respuesta acomodativa

Purpose: To assess the changes in the accommodative response of the eye while reading a text under different contrast polarity conditions: black letters on white background (BoW condition) and white letters on black background (WoB condition). Methods: Eighteen subjects with ages ranging from 21 to 41 years participated in this experimental study. The accommodative response (AR) of the eye while reading a text with BoW or WoB contrast polarity was obtained objectively with an adaptive optics system that corrected all aberrations but subject's own. Two different letter sizes (visual acuity conditions), shown on a microdisplay, were tested. The AR of each eye was measured with its natural pupil diameter at 0-3D of accommodative demand from the far point of the eye, with a step of 0.5D. The slope of the stimulus-response curve was calculated for each subject and condition. Results: The averaged maximum pupil size was bigger for reverse (WoB) than for normal (BoW) contrast with statistical significance. The slopes for the ARs of the four conditions were not significantly different from each other. Conclusions: Contrast polarity does not seem to influence the accommodative response when reading text from an electronic microdisplay

Objetivo: Evaluar los cambios de la respuesta acomodativa del ojo al leer un texto en diferentes condiciones de polaridad de contraste: letras negras sobre fondo blanco (condición BoW) y letras blancas sobre fondo negro (condición WoB). Métodos: En este estudio experimental participaron dieciocho sujetos de edades comprendidas entre 21 y 41 años. Se obtuvo objetivamente la respuesta acomodativa (AR) del ojo al leer un texto con polaridad de contraste BoW o WoB con un sistema de óptica adaptativa que corregía todas las aberraciones salvo las propias del sujeto. Se estudiaron dos tamaños de letra diferentes (condiciones de agudeza visual), mostrados en una micropantalla. Se midió la AR de cada ojo con su diámetro de pupila natural con 0 a 3 D de demanda acomodativa desde el punto remoto del ojo, en intervalos de 0,5 D. Se calculó la pendiente de la curva estímulo-respuesta para cada sujeto y condición. Resultados:El tamaño máximo medio de la pupila fue mayor para el contraste inverso (WoB) que para el normal (BoW), con significación estadística. Las pendientes de las AR para las cuatro condiciones no difirieron significativamente entre ellas. Conclusiones: La polaridad de contraste no parece influir en la respuesta acomodativa al leer un texto en una micropantalla electrónica

Effect of phenylephrine on static and dynamic accommodation.

PURPOSE: We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics. METHODS: We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size. RESULTS: When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p<0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were -0.20±0.51 D, and -0.05±0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p>0.313). CONCLUSIONS: The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation.

Influence of contrast polarity on the accommodative response.

PURPOSE: To assess the changes in the accommodative response of the eye while reading a text under different contrast polarity conditions: black letters on white background (BoW condition) and white letters on black background (WoB condition). METHODS: Eighteen subjects with ages ranging from 21 to 41 years participated in this experimental study. The accommodative response (AR) of the eye while reading a text with BoW or WoB contrast polarity was obtained objectively with an adaptive optics system that corrected all aberrations but subject's own. Two different letter sizes (visual acuity conditions), shown on a microdisplay, were tested. The AR of each eye was measured with its natural pupil diameter at 0-3D of accommodative demand from the far point of the eye, with a step of 0.5D. The slope of the stimulus-response curve was calculated for each subject and condition. RESULTS: The averaged maximum pupil size was bigger for reverse (WoB) than for normal (BoW) contrast with statistical significance. The slopes for the ARs of the four conditions were not significantly different from each other. CONCLUSIONS: Contrast polarity does not seem to influence the accommodative response when reading text from an electronic microdisplay.

Accommodative stimulus-response curves to low-pass filtered natural images.

PURPOSE: To assess how the monocular steady-state accommodative stimulus-response curve is modified when viewing low-pass filtered natural images. METHODS: Eighteen adult subjects participated in the study. The accommodative stimulus-response curve was objectively assessed by means of a Hartmann-Shack wavefront sensor. Measurements were taken at different accommodative demands. Target images were low-pass filtered versions of a natural image that were obtained applying different digital spatial filters that limited the spatial frequency content of the natural image. Cutoff spatial frequencies were set at 30, 21, and 15 cycles per degree (cy/deg). RESULTS: Mean data obtained for each target were fitted to linear models. For the low-pass filtered natural image with a cutoff spatial frequency at 30 cy/deg, the slope for the averaged stimulus-response curve was 0.614, while the slopes obtained for the other two low-pass filtered images corresponding to cutoff frequencies at 21 and 15 cy/deg were 0.613 and 0.619, respectively (p < 0.01). The determination coefficient was R2 ≥ 0.988 for all targets. An ANCOVA analysis revealed that these slopes were not significantly different from each other (p = 0.82). Additionally, accommodative error indexes were obtained to measure both the discrepancy between the regression lines and the ideal lines and the degree of correlation between the stimulus and the response. CONCLUSIONS: Our study reveals that the low and middle spatial frequencies (< 15 cy/deg) are the most important to guide to the needed levels the accommodative response to natural images.

Accommodative Stimulus-Response Curve with Emoji Symbols.

PURPOSE: To evaluate the static measurement of the accommodative stimulus-response curve with emoji symbols. METHODS: The accommodative stimulus-response curve was measured in 18 subjects using a Hartmann-Shack sensor to obtain the objective accommodative response from the Zernike defocus term. Measurements were acquired at different accommodative demands, from 0 to 3 D with a step of 0.5 D. Detailed and nondetailed emoji targets were used with two different sizes, corresponding to the two most common visual angles used in smartphones. RESULTS: A regression analysis was performed to fit the mean results obtained for each target. The determination coefficient was R2 ≥ 0.988 for all targets. For the detailed targets, the slopes for the averaged stimulus-response curve were 0.65 and 0.66 for the bigger and smaller sizes, respectively. For the nondetailed targets, the slopes were 0.60 and 0.58 for the bigger and smaller sizes, respectively. p values for these slopes were statistically significant for the two types of targets (p < 0.01). CONCLUSIONS: Our results reveal that the replacement of a word or several words by detailed or nondetailed emoji symbols seems not to provoke a different accommodative response in normal subjects and under standard viewing conditions in the use of smartphones.

Human eyes do not need monochromatic aberrations for dynamic accommodation.

PURPOSE: To determine if human accommodation uses the eye's own monochromatic aberrations to track dynamic accommodative stimuli. METHODS: Wavefront aberrations were measured while subjects monocularly viewed a monochromatic Maltese cross moving sinusoidally around 2D of accommodative demand with 1D amplitude at 0.2 Hz. The amplitude and phase (delay) of the accommodation response were compared to the actual vergence of the stimulus to obtain gain and temporal phase, calculated from wavefront aberrations recorded over time during experimental trials. The tested conditions were as follows: Correction of all the subject's aberrations except defocus (C); Correction of all the subject's aberrations except defocus and habitual second-order astigmatism (AS); Correction of all the subject's aberrations except defocus and odd higher-order aberrations (HOAs); Correction of all the subject's aberrations except defocus and even HOAs (E); Natural aberrations of the subject's eye, i.e., the adaptive-optics system only corrected the optical system's aberrations (N); Correction of all the subject's aberrations except defocus and fourth-order spherical aberration (SA). The correction was performed at 20 Hz and each condition was repeated six times in randomised order. RESULTS: Average gain (±2 standard errors of the mean) varied little across conditions; between 0.55 ± 0.06 (SA), and 0.62 ± 0.06 (AS). Average phase (±2 standard errors of the mean) also varied little; between 0.41 ± 0.02 s (E), and 0.47 ± 0.02 s (O). After Bonferroni correction, no statistically significant differences in gain or phase were found in the presence of specific monochromatic aberrations or in their absence. CONCLUSIONS: These results show that the eye's monochromatic aberrations are not necessary for accommodation to track dynamic accommodative stimuli.

Dynamic accommodation without feedback does not respond to isolated blur cues.

The aim of this study was to determine whether dynamic accommodation responds to isolated blur cues without feedback, and without changes in the distance of the object. Nine healthy subjects aged 21-40years were recruited. Four different aberration patterns were used as stimuli to induce blur with (1) the eye's natural, uncorrected, optical aberrations, (2) all aberrations corrected, (3) spherical aberration only, or (4) astigmatism only. The stimulus was a video animation based on computer-generated images of a monochromatic Maltese cross. Each individual video was generated for each subject off-line, after measuring individual aberrations at different accommodation levels. The video simulated sinusoidal changes in defocus at 0.2Hz. Dynamic images were observed through a 0.8mm pinhole placed at a plane conjugated with the eye's pupil, thus effectively removing potential feedback stemming from accommodation changes. Accommodation responses were measured with a Hartmann-Shack aberrometer for the four different aberration patterns. The results showed that seven out of nine subjects did not respond to any stimuli, whereas the response of the other two subjects was erratic and they seemed to be searching rather than following the stimulus. A significant reduction in average accommodative gain (from 0.52 to 0.11) was obtained when the dioptric demand cue was removed. No statistically significant differences were found among the experimental conditions used. We conclude that aberration related blur does not drive the accommodation response in the absence of feedback from accommodation.

Accommodation Responds to Optical Vergence and Not Defocus Blur Alone.

Purpose: To determine whether changes in wavefront spherical curvature (optical vergence) are a directional cue for accommodation. Methods: Nine subjects participated in this experiment. The accommodation response to a monochromatic target was measured continuously with a custom-made adaptive optics system while astigmatism and higher-order aberrations were corrected in real time. There were two experimental open-loop conditions: vergence-driven condition, where the deformable mirror provided sinusoidal changes in defocus at the retina between -1 and +1 diopters (D) at 0.2 Hz; and blur-driven condition, in which the level of defocus at the retina was always 0 D, but a sinusoidal defocus blur between -1 and +1 D at 0.2 Hz was simulated in the target. Right before the beginning of each trial, the target was moved to an accommodative demand of 2 D. Results: Eight out of nine subjects showed sinusoidal responses for the vergence-driven condition but not for the blur-driven condition. Their average (±SD) gain for the vergence-driven condition was 0.50 (±0.28). For the blur-driven condition, average gain was much smaller at 0.07 (±0.03). The ninth subject showed little to no response for both conditions, with average gain <0.08. Vergence-driven condition gain was significantly different from blur-driven condition gain (P = 0.004). Conclusions: Accommodation responds to optical vergence, even without feedback, and not to changes in defocus blur alone. These results suggest the presence of a retinal mechanism that provides a directional cue for accommodation from optical vergence.

The effect of longitudinal chromatic aberration on the lag of accommodation and depth of field.

PURPOSE: Longitudinal chromatic aberration is present in all states of accommodation and may play a role in the accommodation response and the emmetropisation process. We study the change of the depth of field (DOFi) with the state of accommodation, taking into account the longitudinal chromatic aberration. METHODS: Subjective DOFi was defined as the range of defocus beyond which the blur of the target (one line of optotypes of 0.1 logMAR shown on a black-and-white microdisplay, seen through different colour filters) was perceived as objectionable. The subject's eye was paralysed and different, previously-measured accommodative states (corresponding to the accommodative demands of 0D, 2D and 4D) were simulated with a deformable mirror. Different colour conditions (monochromatic red, green and blue and polychromatic (white) were tested. The DOFi was measured subjectively, using a motorised Badal system. RESULTS: Taking as reference the average accommodative response for the white stimulus, the blue response exhibits on average a lead of 0.45 ± 0.09D, the green a negligible lead of 0.07 ± 0.02D and red a lag of 0.49 ± 0.10D. The monochromatic DOFi, calculated by averaging DOFi over the red, green and blue colour conditions for each accommodative demand was 1.10 ± 0.10D for 0D, 1.20 ± 0.08D for 2D, and 1.26 ± 0.40D for 4D. The polychromatic white DOFi were greater than the average monochromatic DOFi by 19%, 9% and 14% for 0D, 2D, and 4D of accommodative demand, respectively. CONCLUSION: The longitudinal chromatic aberration causes a dioptric shift of the monochromatic accommodation response. The study did not reveal this shift to depend on the accommodative demand or to have an effect on the DOFi.

Influence of Ametropia and Its Correction on Measurement of Accommodation.

PURPOSE: Amplitude of accommodation (AA) is reportedly greater for myopic eyes than for hyperopic eyes. We investigated potential explanations for this difference. METHODS: Analytical analysis and computer ray tracing were performed on two schematic eye models of axial ametropia. Using paraxial and nonparaxial approaches, AA was specified for the naked and the corrected eye using the anterior corneal surface as the reference plane. RESULTS: Assuming that axial myopia is due entirely to an increase in vitreous chamber depth, AA increases with the amount of myopia for two reasons that have not always been taken into account. First is the choice of reference location for specifying refractive error and AA in diopters. When specified relative to the cornea, AA increases with the degree of myopia more than when specified relative to the eye's first Gaussian principal plane. The second factor is movement of the eye's second Gaussian principal plane toward the retina during accommodation, which has a larger dioptric effect in shorter eyes. CONCLUSIONS: Using the corneal plane (placed at the corneal vertex) as the reference plane for specifying accommodation, AA depends slightly on the axial length of the eye's vitreous chamber. This dependency can be reduced significantly by using a reference plane located 4 mm posterior to the corneal plane. A simple formula is provided to help clinicians and researchers obtain a value of AA that closely reflects power changes of the crystalline lens, independent of axial ametropia and its correction with lenses.

Optical Characterization Method for Tilted or Decentered Intraocular Lenses.

PURPOSE: To test and validate a method for measuring the optical quality and optical power of monofocal intraocular lenses (IOLs) in the presence of a tilt or a decentration as well as its optical power. METHODS: The experimental system consists of an artificial eye (wet-cell) and a commercial aberrometer with a Shack-Hartmann sensor. Optical image quality parameters such as point spread function (PSF), modulation transfer function (MTF), and the simulations of the retinal image of an extended object are computed from the in vitro wavefront data of an IOL. Repeatability and reproducibility of the system are tested. Mathematical and ray-tracing simulations are used to ascertain the precision and accuracy of the method. Comparison with a standard single-pass method of measurement is also made. Optical properties of four commercial monofocal IOLs are measured under tilts of 0°, 2°, and 4°, and vertical decentrations of 0, 0.2, and 0.4 mm; the results of such measurements are compared with a ray-tracing simulation. RESULTS: Precision and accuracy of the system are in good agreement with theoretical calculations. Reproducibility and repeatability are within standard ISO norms. MTFs obtained with this method and with the standard method are very similar. The precision of the estimation of the IOL's optical power is higher than 98.6%. Commercially tested IOLs show a decrease of optical quality in the presence of decentrations and tilts, and their susceptibility to tilts and decentrations depends on the asphericity. These results are in concordance with the results obtained by ray-tracing simulation. CONCLUSIONS: A simple wavefront-based method for in vitro measurements of the wavefront aberrations and power of an IOL is proposed and tested showing accurate and precise results.

Effect of Phenylephrine on the Accommodative System.

Accommodation is controlled by the action of the ciliary muscle and mediated primarily by parasympathetic input through postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia. During accommodation the pupil constricts to increase the depth of focus of the eye and improve retinal image quality. Researchers have traditionally faced the challenge of measuring the accommodative properties of the eye through a small pupil and thus have relied on pharmacological agents to dilate the pupil. Achieving pupil dilation (mydriasis) without affecting the accommodative ability of the eye (cycloplegia) could be useful in many clinical and research contexts. Phenylephrine hydrochloride (PHCl) is a sympathomimetic agent that is used clinically to dilate the pupil. Nevertheless, first investigations suggested some loss of functional accommodation in the human eye after PHCl instillation. Subsequent studies, based on different measurement procedures, obtained contradictory conclusions, causing therefore an unexpected controversy that has been spread almost to the present days. This manuscript reviews and summarizes the main research studies that have been performed to analyze the effect of PHCl on the accommodative system and provides clear conclusions that could help clinicians know the real effects of PHCl on the accommodative system of the human eye.

Focus correction in an apodized system with spherical aberration.

We performed a theoretical and computational analysis of the through-focus axial irradiance in a system with a Gaussian amplitude pupil function and fourth- and sixth-order spherical aberration (SA). Two cases are analyzed: low aberrated systems, and the human eye containing significant levels of SA and a natural apodization produced by the Stiles-Crawford effect. Results show that apodization only produces a refraction change of the plane that maximized the Strehl ratio for eyes containing significant levels of negative SA.

Opto-mechanical artificial eye with accommodative ability.

The purpose of this study was to describe the design and characterization of a new opto-mechanical artificial eye (OMAE) with accommodative ability. The OMAE design is based on a second-pass configuration where a small source of light is used at the artificial retina plane. A lens whose focal length can be changed electronically was used to add the accommodation capability. The changes in the OMAE's aberrations with the lens focal length, which effectively changes the accommodative state of the OMAE, were measured with a commercial aberrometer. Changes in power and aberrations with room temperature were also measured. The OMAE's higher-order aberrations (HOAs) were similar to the ones of the human eye, including the rate at which fourth-order spherical aberration decreased with accommodation. The OMAE design proposed here is simple, and it can be implemented in an optical system to mimic the optics of the human eye.

Changes in the objective amplitude of accommodation with pupil size.

PURPOSE: We evaluate the effect of pupil size on objectively measured amplitude of accommodation (AA). METHODS: Pupil diameter and wavefront aberrometry were obtained in 15 eyes when stimulus swept across the range of clear vision in steps of 0.5 diopters. Wavefront refraction techniques were used to compute objective AA as the maximum refractive change. Measurements were obtained monocularly under low and high ambient room lighting conditions with a fixed luminance of the fixation target. Amplitude of accommodation computations were performed taking into account just paraxial rays (paraxial AA) or including the effects of the change of spherical aberration during accommodation (minRMS AA). RESULTS: Mean pupil size values at low light level were 6.26 mm (relaxed) and 4.15 mm (maximum accommodation), whereas at the high light level, those values became 4.74 and 3.04 mm, respectively. The effects of both light level on accommodation were significant (p < 0.001), and accommodative miosis was slightly larger at low light levels. Mean minRMS and paraxial AA were always greater by more than 1 diopter in high than in low ambient lighting conditions (p < 0.01), indicating a significant impact of pupil size on AA. CONCLUSIONS: The influence of the ambient lighting on the objective AA is not only attributed to the increased effects of spherical aberration as the pupil dilates but mostly attributed to a decrease in the paraxial accommodation as pupil dilates.

Depth-of-field of the accommodating eye.

PURPOSE: To obtain experimental values of the depth-of-field (DOFi) of the human eye for different accommodative states. METHODS: First, the monochromatic ocular wavefront of seven eyes from young subjects (mean [±SD] age, 29.7 [±7.7] years) was measured at eight different accommodative demands (ADs) (from -1 to 6 diopters [D] in steps of 1 D). Then, in a second part, accommodation was paralyzed and an adaptive optics system was used to correct the aberrations of the paralyzed eye and to simulate, with the aid of an artificial pupil, the wavefront of the accommodated eye. The simulation was performed for each AD measured in the first part of the experiment. A Badal system was used to modify the stimulus vergence so as to obtain three repeated measurements of the subjective DOFi, based on the criterion of an objectionable blur. RESULTS: When increasing AD from 0 to 6 D, the mean intersubject pupil diameter and DOFi changed from 5.70 to 4.62 mm and from 0.85 ± 0.26 D to 1.07 ± 0.19 D, respectively. All subjects presented a similar DOFi for all AD (intrasubject SD never exceeded 0.23 D). Paraxial accommodation response showed a lag that increased with the AD. For the lowest (0 D) and the highest (6 D) values of AD, the refractive state of the eye was close to the nearest and furthermost ends of the DOFi, respectively. CONCLUSIONS: The visual system takes advantage of the DOFi to change the refractive state less than necessary to form the paraxial image at the retina when it comes to focusing a near target (5 to 6 D of AD). This indicates that the main purpose of accommodation is not to maximize retinal image quality but to form one that is good enough.