Biomedical subjects

Edwin J Sarver

Publications and source records attributed to Edwin J Sarver.

5 recordsLinked to original sources

PubMed2003

Interaction between aberrations to improve or reduce visual performance.

PURPOSE: To investigate how pairs of Zernike modes interact to increase or decrease visual acuity. SETTING: Visual Optics Institute, College of Optometry, University of Houston, Houston, Texas, USA. METHODS: Subjects read aberrated and unaberrated visual acuity charts 3 times. Each aberrated chart was produced by convolving an aberrated point-spread function with an unaberrated acuity chart. Point-spread functions were defined by 4 pairs of Zernike modes. For each pair, 9 combinations were used, ranging from all aberration being loaded into the first mode to all aberration being loaded into the second mode. The root mean square (RMS) wavefront error always totaled 0.25 microm (6.0 mm pupil), a level similar to the aberration induced by traditional flying small-spot laser refractive surgeries. RESULTS: For all conditions (except the unaberrated charts), visual acuity decreased. Acuity varied significantly depending on which modes were mixed and the relative contribution of each mode. Modes 2 radial orders apart and having the same sign and angular frequency tended to combine to increase visual acuity. Modes within the same radial order tended to combine to decrease acuity. CONCLUSIONS: For low levels of aberration, the RMS wavefront error is not a good predictor of visual acuity. Clinically, it is important to define how aberrations interact to optimize visual performance. New metrics of optical/neural performance that correlate better with clinical measures of visual performance need to be adopted or developed, as well as new clinically viable measures of visual performance that are sensitive to subtle changes in optical performance.

Adult↗

PubMed2003

Visual acuity as a function of Zernike mode and level of root mean square error.

BACKGROUND: The coefficients of normalized Zernike expansion are orthogonal and reflect the relative contribution of each mode to the total root mean square (RMS) wavefront error. The relationship between the level of RMS wavefront error within a mode and its effect on visual performance is unknown. PURPOSE: To determine for various levels of RMS wavefront error how each mode of the normalized Zernike expansion for the second, third, and fourth orders affect high- and low-contrast acuity. METHODS: Three healthy optimally corrected cyclopleged subjects read aberrated and unaberrated high- and low-contrast logarithm of the minimum angle of resolution acuity charts monocularly through a 3-mm artificial pupil. Acuity was defined by the total number of letters read correctly up to the fifth miss. Aberrated and unaberrated charts were generated using a program called CTView. Six levels of RMS wavefront error were used (0.00, 0.05, 0.10, 0.15, 0.20, and 0.25 microm). Each level of RMS error was loaded into each mode of the second, third, and fourth radial orders individually for a total of 72 charts. Data were normalized by subject, and the normalized data were averaged across subjects. RESULTS: Across modes and within each mode as the level of RMS wavefront error increased above 0.05 microm of RMS wavefront error, visual acuity decreased in a linear fashion. Slopes of the linear fits varied depending on the mode. Modes near the center of the Zernike pyramid had steeper slopes than those near the edge. CONCLUSIONS: Increasing the RMS error within any single mode of the normalized Zernike expansion decreases visual acuity in a linear fashion. The slope of the best fitting linear equation varies with Zernike mode. Slopes near the center of the Zernike pyramid are steeper than those near the edge. Although the normalized Zernike expansion parcels RMS error orthogonally, the resulting effects on visual performance as measured by visual acuity are not orthogonal. New metrics of the combined effects of the optical and the neural transfer functions that are predictive of visual performance need to be developed.

Adult↗

PubMed2002

Inattention to nonsuperimposable midline symmetry causes wavefront analysis error.

BACKGROUND: The nonsuperimposable mirror-image symmetry of the body (enantiomorphism) is reflected in the wavefront error maps of eyes. Averaging the wavefront errors of right and left eyes has the potential to adversely affect correlations made between wavefront error and visual acuity or other factors. Not only are the results of past studies using Zernike terms suspected of being invalid, there is concern about possible errors in the algorithms used to create customized corneal ablations. OBJECTIVE: To compare the results of analysis with and without correction for enantiomorphism. METHODS: Fourteen TMS-1 corneal topographic maps from 7 patients having with-the-rule astigmatism in both corneas were selected for Zernike decomposition to 45 terms. The maps were distributed among 3 groups: 7 right eye maps, 7 left eye maps, and 7 left eye maps in which the topography was transposed about the vertical axial to correct for enantiomorphism (left eye-corrected). The wavefront error difference between the right and left eyes was compared with the difference between the right eyes and the left eyes in which enantiomorphism was corrected (right eye vs left eye-corrected). The left eye wavefront error was then compared with the left eye wavefront error after correction (left eye vs left eye-corrected). RESULTS: Correcting for enantiomorphism produced a statisticially significant difference in the first 5 radial orders of Zernike terms (P=.02). Of the 45 Zernike terms analyzed, 7 terms were significantly different at the P<.05 level in the right eye vs left eye category, compared with 4 terms in the right eye vs left eye-corrected category. Eleven terms were significantly different at the P<.05 level in the left eye vs left eye-corrected category. CONCLUSIONS: Correcting for enantiomorphism makes the Zernike terms in right and left eyes appear more similar. Failure to correct for enantiomorphism causes certain terms to cancel each other when averaged across right and left eyes. Wavefront error studies that do not consider enantiomorphism, including those used to adjust laser surgical nomograms, will introduce significant errors to certain Zernike terms.

Astigmatism↗

PubMedDate not supplied

Are all aberrations equal?

PURPOSE: To determine for a fixed RMS error (25 microm, over a 6-mm pupil) how each mode of the normalized Zernike polynomial (second through the fourth radial order) affects high and low contrast logMAR visual acuity. METHODS: Three healthy volunteers served as subjects. CTView was used to generate optically aberrated logMAR charts. Accommodation was paralyzed and pupils dilated. The foveal achromatic axis of the eye was aligned to a 3-mm pupil and the eye was optimally refracted. Aberrated acuity charts were read until five letters were missed. Data were normalized for each subject to the acuity obtained by reading unaberrated charts and plotted as letters lost as a function of Zernike mode. RESULTS: Defocus (Z2(0)) decreased letter acuity more than astigmatism (Z2(2), Z2(-2)). Coma (Z3(1), Z3(-1)) decreased acuity more than trefoil (Z3(3), Z3(-3)). Spherical aberration (Z4(0)) and secondary astigmatism (Z2(2), Z4(-2)) decreased acuity much more than quadrafoil (Z4(4), Z4(-4)). CONCLUSIONS: 1. For an equal amount of RMS error not all coefficients of the Zernike polynomial induce equivalent losses in high and low contrast logMAR acuity. 2. Wavefront error concentrated near the center of the pyramid adversely affects visual acuity more than modes near the edge of the pyramid. 3. Large changes in chart appearance are not reflected in equally large decreases in visual performance (ie, subjects could correctly identify highly aberrated letters). 4. Interactions between modes complicate weighting each Zernike mode for visual impact.

Adult↗

PubMedDate not supplied

Image quality in myopic eyes corrected with laser in situ keratomileusis and phakic intraocular lens.

PURPOSE: To compare image quality due to higher-order aberrations following laser in situ keratomileusis (LASIK) or implantation of phakic intraocular lens (PIOL) to correct high myopia. METHODS: Postoperative wavefront examinations, normalized to a pupil size of 5.5 mm, were obtained for 19 LASIK and 20 PIOL eyes for the same surgeon over the same time period. Higher-order aberrations and simulated retinal images were compared. RESULTS: For this small sample, the LASIK eyes yielded an average three times more spherical aberration and two times more coma than PIOL eyes. The effects of these differences were visualized using the simulated retinal images. CONCLUSION: Spherical aberration and coma are the major differences between postoperative LASIK and PIOL higher-order aberrations, and simulated retinal images can be used to visualize these effects.

Cornea↗