Biomedical subjects

Jason D Marsack

Publications and source records attributed to Jason D Marsack.

5 recordsLinked to original sources

PubMed2004

Metrics of optical quality derived from wave aberrations predict visual performance.

Wavefront-guided refractive surgery and custom optical corrections have reduced the residual root mean squared (RMS) wavefront error in the eye to relatively low levels (typically on the order of 0.25 microm or less over a 6-mm pupil, a dioptric equivalent of 0.19 D). It has been shown that experimental variation of the distribution of 0.25 microm of wavefront error across the pupil can cause variation in visual acuity of two lines on a standard logMAR acuity chart. This result demonstrates the need for single-value metrics other than RMS wavefront error to quantify the effects of low levels of aberration on acuity. In this work, we present the correlation of 31 single-value metrics of optical quality to high-contrast visual acuity for 34 conditions where the RMS wavefront error was equal to 0.25 microm over a 6-mm pupil. The best metric, called the visual Strehl ratio, accounts for 81% of the variance in high-contrast logMAR acuity.

Humans↗

PubMed2004

A population study on changes in wave aberrations with accommodation.

Wave aberrations were measured with a Shack-Hartmann wavefront sensor (SHWS) in the right eye of a large young adult population when accommodative demands of 0, 3, and 6 D were presented to the tested eye through a Badal system. Three SHWS images were recorded at each accommodative demand and wave aberrations were computed over a 5-mm pupil (through 6th order Zernike polynomials). The accommodative response was calculated from the Zernike defocus over the central 3-mm diameter zone. Among all individual Zernike terms, spherical aberration showed the greatest change with accommodation. The change of spherical aberration was always negative, and was proportional to the change in accommodative response. Coma and astigmatism also changed with accommodation, but the direction of the change was variable. Despite the large inter-subject variability, the population average of the root mean square for all aberrations (excluding defocus) remained constant for accommodative levels up to 3.0 D. Even though aberrations change with accommodation, the magnitude of the aberration change remains less than the magnitude of the uncorrected aberrations, even at high accommodative levels. Therefore, a typical eye will benefit over the entire accommodative range (0-6 D) if aberrations are corrected for distance viewing.

Accommodation, Ocular↗

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↗

PubMedDate not supplied

Measuring visual acuity--mesopic or photopic conditions, and high or low contrast letters?

PURPOSE: To develop single-valued wavefront aberration metrics that correlate strongly with visual performance. The purpose of this study is to explore whether photopic high contrast visual acuity (VA) is an appropriate visual performance reference and whether mesopic and/or low contrast testing provides any advantage. METHODS: Subjects from the Texas Investigation of Cataract Optics study (N = 148) ranged in age from 21.6 to 83.8 years and from clear lens to dense nuclear cataract. Visual acuity was measured under four conditions: photopic high (VA(PHC)) and low (VA(PLC)) contrast, mesopic high (VA(MHC)) and low (VA(MLC)) contrast (photopic = 280 cd/m(2), mesopic = 0.75 cd/m(2)). Variables were tested for compliance with normality (-2.00 < skew and kurtosis < 2.00) and transformed if required. Linear regression and Bland-Altman 95% limits of agreement (+/-1.96 SD) were used to examine relationships between VA variables and between VA and wavefront aberration metrics. RESULTS: The two photopic measures VA(PHC) (skew = 2.57, kurtosis = 12.2) and VA(PLC) (1.67, 5.41) were poorly distributed, but the two mesopic measures VA(MHC) (0.88, 1.67) and VA(MLC) (0.29, -0.18) were normally distributed. Strong correlations existed between the (transformed) VA variables (R: 0.53 to 0.84). However, limits of agreement ranged over 0.30 to 0.42 logMAR, whereas retest data suggested a range of 0.15. All four were offered in stepwise multiple linear regression for 30 wavefront metrics: 20 included VA(MLC) alone, two included VA(PLC), two included VA(MHC) and six included both VA(MLC) and VA(MHC); R(2) averaged 25%. CONCLUSION: Although strongly correlated, low contrast and/or mesopic VA testing provides different information. Wave aberration metrics correlates better with VA(MLC) making this the visual performance test of choice.

Adult↗

PubMedDate not supplied

Quantifying scatter in Shack-Hartmann images to evaluate nuclear cataract.

PURPOSE: Quantify and localize lenticular forward scatter using Shack-Hartmann wavefront sensing (SHWS) as single-valued metrics and a scatter map, and to examine the relationships between forward scatter and backscatter metrics and visual acuity. METHODS: We obtained SHWS images from 148 patients in the Texas Investigation of Cataract Optics study. Patient age was 22 to 84 years, with Lens Opacities Classification System III (LOCS III) nuclear opalescence (NO) scores ranging from 0.8 to 5.6. Visual acuities were measured at photopic (280 cd/m2) high (VA(PHC)) and low contrast (VA(PLC)) and mesopic (0.75 cd/m2) high (VA(MHC)) and low contrast (VA(MLC)). Scattering was described in a scatter map and by five single-valued metrics characterizing SHWS lenslet point spread functions. The relationships between scatter and visual acuity were tested using linear regression. RESULTS: Visual acuities decreased proportional to both LOCS III NO (R2=up to 39%) and scatter metrics (R2=up to 21%). Stepwise multiple linear regression improved visual acuity prediction by including a backscatter and a forward scatter metric (R2 up to 51.2%). For the subjects over age 60 years (N=46, 68.8+/-6.12 years), the forward scatter metrics explain as much variance in visual acuities (R2=up to 29%) as LOCS III NO (R2=up to 26%). Combined they accounted for up to 48.8% of visual acuity variance. CONCLUSION: Forward light scatter can be quantified using SHWS and the resulting metrics explain significant variance in visual acuity, especially in the aging eye. Together with a backscatter metric they explain approximately 50% of the variance in VA.

Adult↗