Cognitive Science Program

Statistical variation of aberration structure and image quality in a normal population of healthy eyes (2002)
Larry N Thibos, Xin Hong, Arthur Bradley and Xu Cheng
JOSA A, 19 (12), 2329-2348

Abstract

A Shack–Hartmann aberrometer was used to measure the monochromatic aberration structure along the primary line of sight of 200 cyclopleged, normal, healthy eyes from 100 individuals. Sphero-cylindrical refractive errors were corrected with ophthalmic spectacle lenses based on the results of a subjective refraction performed immediately prior to experimentation. Zernike expansions of the experimental wave-front aberration functions were used to determine aberration coefficients for a series of pupil diameters. The residual Zernike coefficients for defocus were not zero but varied systematically with pupil diameter and with the Zernike coefficient for spherical aberration in a way that maximizes visual acuity. We infer from these results that subjective best focus occurs when the area of the central, aberration-free region of the pupil is maximized. We found that the population averages of Zernike coefficients were …

Accuracy and precision of objective refraction from wavefront aberrations (2004)
Larry N Thibos, Xin Hong, Arthur Bradley and Raymond A Applegate
Journal of vision, 4 (4), 9-Sep

Abstract

We determined the accuracy and precision of 33 objective methods for predicting the results of conventional, sphero-cylindrical refraction from wavefront aberrations in a large population of 200 eyes. Accuracy for predicting defocus (as specified by the population mean error of prediction) varied from− 0.50 D to+ 0.25 D across methods. Precision of these estimates (as specified by 95% limits of agreement) ranged from 0.5 to 1.0 D. All methods except one accurately predicted astigmatism to within±1/8D. Precision of astigmatism predictions was typically better than precision for predicting defocus and many methods were better than 0.5 D. Paraxial curvature matching of the wavefront aberration map was the most accurate method for determining the spherical equivalent error whereas least-squares fitting of the wavefront was one of the least accurate methods. We argue that this result was obtained because curvature matching is a biased method that successfully predicts the biased endpoint stipulated by conventional refractions. Five methods emerged as reasonably accurate and among the most precise. Three of these were based on pupil plane metrics and two were based on image plane metrics. We argue that the accuracy of all methods might be improved by correcting for the systematic bias reported in this study. However, caution is advised because some tasks, including conventional refraction of defocus, require a biased metric whereas other tasks, such as refraction of astigmatism, are unbiased. We conclude that objective methods of refraction based on wavefront aberration maps can accurately predict the results of subjective refraction …

Optical and visual impact of tear break-up in human eyes (2000)
Ron Tutt, Arthur Bradley, Carolyn Begley and Larry N Thibos
Investigative ophthalmology & visual science, 41 (13), 4117-4123

Abstract

The purpose of this study was to examine the optical and visual impact of tear break-up.methods. Optical quality of the eye was assessed during periods of nonblinking by quantifying vessel contrast in the fundus image and by monitoring the psychophysical contrast sensitivity and the spatial distribution of tear thickness changes by retroillumination. All measures were obtained from three eyes either with or without a soft contact lens.results. A noticeable decrease in retinal vessel contrast and contrast sensitivity were observed soon after a blink. Both of these measures of optical quality of the eye showed a similar pattern of image degradation both with and without a soft contact lens. Although trial-to-trial variability was considerable, sample means show that image contrast in the low spatial frequency range can drop to between 20% and 40% of initial values after 60 seconds of nonblinking. Retroillumination of the tear film showed local intensity fluctuations that progressively spread across the pupil with increasing time after the blink.conclusions. Optical aberrations created by tear break-up contribute to the decline in image quality observed objectively and psychophysically. The decline in image quality that accompanies tear break-up may be a direct cause of the blurry vision complaints commonly encountered in dry-eye patients.

The chromatic eye: a new reduced-eye model of ocular chromatic aberration in humans (1992)
Larry N Thibos, Ming Ye, Xiaoxiao Zhang and Arthur Bradley
Applied optics, 31 (19), 3594-3600

Abstract

New measurements of the chromatic difference of focus of the human eye were obtained with a two-color, vernier-alignment technique. The results were used to redefine the variation of refractive index of the reduced eye over the visible spectrum. The reduced eye was further modified by changing the refracting surface to an aspherical shape to reduce the amount of spherical aberration. The resulting chromatic-eye model provides an improved account of both the longitudinal and transverse forms of ocular chromatic aberration.

The effects of contrast on visual orientation and spatial frequency discrimination: a comparison of single cells and behavior (1987)
Bernt C Skottun, Arthur Bradley, Gary Sclar, Izumi Ohzawa and Ralph D Freeman
Journal of neurophysiology, 57 (3), 773-786

Abstract

We have compared the effects of contrast on human psychophysical orientation and spatial frequency discrimination thresholds and on the responses of individual neurons in the cat's striate cortex. Contrast has similar effects on orientation and spatial frequency discrimination: as contrast is increased above detection threshold, orientation and spatial frequency discrimination performance improves but reaches maximum levels at quite low contrasts. Further increases in contrast produce no further improvements in discrimination. We measured the effects of contrast on response amplitude, orientation and spatial frequency selectivity, and response variance of neurons in the cat's striate cortex. Orientation and spatial frequency selectivity vary little with contrast. Also, the ratio of response variance to response mean is unaffected by contrast. Although, in many cells, response amplitude increases approximately linearly …

Theory and measurement of ocular chromatic aberration (1990)
LN Thibos, A Bradley, DL Still, X Zhang and PA Howarth
Vision research, 30 (1), 33-49

Abstract

We have determined the transverse chromatic aberration of the human eye by measuring the apparent offset of a two-color vernier viewed foveally through a displaced, pinhole aperture. For the same subjects, we also determined the longitudinal chromatic aberration for foveal viewing by the method of best focus. In both cases, the results were closely predicted by a simple, reduced-eye optical-model for which transverse and longitudinal chromatic aberration are directly proportional, with the constant of proportionality being the amount of displacement of the pinhole from the visual axis. Further measurements revealed that the natural pupil was closely centered on the visual axis for two subjects and slightly displaced in the temporal direction for three other subjects. One implication of these results is that, although the eye has substantial chromatic aberration, the pupil is positioned so as to minimize the transverse …

Predicting subjective judgment of best focus with objective image quality metrics (2004)
Xu Cheng, Arthur Bradley and Larry N Thibos
Journal of Vision, 4 (4), 7-Jul

Abstract

<h3 class="gsh_h3">Purpose:</h3> To determine the impact of higher-order monochromatic aberrations on lower-order subjective sphero-cylindrical refractions. <h3 class="gsh_h3">Methods:</h3> Computationally-aberrated, monochromatic Sloan letters were presented on a high luminance display that was viewed by an observer through a 2.5 mm pupil. Through-focus visual acuity (VA) was determined in the presence of spherical aberration (Z 4 0) at three levels (0.10, 0.21 and 0.50 D). Analogous through-astigmatism experiments measured visual acuity in the presence of secondary astigmatism (Z 4±2) or coma (Z 3− 1). Measured visual acuity was correlated with 31 different metrics of image quality to determine which metric best predicts performance for degraded retinal images. The defocus and astigmatism levels that optimized each metric were compared with those that produced best visual acuity to determine which metric best predicts subjective refraction. <h3 class="gsh_h3">Results:</h3> Spherical aberration, coma and secondary astigmatism all reduced VA and increased depth of focus. The levels of defocus and primary astigmatism that produced the best performance varied with levels of spherical aberration and secondary astigmatism, respectively. The presence of coma, however, did not affect cylindrical refraction. Image plane metrics, especially those that take into account the neural contrast sensitivity threshold (eg the visual Strehl ratio, VSOTF), are good predictors of visual acuity in both the through-focus and through-astigmatism experiments (R=− 0.822 for VSOTF). Subjective sphero-cylindrical refractions were accurately predicted by some image-quality metrics (eg, pupil fraction, VSOTF and standard …

A link between tear instability and hyperosmolarity in dry eye (2009)
Haixia Liu, Carolyn Begley, Minhua Chen, Arthur Bradley, Joseph Bonanno, Nancy A McNamara ...
Investigative ophthalmology & visual science, 50 (8), 3671-3679

Abstract

purpose. Tear film instability and tear hyperosmolarity are considered core mechanisms in the development of dry eye. The authors hypothesize that evaporation and instability produce transient shifts in tear hyperosmolarity that lead to chronic epithelial stress, inflammation, and symptoms of ocular irritation. The purpose of this study was to provide indirect evidence of short-term hyperosmolar conditions during tear instability and to test whether the corneal epithelium responds to transient hyperosmolar stress.methods. Five subjects kept one eye open as long as possible, and overall discomfort and sensations associated with tear break-up were scaled. Later, the same subjects used the same scales to report discomfort sensations after instillation of NaCl and sucrose hyperosmolar drops (300–1000 mOsM/kg). A two-alternative, forced-choice experiment was used to obtain osmolarity thresholds. In the second experiment, primary cultured bovine corneal epithelial cells were transiently stressed with the same range of hyperosmolar culture medium, and proinflammatory mitogen-activated protein kinase (MAPKs) were measured by Western blot analysis.results. Tear instability led to an average discomfort rating of 6.13 and sensations of burning and stinging. These sensations also occurred with hyperosmolar solutions (thresholds, 450–460 mOsM/kg) that required 800 to 900 mOsM/kg to generate the same discomfort levels reported during tear break-up. MAPK was activated at 600 mOsM/kg of transient hyperosmolar stress.conclusions. These experiments provide a link between hyperosmolarity and tear instability, suggesting that hyperosmolar …

Contrast dependence and mechanisms of masking interactions among chromatic and luminance gratings (1988)
Eugene Switkes, Arthur Bradley and Karen K De Valois
JOSA A, 5 (7), 1149-1162

Abstract

The contrast dependence of simultaneous masking has been measured using isochromatic yellow–black luminance sinusoids and isoluminant red–green chrominance gratings. Masking functions for all four combinations of chromatic and luminance masks and tests are reported. In the two same-on-same conditions (luminance mask/luminance test and chromatic mask/chromatic test) these functions (increment threshold contrast versus mask contrast) have the typical dipper shape and are almost identical when test and mask contrasts are normalized to the unmasked contrast thresholds. The contrast dependence of the luminance mask/color test and color mask/luminance test functions are quite different. The luminance mask/color test shows facilitation over a broad range of both subthreshold and suprathreshold contrasts of the luminance mask. In the color mask/luminance test condition facilitation is never …

Visual orientation and spatial frequency discrimination: a comparison of single neurons and behavior (1987)
Arthur Bradley, Bernt C Skottun, IZUMI Ohzawa, Gary Sclar and RALPH D Freeman
Journal of Neurophysiology, 57 (3), 755-772

Abstract

Neurons in the visual cortex respond selectively to stimulus orientation and spatial frequency. Changes in response amplitudes of these neurons could be the neurophysiological basis of orientation and spatial frequency discrimination. We have estimated the minimum differences in stimulus orientation and spatial frequency that can produce reliable changes in the responses of individual neurons in cat visual cortex. We compare these values with orientation and spatial frequency discrimination thresholds determined behaviorally. Slopes of the tuning functions and response variability determine the minimum orientation and spatial frequency differences that can elicit a reliable response change. These minimum values were obtained from single cells using receiver operating characteristic (ROC) analysis. The average minimum orientation and spatial frequency differences that could be signaled reliably by cells …

Relationship between refractive error and monochromatic aberrations of the eye (2003)
Xu Cheng, Arthur Bradley, Xin Hong and Larry N Thibos
Optometry and Vision Science, 80 (1), 43-49

Abstract

Purpose. To examine the relationship between ametropia and optical aberrations in a population of 200 normal human eyes with refractive errors spanning the range from+ 5.00 to− 10.00 D.Methods. Using a reduced-eye model of ametropia, we tested the hypothesis that the optical system of the eye is uncorrelated with the degree of ametropia. These predictions were evaluated experimentally with a Shack-Hartmann aberrometer that measured the monochromatic aberrations across the central 6 mm of the dilated pupil in well-corrected, cyclopleged eyes.Results. Optical theory predicted, and control experiments on a model eye verified, that Shack-Hartmann measurements of spherical aberration will vary with axial elongation of the eye even if the dioptric components of the eye are fixed. Contrary to these predictions, spherical aberration was not significantly different from emmetropic eyes. Root mean square of …

Contrast sensitivity in anisometropic amblyopia (1981)
A Bradley and RD Freeman
Investigative ophthalmology & visual science, 21 (3), 467-476

Abstract

Contrast sensitivity functions were measured for sinusoidal gratings from a sample of 10 anisometropic amblyopes. A high spatial frequency deficit was found from tests of the amblyopic eyes of all subjects. This defect decreased with spatial frequency and was correlated with the magnitude of anisometropia. Controls were instituted to rule out psychophysical method and residual defocus as possible causes of these effects. At low spatial frequencies, there were small differences between the two eyes. For some subjects, sensitivities of the amblyopic eyes appeared actually higher than normal whereas the reverse was found for most of the others. Additional tests demonstrated that the low-frequency differences could be accounted for by magnification differences (aniseikonia) between the two eyes. These findings are consistent with the idea that monocular contrast deprivation is the causal agent in anisometropic amblyopia.

Blinking and tear break-up during four visual tasks (2009)
Nikole L Himebaugh, Carolyn G Begley, Arthur Bradley and Jenni A Wilkinson
Optometry and Vision Science, 86 (2), E106-E114

Abstract

Purpose. This study investigates the relationship between blinking, tear film break-up, and ocular symptoms for normal and dry eye subjects performing four different visual tasks.Methods. Sixteen control and sixteen dry eye subjects performed four visual tasks (looking straight ahead, watching a movie, identifying rapidly changing letters, and playing a computer game) while blink patterns and fluorescein images of the tear film were videotaped. Pre and posttesting symptom questionnaires, querying the intensity of nine symptoms of ocular irritation, were completed by all subjects. Blink rate and blink amplitude were computed from digitized videos. The percentage of tear film break-up before the blink was calculated.Results. Dry eye subjects had a significantly higher blink rate (p= 0.017, t-test). Both groups blinked significantly less during the game and letter tasks (p&lt; 0.04, t-test). Partial blinks were common as were …

A statistical model of the aberration structure of normal, well‐corrected eyes (2002)
Larry N Thibos, Arthur Bradley and Xin Hong
Ophthalmic and Physiological Optics, 22 (5), 427-433

Abstract

A statistical model of the wavefront aberration function of the normal, well‐corrected eye was constructed based on normative data from 200 eyes which show that, apart from spherical aberration, the higher‐order aberrations of the human eye tend to be randomly distributed about a mean value of zero. The vector of Zernike aberration coefficients describing the aberration function for any individual eye was modelled as a multivariate, Gaussian, random variable with known mean, variance and covariance. The model was verified by analysing the statistical properties of 1000 virtual eyes generated by the model. Potential applications of the model include computer simulation of individual variation in aberration structure, retinal image quality, visual performance, benefit of novel designs of ophthalmic lenses, or outcome of refractive surgery.

Orientation and spatial frequency selectivity of adaptation to color and luminance gratings (1988)
Arthur Bradley, Eugene Switkes and Karen De Valois
Vision research, 28 (7), 841-856

Abstract

Prolonged viewing of sinusoidal luminance gratings produces elevated contrast detection thresholds for test gratings that are similar in spatial frequency and orientation to the adaptation stimulus. We have used this technique to investigate orientation and spatial frequency selectivity in the processing of color contrast information. Adaptation to isoluminant red-green gratings produces elevated color contrast thresholds that are selective for grating orientation and spatial frequency. Only small elevations in color contrast thresholds occur after adaptation to luminance gratings, and vice versa. Although the color adaptation effects appear slightly less selective than those for luminance, our results suggest similar spatial processing of color and luminance contrast patterns by early stages of the human visual system.