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Biomedical subjects

D A Atchison

Publications and source records attributed to D A Atchison.

At least 19 recordsLinked to original sources
PubMed2002

Age-related changes in refractive index distribution and power of the human lens as measured by magnetic resonance micro-imaging in vitro.

We report a new technique for non-invasively mapping the refractive index distribution through the eye lens using magnetic resonance micro-imaging. The technique is applied to map the refractive index distribution throughout the sagittal plane of 18 human eye lenses ranging in age from 14 to 82 years in vitro. The results are compared with standard models for the human eye lens. They confirm that the refractive index distribution, when plotted as a function of normalised lens radius, is a function of lens age and differs both between the equatorial and axial directions and between the anterior and posterior halves of the optical axis. The refractive index of the lens nucleus exhibits a significant reduction with age amounting to 3.4+/-0.6 x 10(-4) years(-1). The contribution of the gradient index (GRIN) to the lens power decreases by 0.286+/-0.067 D/year, accounting almost entirely for the estimated overall change in lens power with age for these lenses, which were probably in their most accommodated state. The results provide experimental verification of hypothesised changes in the GRIN that have previously been invoked as contributing to presbyopia and support the hypothesis that changes in the GRIN are sufficient to offset effects of increasing curvature of human lenses with age in their unaccommodated state.

Accommodation, Ocular↗
PubMed2001

Description of a method for neutralising the Stiles-Crawford effect.

The influence of the Stiles-Crawford effect on visual performance can be investigated by filters based on the apodisation model of the Stiles-Crawford effect. We describe the development of practical filters to achieve neutralisation. We present some results of the Stiles-Crawford function showing that the filters work well for expected errors in aligning filters in front of the eye.

Equipment Design↗
PubMed2001

The effect of under and over refractive correction on visual performance and spectacle lens acceptance.

As a follow-on from a previous study by Miller, Kris and Griffiths (1997, Optom. Vis. Sci. 84, 521-526), we investigated the effect of small prescription errors on spatial visual performance and spectacle lens acceptability. We included both negative and positive errors and binocular and monocular errors. Data were collected on 15 young adult subjects. Clinical measures were taken of pupil size, ocular dominance, binocular visual acuity, negative and positive relative accommodation, distance and near heterophorias, and stereopsis. Visual performance was measured with the best correction and for +/-0.50 D spherical binocular and monocular errors. Subjects wore spectacles, each containing a different error in turn, for 2 days and compared them with a reference pair. Following the wearing period subjects assessed the performance of the spectacles by completing a short questionnaire. The only ocular tests for which these small prescription errors had significant effects were binocular visual acuity and negative relative accommodation. No significant relationship was observed between any of the clinical test results and overall lens acceptance. Despite this, the reference pair was generally perceived as better than the test pairs containing errors, and a considerable proportion of subjects reported pairs with errors as being unacceptable. In conclusion, the questionnaire findings support the need for both accurate monocular and binocular refractions. Subjects differed in their criteria for judging lens acceptability.

Accommodation, Ocular↗
PubMed2001

The gradient index and spherical aberration of the lens of the human eye.

We present equations for analysing the primary spherical aberration of the crystalline lens, both as a whole and of its contributing parts. We apply these equations to two examples of gradient index lenses. The spherical aberration of one lens is well outside that expected for real lenses, but that of the other is within this range. We assess the sensitivity of the aberration to changes in surface shape and gradient index distribution. The process is applicable to other models of the gradient index of the lens.

Adolescent↗
PubMed2001

Influence of Stiles-Crawford effect apodization on spatial visual performance with decentered pupils.

Using theoretical estimates of the optical-transfer function and line-spread function as image-quality criteria, we predicted the influence of the Stiles-Crawford effect (SCE) on both optical performance of the eye and subjective measurements of transverse aberrations when pupils are decentered. The SCE was modeled as a pupil apodization. The SCE appears to improve image quality by providing compensation for aberrations induced by pupil decentration, but this improvement is usually small. When a criterion of the placement of the image is used as the centroid of the line-spread function, an average SCE reduces the influence of pupil decentration on subjective transverse chromatic aberrations (TCA's) for 5-mm-diameter pupils by 30%. This reduction is much less than that obtained by previous experimental studies of TCA, and possible reasons for this discrepancy are discussed. Decentering the SCE produces an appreciable shift in subjective TCA for 5-mm-diameter pupils of 1.4 arc min per 1-mm decentration (at wavelengths 433 and 622 nm).

Color Perception↗
PubMed2000

Measuring contrast sensitivity with inappropriate optical correction.

Spatial frequency-selective minima (notches) in the contrast sensitivity function (CSF) because of defocus can mimic those that occur with ocular disease. We examined the influence of measurement conditions on CSF shape in simulated clinical testing. CSF notches occurred with almost all levels of defocus for all subjects. Multiple notches were found under some conditions. Notches were found with defocus as small as 0.50 D. Effects of induced astigmatism depended on the orientation of the target. Notches were apparent in defocus conditions after stimulus size and room illuminance were modified and when subjects had insufficient accommodation to compensate for hypermetropic defocus. The equivalent of notches was not noted with the Pelli-Robson chart. As defocus-induced CSF notches may be mistaken for functional loss, careful refractive correction should be conducted prior to clinical or experimental CSF measurement, even at low spatial frequencies.

Adult↗
PubMed1999

Effects of defocus and pupil size on human contrast sensitivity.

Defocus lowers the contrast sensitivity function (CSF), producing a complex function with local dips and peaks. Previously, we were able to predict the shape of the CSF with large pupils from measured transverse aberrations with hypermetropic defocus but not with myopic defocus (Atchison et al., 1998c, J. Opt. Soc. Am. A. 15, 2536). As there is no reason that myopic defocus should be more difficult to predict than hypermetropic defocus, we modified the procedure to try to improve CSF predictions with myopic defocus. Also, we extended the study to consider a range of pupil sizes. CSFs were measured for three subjects at three defocus levels (in-focus, -2D and +2D) and three pupil sizes (2 mm, 4 mm and 6 mm). Using a diffraction optics model, transverse aberration measures and in-focus CSF measures, we predicted the defocused CSFs. The predicted defocused CSFs were lower than the in-focus CSF as expected, and had complex shapes that varied with defocus and pupil size and between subjects. While a few predictions were poor, generally, the overall magnitude and shape of the defocused CSFs were well predicted and similarly so for myopic and hypermetropic defocus. Some further improvements in technique are indicated.

Accommodation, Ocular↗
PubMed1998

First- and third-order optical theory of gradient index materials, with application to contact lenses.

PURPOSE: To investigate the feasibility of using gradient index media in contact lenses, we developed simple methods which we used to derive the power and aberrations associated with the contact lenses. METHODS: In one method, we assume that the height of a ray does not change as it passes through the lens. We describe a second method in which the ray is assumed to follow a parabolic path as it passes through the lens. We use sophisticated third-order theory and finite raytracing for comparison with these methods. RESULTS: The methods are compared for contact lenses with parabolic radial gradient index media. Without the gradient index, these lenses would have zero power. The formula for power which assumes no change in ray height gives errors of approximally 0.8 and 1.5% for lenses of thicknesses 0.18 and 0.36 mm. However, the formula for third-order spherical aberration which uses the same assumption gives poor estimations. The method for calculating power using the parabolic ray path is very accurate. The sophisticated third-order aberration theory was reasonably accurate out to 2.5 mm ray height. The contact lenses with the gradient index media have much smaller aberration in air than do conventional contact lenses of the same powers, with the sign of the aberration being reversed. CONCLUSIONS: Our simple procedures give good estimations of powers of contact lenses with gradient index media. The approximate method, which assumes that the height of a ray does not change as it passes through the lens, should not be used for finding the spherical aberration of such a lens. Contact lenses with gradient index media have potential for minimizing spherical aberration.

Contact Lenses↗
PubMed1998

Assessment of the accuracy of the crossed-cylinder aberroscope technique.

Simulations of the optics of the Howland crossed-cylinder aberroscope technique show that errors in alignment, data collection, and analysis can lead to unexpected asymmetries of the determined aberrations in a rotationally symmetric system. In particular, coma can be incorrectly indicated. The magnitude of the error in aberration measurement depends on the magnitude of the alignment, data collection, and alignment errors. These findings indicate that the tolerances for setting up the technique and data collection should be analyzed thoroughly before quantitative significance is given to the determined aberration coefficients.

Computer Simulation↗
PubMed1998

Predicting the effects of optical defocus on human contrast sensitivity.

We used diffraction modulation transfer functions and model eyes to predict the effect of defocus on the contrast sensitivity function (CSF) and compared these predictions with previously published experimental data. Using the principle that optically induced changes in the modulation transfer function should be paralleled by identical changes in the CSF, we used the modulation transfer function calculations with the best-focus CSF measurements to predict the defocused CSF. An aberration-free model predicted the effects of defocus well when the CSF was measured with small pupils (e.g., 2 mm) but not with larger pupils (6-8 mm). When the model included average aberrations, prediction of the defocused CSF with large pupils was better but remained inaccurate, failing, in particular, to reflect differences between individual subjects. Inclusion of measured aberrations for individual subjects provided accurate predictions in the shape of the monochromatic CSF of two of three subjects with hyperopic defocus and good predictions of the polychromatic CSF of two subjects with hyperopic defocus. Prediction of the effects of myopic defocus by use of measured individual aberrations of one subject were less successful. Hence a diffraction optics model can provide good predictions of the effects of defocus on the human CSF, given that one has knowledge of the individual ocular aberrations. These predictions are dependent on the quality of the aberration measurements.

Contrast Sensitivity↗
PubMed1998

Influence of Stiles-Crawford effect apodization on spatial visual performance.

The Stiles-Crawford effect is often invoked by vision scientists when predictions of the effects of aberrations and defocus on spatial visual performance are not borne out experimentally. Modeling the Stiles-Crawford effect as an apodization, we investigated the expected influence that it would have on spatial visual performance in the presence of 1-diopter primary spherical aberration at the edge of a 6-mm-diameter centered pupil. The changes in refraction produced by a high Stiles-Crawford effect, according to various criteria, were small at approximately 0.10 diopter. The Stiles-Crawford effect has only a small capability to compensate for defocus and spherical aberration. These results indicate that the Stiles-Crawford effect has little influence on spatial visual performance in the case of centered pupils. We suggest that the faith that has often been placed in the Stiles-Crawford effect to account for discrepancies between experimental results and expected results is not justified, at least for well-centered pupils and Stiles-Crawford effects.

Light↗
PubMed1998

Errors in determining the direction of the visual axis in the presence of defocus.

The visual axis is the ray path from the fixation point to the fovea by way of the nodal points. Such a ray path does not exist in unaccommodated vision when the fixation point does not coincide with the far point of the eye because of induced or natural defocus. Nevertheless an approximation to the visual axis can be obtained by the position of a small pupil close to the cornea for which a bichromatic vernier target appears correctly aligned. When the visual axis is determined, an eye rotation from the visual fixation position must occur, which is reversed upon removing the small pupil. Upon engaging in any experiments for which the visual axis is then used as a reference, there is an error in this reference position. We develop simple paraxial equations to estimate this error. We show that these equations have good accuracy. The error associated with locating the visual axis at the cornea is 0.002 mm per dioptre of defocus, which is small enough to be ignored.

Convergence, Ocular↗
PubMed1997

Subjective depth-of-focus of the eye.

An experiment is described in which the subjective depth-of-focus (DOF) of the eye, defined as the range of focusing errors for which the image of the target appears to have the same clarity, contrast, and form as the optimal in-focus image, was measured as a function of the size of high contrast (99%) Snellen Es for 5 trained subjects under cycloplegia. Mean DOF increased by approximately 60% as the size of the letter detail increased from -0.2 to 0.87 log min arc (Snellen equivalent: 6/3.8 to 6/45), although there were considerable intersubject variations. DOF declined with increasing pupil diameter, the mean total DOFs being 0.86, 0.59, and 0.55 D for 2-, 4-, and 6-mm pupils, respectively. In a second experiment, use of low (21%) contrast letters with a 4-mm pupil and 4 subjects marginally increased the DOF (by 0.08 +/- 0.05 D); refraction also shifted in a myopic direction by a mean of 0.15 +/- 0.06 D compared with the high contrast letters. A third experiment with four less-experienced subjects demonstrated the importance of instruction and training in any measurement involving judgment of just-perceptible defocus blur. The clinical implications of the results for measurements of refraction and amplitude of accommodation are discussed.

Accommodation, Ocular↗
PubMed1997

Equivalent power of the crystalline lens of the human eye: comparison of methods of calculation.

We present four methods, with different levels of sophistication and precision, for calculating the refractive power of the ocular lens from its optical structure. The first method uses finite ray tracing but simulates a paraxial ray by using small ray heights. The second method involves a recursive paraxial ray-tracing procedure. The other two methods do not depend on any ray-tracing procedure but use much simpler, approximate equations. In the third method the ray height is assumed not to change within the lens, and in the fourth method the ray path is assumed to be parabolic. The fourth method, but not the third method, can separately calculate the power of the surfaces and the gradient-index lens bulk, which are then used in the three-lens equation to calculate the power of the lens as a whole.

Humans↗
PubMed1997

Pupil size, mean accommodation response and the fluctuations of accommodation.

We wished to determine how pupil size and mean accommodation response level interact to influence the fluctuations of accommodation. A dynamic infra-red optometer was used to record accommodation responses while subjects viewed a steady target at two stimulus levels (1.5 and 3 D) through four pupils (1, 2, 4 and 6 mm). It was found for most subjects that the fluctuations of accommodation increase at higher mean accommodation response levels, and small pupils lead to an increase in the low frequency (but not the high frequency) fluctuations of accommodation. The effects of mean accommodation response are independent of pupil size, and the effects of pupil size are independent of mean response level.

Accommodation, Ocular↗
PubMed1996

Consequences of monocular diplopia for the contrast sensitivity function.

Though the human eye generally creates a single image on the retina, the literature contains many examples showing perceptual monocular diplopia. Previously, monocular diplopia resulting from astigmatic defocus has been demonstrated to cause a notch (local minimum) in the contrast sensitivity function (CSF). We examine Verhoeff's (1900) model which explains how monocular diplopia can occur through an interaction between defocus and common ocular aberrations. From the measured ocular transverse aberration function and from the measured monocular diplopia of three cyclopleged subjects we predicted multiple notches in the CSF with hyperopic spherical defocus. Monochromatic and polychromatic CSF were measured for vertical gratings with best refraction and with simulated myopia and hyperopia. Multiple notches in CSF were observed experimentally. Notches in the polychromatic CSF were smaller and broader than those found in the monochromatic CSF. Our aberration model was successful in predicting notches in the CSF with hyperopic spherical defocus. The implications for clinical measurement of CSF are discussed.

Adult↗
PubMed1996

Monocular diplopia caused by ocular aberrations and hyperopic defocus.

As a single aperture, approximately monofocal optical system, the human eye generally creates a single image on the retina. However, the literature contains many reports of perceptual monocular diplopia. While it is easy to understand how distortion may produce monocular diplopia, its reported high incidence in normal eyes is less easily understood. We examine a model which ascribes monocular diplopia to an interaction between defocus and ocular spherical aberration. Using a psychophysical hyperacuity-based alignment procedure we measured the transverse aberration function in 0.5 mm steps horizontally across the pupil in the eyes of three cyclopleged subjects. Ocular transverse aberration functions were derived with best refraction and with simulated myopia and hyperopia. Monocular diplopia was also measured under the same conditions. All three subjects showed significant, but different, degrees of positive spherical aberration. The measured ocular transverse aberration functions were predictably modified by the hyperopic and myopic defocus. Hyperopic defocus combined with positive (myopic) spherical aberration changes a monotonic transverse aberration function with a single inflection point into a biphasic function with two inflection points. The locations of the inflections predict the presence and magnitude of the perceived diplopia. These experimental results confirm Verhoeff's (1900) hypothesis for the ocular cause of monocular diplopia.

Adult↗