Statistical variation of aberration structure and image quality in a normal population of healthy eyes

Higher-order aberrations in young refractive surgery candidates in India: establishment of normal values and comparison with white and Chinese Asian populations

PURPOSE

To establish normal values of higher-order aberrations (HOAs) in young candidates in an Indian population and compare them with results in white and Asian (Chinese) populations.

SETTING

Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India.

METHODS

This prospective trial comprised 206 consecutive candidates (412 eyes) for customized laser in situ keratomileusis. Exclusion criteria were previous ocular surgery, corneal ectasia, rigid gas-permeable contact lens use, corneal scar, or cataract. Higher-order aberrations were analyzed on a Zywave workstation (Bausch & Lomb Surgical).

RESULTS

The mean age was 23.63 years +/- 1.99 (SD); the mean refractive error, -2.97 +/- 4.0 diopters (D) sphere and -1.73 +/- 3.6 D cylinder; and the mean 6.0 mm HOA root mean square (RMS), 0.36 +/- 0.26 microm. The most predominant HOAs were 3rd order (coma, trefoil). The summated RMS of the 3rd order was the most predominant (mean 0.23 +/- 0.15 microm), followed by the 4th and 5th orders (P<.0001). The ratio between summated RMS means was 1:0.7:0.3, similar to that in white populations (1:0.7:0.3) and different than in Asian (Chinese) populations (1: ~ 0.8:0.002). Third- and 4th-order aberrations were between 60% and 70% higher in Asian (Chinese) eyes. The difference between data in this study and those in others was less than 10% in 3rd and 4th order.

CONCLUSION

The normative data for HOA in Indian eyes closely matched that in white populations but was different from that in Asian (Chinese) populations, which may provide help in nomogram modifications.

Design and validation of an infrared Badal optometer for laser speckle

PURPOSE

To validate the design of an infrared wavefront aberrometer with a Badal optometer employing the principle of laser speckle generated by a spinning disk and infrared light. The instrument was designed for subjective meridional refraction in infrared light by human patients.

METHODS

Validation employed a model eye with known refractive error determined with an objective infrared wavefront aberrometer. The model eye was used to produce a speckle pattern on an artificial retina with controlled amounts of ametropia introduced with auxiliary ophthalmic lenses. A human observer performed the psychophysical task of observing the speckle pattern (with the aid of a video camera sensitive to infrared radiation) formed on the artificial retina. Refraction was performed by adjusting the vergence of incident light with the Badal optometer to nullify the motion of laser speckle. Validation of the method was performed for different levels of spherical ametropia and for various configurations of an astigmatic model eye.

RESULTS

Subjective measurements of meridional refractive error over the range -4D to +4D agreed with astigmatic refractive errors predicted by the power of the model eye in the meridian of motion of the spinning disk.

CONCLUSIONS

Use of a Badal optometer to control laser speckle is a valid method for determining subjective refractive error at infrared wavelengths. Such an instrument will be useful for comparing objective measures of refractive error obtained for the human eye with autorefractors and wavefront aberrometers that employ infrared radiation.

Changes in through-focus spatial visual performance with adaptive optics correction of monochromatic aberrations

We determined the influence of adaptive optics correction on through-focus illiterate-E visual acuity and through-focus contrast sensitivity under monochromatic conditions. In two subjects, adaptive optics improved high and low (12%) contrast in-focus visual acuity by 0.1 to 0.15logMAR, but resulted in more rapid and more symmetrical deterioration in visual acuity away from in-focus. In one subject, adaptive optics improved in-focus contrast sensitivity and resulted in more symmetrical and greater loss of contrast sensitivity about the peak sensitivity because of correction of higher-order aberrations. The results show that full correction of higher-order aberrations may worsen spatial visual performance in the presence of some defocus.

Predicting visual acuity from wavefront aberrations

It is now possible to routinely measure the aberrations of the human eye, but there is as yet no established metric that relates aberrations to visual acuity. A number of metrics have been proposed and evaluated, and some perform well on particular sets of evaluation data. But these metrics are not based on a plausible model of the letter acuity task and may not generalize to other sets of aberrations, other data sets, or to other acuity tasks. Here we provide a model of the acuity task that incorporates optical and neural filtering, neural noise, and an ideal decision rule. The model provides an excellent account of one large set of evaluation data. Several suboptimal rules perform almost as well. A simple metric derived from this model also provides a good account of the data set.

Vergence-accommodation conflicts hinder visual performance and cause visual fatigue

Three-dimensional (3D) displays have become important for many applications including vision research, operation of remote devices, medical imaging, surgical training, scientific visualization, virtual prototyping, and more. In many of these applications, it is important for the graphic image to create a faithful impression of the 3D structure of the portrayed object or scene. Unfortunately, 3D displays often yield distortions in perceived 3D structure compared with the percepts of the real scenes the displays depict. A likely cause of such distortions is the fact that computer displays present images on one surface. Thus, focus cues-accommodation and blur in the retinal image-specify the depth of the display rather than the depths in the depicted scene. Additionally, the uncoupling of vergence and accommodation required by 3D displays frequently reduces one's ability to fuse the binocular stimulus and causes discomfort and fatigue for the viewer. We have developed a novel 3D display that presents focus cues that are correct or nearly correct for the depicted scene. We used this display to evaluate the influence of focus cues on perceptual distortions, fusion failures, and fatigue. We show that when focus cues are correct or nearly correct, (1) the time required to identify a stereoscopic stimulus is reduced, (2) stereoacuity in a time-limited task is increased, (3) distortions in perceived depth are reduced, and (4) viewer fatigue and discomfort are reduced. We discuss the implications of this work for vision research and the design and use of displays.

[Application of wavefront analysis in clinical and scientific settings. From irregular astigmatism to aberrations of a higher order--Part II: examples]

In recent years, wavefront analysis has ceased to be purely a laboratory application and emerged as a method used in ophthalmological diagnosis. This development has been promoted mainly by the widespread use of wavefront-guided LASIK (laser in situ keratomileusis). However, aberrometry is still not a common diagnostic technique, and for many ophthalmologists interpretation of the results is difficult. The second part of this serial paper reviews findings that are relevant for the ophthalmological community and highlights current scientific applications in this area.

Association between Offset of the Pupil Center from the Corneal Vertex and Wavefront Aberration

Purpose

To investigate the influence of offsets of the pupil center from the corneal vertex on wavefront aberrations in the anterior cornea and the whole eye.

Methods

Both right and left eyes of 103 subjects were measured for the wavefront aberrations in the anterior cornea, along with the offset of the pupil center relative to the corneal vertex, using a Humphrey corneal topographer, and for the wavefront aberration in the whole eye using a WASCA wavefront sensor. Correlations of the pupil center offsets with the Zernike aberrations were tested.

Results

X-axis shift of the pupil center from the corneal vertex was significantly correlated to horizontal coma for both the right (r = 0.54, P<0.0001) and left eyes (r=0.48, P<0.0001) in the cornea, but was weakly correlated to the coma in the whole eye (r=0.17, P=0.04 for OD; and r=0.17, P=0.05 for OS). Significant but weak correlations with the x-axis pupil center shift were also found for several other Zernike aberrations, including the oblique astigmatism, vertical trefoil and secondary astigmatism. Very few Zernike aberrations were significantly correlated to y-axis pupil center shift. Most Zernike aberrations were significantly correlated between the right and left eyes to produce bilateral symmetry in the cornea and the whole eye.

Conclusions

The results suggest that offset of the pupil center from the corneal vertex plays an important role in determining horizontal coma and few other Zernike aberrations. Factors controlling bilateral symmetry of the wavefront aberrations between the two eyes could make important contributions to wavefront aberrations in the human eye.

Use of adaptive optics to determine the optimal ocular spherical aberration

PURPOSE

To explore the impact of spherical aberration (SA) on contrast sensitivity using an adaptive optics vision simulator to determine the optimal amount of SA to include in customized corrections of wavefront aberrations.

SETTING

Laboratorio de Optica, Universidad de Murcia, Murcia, Spain, and AMO Groningen BV, Groningen, The Netherlands.

METHODS

An adaptive optics vision simulator consisting of a wavefront sensor, a 97-segmented deformable mirror to induce and correct aberrations of the eye, and a visual testing path was constructed for this study. The deformable mirror allows the effective ocular wavefront aberration to be manipulated and the resulting visual performance to be measured simultaneously. Subjective measurements of contrast sensitivity at 15 cycles per degree were performed with a 4.8 mm pupil in 5 subjects with different levels of naturally occurring SA. Contrast sensitivity was measured when SA values of -0.09 microm, 0.0 microm, 0.09 microm, and 0.182 microm were induced when the other natural aberrations of the eye were present, when the aberrations were corrected, and at defocus values of +/-0.25 diopter (D) and +/-0.50 D.

RESULTS

Subjects experienced peak contrast sensitivity performance with varying levels of SA when their natural aberrations were present; however, average contrast performance peaked at 0 mum of SA. When all higher-order aberrations were corrected, all 5 subjects' peak performance occurred at 0 microm of SA.

CONCLUSIONS

The adaptive optics vision simulator reduced the root-mean-square wavefront aberration of the eye by up to a factor of 4 and allowed noninvasive testing of the visual performance resulting from any ocular wavefront aberration introduced by customized correction procedures. This study showed that, on average, contrast performance peaked when SA was completely corrected.

Ocular aberrations in barn owl eyes

Optical quality in barn owl eyes is presented in terms of measuring the ocular wavefront aberrations with a standard Tscherning-type wavefront aberrometer under natural viewing conditions. While accommodative state was uncontrolled, all eyes were focused within 0.4D with respect to the plane of the aberrometer. Total RMS wavefront error was between 0.06 and 0.15 microm (mean: 0.10 microm, STD: 0.03 microm, defocus cancelled) for a 6 mm pupil. The results suggest that image quality in barn owl eyes is excellent.

Wave aberrations in rhesus monkeys with vision-induced ametropias

The purpose of this study was to investigate the relationship between refractive errors and high-order aberrations in infant rhesus monkeys. Specifically, we compared the monochromatic wave aberrations measured with a Shack-Hartman wavefront sensor between normal monkeys and monkeys with vision-induced refractive errors. Shortly after birth, both normal monkeys and treated monkeys reared with optically induced defocus or form deprivation showed a decrease in the magnitude of high-order aberrations with age. However, the decrease in aberrations was typically smaller in the treated animals. Thus, at the end of the lens-rearing period, higher than normal amounts of aberrations were observed in treated eyes, both hyperopic and myopic eyes and treated eyes that developed astigmatism, but not spherical ametropias. The total RMS wavefront error increased with the degree of spherical refractive error, but was not correlated with the degree of astigmatism. Both myopic and hyperopic treated eyes showed elevated amounts of coma and trefoil and the degree of trefoil increased with the degree of spherical ametropia. Myopic eyes also exhibited a much higher prevalence of positive spherical aberration than normal or treated hyperopic eyes. Following the onset of unrestricted vision, the amount of high-order aberrations decreased in the treated monkeys that also recovered from the experimentally induced refractive errors. Our results demonstrate that high-order aberrations are influenced by visual experience in young primates and that the increase in high-order aberrations in our treated monkeys appears to be an optical byproduct of the vision-induced alterations in ocular growth that underlie changes in refractive error. The results from our study suggest that the higher amounts of wave aberrations observed in ametropic humans are likely to be a consequence, rather than a cause, of abnormal refractive development.

Optical Quality of the Eye Degraded by Time-Varying Wavefront Aberrations with Tear Film Dynamics

PURPOSE

Wavefront aberrations (WFAs) of the eye vary with time because of the tear film dynamics. We investigated, using a simulation method, the variation of optical quality with time-varying wavefront measurements of 13 eyes with different refractions.

METHODS

WFAs of 13 normal eyes of 13 subjects were measured every second for 10 s. First, we simulated WFAs with conventional corneal laser refractive surgery by subtracting the second-order aberrations of the least aberrated measurement from measured consecutive WFAs. Second, we simulated customized refractive surgery by subtracting the second- to sixth-order aberrations of the least aberrated measurement from measured consecutive WFAs. We calculated Strehl ratios and retinal images from these corrected consecutive WFAs.

RESULTS

In one eye, the root mean square (RMS) values of WFAs with a second-order correction were sometimes smaller than those of WFAs with a second- to sixth-order correction, when these were compared at the same time point after a blink. However, in the other 12 eyes, the RMS values with second- to sixth-order corrections were smaller than those with only a second-order correction. In eight eyes, the Strehl ratios with second- to sixth-order corrections were larger than those with second-order corrections. In the remaining five eyes, Strehl ratios with second- to sixth-order corrections were sometimes smaller than those with second-order corrections.

CONCLUSIONS

In a simulation, the correction of time-invariant higher order aberrations usually reduced RMS values, but it did not always result in higher Strehl ratios than those obtained with only second-order corrections.

Wavefront aberration and its association with intraocular pressure and central corneal thickness in myopic eyes

PURPOSE

To investigate the association between intraocular pressure (IOP), central corneal thickness (CCT), or both and wavefront aberrations in the anterior cornea and the whole eye in myopia.

SETTING

Wenzhou Medical College, Wenzhou, Zhejiang, China.

METHODS

Seventy myopic subjects were tested with a Humphrey corneal topographer for wavefront aberrations in the anterior corneal surface and with a wavefront aberration-supported cornea ablation wavefront analyzer (Complete Ophthalmic Analysis System, Carl Zeiss Meditec) for wavefront aberrations in the whole eye. The IOP and CCT were measured with a noncontact Canon tonometer and a Tomey pachymeter, respectively. Relationships between the wavefront aberrations and the IOP and CCT were analyzed.

RESULTS

Four corneal Zernike aberrations were significantly correlated with the IOP, including defocus (Z4) (r = 0.36, P<.01 for right eye; r = 0.31, P<.05 for left eye), y-axis coma (Z7) (r = 0.32, P<.05 for right eye), spherical aberration (Z12) (r = 0.34, P<.02 for both eyes), and secondary astigmatism (Z13) (r = -0.35, P<.01 for right eye; r = -0.37, P<.01 for left eye). In the whole eye, 3 Zernike aberrations were significantly correlated with IOP: trefoil (Z6) (r = 0.31, P<.05 for right eye), spherical aberration (Z12) (r = 0.34, P<.02 for right eye; r = 0.30, P<.05 for left eye), and secondary astigmatism (Z13) (r = -0.45, P<.01 for right eye; r = -0.30, P<.05 for left eye). However, only a single Zernike aberration (Z11) was significantly correlated with CCT (r = -0.29, P<.05 for right eye).

CONCLUSIONS

Intraocular pressure was associated with wavefront aberrations in both the cornea and the whole eye, and its association was not limited to symmetrical aberrations but included asymmetric higher-order aberrations as well. In contrast, CCT was very weakly associated with wavefront aberrations.

Wide-field schematic eye models with gradient-index lens

We propose a wide-field schematic eye model, which provides a more realistic description of the optical system of the eye in relation to its anatomical structure. The wide-field model incorporates a gradient-index (GRIN) lens, which enables it to fulfill properties of two well-known schematic eye models, namely, Navarro's model for off-axis aberrations and Thibos's chromatic on-axis model (the Indiana eye). These two models are based on extensive experimental data, which makes the derived wide-field eye model also consistent with that data. A mathematical method to construct a GRIN lens with its iso-indicial contours following the optical surfaces of given asphericity is presented. The efficiency of the method is demonstrated with three variants related to different age groups. The role of the GRIN structure in relation to the lens paradox is analyzed. The wide-field model with a GRIN lens can be used as a starting design for the eye inverse problem, i.e., reconstructing the optical structure of the eye from off-axis wavefront measurements. Anatomically more accurate age-dependent optical models of the eye could ultimately help an optical designer to improve wide-field retinal imaging.

Requirements for discrete actuator and segmented wavefront correctors for aberration compensation in two large populations of human eyes

Numerous types of wavefront correctors have been employed in adaptive optics (AO) systems for correcting the ocular wavefront aberration. While all have improved image quality, none have yielded diffraction-limited imaging for large pupils (>/=6 mm), where the aberrations are most severe and the benefit of AO the greatest. To this end, we modeled the performance of discrete actuator, segmented piston-only, and segmented piston/tip/tilt wavefront correctors in conjunction with wavefront aberrations measured on normal human eyes in two large populations. The wavefront error was found to be as large as 53 microm, depending heavily on the pupil diameter (2-7.5 mm) and the particular refractive state. The required actuator number for diffraction-limited imaging was determined for three pupil sizes (4.5, 6, and 7.5 mm), three second-order aberration states, and four imaging wavelengths (0.4, 0.6, 0.8, and 1.0 microm). The number across the pupil varied from only a few actuators in the discrete case to greater than 100 for the piston-only corrector. The results presented will help guide the development of wavefront correctors for the next generation of ophthalmic instrumentation.

Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor

PURPOSE

The purpose of this study was to characterize aberrations in 2 populations of eyes, namely those with keratoconus (KC) and those having undergone penetrating keratoplasty (PK), using a large-dynamic range Shack-Hartmann wavefront sensor.

DESIGN

Prospective comparative case series.

PARTICIPANTS

Twenty-one people with ocular pathologic features (either KC or PK) were recruited for this study. A previously compiled population of 190 people with no pathologic features other than refractive error was used as a means for comparison.

METHODS

Thirty-three abnormal eyes (19 with KC and 14 PK) were measured using a high-dynamic range wavefront sensor, and Zernike coefficients were computed over a 6-mm pupil. The data then were used to characterize the populations by themselves, as well as to compare them with the population of normal eyes.

MAIN OUTCOME MEASURES

Root mean square (RMS) higher-order aberration (HOA), percent of higher-order or total aberration variance, and magnitude of individual Zernike modes (in micrometers). Visual benefit of correcting higher-order aberrations was used when comparing pathologic and normal populations.

RESULTS

The keratoconic eyes exhibited 2.24 microm of HOA RMS on average. Vertical coma accounted for 53+/-32% (mean+/-standard deviation [SD]) of the HOA variance and was the most dominant higher-order aberration. The PK subjects had an average higher-order RMS of 2.25 microm, and trefoil dominated in this population with an average HOA variance contribution of 38+/-23% (mean+/-SD). The KC and PK higher-order aberrations represented 16+/-20% and 16+/-13% (mean+/-SD) of the total aberration variance, whereas the ratio was only 1+/-1% in the normal population. A visual benefit calculation on 15 KC eyes and 14 PK eyes yielded a result of 4.4+/-2.0 and 6.0+/-1.5 (mean+/-SD), respectively, whereas the normal population had a visual benefit of only 2.1+/-0.4.

CONCLUSIONS

Eyes with KC and PK have higher-order aberrations that are approximately 5.5 times more than what is typical in normal eyes. Vertical coma is the dominant higher-order aberration in people with KC, whereas PK eyes are dominated by trefoil, spherical aberration, and coma. Correcting these aberrations may provide substantial improvements in vision beyond what is possible with conventional correction methods.

Effect of aberrations and scatter on image resolution assessed by adaptive optics retinal section imaging

The effect of increased high-order wavefront aberrations on image resolution was investigated, and the performance of adaptive optics (AO) for correcting wavefront error in the presence of increased light scatter was assessed in a model eye. An AO section imaging system provided an oblique view of a model retina and incorporated a wavefront sensor and deformable mirror for measurement and compensation of wavefront aberrations. Image resolution was quantified by the width of a Lorentzian curve fitted to a laser line image. Wavefront aberrations were significantly reduced with AO, resulting in improvement of image resolution. In the model eye, image resolution was degraded with increased high-order wavefront aberrations (horizontal coma and spherical) and improved with AO correction of wavefront error in the presence of increased light scatter. The findings of the current study suggest that AO imaging systems can potentially improve image resolution in aging eyes with increased aberrations and scatter.

Characterizing image quality in a scanning laser ophthalmoscope with differing pinholes and induced scattered light

We quantify the effects on scanning laser ophthalmoscope image quality of controlled amounts of scattered light, confocal pinhole diameter, and age. Optical volumes through the optic nerve head were recorded for a range of pinhole sizes in 12 subjects (19-64 years). The usefulness of various overall metrics in quantifying the changes in fundus image quality is assessed. For registered and averaged images, we calculated signal-to-noise ratio, entropy, and acutance. Entropy was best able to distinguish differing image quality. The optimum confocal pinhole diameter was found to be 50 microm (on the retina), providing improved axial resolution and image quality under all conditions.

Objective refraction from monochromatic wavefront aberrations via Zernike power polynomials

With the increased popularity of wavefront sensor devices there has been significant interest in relating the subjectively measured sphero-cylindrical refractive error to the objectively measured wavefront aberrations. We investigate the applicability of four different measures for objective refraction assessment that are derived from a set of estimated Zernike wavefront coefficients. Two of the measures are based on the second- and higher-order Zernike polynomials while the other two measures are based on the estimated dioptric power representations. For the latter, closed-form expressions for dioptric powers based on the estimated focal length and the wavefront curvature are derived. We call these two new representations the 'refractive Zernike power polynomials' and 'curvature Zernike power polynomials'. Data from 120 eyes from 60 normal subjects were used to assess the correlation between the subjective and objective refractions. The results indicate that the objective sphero-cylindrical refraction calculated from the estimated refractive power map via the Zernike power polynomials is superior to the other considered representations and achieves best correlation with subjective sphero-cylindrical refraction.

High-resolution adaptive optics scanning laser ophthalmoscope with dual deformable mirrors

Adaptive optics scanning laser ophthalmoscopes have been used to produce noninvasive views of the human retina. However, the range of aberration compensation has been limited by the choice of deformable mirror technology. We demonstrate that the use of dual deformable mirrors can effectively compensate large aberrations in the human eye while maintaining the quality of the retinal imagery. We verified experimentally that the use of dual deformable mirrors improved the dynamic range for correction of the wavefront aberrations compared with the use of the micro-electro-mechanical-system mirror alone and improved the quality of the wavefront correction compared with the use of the bimorph mirror alone. We also demonstrated that the large-stroke bimorph deformable mirror improved the capability for axial sectioning with the confocal imaging system by providing an easier way to move the focus axially through different layers of the retina.

The effect of dioptric blur on reading performance

Little is known about the systematic impact of blur on reading performance. The purpose of this study was to quantify the effect of dioptric blur on reading performance in a group of normally sighted young adults. We measured monocular reading performance and visual acuity for 19 observers with normal vision, for five levels of optical blur (no-blur, 0.5, 1, 2, and 3D). Dioptric blur was induced using convex trial lenses placed in front of the testing eye, with the pupil dilated and in the presence of a 3mm artificial pupil. Reading performance was assessed using eight versions of the MNREAD Acuity Chart. For each level of dioptric blur, observers read aloud sentences on one of these charts, from large to small print. Reading time for each sentence and the number of errors made were recorded and converted to reading speed in words per minute. Visual acuity was measured using 4-orientation Landolt C stimuli. For all levels of dioptric blur, reading speed increased with print size up to a certain print size and then remained constant at the maximum reading speed. By fitting nonlinear mixed-effects models, we found that the maximum reading speed was minimally affected by blur up to 2D, but was approximately 23% slower for 3D of blur. When the amount of blur increased from 0 (no-blur) to 3D, the threshold print size (print size corresponded to 80% of the maximum reading speed) increased from 0.01 to 0.88 logMAR, reading acuity worsened from -0.16 to 0.58 logMAR, and visual acuity worsened from -0.19 to 0.64 logMAR. The similar rates of change with blur for threshold print size, reading acuity and visual acuity implicates that visual acuity is a good predictor of threshold print size and reading acuity. Like visual acuity, reading performance is susceptible to the degrading effect of optical blur. For increasing amount of blur, larger print sizes are required to attain the maximum reading speed.

Theoretical Comparison of Aberration-correcting Customized and Aspheric Intraocular Lenses

PURPOSE

To assess the performance and optical limitations of standard, aspheric, and wavefront-customized intraocular lenses (IOLs) using clinically verified pseudophakic eye models.

METHODS

White light pseudophakic eye models were constructed from physical measurements performed on 46 individual cataract patients and subsequently verified using the clinically measured contrast sensitivity function (CSF) and wavefront aberration of pseudophakic patients implanted with two different types of IOLs. These models are then used to design IOLs that correct the astigmatism and higher order aberrations of each individual eye model's cornea and to investigate how this correction would affect visual benefit, subjective tolerance to lens misalignment (tilt, decentration, and rotation), and depth of field.

RESULTS

Physiological eye models and clinical outcomes show similar levels of higher order aberration and contrast improvement. Customized correction of ocular wavefront aberrations with an IOL results in contrast improvements on the order of 200% over the control and the Tecnis IOLs. The customized lenses can be, on average, decentered by as much as 0.8 mm, tilted > 10 degrees , and rotated as much as 15 degrees before their polychromatic modulation transfer function at 8 cycles/degree is less than that of the Tecnis or spherical control lens. Correction of wavefront aberration results in a narrower through focus curve but better out of focus performance for +/- 0.50 diopters.

CONCLUSIONS

The use of realistic eye models that include higher order aberrations and chromatic aberrations are important when determining the impact of new IOL designs. Customized IOLs show the potential to improve visual performance.

Effect of third-order aberrations on dynamic accommodation

We investigate the potential for the third-order aberrations coma and trefoil to provide a signed cue to accommodation. It is first demonstrated theoretically (with some assumptions) that the point spread function is insensitive to the sign of spherical defocus in the presence of odd-order aberrations. In an experimental investigation, the accommodation response to a sinusoidal change in vergence (1-3D, 0.2Hz) of a monochromatic stimulus was obtained with a dynamic infrared optometer. Measurements were obtained in 10 young visually normal individuals with and without custom contact lenses that induced low and high values of r.m.s. trefoil (0.25, 1.03 microm) and coma (0.34, 0.94 microm). Despite variation between subjects, we did not find any statistically significant increase or decrease in the accommodative gain for low levels of trefoil and coma, although effects approached or reached significance for the high levels of trefoil and coma. Theoretical and experimental results indicate that the presence of Zernike third-order aberrations on the eye does not seem to play a crucial role in the dynamics of the accommodation response.

Potential Higher-Order Aberration Cues for Sphero-Cylindrical Refractive Error Development

PURPOSE

To investigate analytically whether higher-order wavefront errors comprising combinations of trefoil along 30 degrees (trefoil30), vertical coma, and spherical aberration could provide cues to sphero-cylindrical refractive error development.

METHODS

A total of 25 test wavefronts, subdivided into five different types and five levels of higher-order root mean square errors (HO-RMS), were created for the study. One type contained spherical aberration only, producing HO-RMS levels between 0.1 and 0.5 microm. Four wavefront types contained coma, trefoil, and spherical aberration of various sign combinations also producing HO-RMS levels between 0.1 and 0.5 microm. From the 25 wavefronts, refractive power maps were created and 2025 different sphero-cylindrical combinations were added to each refractive power map. For each sphero-cylinder combination, the visual Strehl ratio based on the modulation transfer function (VSMTF) was calculated. Retinal images and refractive power histograms were calculated for the refractive power maps corresponding to the peak of the VSMTF.

RESULTS

Spherical aberration affected the best focal plane thereby inducing spherical or defocus cues. The VSMTF produced by vertical coma and trefoil30, in combination with spherical aberration, could be improved with sphero-cylinders of various magnitudes and directions (i.e., with-the-rule, against-the rule, myopic astigmatism, or hyperopic astigmatism). Clinical significance of sphero-cylinders (i.e., >or=0.25 D) was reached at HO-RMS levels between 0.2 and 0.3 microm for a 5-mm pupil zone.

CONCLUSIONS

In the context of compensatory blur driven eye growth, commonly occurring combinations of the three considered higher-order aberrations have the potential to produce cues to eye growth resulting in myopia and with-the-rule astigmatism.

Long-term optical quality of the photoablated cornea

High-order coreal wavefront analysis was performed in a population of 60 myopic eyes that underwent photorefractive keratectomy. Corneal aberration data over 3, 5, and 7 mm pupils were collected for up to three years after surgery. The optical performance of the anterior cornea was characterized by estimation of the modulation transfer function (MTF) and the point-spread function. The high-order corneal wavefront aberrations were shown to stabilize one year after surgery. Over photopic pupils, after an early slight increase, corneal RMS-high-order aberrations (HOA) tended to decrease toward preoperative values. On the other hand, over mid- and large-pupil sizes, corneal HOA significantly increased compared with the preoperative state, while the optical performance of the cornea was diminished. The MTF ratio showed a distinct decline in the optical quality of postoperative corneas at low and middle spatial frequencies over larger pupils in the range between 6 and 19 c/deg, especially for deeper ablations.

Three-dimensional relationship between high-order root-mean-square wavefront error, pupil diameter, and aging

We report root-mean-square (RMS) wavefront error (WFE) for individual aberrations and cumulative high-order (HO) RMS WFE for the normal human eye as a function of age by decade and pupil diameter in 1 mm steps from 3 to 7 mm and determine the relationship among HO RMS WFE, mean age for each decade of life, and luminance for physiologic pupil diameters. Subjects included 146 healthy individuals from 20 to 80 years of age. Ocular aberration was measured on the preferred eye of each subject (for a total of 146 eyes through dilated pupils; computed for 3, 4, 5, 6, and 7 mm pupils; and described with a tenth-radial-order normalized Zernike expansion. We found that HO RMS WFE increases faster with increasing pupil diameter for any given age and pupil diameter than it does with increasing age alone. A planar function accounts for 99% of the variance in the 3-D space defined by mean log HO RMS WFE, mean age for each decade of life, and pupil diameter. When physiologic pupil diameters are used to estimate HO RMS WFE as a function of luminance and age, at low luminance (9 cd/m2) HO RMS WFE decreases with increasing age. This normative data set details (1) the 3-D relationship between HO RMS WFE and age for fixed pupil diameters and (2) the 3-D relationship among HO RMS WFE, age, and luminance for physiologic pupil diameters.

[Distribution and enantiomorphism of higher-order ocular optical aberrations]

PURPOSE

To characterize and investigate the higher-order optical aberrations (HOAs) and their distribution and symmetry between both eyes (enantiomorphism).

PATIENTS AND METHODS

Wave-front measurements were taken using Hartmann-Shack aberrometry and Zernike polynomial terms reconstruction of the total ocular wave front. Optical aberrations were quantitatively analyzed using the RMS (root mean square) Zernike coefficient term value. Symmetry between both eyes was assessed for each aberration according to two criteria: the RMS coefficient value and aberration magnitude.

RESULTS

Sixty patients were included in the study. The HOAs presenting the highest magnitude were fourth-order spherical aberration (Z40; M=0.3038 microm), primary coma (Z31; magnitude M=0.2285 microm) and trefoil (Z33; M=0.1870 microm). A significant mirror symmetry between both eyes was present for high-order aberration orientations. Considering the magnitude of each aberration, symmetry was present in seven of eight aberrations (all but primary coma). HOAs were higher for ametropes than emmetropes. The hyperoptic eyes had more spherical aberrations than myopic and emmetropic eyes.

CONCLUSION

Symmetry between the right and left eye is a less common characteristic of HOAs. The difference in the mean age of the studied population may account for the differences in the total high-order and spherical aberration levels found in our study.

Fluctuations in intraocular pressure and the potential effect on aberrations of the eye

OBJECTIVE

To investigate the fluctuations in intraocular pressure during the day and to see if these are associated with changes in corneal shape and in the patterns of ocular aberrations.

METHODS

Intraocular pressure, corneal curvature, refractive error, spherical equivalent and aberrations (defocus (sphere); cylinder (astigmatism); coma, trefoil and third order spherical aberration) were measured in 17 healthy subjects three times during the day. The first measurement was made between 9:00 and 9:30, the second at midday (12:30-13:00) and the third in the afternoon (17:00-17:30). Aberrations, corneal shape, refractive error and pupil size (for which correction was made) were measured with an Irx3 Dynamic Wavefront Aberrometer. Intraocular pressures were measured using a non-contact tonometer (Cambridge Instruments Inc.) and calibrated with the Goldmann applanation tonometer.

RESULTS

Variations in intraocular pressures were unrelated to age or refractive error. Statistically significant differences in intraocular pressure between morning and midday as well as between midday and afternoon were found. Intraocular pressure variations between midday and afternoon were associated with changes in spherical equivalent, corneal radius of curvature and aberrations (defocus, cylinder, coma, trefoil and spherical aberration) over the same time period. Aberration patterns varied between individuals, and no association was found between two eyes of the same subject.

CONCLUSIONS

Changes in intraocular pressure have no noticeable effect on image quality. This could be because the eye has a compensating mechanism to correct for any effect of ocular dynamics on corneal shape and refractive status. Such a mechanism may also affect the pattern of aberrations or it may be that aberrations alter in a way that offsets any potentially detrimental effects of intraocular pressure change on the retinal image. Variations in patterns of aberrations and how they may be related to ocular dynamics need to be investigated further before attempts at correction are made.

Optimizing higher-order aberrations with intraocular lens technology

PURPOSE OF REVIEW

Aspheric intraocular lens technology has been implemented during the past 5 years, and more and more intraocular lenses with different amounts of asphericity are becoming available. Despite the efficacy in the correction of spherical aberration and the good results on implanted eyes, the theoretical advantages of aspheric intraocular lenses are still controversial.

RECENT FINDINGS

All investigations showed the ability of the aspheric intraocular lenses to correct the positive spherical aberration of the cornea, with variable impact on the total eye wavefront but with constant advantages in the optical quality of the eyes as measured by the Modulation Transfer Function, the Point Spread Function, and the contrast sensitivity of implanted patients. Theoretical studies on model eyes underlined some possible limitations of aspheric intraocular lenses, especially sensitivity to decentration. In addition, the actual optical quality in implanted eyes is also affected by light scattering, a parameter missed by simple aberration analysis.

SUMMARY

Aspheric intraocular lenses effectively reduce spherical aberration in implanted eyes, with improvement in optical quality over the parent spherical intraocular lens. The advantages for implanted eyes could be limited by decentration, by small pupil diameter, and by reduced media transparency.

Incremental retinal-defocus theory of myopia development--schematic analysis and computer simulation

Previous theories of myopia development involved subtle and complex processes such as the sensing and analyzing of chromatic aberration, spherical aberration, spatial gradient of blur, or spatial frequency content of the retinal image, but they have not been able to explain satisfactorily the diverse experimental results reported in the literature. On the other hand, our newly proposed incremental retinal-defocus theory (IRDT) has been able to explain all of these results. This theory is based on a relatively simple and direct mechanism for the regulation of ocular growth. It states that a time-averaged decrease in retinal-image defocus area decreases the rate of release of retinal neuromodulators, which decreases the rate of retinal proteoglycan synthesis with an associated decrease in scleral structural integrity. This increases the rate of scleral growth, and in turn the eye's axial length, which leads to myopia. Our schematic analysis has provided a clear explanation for the eye's ability to grow in the appropriate direction under a wide range of experimental conditions. In addition, the theory has been able to explain how repeated cycles of nearwork-induced transient myopia leads to repeated periods of decreased retinal-image defocus, whose cumulative effect over an extended period of time results in an increase in axial growth that leads to permanent myopia. Thus, this unifying theory forms the basis for understanding the underlying retinal and scleral mechanisms of myopia development.

Recent developments in optical coherence tomography for imaging the retina

Optical coherence tomography (OCT) was introduced in ophthalmology a decade ago. Within a few years in vivo imaging of the healthy retina and optic nerve head and of retinal diseases was a fact. In particular the ease with which these images can be acquired considerably changed the diagnostic strategy used by ophthalmologists. The OCT technique currently available in clinical practice is referred to as time-domain OCT, because the depth information of the retina is acquired as a sequence of samples, over time. This can be done either in longitudinal cross-sections perpendicular to, or in the coronal plane parallel to the retinal surface. Only recently, major advances have been made as to image resolution with the introduction of ultrahigh resolution OCT and in imaging speed, signal-to-noise ratio and sensitivity with the introduction of spectral-domain OCT. Functional OCT is the next frontier in OCT imaging. For example, polarization-sensitive OCT uses the birefringent characteristics of the retinal nerve fibre layer to better assess its thickness. Blood flow information from retinal vessels as well as the oxygenation state of retinal tissue can be extracted from the OCT signal. Very promising are the developments in contrast-enhanced molecular optical imaging, for example with the use of scattering tuneable nanoparticles targeted at specific tissue or cell structures. This review will provide an overview of these most recent developments in the field of OCT imaging focussing on applications for the retina.

Normal-eye Zernike coefficients and root-mean-square wavefront errors

PURPOSE

To compare aberrometry measurements from multiple sites and compute mean Zernike coefficients and root-mean-square (RMS) values for the entire data pool to serve as a reference set for normal, healthy adult eyes.

SETTING

Northeastern State University, Tahlequah, Oklahoma, USA.

METHODS

Data were collected from 10 laboratories that measured higher-order aberrations (HOAs) in normal, healthy adult eyes using Shack-Hartmann aberrometry (2560 eyes of 1433 subjects). Signed Zernike coefficients were scaled to pupil diameters of 6.0 mm, 5.0 mm, 4.0 mm, and 3.0 mm and corrected to a common wavelength of 550 nm. The mean signed and absolute Zernike coefficients across data sets were compared. Then, the following were computed: overall mean values for signed and absolute Zernike coefficients; polar Zernike magnitudes and RMS values for coma-like aberrations (Z(3)(+/-1) and Z(5)(+/-1) combined); spherical-like aberrations (Z(4)(0) and Z(6)(0) combined); and 3rd-, 4th-, 5th-, and 6th-order, and higher-order aberrations (orders 3 to 6).

RESULTS

The different data sets showed good agreement for Zernike coefficients values across most higher-order modes, with greater variability for Z(4)(0) and Z(3)(-1). The most prominent modes and their mean absolute values (6.0-mm pupil) were, respectively, Z(3)(-1) and 0.14 microm, Z(4)(0) and 0.13 microm, and Z(3)(-3) and 0.11 microm. The mean total higher-order RMS was 0.33 microm.

CONCLUSIONS

There was a general consensus for the magnitude of HOAs expected in normal adult human eyes. At least 90% of the sample had aberrations less than double the mean values reported here. These values can serve as a set of reference norms.

Topographic- and Wavefront-guided Customized Ablations With the NIDEK-EC5000CXII in LASIK for Myopia

PURPOSE

To assess refractive outcomes, changes in the total higher order root mean square (RMS) aberration, and changes in higher order wavefront aberrations after LASIK for myopia and myopic astigmatism with the NIDEK Advanced Vision Excimer Laser platform (NAVEX) using either an aspheric or topography-based or whole eye wavefront ablation algorithm.

METHODS

This was a retrospective study of 1459 eyes that underwent LASIK for myopia and myopic astigmatism. The mean preoperative spherical equivalent refraction was -4.68 diopters (D) (range: -0.50 to -9.63 D) with astigmatism up to -4.50 D. Treatments were classified into three categories depending on the type of ablation algorithm used--optimized aspheric transition zone (OATz) denoted eyes that underwent aspheric treatment zones; customized aspheric treatment zone (CATz) denoted eyes that underwent customized ablations based on corneal topography; and OPDCAT denoted eyes that underwent customized ablation based on the whole eye wavefront profile. Follow-up data are reported at 3 months (69%) and 12 months (17%) postoperatively.

RESULTS

Three months after LASIK, the predictability (10.5 D from target refraction) was 80% for OATz, 91% for CATz, and 76% for OPDCAT. Of all eyes, 96% were within +/- 1.0 D of intended refraction 3 months postoperatively and 100% after 12 months (87% +/- 0.5 D). In the aspheric and custom groups, a notable improvement of uncorrected visual acuity was noted between 3 and 12 months after LASIK. No eye lost >1 line of best spectacle-corrected visual acuity. Mean higher order RMS increased in all groups.

CONCLUSIONS

The data support that the treatment of myopia and myopic astigmatism is safe and effective with NAVEX. Customized ablation based on corneal topography rather than on total wavefront error was more predictable.

Principles of modern low vision rehabilitation

Low vision rehabilitation is a new emerging subspecialty drawing from the traditional fields of ophthalmology, optometry, occupational therapy, and sociology, with an ever-increasing impact on our customary concepts of research, education, and services for the visually impaired patient. A multidisciplinary approach and coordinated effort are necessary to take advantage of new scientific advances and achieve optimal results for the patient. Accordingly, the intent of this paper is to outline the principles and details of a modern low vision rehabilitation service. All rehabilitation attempts must start with a first hand interview (the intake) for assessing functionality and priority tasks for rehabilitation, as well as assessing the patient's all-important cognitive skills. The assessment of residual visual functions follows the intake and offers a unique opportunity to measure, evaluate, and document accurately the extent of functional loss sustained by the patient from disease. An accurate assessment of residual visual functions includes assessment of visual acuity, contrast sensitivity, binocularity, refractive errors, perimetry, oculomotor functions, cortical visual integration, and light characteristics affecting visual functions. Functional vision assessment in low vision rehabilitation measures how well one uses residual visual functions to perform routine tasks, using different items under various conditions, throughout the day. Of the many functional vision skills known, reading skills is an obligatory item for all low vision rehabilitation assessments. Results of assessment guide rehabilitation professionals in developing rehabilitation plans for the individual and recommending appropriate low vision devices. The outcome from assessing residual visual functions is detection of visual functions that can be improved with the use of optical devices. Methods for prescribing devices such as image relocation with prisms to a preferred retinal locus, field displacement to primary gaze position, field expansion, and manipulation of light are practiced today in addition to, or instead of, magnification. Correction of refractive errors, occlusion therapy, enhancement of oculomotor skills, and field restitution are additional methods now available for prescribing devices leading to rehabilitation of visual functions. The outcome from assessing residual functional vision is detection of functional vision that can be improved with the use of vision therapy training. After restoration of optimal residual visual functions is achieved with optical devices, one can follow with training programs for restoration of lost vision-related skills. If an optical dispensary is available where prescribing of low vision devices routinely take place, this will help ensure familiarity and specialization of the dispensary and staff with low vision devices and their special dispensing requirements. The dispensing of low vision devices is an opportunity to introduce the device to the patient, train the patient in the correct use of the device for the task selected, and create a direct and continuous connection with the patient until the next encounter. Following assessment, prescribing, and dispensing of devices, a low vision practitioner, ophthalmologist or optometrist, is responsible for recommending and prescribing vision therapy training to improve residual functional vision. An attempt to present a template for a comprehensive modern low vision rehabilitation practice is made here by summarizing scientific developments in the field and stressing the multidisciplinary involvement required for this kind of practice. It is hoped that this paper and other initiatives from colleagues, the public, and government will promote and raise awareness of modern low vision rehabilitation for the benefit of all.

Einfluss des Alters auf die optischen Aberrationen des menschlichen Auges

BACKGROUND

Currently the influence of age on corneal and ocular aberrations is still insufficiently known. The aim of this clinical study was to compare age-related aberrations of human eyes.

METHODS

In a prospective study 98 eyes of 49 healthy patients ranging from 17 to 65 years of age (38.6+/-10.0 years) were consecutively examined. The best corrected visual acuity ranged from 0.8 to 1.6; 48 eyes were emmetropic (SE+/-0.5 D), 42 eyes myopic (SE <-0.5 to -6.75 D), and 8 eyes hyperopic (SE >+0.5 to +3.88 D). The corneal aberrations were derived from corneal topography (Keratron Scout, Optikon). The measurement of ocular aberrations was performed with a Tscherning wavefront aberrometer (ORK, Schwind). The aberrations of the Zernike coefficients and RMS values (1st to 4th order) were determined.

RESULTS

The mean corneal and ocular Zernike coefficients of higher order were smaller than 0.2 microm. There was an evident decrease of wavefront aberrations with increasing order. Higher order corneal aberrations were larger than the corresponding ocular aberrations. With increasing age higher optical errors increased in complexity, and the correlation of corneal and ocular aberrations decreased with significant differences. Although the corneal ocular RMS value of the 3rd and 4th order correlated in the younger group (r=0.51, p=0.0001), there was no correlation in the older group (r=-0.48, p=0.832). The influence of age caused a significant increase of ocular aberrations of the 3rd and 4th order, in particular a tenfold extension of coma (C07) (p=0.002), a twofold extension of spherical aberration (C12) (p=0.0001), and an increase of the 3rd and 4th order RMS values (p=0.001).

CONCLUSIONS

Increased age induced an increase in optical aberrations of the eye, which demonstrates the influence of the lens on ocular aberrations. The combination of corneal and ocular diagnostic methods is recommendable for a better understanding of visual performance.

Ocular aberrations after cataract surgery with hydrophobic and hydrophilic acrylic intraocular lenses

PURPOSE

To compare tilt and some higher-order aberrations (HOAs) in patients who had cataract surgery and implantation of 2 types of acrylic intraocular lens (IOL), the hydrophobic MA60AC (Alcon) or the hydrophilic XLSTABI (Ioltech).

SETTING

Fondation Ophtalmologique Adolphe de Rothschild, Department of Pr Hoang-Xuan, Paris, France.

METHODS

Sixty eyes were included in this study. Eyes in group 1 (n = 30) received the acrylic hydrophobic MA60AC IOL, and eyes in group 2 (n = 30) received the acrylic hydrophilic XLSTABI IOL. Optical aberrations were measured with Nidek OPD-Scan at least 1 month after cataract surgery for a 6.0 mm pupil diameter using a 6th-order Zernike polynomials decomposition.

RESULTS

In group 1, the mean root-mean-square (RMS) coefficient for the total higher aberration was 0.86 microm +/- 0.38 (SD). The mean RMS of the tilt, coma, trefoil, tetrafoil, spherical aberration, and secondary astigmatism was 0.81 +/- 0.45 microm, 0.38 +/- 0.30 microm, 0.55 +/- 0.18 microm, 0.13 +/- 0.23 microm, 0.30 +/- 0.13 microm, and 0.17 +/- 0.21 microm, respectively. In group 2, the mean RMS coefficient for the total higher aberration was 0.72 +/- 0.31 microm. The mean RMS of the tilt, coma, trefoil, tetrafoil, spherical aberration, and secondary astigmatism was 0.55 +/- 0.30 microm, 0.24 +/- 0.19 microm, 0.50 +/- 0.23 microm, 0.07 +/- 0.22 microm, 0.32 +/- 0.13 microm, and 0.14 +/- 0.14 microm, respectively. There was a statistical difference in aberrations between the 2 groups for the tilt and the coma aberrations (P<.05). There were more aberrations in the MA60AC IOL group.

CONCLUSIONS

The design of the IOL influenced ocular aberrations after cataract surgery. There were more tilt and coma aberrations in the MA60AC IOL group.

Monochromatic ocular wave aberrations in young monkeys

High-order monochromatic aberrations could potentially influence vision-dependent refractive development in a variety of ways. As a first step in understanding the effects of wave aberration on refractive development, we characterized the maturational changes that take place in the high-order aberrations of infant rhesus monkey eyes. Specifically, we compared the monochromatic wave aberrations of infant and adolescent animals and measured the longitudinal changes in the high-order aberrations of infant monkeys during the early period when emmetropization takes place. Our main findings were that (1) adolescent monkey eyes have excellent optical quality, exhibiting total RMS errors that were slightly better than those for adult human eyes that have the same numerical aperture and (2) shortly after birth, infant rhesus monkeys exhibited relatively larger magnitudes of high-order aberrations predominately spherical aberration, coma, and trefoil, which decreased rapidly to assume adolescent values by about 200 days of age. The results demonstrate that rhesus monkey eyes are a good model for studying the contribution of individual ocular components to the eye's overall aberration structure, the mechanisms responsible for the improvements in optical quality that occur during early ocular development, and the effects of high-order aberrations on ocular growth and emmetropization.

The Charles F. Prentice Award Lecture 2005: Optics of the Human Eye: Progress and Problems

The history of measurements of ocular aberration is briefly reviewed and recent work using much-improved aberrometers and large samples of eyes is summarized. When on-axis, higher-order, monochromatic aberrations are averaged, undercorrected, positive, fourth-order spherical aberration dominates; other Zernike wavefront aberration coefficients have average values near zero. Individually, however, many eyes show substantial amounts of third-order and other fourth-order aberrations; the value of these varies idiosyncratically about zero. Most normal eyes show only small amounts of axial monochromatic aberration for photopic pupils up to around 3 mm; the limits to retinal image quality are then usually set by diffraction, uncorrected or imperfectly corrected spherocylindrical refractive error, accommodation error, and chromatic aberration. Longitudinal chromatic aberration varies very little across the population. With larger mesopic and scotopic pupils, monochromatic aberration plays a more important optical role, but overall visual performance is increasingly dominated by neural factors. Some remaining problems in measuring and modeling the eye's optical performance are discussed.

Effects of interactions among wave aberrations on optical image quality

Wave aberrations degrade the optical quality of the eye relative to the diffraction limit, but there are situations in which having slightly aberrated optics can provide some relative visual benefits. This fact led us to consider whether interactions among aberrations in the eye's wavefront produce an advantage for image quality relative to wavefronts with randomized combinations of aberrations with the same total RMS error. Total ocular wave aberrations from two experimental groups and corneal wave aberrations from one group were measured and expressed as Zernike polynomial expansions through the seventh-order. In a series of Monte Carlo simulations, modulation transfer functions (MTFs) for the measured wave aberrations were compared to distributions of artificial MTFs for wavefronts created by randomizing the sign or orientation of the aberrations, while maintaining the RMS error within each Zernike order. In a control condition, "synthetic" model eyes were produced by choosing each individual aberration term at random from individuals in the experimental group, and again MTFs were compared for original and randomized signs. Results were summarized by the MTF ratio: real MTF/mean simulated MTF, as a function of spatial frequency. For a 6mm pupil, the mean MTF ratio for total ocular aberrations was greater than 1.0 up to 60 cycles per degree, suggesting that the eye's aberrations are not independent and that there may be a positive functional consequences to their interrelations. This positive relation did not hold for corneal aberrations alone, or for the synthetic eyes.

Perception of very small visual stimuli in the fovea: normative data for the Rarebit Foveal Test

BACKGROUND

Conventional visual tests are not sensitive enough to detect low degree neural damage, as 6/6 (1.0) visual acuity can be upheld with less than two-thirds of the normal number of optic nerve axons. The aim of the current study was to evaluate the physiologic properties of a new computerised test, the Rarebit Fovea Test (RFT), using very small stimuli, by quantifying the effect of age and binocular summation in relation to stimulus luminance.

METHODS

The RFT relies on the perception of very small (less than 0.5 minutes of arc) bright stimuli. Two different experiments were performed. 1. Thirty-five subjects (age 19 to 63 years) were tested with five different stimulus luminances, 158, 64, 53, 41 and 33 cd/m(2). 2. Nineteen subjects (age 19 to 63 years) were tested using binocular stimulation to define the binocular summation.

RESULTS

Significantly reduced median hit rates were observed at luminances of 53 cd/m(2) or below. Age and mean hit rate correlated negatively at all luminance levels below 158 cd/m(2). The mean hit rate from binocular stimulation, compared to the highest value from monocular stimulation in the same subject, was increased by a factor of 1.54 +/- 0.45 (SD). No age effect was found regarding binocular summation.

CONCLUSIONS

The results in the current study indicate that RFT can identify some of the well-known features of the visual system, that is, the effects of age and binocular summation, provided that the stimulus luminance is adequately selected.

Interocular high-order corneal wavefront aberration symmetry

The interocular symmetry of the high-order corneal wavefront aberration (WA) in a population of myopic eyes was analyzed before and after photorefractive keratectomy (PRK). The preoperative and one-year postoperative corneal aberration data (from third to seventh Zernike orders) for 4- and 7-mm pupils from right and left eyes were averaged after correcting for the effects of enantiomorphism to test for mirror symmetry. Also, the mean corneal point-spread function (PSF) for right and left eyes was calculated. Preoperatively, a moderate and high degree of correlation in the high-order corneal WA between eyes was found for 4- and 7-mm pupils, respectively. Myopic PRK did not significantly change the interocular symmetry of corneal high-order aberrations. No discernible differences in the orientation PSF between eyes were observed one year after surgery in comparison with the preoperative state over the two analyzed pupils.