Download Variations in image optical quality of the eye and the sampling limit

ARTICLE
Variations in image optical quality of the eye
and the sampling limit of resolution of the cone
mosaic with axial length in young adults
Marco Lombardo, MD, PhD, Sebastiano Serrao, MD, PhD,
Pietro Ducoli, MD, Giuseppe Lombardo, MEng, PhD
PURPOSE: To evaluate the variation in higher-order ocular wavefront aberrations and the Nyquist
limit of resolution of the cone mosaic (Nc) in a population of young healthy subjects and the relation
to axial length (AL).
SETTING: Fondazione G.B. Bietti IRCCS, Rome, Italy.
DESIGN: Case series.
METHODS: An adaptive optics retinal camera prototype (rtx1) was used to image the cone mosaic.
Cone density and Nc were calculated at fixed eccentricity between 260 mm and 600 mm from the
foveal center. Ocular higher-order wavefront aberrations were measured using the OPD Scan II
device. The coefficient of variation (CoV) was used to analyze the variation in optical and retinal
parameters. The correlation of optical and retinal parameters with AL was performed using
Pearson analysis.
RESULTS: Twelve subjects (age 24 to 38 years; AL 22.61 to 26.63 mm) were evaluated. A high
interindividual variation in the higher-order wavefront aberrations was found, ranging from 26%
for corneal higher-order aberrations (HOAs) to 41% for intraocular HOAs. The CoV of cone
density and Nc were 16% and 5%, respectively. The decline in cone density and Nc with AL was
statistically significant at all retinal eccentricities (R2 > 0.44, P<.001).
CONCLUSIONS: Although there appeared to be random variation in the eye’s optical wavefront
aberration from subject to subject, the cone-packing density and Nc were highly correlated with
AL. Although the eye’s overall image optical quality in the emmetropic group and the myopic
group was comparable, the spatial sampling of the cone mosaic decreased with increasing AL.
Financial Disclosure: No author has a financial or proprietary interest in any material or method
mentioned.
J Cataract Refract Surg 2012; 38:1147–1155 Q 2012 ASCRS and ESCRS
Supplemental material available at www.jcrsjournal.org.
Individual visual performance is the result of equilibrium between the ability of the eye to focus a detail
of the external world on the retina and the sampling
frequency of the visual system. It has long been known
that the major limit to obtaining sharp images of the
external world is the optical system of the eye.1,2 The
foveal cone mosaic has been estimated to resolve details up to 80 cycles per degree (cpd), corresponding
to a visual acuity of 20/10,3–6 although with high interindividual differences. This variation in visual acuity
is not surprising because of the inherent differences
in foveal cone-packing density and the location of
Q 2012 ASCRS and ESCRS
Published by Elsevier Inc.
the steady fixation region between individuals.7–12
Studies13–15 have also shown that the receptive fields
of the retinal ganglion cells limit spatial resolution outside the foveal pit, adding to the debate on whether
there is a need to develop improved models of the
human eye's visual performance.
Even if the lower-order aberrations (LOAs) of the
eye can be corrected with optical treatments or laser
surgery, higher-order aberrations (HOAs) continue
to impose constraints on the sharpness and intensity
of the images of the external world projected onto
the retina. Nevertheless, people are not aware of the
0886-3350/$ - see front matter
doi:10.1016/j.jcrs.2012.02.033
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VARIATIONS IN THE EYE’S IMAGE OPTICAL QUALITY
limits imposed by their optical aberrations because
most visual tasks in our surrounding world are limited
to seeing details in the range of low frequencies
(between 5 cpd and 19 cpd). In general, the highest
frequencies detectable by normal subjects viewing
natural scenes are distributed between 20 cpd and
32 cpd.16
With the advent of adaptive optics technology in
vision science, it is now feasible to understand the relationship between optical quality and the sampling
limit of resolution of the cone mosaic in the same
subject. The in vivo analysis of the spatial distribution
of cone photoreceptors can provide new information
on the physical aspects of visual sampling. Researchers can now directly compare the optical and
retinal performance in the same eye and further understand the basis of the variation in visual performance
in the normal population.
Previous studies determined the photoreceptor cone
density9–12 and the optical quality16–22 in healthy emmetropic and myopic eyes separately. In this study,
we measured the interindividual and intrasubject
variations in the optical quality of the eye and the
sampling limit of resolution of the cone mosaic in
a population of young adults. We sought to determine
whether the optical and retinal parameters of the eye
are correlated with the eye's axial length (AL).
SUBJECTS AND METHODS
Healthy volunteers participated in this study and signed an
informed consent after receiving a full explanation of the
procedure. All subjects were free of ocular or systemic disease, had 20/20 or better uncorrected or corrected distance
visual acuity, and a scotopic pupil diameter of 6.0 mm or
larger in both eyes. The study protocol had the approval of
the local ethical committee and adhered to the tenets of the
Declaration of Helsinki.
All subjects received a complete eye examination including noncontact ocular biometry (IOLMaster, Carl Zeiss
Meditec AG), retinal imaging by scanning laser ophthalmoscopy–optical coherence tomography (Spectralis,
Heidelberg Engineering GmbH), and ocular and corneal
aberrometry (OPD-Scan II, Nidek Co., Ltd.; wavelength of
light source centered at 808 nm).
Submitted: November 30, 2011.
Final revision submitted: February 4, 2012.
Accepted: February 8, 2012.
From Fondazione G.B. Bietti IRCCS (M. Lombardo, Serrao, Ducoli),
Vision Engineering (G. Lombardo), Rome, and CNR-IPCF Unit
of Support Cosenza, LiCryL Laboratory, University of Calabria,
(G. Lombardo), Rende, Italy.
Corresponding author: Marco Lombardo, MD, PhD, Fondazione
G.B. Bietti IRCCS, Via Livenza 3, 00198 Rome, Italy. E-mail:
[email protected].
An adaptive optics retinal camera prototype (rtx1,
Imagine Eyes) was used to evaluate the cone mosaic. The
apparatus' core components include a wavefront sensor
(HASO 32-eye, Imagine Eyes), a correcting element (MIRAO
52-e, Imagine Eyes), and a low-noise, high-resolution,
charge-coupled device camera (Rope Scientific). The adaptive optics system uses a low-coherence superluminescent
diode centered at 750 nm to measure and correct optical distortions and simultaneously control the focus at the retinal
layers. The second source (a light-emitting diode with wavelength centered at 850 nm) provides uniform illumination of
the retinal field to be imaged (4 degree 4 degree field).
Adaptive optics imaging sessions were performed after
the pupils were dilated with 1 drop of tropicamide 0.5%.
Both eyes of each subject were imaged; a video (40 frames)
was recorded at various locations surrounding fixation.
During adaptive optics imaging, fixation was maintained
by having the patient fixate on a yellow cross that the investigator moved at fixed retinal locations.
After image acquisition, a manufacturer-provided program was used to correlate and average the captured image
frames to reduce noise artifacts and produce a final image
with an enhanced signal-to-noise ratio.
Data Processing and Analysis
Image analysis of the photoreceptor mosaic was performed using Image J (version 1.45a, U.S. National Institutes
of Health). For each eye, the acquired images were stitched
together to create a larger montage (approximately 9 degrees
9 degrees) of the photoreceptor mosaic using commercial
software (Photoshop version CS3, Adobe Systems, Inc.).
Each montage was verified by comparing it with the subject's
fundus image taken by the scanning laser ophthalmoscopy–
optical coherence tomography device. The foveal center
reference point for retinal coordinates was determined by
finding the center of the image taken when the subject fixated on a yellow cross at a 0-degree angle compared with
overlapping area of the 4 images when the subject fixated
on a 1.5-degree upper nasal and temporal angle and on
a 1.5-degree lower nasal and temporal angle. Cone density
(cells/mm2) was estimated at fixed locations, along the temporal and nasal meridians, at specific eccentricities from the
foveal center (G260 mm, G400 mm, and G600 mm) throughout the montages. The program automatically identified the
cone within two 50 mm 50 mm windows (Figure 1). If
required, the user performed manual editing to displace
the window position in areas devoid of blood vessels and
where cones were well resolved. Center-to-center cone spacing (intercone distance [ICD], minutes per arc [arcmin]) was
calculated from the identified cone center considering that
the photoreceptors are hexagonally arranged, as previously
described.23 Accordingly, the Nyquist sampling limit of the
resolution of the cone mosaic, Nc (cpd), was calculated as
pffiffiffi
3
ICD
2
Details of the model used in this study are given in the
Appendix (available at http://jcrsjournal.org).
The retinal magnification factor was estimated using the
nonlinear formula of Drasdo and Fowler,24 taking into account the AL and the corresponding posterior nodal point
distance in each eye. The spectacle-corrected magnification
factor was then estimated for each eye by considering the
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Figure 1. Cone-density packing distribution in an emmetropic eye (AL 23.84 mm). A: A 892 mm 1422 mm area of the parafoveal cone photoreceptor mosaic centered 415 mm from the foveal center. B: Image section encloses a 175 mm 765 mm area centered 260 mm from the fovea. The
section represents the results after the image was filtered to improve the overall contrast of the original image and leave the higher frequency
data, which tend to be cone photoreceptors. C and D: Cone-density estimates are taken using a fixed 50 mm 50 mm window area.
spherical equivalent (SE) refraction (corresponding to the
trial lens added to the system).25 The spectacle vertex distance was set at 14.0 mm for all eyes. The radial and area
magnification factors were computed for each eye.
The ocular, corneal, and internal aberration data output
for each eye were exported from the aberrometry system
and processed using purpose-written software in Matlab
software (version 7.0, The Mathworks, Inc.) for analysis.
Excluding LOAs, 3rd- to 6th-order aberration data were
computed by averaging 3 consecutive measurements over
a simulated 6.0 mm pupil. The root mean square (RMS)
values of the higher-order ocular, corneal, and internal wavefront aberrations were computed from the Zernike
coefficients, and the more recent recommended notation
was used.1 The modulation transfer function (MTF) and
the point-spread function (PSF) in the eye, the anterior
cornea and the internal optics were computed from the
higher-order wavefront aberration (wavelength Z 808 nm).
The Stiles-Crawford effect1 was incorporated into the calculation, using r Z 0.05 mm2 as the shape factor value.
Statistical Analysis
The cone density was calculated by dividing the number
of cones by the area of sampling window at each eccentricity.
Representative cone-density measurements at different
eccentricities were computed by averaging the density
estimates at the temporal and nasal meridians. Two-way
analysis of variance (ANOVA) was used to analyze the
differences in cone density and Nc at different eccentricities.
The interindividual variance in the optical and retinal parameters was calculated using the coefficient of variation
(CoV). The intraindividual variance was calculated using
the intereye ratio (expressed in %); that is, the ratio between
the cone density or Nc differences in fellow eyes and the
bilateral mean
Reye Leye
MeanReye Leye
where Reye and Leye are the right and left data, respectively. The bilateral differences in refractive data, cone
density, and Nc were evaluated using the paired Student
t test.
The differences between emmetropic eyes and myopic
eyes were evaluated using the unpaired Student t test. The
correlation of optical and retinal parameters with AL was
performed using the Pearson analysis. Statistical analysis
was performed using SPSS software (version 17.0, SPSS,
Inc.). Differences with a P value less than 0.05 were considered statistically significant for all the tests.
RESULTS
The study evaluated 12 subjects (3 men, 9 women). The
patients' age ranged from 24 to 38 years.
The mean SE refractive error was 1.77 diopters (D)
G 2.26 (SD) (range C0.25 to 5.50 D) with astigmatism less than 1.50 D when referenced to the spectacle plane. The AL was between 22.61 mm and
26.63 mm. In the emmetropic group, the mean age
was 30.57 G 5.00 years, the mean SE refractive error
was 0.14 G 0.16 D (range C0.25 to 0.25 D), and
the mean AL was 23.37 G 0.46 mm. In the myopic
group, the mean age was 30.00 G 4.43 years, the
mean SE refractive error was 4.41 G 0.88 D (range
3.00 to 5.50 D), and the mean AL was 25.02 G
0.85 mm. No differences in SE refractive error or AL
were found between right eyes and left eyes (PO.05,
t test).
In general, the total amount of the whole-eye higherorder RMS was less than values of the anterior cornea
or the internal optics except in 6 eyes (6/24 eyes, 25%).
The ocular higher-order RMS values ranged from 0.20
to 0.53 mm (CoV, 29%). Corneal HOAs ranged from
0.21 to 0.59 mm (CoV, 26%). Intraocular HOAs ranged
from 0.23 and 0.87 mm (CoV, 41%). Table 1 shows the
mean values of the higher-order wavefront aberration
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Table 1. Mean AL, refractive error and RMS of ocular HOAs.
Mean G SD
Group
All eyes (n Z 24)
Emmetropic RE (n Z 7)
Emmetropic LE (n Z 7)
Myopic RE (n Z 5)
Myopic LE (n Z 5)
AL (mm)
SE (D)
Total HOAs (mm)
Corneal HOAs (mm)
Intraocular HOAs* (mm)
24.13 G 1.06
23.35 G 0.47
23.29 G 0.49
25.06 G 0.76
24.97 G 0.99
2.15 G 2.30
0.11 G 0.20
0.18 G 0.12
4.63 G 0.86
4.29 G 0.87
0.33 G 0.10
0.32 G 0.09
0.32 G 0.13
0.33 G 0.05
0.33 G 0.11
0.38 G 0.10
0.40 G 0.12
0.40 G 0.10
0.37 G 0.09
0.34 G 0.08
0.43 G 0.16
0.35 G 0.08
0.38 G 0.08
0.51 G 0.21
0.49 G 0.18
AL Z axial length; HOAs Z higher-order aberrations; SE Z spherical equivalent
*P!.05 between emmetropic eyes and myopic eyes (t test)
data. Figure 2 shows the mean higher-order wavefront
aberration maps. The corresponding mean PSFs and
MTFs are plotted in Figure 3 and Figure 4,
respectively.
There were no statistically significant differences in
the mean total HOA values (mean difference 0.01 G
0.08 mm), mean corneal HOA values (mean difference
0.02 G 0.10 mm), or mean intraocular HOA values
(mean difference 0.01 G 0.14 mm) between right eyes
and left eyes (PO.05, t test).
There were no significant differences in total HOAs
(mean difference 0.01 G 0.01 mm) or corneal HOAs
(mean difference 0.05 G 0.02 mm) between emmetropic eyes and myopic eyes (PO.05, t test); the
amount of intraocular HOAs was, however, statistically significantly higher in myopic eyes than in
emmetropic eyes (mean difference 0.13 G 0.01 mm)
(P!.05, t test). There was no statistically significant
correlation between the amount of HOAs and the AL
(r2 Z 0.005 for total HOAs; r2 Z 0.11 for corneal
Figure 2. Higher-order corneal wavefront aberration maps for average right eyes and left eyes in the emmetropic group and myopic group. The
software overlaps the wavefront data for averaging with respect to the line of sight. The fixed color scale bar is in micrometers. The mean differences in RMS values between fellow eyes were lower than 0.03 mm both in the emmetropic and myopic groups. There is evidence of mirror
symmetry in higher-order wavefront aberration between fellow eyes22 (HOA Z higher-order aberrations).
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Figure 3. Mean PSFs in the right eyes and left eyes computed from the higher-order wavefront aberrations (6.0 mm pupil). The PSFs subtend to
a visual angle of 20 arcmin. Total, corneal, and intraocular PSFs in the emmetropic group and myopic group are shown. Excluding defocus and
astigmatism, 3rd-order aberrations and spherical aberration together contributed to degrade most of the image optical quality of the corneal and
intraocular optics with a 6.0 mm pupil. The combined effect of corneal and intraocular optics improved the optical performance of the whole eye
in the emmetropic group and the myopic group (HOA Z higher-order aberrations).
HOAs, and r2 Z 0.06 for intraocular HOAs) (all
PO.05).
Cone Density
Figure 5 shows the mean cone density and the mean
Nc as a function of retinal eccentricity. The mean cone
density dropped from 49393 G 7941 cells/mm2 (peak
65 220 cells/mm2) at 260 mm eccentricity to 30 049
G 4954 cells/mm2 at 600 mm eccentricity. The mean
Nc was 33 G 2 cpd at 260 mm (peak 38 cpd) and
26 G 2 cpd at 600 mm eccentricity. The CoV of the
cone density and Nc was 16% and 5%, respectively,
at each retinal eccentricity, with higher values in
myopic eyes (18% and 6%, respectively) than in emmetropic eyes (12% and 4%, respectively).
The intereye ratio of cone density was 6% at each eccentricity (%370 cells/mm2; PO.05). The intereye ratio
of Nc was 3% (%0.2 cpd; PO.05) at each eccentricity.
There were statistically significant differences in cone
density and Nc between emmetropic eyes and myopic
eyes at the specified eccentricities (P!.01, t test)
(Figure 5). Cone density and Nc were statistically significantly correlated with AL (Figure 6). No correlation
was found between the amount of HOAs and Nc at
any eccentricity (PO.05).
DISCUSSION
It is still under debate whether an improvement in the
optical quality of the eye can optimize the visual resolution of the individual. Previous studies3,16,26 have
shown that the main constraint to high-resolution
visual acuity is the optics of the eye, mainly in cases
of large pupils. However, there is a lack of knowledge
about the variation in the theoretical maximum retinal
resolution in the population. Cone density varies
significantly among individuals; therefore, a direct
correlation between optical quality and the retinal
sampling in the same subject/eye would overcome
the uncertainty caused by intersubject variability.
With the advent of adaptive optics technology in
clinical ophthalmology, it is feasible to estimate the
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Figure 4. Averaged 1-dimensional radial profiles of the MTFs in right eyes and left eyes (6.0 mm pupil) with the LOAs corrected. The solid line
represents the diffraction-limited MTF. The long, dashed curve represents the MTF of total higher-order wavefront aberrations. The dotted curve
represents the MTF of corneal higher-order wavefront aberration. The short, dashed curve represents the MTF of intraocular higher-order wavefront aberration. On average, corneal and intraocular HOAs interacted with each other to improve the whole ocular MTF in emmetropic eyes and
in myopic eyes. The effect of compensation on the image optical quality was higher at almost all spatial frequencies (c/deg Z cycles per degree;
MTF Z modulation transfer function).
Nyquist limit of resolution of the cone mosaic (Nc) by
imaging the photoreceptor layer in vivo.
In this study, we evaluated the variation in the eye's
optical image quality and the Nyquist limit of the cone
mosaic in a population of healthy young adults with
SE refractive errors ranging between C0.25 D and
5.50 D and ALs ranging between 22.61 mm and
26.63 mm. We aimed to correlate the optical and
retinal parameters of the eye to the AL. We believe
that we therefore provide for the first time a detailed
comparison of the optical and retinal performance
between emmetropic eyes and myopic eyes.
Previous studies1,17,27,28 found high variation in
higher-order wavefront aberrations. The interindividual variation in ocular aberrations, here expressed by
CoV, ranged from 26% for corneal HOAs to 29% for total HOAs and 41% for intraocular HOAs. He et al.18
found a variation between 22% and 38% in the mean
RMS values of corneal, intraocular, and total HOAs
in 45 young subjects (age range 9 to 29 years; SE refractive error between C0.50 D and 7.75 D), which is in
agreement with our data. The mean intraindividual
differences in the higher-order RMS values were less
than 0.02 mm. We can therefore confirm that although
there appears to be random variation in the eye's aberration from subject to subject, the variation in HOAs
between fellow eyes is relatively low.1,17,21,22
In general, the optical quality of the whole eye, here
expressed by pupil and image-quality metrics, was
better than that provided by corneal and intraocular
optics alone, as expected.1,18,19 In the young healthy
eye, the result of the compensatory processing
between the cornea and the lens has been shown to
improve image optical quality at the fovea.1,20,21 No
differences between emmetropic eyes and myopic
eyes were found except for a higher amount of intraocular HOAs in myopic eyes. Data in the literature surrounding this issue are controversial. Some studies21
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Figure 5. A and B: The distribution of cone density and Nyquist limit
of resolution. The vertical bars represent 1 standard deviation (SD) in
the spread of cone density and Nc at the specified eccentricity. Left
eye values are underlined. The decline in cone density and Nc
with increasing retinal eccentricity was statistically significant
(P!.001. ANOVA). The mean difference in cone density between
emmetropic eyes and myopic eyes was 7709 cells/mm2, 5122
cells/mm2, and 4536 cells/mm2 at 260 mm, 400 mm, and 600 mm
eccentricities, respectively. The SD was higher in myopic eyes. The
mean difference in Nc between emmetropic eyes and myopic eyes
was 1.8 cpd, 1.2 cpd, and 1.4 cpd at 260 mm, 400 mm, and 600 mm
eccentricity, respectively.
found that the optical quality was worse in myopic
eyes than in emmetropic eyes (expressed by
increased HOAs), whereas others22,26 found no correlation between refractive error and monochromatic
aberrations. No clear evidence supports the relationship between emmetropization and ocular aberrations.1 We cannot draw conclusions from our results
because the intraocular HOAs showed the largest variance in this study. The influence of the StilesCrawford effect was incorporated into the calculation
of the PSF and MTF to properly model the optical effect of HOAs. This phenomenon has a retinal basis in
the optical behavior of cone photoreceptors because
the photoreceptors act as optical fibers and have typical waveguide properties.1,29 From the data shown,
the eye's optics play a primary role in filtering out
1153
Figure 6. A and B: The correlation between cone density and Nc with
AL. A moderate, although statistically significant correlation between cone density or Nc and AL was found at each specified eccentricity (P!.001) (c/deg Z cycles per degree).
high-frequencies and therefore limiting the resolution
in the normal eye, even when the correction for defocus and astigmatism is optimum (as modeled in our
study). The overall image optical quality in the eye
in the emmetropic group and the myopic group was
comparable.
Along the horizontal meridian, cone density dropped from 49 393 G 7941 cells/mm2 at 260 mm from
the foveal center to 30 049 G 4954 cells/mm2 at
600 mm from the foveal center. The interindividual variation in cone density and Nc was 16% and 5%, respectively, between 260 mm and 600 mm eccentricity from
the foveal center. The mean differences in cone density
or Nc between fellow eyes was 5% and 3%, respectively, and did not achieve statistical significance. A
statistically significant difference in cone density between emmetropic eyes and myopic eyes at each specified eccentricity was found. The cone density
estimates in the present study are in agreement with
those in previous studies.5,9–12 In a population of 18
eyes of 18 healthy subjects (age range 23 to 43 years;
AL range 22.86 to 28.31 mm), Li et al.10 found that
cone density decreased significantly with increasing
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AL at eccentricities between 100 mm and 300 mm from
the foveal center (r2 Z 0.56, r2 Z 0.65, and r2 Z 0.75 at
0.3 degrees, 0.7 degrees, and 1.0 degree eccentricity, respectively). In 11 eyes of 11 healthy subjects (age range
21 to 31 years), cone density in moderately myopic
eyes (up to 7.50 D) was significantly lower than in
emmetropic eyes within 2.0 mm from the fovea.11
The interindividual variation in cone density was, on
average, 18% between 0.3 mm and 2.0 mm from the
fovea in previous adaptive optics studies.9,12
In the current study, the mean Nyquist limit sampling of resolution of the cone mosaic was estimated
to be 33 G 2 cpd at 260 mm eccentricity, declining to
26 G 2 cpd at 600 mm eccentricity. A statistically significant difference in Nc between emmetropic eyes and
myopic eyes was found, as previously reported.9,11
The Nc was calculated to be 34 cpd at 1-degree eccentricity (approximately 270 mm) by Chui et al.11 Coletta
and Watson25 estimated the Nc to range between
50 cpd and 42 cpd at the fovea and between 24 cpd
and 22 cpd at 4 degrees eccentricity (approximately
1.1 mm) in a population with SE refractive errors
ranging between 0.0 D and 14.0 D. The lower cone
density estimated in myopic eyes has been postulated
to be caused by retinal stretching resulting from the
increased AL in the eye.11,25
A limitation of our study is that we were not able to
resolve cones at the foveal center. Variability in cone
density at close to the fovea (!0.2 mm) could be
even greater between emmetropic eyes and myopic
eyes based on histologic data.5,7 Beyond optical
aberrations, scatter and dispersion of light, which
were not measured in this study, can degrade the optical image quality in the individual eye at the retinal
plane. We therefore were not able to measure the overall optical quality of the eye and model the Nyquist
limit of resolution of the foveal cones.
Individual visual performance is the result of how
the image is focused on the retina and how the visual
system processes and integrates the information. The
resolution depends on the eye's optical factors, the
structural properties of the cone mosaic, and the functional properties of the post-receptorial structures. The
cone-packing density within 600 mm eccentricity from
the fovea showed less variation than the eye's optical
aberrations in a population of young adults and was
statistically significantly correlated with the AL. The
spatial vision of the cone mosaic was slightly reduced
with increasing AL from 22.60 to 26.60 mm. Our results indicate that optical improvement in the eye
will likely benefit young emmetropic patients more
than patients with moderate myopia. Previous studies9,11,25 support the hypothesis that retinal stretching
at the posterior pole in myopic eyes reduces retinal
sampling density. Although a difference less than
2 cpd might not be considered clinically meaningful,
researchers have found that myopic eyes perform
slightly worse than emmetropic eyes when acuity
limits were tested with grating stimuli generated
with high-contrast laser interference fringes and the effect of optical degradation was compensated for.25,29
The visual acuity in myopic eyes has been found to
be even worse when measured with letter targets
rather than with grating stimuli.25,29 A greater difference between myopic eyes and emmetropic eyes also
occurred when the acuity limits were expressed in retinal units (c/mm) rather than in angular units (cpd).
The discord between resolution and Nc expressed in
retinal acuity was primarily considered to be the result
of the psychophysical and neural factors, including the
neural sampling rate; that is, the receptive field density
of the retinal ganglion cells.13–15 When modeled, the
Nyquist limit of the retinal ganglion cells mosaic was
estimated to be lower than the Nyquist limit of the
cone mosaic and to be better correlated to visual acuity
up to 5 degrees eccentricity from the fovea.13,15 Theoretically, the differences in acuity tasks between emmetropic eyes and myopic eyes might be even higher
considering retinal stretching, which may ultimately
decrease the neural sampling density of the myopic
retina.14,15,25 The contribution of post-receptor neural
factors to the individual visual function should be
studied to determine in detail the spatial filtering
properties of the human neuroretinal system.
WHAT WAS KNOWN
The higher-order wavefront aberrations in the eye vary
largely between individuals. The correction of HOAs is
believed to benefit emmetropic eyes and myopic eyes to
the same extent.
WHAT THIS PAPER ADDS
Although the optical image quality in emmetropic eyes
and myopic eyes was comparable, the Nyquist limit of
resolution of the retinal cone mosaic was reduced with
increasing AL.
The theoretical full correction of the wavefront aberrations
should therefore benefit emmetropic eyes more than
myopic eyes.
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J CATARACT REFRACT SURG - VOL 38, JULY 2012
First author:
Marco Lombardo, MD, PhD
IRCCS Fondazione G.B. Bietti,
Rome, Italy