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Increased accommodation following adaptation to image blur in myopes
Abstract
Prolonged exposure to blurred images produces perceptual adaptation (M. A. Webster, M. A., Georgeson, & S. M. Webster, 2002). The purpose of this study is to test whether in addition to the reported change in perceived blur there is also a change in accommodation. Young adult (aged 18 to 31 years) myopic (n = 23) and emmetropic (n = 17) subjects participated in the study. Myopes were tested with contact lenses and had corrected monocular visual acuity of 20/20 or better. Accommodation was measured binocularly with a PowerRefractor, an eccentric infrared photorefractor. Accommodation for a near target (high-contrast text at 0.33 m) was measured for 2 min before and immediately after 3 min of blur exposure. Blur was induced using 0.2 Bangerter diffusing filters in front of both eyes. In addition, accommodation was measured for a far target (high-contrast letters at 4.0 m) before and after the near measurements, with each subject’s initial far readings used as a baseline for calculating the accommodative responses at near. Compared to the pre-adaptation level, myopes showed a significant (p < .01) increase in the near accommodative response after 3 min of blur adaptation, while accommodation to the near target in emmetropes did not change. In a second experiment using monocular viewing, the increase of accommodation found in myopes was shown to occur during the period of blur exposure. The refractive group differences in the accommodative response may be related to differences in the habitual response to image clarity between myopes and emmetropes under normal viewing conditions.
History
Received September 9, 2003; published December 28, 2004
Citation
Vera-Diaz, F. A., Gwiazda, J., Thorn, F., & Held, R. (2004). Increased accommodation following adaptation to image blur in myopes.
Journal of Vision, 4(12):10, 1111-1119,
Keywords
blur, adaptation, accommodation, myopia, emmetropia, oculomotor, neurosensory
Recent models of human myopia propose retinal defocus
as a causative factor in refractive error development (Flitcroft, 1998; Jiang & Morse, 1999; Hung & Ciuffreda, 1999, 2000). In these models, the growing
eye works as a feedback system designed to maintain the clarity of the retinal
image by modulating eye growth according to the magnitude of retinal defocus.
Retinal defocus would, therefore, serve as a stimulus regulating the rate of
axial elongation in myopia.
During normal visual development the eye achieves a
close match between the power of its optics and its axial length with the result
that far images are focused on the retina without accommodative effort
(emmetropia). This emmetropization process is partly an optical consequence of
proportional eye growth, and thus passive in nature. However, experimental
models of myopia also provide strong evidence for an active role of defocus in
the emmetropization process (Wallman & Adams, 1987; for a review, see Wildsoet, 1997). Degrading a retinal image by frosted
eye occluders produces elongated eyes and ”form-deprivation myopia”
in a variety of animal species, providing evidence for a feedback system that
correlates eye growth and the magnitude of retinal defocus. Partial frosting
degrades the retinal image in a more subtle manner, leading to the development
of lesser amounts of myopia (Bartmann & Schaefffel, 1994; Smith & Hung, 1999). In addition, a number of studies have
demonstrated that experimental myopia may be induced by placing negative lenses
before the eyes in various animal species (e.g., chick, guinea pig, tree shrew,
and marmosets) (for reviews, see Edwards, 1996, and Norton, 1999). Although the exact mechanism
controlling emmetropization remains uncertain, a number of studies (Graham &
Judge, 1999; Schaeffel & Diether,
1999; Smith & Hung, 1999) highlight the fundamental role
played by blur in the regulation of eye growth.
Disruption of the emmetropization process results in
the development of refractive errors, of which myopia is the most common. Myopia
is a highly significant problem, not only because of its increasing
prevalence-more than 80% in some Asian countries-(Lin et al., 2001), but also because it is a high risk factor
for vision-threatening conditions (e.g., retinal detachment and glaucoma). These
conditions are due to the stresses produced in the posterior segment of the eye
as a result of the excessive increase in axial length.
According to clinical observations, the visual
performance (e.g., visual acuity [VA] and
contrast sensitivity function [CSF]) of corrected myopic subjects improves after
a period of uncorrected vision compared to the performance when the correction
is worn at all times (Pesudovs & Brennan, 1993). This phenomenon can be
interpreted as an increased tolerance to blur (learning process), or an
improvement in vision due to neural or optical adjustments within the visual
system (Mon-Williams, Tresilian, Strang, Kochhar, & Wann, 1998).
Myopes normally have reduced sensitivity to blur in
comparison to emmetropes. Rosenfield and Abraham-Cohen (1999) showed that on average,
myopes have increased blur thresholds. Various models of human myopia (Jiang, 1997; Flitcroft, 1998; Hung & Ciuffreda, 1999) suggest that higher blur
thresholds may be related to increased accommodative errors and the development
of myopia. The increased accommodative lag found in some myopic subjects
(Gwiazda, Thorn, Bauer, & Held, 1993; Jiang, 1997; Abbott, Schmid, & Strang, 1998) would produce a hyperopic retinal
defocus that may play a significant role in myopia development and/or
progression.
More recently, prolonged exposure to blurred images has
been shown to produce perceptual adaptation. Webster, Georgeson, and Webster (2002) found that exposure to a blurred
image caused the original image, which had previously been interpreted as clear,
to appear to be too sharp. These aftereffects appeared after brief periods (a
few seconds) of adaptation. Although the authors did not identify the refractive
errors of the subjects, they did note that “these adaptation effects are
thus important for understanding. . . how vision changes during development and
with refractive errors.” Judgments of focus are strongly biased by adaptation to blurred or sharpened versions of an image. Adaptive tuning may be important in calibrating and maintaining the correlation between the image processing in the visual cortex and natural visual stimuli during visual development. Variations of the environment and/or the observer, such as in refractive errors, may alter this correlation. The adjustments taking place by adaptation to blur may be important in maintaining a constant perception of the world. Furthermore, these adaptation effects can potentially alter the accommodative response to the image, by altering sensitivity or responsiveness of the accommodative system to blur.
The present study was designed to test whether the
reported adaptation in perceived blur produced by exposure to blurred images is
accompanied by a change in accommodation, and whether that change differs
between emmetropes and myopes.
Forty young adult (mean age = 24.88 ± 3.23 years)
myopic (mean refraction = –3.46 ± 1.86 D;
n = 23) and emmetropic (mean refraction
= +0.03 ± 0.15 D; n = 17) subjects
participated in the study. Myopes were tested with soft contact lenses and all
subjects had corrected monocular visual acuities of 20/20 or better. Subjects
with astigmatism > 1.00 D were not included in the study. The research
followed the tenets of the Declaration of Helsinki, and informed consent was
obtained from the subjects after explanation of the nature and possible
consequences of the
study.
The accommodative response was measured binocularly
with the PowerRefractor (PlusOptixs,
Germany), an eccentric infrared photorefractor that converts the slopes of the
brightness distributions in the pupil into refractive error. Figure 1 shows the appearance of the screen of
the PowerRefractor during the dynamic measurement of accommodation. The
PowerRefractor can record the pupil sizes, the refraction in the vertical
meridian of both eyes, and the angle of convergence of the pupils’ axes of
both eyes. In its binocular mode, the PowerRefractor measures the slope of the
pupil distributions in the vertical meridian every 0.04 s. Detailed descriptions
of the PowerRefractor can be found in Choi et al. (2000) and Seidemann and Schaeffel (2002).
Figure
1. Appearance of the screen of the
PowerRefractor during dynamic measurement of accommodation. The PowerRefractor
can record pupil sizes, the refraction in the vertical meridian of both eyes,
and the angle of convergence of the pupils’ axes of both eyes.
The PowerRefractor uses a built-in calibration function
to determine refractive state from the change of the pixel intensities across
the vertical meridian of the pupil (Seidemann & Schaeffel, 2002; Seidemann and Schaeffel 2003). To insure accurate
readings, particularly for larger refractive errors, re-calibration of the data
was performed in the present study using the method described by Harb, Troilo,
and Thorn (2003).
Diffusing lenses (0.2 Bangerter Occlusion Foils; The Fresnel Prism and Lens
Co., LLC) that induce scatter blur were used to produce an adaptation to blur
similar in magnitude to that induced by Webster et al. (2002). The Bangerter Foil induced a reduction of contrast of ~75% on
the target used in the experiment. It is primarily a low-pass filter with the
transmission characteristics shown in Figure 2. Previous reports have shown that the peak of accommodative responses is
found in the region of 5 c/deg, with open-loop conditions being initiated in the
region of 0.5 c/deg (Ward, 1987; Matthews
& Kruger, 1994; Niwa &
Tokoro, 1998).
Figure
2. Modulation transfer function (MTF)
reduction by the diffusing lenses for various sine wave frequencies. Calculation
of the characteristics of the diffusing lenses was done by photographing sine
waves simulating the experimental conditions. The digital pictures were Fourier
transformed to get amplitude spectra. The amplitude of the blurred sine wave
spectrum was divided by the amplitude of the nonblurred sine wave spectrum for
the given sine wave frequencies. Due to the abrupt change in the lower frequency
region of the MTF, the curve was fitted with two second-order polynomials
splined together at 0.28 c/deg. The fitting curves are presented for a visual
guide to show the overall shape of the MTF.
Initially, accommodation was measured binocularly for a
far (4 m) target (high-contrast 90% letters; logMAR 0.3) for 1 min.
Accommodation was then measured while the subjects read a paragraph of
high-contrast text (85%) with letter size 10 point at 0.33 m (logMAR 0.5) for 2
min. After this period, blur was induced by the scattering filters during a
3-min period. This time period was chosen as consistent with the adaptation to
blur that has been found following this length of blur exposure (Webster et al.,
2002). Subjects continued looking at
the same text target at 0.33 m during this period of blur adaptation. The text
blurred with scattering filters provided a degraded accommodative stimulus
during the adaptation period (see movie
simulation). Subjects reported that they could not discriminate the letters
of the text during this period.
Immediately after the blur exposure period,
accommodation at near was measured for a further 2-min period. Lastly,
accommodation was measured while subjects viewed the far target (4 m) for 1
min.
Individual raw data were calibrated as described
earlier and divided into 10-s intervals. Analyses of mean data for Figure 5 were carried out averaging all
post-adaptation data. For purposes of comparison and statistical analysis, each
subject’s far readings were used as a baseline for calculating the
accommodative responses at near.
Data taken during the blur exposure condition were not
valid because the diffusing filter was placed between the PowerRefractor and the
eye. To understand what was happening to the accommodative response during the
period of blur exposure, two additional experiments were carried out.
The aim of this experiment was to assess accommodative
responses during the blur exposure condition. It was performed in a subgroup of
subjects (5 myopes and 5 emmetropes). The procedures were the same as described
above except that the right eye (measured eye) was covered with an
infrared (IR)-only transmitting filter (peak at
720 nm), while the left eye was the fixating eye throughout the experiment.
Note that this experiment differs from the main experiment in that it occurs
under monocular viewing. Analysis of the data was carried out as formerly
described.
The aim of this experiment was to compare accommodative
adaptation following a period of darkness to accommodative adaptation following
blur exposure described in the main experiment. It was performed in a subgroup
of subjects (4 myopes and 2 emmetropes). The procedures were the same as
described above except that the 3 min of blur were replaced with a 3-min
open-loop accommodation condition, with the subjects in complete darkness.
Analysis of the data was carried out as described above.
Figure 3a presents
the accommodative responses of all emmetropes during 10-s intervals. These data
revealed stable responses over time for each of the conditions tested. For
distance viewing (4 m), accurate accommodative responses were found for all
emmetropic subjects, with values near zero (ranging from –0.52 D to +0.46
D). Accommodative responses at near gave similar values pre- (between
–1.21 and –2.62 D) and post- (between –1.25 and –2.77 D)
exposure to blur, indicating that all subjects showed a lag of accommodation to
the 3.00 D stimulus that was unaffected by exposure to blur.
Figure 3a-b.
Individual (a) and mean (b) accommodation responses for the emmetropes at each
of the conditions tested: far viewing (4 m); initial near condition (0.33 m);
near condition following blur adaptation (0.33 m); and far condition (4 m).
Error bars show ± 1 SEM.
Figure 3b shows mean
data for the emmetropic subjects, where it is clear that the accommodative
response to the near target following the blur adaptation period remained
unchanged from the initial condition
(t-paired =
–1.48; p = .29). In addition, the
distance viewing values remained unchanged, with values slightly negative for
both the initial (mean ± SD =
–0.02 ± 0.07 D) and the last (mean ±
SD = –0.01 ± 0.08 D)
conditions.
Myopic subjects also showed stable responses over time
for each condition (Figure 4a). For pre-task
distance viewing, the myopes’ responses ranged between +0.82 and
–0.83 D. For near viewing, all subjects except one showed a lag of
accommodation, with accommodative responses ranging between –1.12 and
–3.09 D for the 3.00 D target. Following the 3-min blur exposure, an
enhanced accommodative response at near toward more negative values can be
observed. Mean data show a significant increase in the accommodation of myopes
following blur exposure (t-paired =
7.32; p < .001) (Figure 4b). This shift in accommodation remains
throughout the 2-min near period and persists during the 1 min of far viewing.
No correlation was found between the baseline accommodative response and the
accommodative adaptation following blur adaptation
(r2 < 0.01;
t-paired = 1.72; p <
.01).
Figure 4a-b.
Individual (a) and mean (b)
accommodation responses for the myopes at each of the conditions tested:
far viewing (4 m); initial near condition (0.33 m); near condition following
blur adaptation (0.33 m); and far condition (4 m). Error bars show ± 1
SEM.
No significant differences in pre-blur baseline accommodation at near were found between the refractive groups (mean emmetropes = 1.98 ± 0.13 D; mean myopes = -2.14 ± 0.12 D) (factorial ANOVA, F1, 38 = 0.90;
p = .35). Analysis of the far (4 m)
accommodative response following near viewing shows a myopic shift in the myopic
subjects (mean ± SD = –0.19
± 0.07 D) compared to the emmetropes (mean ±
SD = +0.01 ± 0.03 D). The
difference of 0.20 D between the refractive groups was statistically significant
(factorial ANOVA, F1, 38 =
5.00; p = .03).
Analysis of individual data demonstrates (Figure 5) that all myopes showed an increase in
their near accommodative response following blur exposure, while all but one of
the emmetropes remained unchanged or showed a slight decrease in the
accommodative response. In addition, Figure 5
shows mean data from both refractive groups. There was a significant increase
(–0.29 D) in the mean accommodative response at near after blur adaptation
in myopes (factorial ANOVA, F1, 44
= 4.87; p = .01) and no
significant change in the accommodative response in emmetropes (+0.06 D)
(factorial ANOVA, F1, 32 =
0.12; p = .73). The mean shift of
accommodation is significantly higher for myopes in comparison to emmetropes
(t-paired = 6.44;
p < .001).
Figure 5.
Accommodation change at near (D) following blur exposure as a function of
refractive error; individual
(
 )
and mean grouped data for myopes
(▲) and emmetropes
(n). Figure 6 presents the mean accommodative responses of emmetropes and myopes during 10-s intervals for the eye covered with the IR-transmitting filter (right eye), while the left eye fixated the target throughout this control experiment. These data reveal differences between the responses of emmetropes and myopes during the blur adaptation period. Emmetropic subjects show a stable pattern in their responses for each of the conditions tested, including the blur condition. However, myopic subjects show a progressive increase in their accommodative response over the 3 min blur period. Following the blur condition, the increase in the accommodative response appears to continue during part of the 2-min near measurement period and then regresses to the baseline level of the near accommodative response.
Figure 6.
Mean accommodative responses for the eye
with the IR transmitting filter at each of the conditions tested [(1) far
viewing (4 m); (2) initial near condition (0.33 m); (3) near condition during
blur adaptation (0.33 m); (4) near condition following blur adaptation (0.33 m);
and (5) far condition (4 m)] as a function of refractive group [emmetropes
(o) and myopes
(▲)]. Dotted vertical lines represent
landmarks for each condition. Error bars show +/- 1
SEM.
A repeated measures regression model (GEE), which
included accommodation, refractive error, time, and the interaction of time with
refractive error, revealed that the slopes for emmetropes and myopes were
different during the blur adaptation period
(p < .001). The model showed that
myopes’ accommodation increased 0.02 D every 10 s (a total of 0.36 D after
3 min) compared to emmetropes. Repeated measures ANOVA revealed no significant
differences between refractive groups for the baseline near accommodative
response (F1, 119 = 0.02;
p = .78).
None of the subjects showed accommodative adaptation
following the open-loop condition, with mean adaptation values for each
refractive group near zero (emmetropes: mean ±
SD = 0.02 ± 0.07; myopes: mean
± SD = –0.01 ± 0.06).
Accommodative adaptation values following the open-loop condition were not
correlated with the adaptation following the blur condition in the main
experiment (r2 = 0.08;
t-paired = 2.72;
p = .13).
Our results reveal for the
first time that myopic, but not emmetropic, young adults show an increase in
accommodation after experiencing 3 min of adaptation to a near target that has
been blurred with a scattering filter. This finding builds on previous work,
some from this laboratory, showing differences in accommodation between myopes
and emmetropes (Gwiazda et al., 1993;
Jiang, 1997; Abbott et al., 1998). The present results are also
consistent with previous studies of blur sensitivity, showing that myopes
interpret and adapt to blur differently than emmetropes (Rosenfield &
Abraham-Cohen, 1999; Oen,
Lim, & Cheng, 1994; Wu, Lim, Seet,
& Chew, 1997; Thorn, Cameron, Arnel,
& Thorn, 1998; Strang, Winn, &
Bradley 1998; Turatto et al., 1999). To relate accommodative
adaptation to myopia, we suggest that adaptation to blur enhances the gain of
the stimulus for accommodation in myopic individuals.
The finding that myopic children show reduced
accommodative responses to negative lens-induced blur has contributed to a new
understanding of the role of accommodation in the etiology of myopia (Gwiazda et
al., 1993; Gwiazda, Bauer, Thorn, & Held,
1995). Accommodation may be an
important factor in mediating the amount of defocus that the retina experiences
when near objects are viewed. A consequence of habitual reduced accommodation is
that near targets are partially blurred with a hyperopic defocus. Extended
periods of such blur may contribute to the development and progression of
myopia.
Although the accommodation of myopes is reduced during
the progression phase of myopia, (Gwiazda et al., 1993; Jiang, 1997; Abbott et al., 1998), it returns to the level of
emmetropes when myopia stabilizes (Gwiazda et al., 1995). The mechanism underlying
myopes’ improvement in accommodation during childhood may be the same
mechanism that explains our present results. In the present study, no differences in baseline accommodation levels for
near viewing were found between the two refractive groups. This is not
surprising given that refractive histories, available from approximately half of
our subjects, revealed that their myopia was stable.
Although it is not well understood how image blur
contributes to eye growth and myopia, perceived blur is known to be influenced
by adaptation (Mon-Williams et al., 1998). Myopes often report that
their vision is poorer immediately after spectacle removal compared to their
performance following a prolonged period without spectacles. Pseudovs and
Brennan (1993) investigated this
phenomenon and found increased visual acuity in low myopes following a period of
uncorrected vision. They suggested that improved VA indicates a sensory
adaptation to blur and/or differences in their ability to perceive blur. Data
from our first control experiment support this hypothesis. The accommodative
changes occurring in myopes during the blur exposure period may reflect a
sensory adaptation in those subjects.
Two recent abstracts reported similar improvements in
visual resolution after defocus-induced blur adaptation to those found by
Pseudovs and Brennan (1993) and
showed that adaptation to blur is a robust and long-lasting phenomenon (Portello
& Rosenfield, 2002;
Rosenfield, Hong, Ren, & Ciuffreda, 2002). Georgeson and Sullivan (1975) hypothesized that everyday
vision is chronically altered by a lifetime of experiencing optical degradation
from normal visual optics. This experience leads to adaptation in which an
observer can perceive suprathreshold high spatial frequencies as having contrast
equal to low spatial frequency targets with the same physical contrast, even
though high frequencies give much higher thresholds due, in part, to optical
degradation. Further suprathreshold equalization of high spatial frequency
contrast is reported to occur after a few minutes of adaptation either to
defocus caused by positive lenses or to blur induced by the same diffusing
filters that are used in the present experiment (Comerford, Thorn, & Chuang,
2002; Hendricks, Comerford, &
Thorn, 2003).
In contrast to our results, the only previous study
investigating blur-induced adaptation of accommodation demonstrated no
significant change in the static accommodative response after three hours of
viewing the world through +2.50 D lenses over the subjects’ distance
refraction (George & Rosenfield, 2002). However, perceptual
adaptation did occur. A possible explanation for the discrepant results is that
George and Rosenfield (2002)
used dioptric blur induced by convex lenses while subjects watched television at
4 m, rather than using scatter blur as in this study. Scatter blur was
necessary for the purpose of this study as it does not provide cues for
accommodation. Because convex lenses signal the accommodative system to relax
accommodation, it is not surprising that accommodation did not
increase in the earlier study.
The literature reviewed above suggests that myopes,
most likely both stable and progressing myopes, show reduced perceptual
sensitivity to blur. In addition, accommodation differs between progressing and
stable myopes, with progressing myopes showing reduced accommodation that
improves with the stabilization of myopia to the same levels shown by emmetropes
(Gwiazda et al., 1995). The
improvement in accommodation must involve an active long-term adaptation process
within the accommodative system itself because perceptual sensitivity to blur
continues to be deficient in stable myopes (Rosenfield & Abraham-Cohen, 1999).
Results from the monocular viewing control experiment
show that myopes accommodate differently than emmetropes during the blur
exposure period. Their accommodative response increases over time, becoming more
accurate, whereas the emmetropes’ response is stable. This accommodative
increase in myopes extends into the post-blur period of near vision. On the
other hand, a complete elimination of blur feedback as in the open-loop control
experiment leads to no increase in accommodation for either refractive group.
This suggests that the accommodation enhancement is specifically due to a
blurred stimulus as opposed to no stimulus and that it is the difference in the
two refractive groups’ use of sensory blur cues that determines the
difference in their accommodation adaptation.
The improvement in the accommodative response with the
myopia stabilization reported in previous studies may be a consequence of the
development of a prolonged blur adaptation mechanism, which results in a
habitually more accurate accommodative response. This mechanism may enhance
accommodation in the same way when a myope is presented with the blurred target
used in the present study. This strong blur cue would further enhance the
accommodative signal and, therefore, the accommodative response. On the other
hand, emmetropes habitually have strong sensory cues for accommodation. They
have not developed the strong blur adaptation mechanism posited for myopes; and,
thus, they show no accommodative adaptation when presented with the blurred
target in the present study.
Accommodation adaptation mechanisms may use a change in spatial frequency
channel responses or a shift in the stimulus-response range to strengthen blur
cues. Blur adaptation may be due to spatial frequency-specific adaptation
(Blakemore & Sutton, 1969) in
which low frequency channels become fatigued relative to high frequency channels
when looking at the blurred text and are thereby prevented from responding to
their normal potential. Therefore, in the present study, when the diffusing
lenses are removed from in front of the eyes, the high spatial frequency
channels may be relatively more responsive than the low frequency channels, and
the myopic subjects may perceive primarily high spatial frequency information.
However, there is little evidence showing refractive group differences in the
balance between high and low frequency channels (Thorn, Corwin, & Comerford,
1986; Comerford, Thorn, & Corwin,
1987; Liou & Chiu, 2001) or strong long-term effects from this
type of adaptation. Another possibility is that a long-term increase in the gain
of high frequency channels is used to counteract suprathreshold blur and maintain
contrast constancy (Georgeson & Sullivan, 1975; Comerford et al., 2002; Hendricks et al., 2003). In addition, myopes may have a shift in the range of stimuli responded to by the
accommodative system. Jiang’s (1997)
model of myopic accommodation includes an elevated accommodative threshold with
no change in the stimulus-response gain. An overall stimulus-response function
shift may allow myopes to respond to the higher amounts of blur viewed in the
present study.
Webster et al. (2002) proposed another possible
mechanism mentioned in the Introduction. They proposed that adaptation to blur
is consistent with an adjustment that recalibrates the neural response to blur
according to the prevailing image, and that it occurs at the level of the visual
cortex or higher. At present it is not clear what is causing the increased accommodation following adaptation to image blur in myopes. However, the underlying mechanism may have implications for the understanding of myopia development.
We are currently examining perceptual blur adaptation
and accommodation adaptation in parallel in myopes and emmetropes. If the basis
for the present result is a poor sensory signal to blur for driving
accommodation in myopes, then myopes should demonstrate a greater perceptual
adaptation to blur under our testing conditions. In addition, to investigate
what may be occurring in early stages of myopia development, we propose to study
this phenomenon longitudinally in young children who are likely to become myopes
as well as in progressing
myopes.
This movie shows a demonstration
of blur adaptation following a protocol similar to that used by Webster et al.
(2002) but with a stimulus similar to
that used in the present study. The movie shows two images: The bottom image is
used as a reference and the top image simulates how the target was perceived by
the subjects during our experimental set up, before and after adapting to blur.
The amount of blur induced in the top image corresponds to the blur induced by
the fogging lenses that were used in this study. The aftereffects are best
observed by fixating on the center cross. The image on the top is perceived as
sharper following adaptation compared to the reference image at the
bottom.
Movie simulation.
This research was supported by National Institutes of Health Grants EY-01191 (JG), and Grant EY-014817.
We thank Michael A. Webster for his technical
assistance and helpful discussions. We also thank Hyunjeong Han and Sarah Hill
for their technical assistance.
Commercial relationships: none.
Corresponding author: Fuensanta A. Vera-Diaz.
Email: vera-diazf@neco.edu.
Address: The New England College of Optometry, 424 Beacon Street, Boston, MA 02115, USA.
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