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(Investigative Ophthalmology and Visual Science. 2007;48:2932-2939.)
© 2007 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.06-0836
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Effect of Blur Adaptation on Blur Sensitivity and Discrimination in Emmetropes and Myopes

Matthew P. Cufflin, Alex Mankowska, and Edward A. H. Mallen

From the Department of Optometry, University of Bradford, Bradford, United Kingdom.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To determine whether blur adaptation influences blur sensitivity and blur discrimination thresholds in young adult myopes and emmetropes. In addition, to determine whether there is a differential effect of blur adaptation on blur sensitivity and discrimination between refractive error groups.

METHODS. Proximal and distal blur sensitivity thresholds and blur discrimination thresholds were measured under cycloplegia with a Badal optometer in 24 young adult subjects (8 emmetropes [EMM], 8 early-onset myopes [EOM], and 8 late-onset myopes [LOM]). Adaptation to 1 D of myopic refractive blur was then undertaken for 30 minutes. Blur sensitivity and discrimination thresholds were then remeasured.

RESULTS. After blur adaptation, blur sensitivity, and blur discrimination thresholds were found to be elevated. Blur adaptation had a significant effect on distal blur sensitivity threshold, with the largest effect being observed in the EOMs. Mean changes in distal blur sensitivity thresholds were EMMs +0.03 ± 0.14 D, EOMs +0.30 ± 0.21 D, and LOMs +0.08 ± 0.13 D.

CONCLUSIONS. Adaptation to a degraded stimulus modifies the blur detection mechanisms of the visual system in young adults. Depth of focus is expanded by prolonged exposure to defocus. EOMs are more susceptible to this phenomenon than are LOMs and EMMs.

The human depth of focus (DOF) is commonly defined as the maximum level of defocus that can be applied to an optimally focused image without subjective detection.1 DOF is measured in diopters (D) and quoted in both positive and negative terms, to represent myopic and hyperopic defocus, respectively. The DOF provides a direct measurement of a subject’s blur sensitivity,2 with low dioptric thresholds representing high subjective blur sensitivity and vice versa. Several variables have been found to influence the DOF including pupil size and target luminance,3 target contrast,4 target size,4 5 refractive error,2 and retinal eccentricity.6 A method of adjustment is commonly used to resolve the limits of the DOF3 4 7 8 and enabled Campbell3 to conclude that the DOF of the human eye was ±0.30 D for a 3-mm pupil diameter. The DOF was found to increase as target contrast, luminance, and pupil diameter were all decreased.

Badal optometers have become the instrument of choice for recording subjective DOF measurements6 9 and allow for changes in target vergence, while minimizing changes in target contrast and size, both of which influence DOF.3 Atchison et al.4 used a modified Badal system to investigate the limits of subjective clear vision. Accommodation was controlled using cyclopentolate hydrochloride (1%) and the position of a –6-D auxiliary lens was adjusted to gradually induce defocus on a target. The DOF via a 4- and 6-mm diameter pupil was found to be ±0.295 and ±0.275 D, respectively. Increasing the target size from 6/4 to 6/45 Snellen equivalent increased the magnitude of the DOF by approximately 60%, and a reduction in target contrast produced an increase in DOF of +0.08 ± 0.05 D. Wang and Ciuffreda6 used a similar methodology to investigate the DOF centrally and at the near periphery of the retina. DOF was determined as ±0.445 D foveally and increased steadily with eccentricity to ±1.755 D at 8°. These values are slightly higher than those determined by Campbell3 and Atchison et al.4 They were explained by target characteristics and method of threshold determination. The target used by Wang and Ciuffreda6 was an annulus containing less high-spatial-frequency content than the optotypes used by Atchison et al.4 and also possessed a lower target contrast. Both of these characteristics would increase the DOF. Also, Wang and Ciuffreda6 found the blur threshold from the point of best focus, then altered target vergence by 1.5 D and found the threshold from the suprablur threshold direction. This method results in a mean value higher than if just the clear to blur direction were used.

Jiang10 postulated that myopes would possess a greater DOF than emmetropes. Before this, Green et al.5 had derived a formula for DOF from animal axial length data in which DOF is inversely proportional to the square of the axial length. Myopes tend to have larger axial lengths than emmetropes and hyperopes,11 and thus myopes would have a reduced DOF than other refractive groups. This was found not to be the case when Rosenfield and Abraham-Cohen2 directly compared the blur sensitivities of 12 myopes and 12 emmetropes. A bipartite target was viewed via a Badal optometer after cycloplegia. The mean blur detection threshold was significantly greater in the myopic subjects than in the emmetropic subjects (±0.19 and ±0.11 D, respectively). In Campbell,3 DOF was determined using artificial pupils between 2 and 7 mm diameter. The DOFs in Rosenfield and Abraham-Cohen2 are smaller than those found by Campbell using a 2-mm pupil diameter.3 The DOFs in Rosenfield and Abraham-Cohen2 appear to be relatively low considering that a 2-mm pupil was used. It has been suggested that the absence of adequate accommodative control led to an overestimation of DOF in Campbell’s original study.3 Also, subject instructions for the reporting of target blur differed between these two studies. Two further studies failed to find any significant difference between blur detection thresholds in myopes and emmetropes.12 13

Prior exposure to larger levels of positive defocus (typically, +1 to +3 spherical diopters [DS]) has been shown to improve a subject’s tolerance to blur.14 15 Variation in blur detection has only been measured indirectly, by tracking the changes in visual acuity during exposure to myopic defocus. These changes have been termed blur adaptation and are defined as an improvement in visual resolution after exposure to defocus, which is unaccompanied by a change in refractive error,15 pupil size, or palpebral aperture size. Mon-Williams et al.14 provided +1-DS lenses for 15 emmetropes to wear binocularly for 30 minutes. After this blur exposure, mean improvements in visual acuity with the plus lenses of 0.12 logarithm of the minimum angle of resolution (logMAR; OD), 0.094 logMAR (OS), and 0.089 logMAR (OU) were observed. Contrast sensitivity measurements before and after blur exposure showed that adaptation caused a decrease in sensitivity to spatial frequencies between 5 and 25 cyc/deg. It was also shown that monocular blur adaptation produced an improvement in the contralateral unaided acuity of ~35% of that in the adapted eye, suggesting contribution from higher levels of the visual system. Portello and Rosenfield16 detected an acuity improvement of 0.12 logMAR units when 12 subjects wore +2.50-D lenses for 1 hour. One, 5, and 10 minutes of clear vision after adaptation were found to have no effect on the improvement in visual resolution, indicating the robust nature of this neural adaptation.

George and Rosenfield15 directly compared the effect of blur adaptation on grating and Landolt C acuity in myopes and emmetropes. Two hours of blur adaptation to a +2.50-D lens produced a significant improvement in Landolt C acuity in both groups. The myopes exhibited a slightly greater improvement in acuity than did the emmetropes (0.27 ± 0.20 vs. 0.13 ± 0.22 logMAR units), although this difference failed to reach significance. However, the myopes did display a greater improvement than the emmetropes in grating acuity at low contrast. This implies that blur adaptation improvements may differ between refractive groups. Chronic exposure to defocus is more likely to occur in myopes, and these individuals are found to retain better visual acuity than emmetropes when viewing through myopic defocus.17 If myopes retain better visual resolution and adapt to defocused images to a greater extent than emmetropes, the retina may be subjected to a wider range of blurred images than in the emmetropic eye. This blurring could be in the form of inaccurate accommodative responses or failure to detect myopic shifts in vision, which could then exacerbate any myopia progression.

Most recently, Wang et al.18 showed that a 1-hour adaptation period to a +2.50-D lens produced an improvement in blur sensitivity. Eight myopes exhibited a mean decrease in blur sensitivity thresholds of between –0.15 and –0.19 D for a single optotype target, but no significant change in blur sensitivity to equivalent-sized optotypes in a line configuration. The authors propose that lateral inhibition due to the generally higher blur sensitivity threshold of the peripheral retina contributed to this effect.

To date, the potential for a differential effect of blur adaptation on blur sensitivity and discrimination thresholds between emmetropes and myopes has not been investigated. We have used a Badal optometer with bipartite field to determine whether any interrefractive differences exist in blur sensitivity or blur discrimination after adaptation to positive defocus in myopes and emmetropes.

Blur adaptation invariably produces a significant improvement in visual acuity,14 15 16 yet the work of Wang et al.18 suggests that blur adaptation increases subjective blur sensitivity. Without exception, the level of defocus has remained constant throughout the blur adaptation period in all previous studies. If subjective blur sensitivity were to increase after blur adaptation, then it is reasonable to expect that the deleterious effect of the adapting defocus level would cause a relative decline (not improvement) in visual acuity after blur adaptation. Therefore, our hypothesis states that blur adaptation decreases the subjective blur sensitivity and hence increases the subjective DOF. The resultant reduced sensitivity to blur and an expanded DOF reduces the potency of the induced defocus and supplements the improvement in visual resolution due to higher-order processes after a period of adaptation to constant defocus.

Direct measurements of the proximal and distal limits of the subjective DOF must be determined, along with a measurement of the blur discrimination threshold. Blur discrimination represents the ability to detect a subjective decrease in target clarity when the target is already subjected to a detectable level of defocus. The subjective detection of blur is not solely limited to targets that are optimally focused, but can be broadened to include targets that possess baseline levels of defocus. The ability to detect changes in target defocus is known to improve in the presence of baseline target defocus,19 20 21 22 which results in greater subjective appreciation of defocus changes for blurred targets than for initially clear targets. This second, more sensitive, threshold must also be measured pre- and postblur adaptation, to gain a full picture of any changes in blur detection that may occur. This method will result in two different measures of blur detection:

Blur sensitivity threshold is measured from both the proximal and distal directions, and the aggregate of these thresholds represents the DOF. The myopic (distal) and hyperopic (proximal) blur sensitivity thresholds are the change in target vergence necessary to produce a subjective change in target clarity and are measured from the subjective point of best focus.

Blur discrimination is the measure of blur detection with a convex lens (+1 DS) used for blur adaptation in the spectacle plane. This lens produces a blurred perception of the target. The change in target vergence necessary to produce a perceived worsening of target clarity is determined in the distal direction only.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Subjects
Twenty-four participants were recruited from the Department of Optometry, University of Bradford. All participants gave informed consent to take part after receiving information on the experimental procedure and possible consequences. All procedures were conducted in accordance with the Declaration of Helsinki, and the experimental procedure was granted approval by the Institutional Ethical Committee. The observer cohort consisted of eight emmetropes (EMMs), eight early-onset myopes (EOMs; myopia onset before 15 years of age), and eight late-onset myopes (LOMs; myopia onset after 15 years of age). Table 1 shows mean age, gender, and mean spherical equivalent refractive error (SER) for EMMs, EOMs, and LOMs. The cohort of 24 subjects was made up of 17 white Europeans, 4 Asians of Indian or Pakistani descent, 2 Southeast Asians, and 1 Afro-Caribbean. All participants had minimum best corrected visual acuity of 0.00 logMAR, were free from ocular disease and had no history of ocular surgery. Emmetropes were defined as subjects having SER between –0.49 and +0.49 DS, and astigmatism not exceeding 0.75 DC. An SER of at least –0.75 DS was used as the definition of myopia. Ametropic subjects habitually wore either spectacle or contact lens correction of their full refractive error and had been wearing their correction immediately before the experiment.


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  		TABLE 1. Group Details

 
Accommodation of the right eye was paralyzed using two drops of cyclopentolate hydrochloride (0.5%), separated by 5 minutes. Anterior diameter angles were checked by using the Van Herick technique before drug instillation.23 Forty minutes after instillation, autorefraction measurements were made with an infrared optometer (SRW-5000; Shin-Nippon, Tokyo, Japan), with subjects fixating at distance (6 m) and then near (20 cm), to confirm that cycloplegia had occurred.18

Apparatus
The experimental apparatus consisted of a chin rest, trial frame, 4-mm artificial pupil, +5.00-D Badal optometer,24 and bipartite target with rack-and-pinion gear system to move one half of the target relative to the other half (see Fig. 1 ). Printed on each half of the bipartite target were Sloan letters arranged in a 4 x 4 grid. Each letter subtended a visual angle of 15 minutes of arc at the subject’s eye when viewed via the Badal optometer. Subjects were given instruction and allowed practice trials, on adjustment of the bipartite target. In each trial, the moveable half of the target was racked in distal (away from the subject) and proximal (toward the subject) directions to find the position of just-noticeable blur of the letters relative to the letters on the fixed half of the target (i.e., blur sensitivity). Practice sessions were also conducted with +1.00 DS in place to blur the target. In this condition, the subject was instructed to adjust the position of one half of the bipartite target until a just-noticeable difference in blur was observed relative to the fixed half of the target (i.e., blur discrimination). The separation of the two halves of the bipartite target could be read from a dioptric scale, calibrated to the power of the Badal optometer lens, which provided a resolution of 0.025 D. The trial frame was used to support distance vision-correcting lenses.


Figure 1
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  		FIGURE 1. Diagram of experimental apparatus.

 
Procedure
Once adequate cycloplegia was obtained, refractive error in the right eye was measured with an open-view infrared autorefractor (average of 10 readings; SRW-5000; Shin-Nippon). After this, a subjective refraction was conducted on the right eye with a 4-mm artificial pupil in place. The subjective point of best focus was determined by the average of six readings: three with the target moving distally and three moving proximally. The subject was instructed to position the whole bipartite target at the point of maximum clarity. Baseline measurements of blur sensitivity before blur adaptation were made (average of five individual measurements) with optimal refractive correction before the right eye. The subject was instructed to "turn the dial toward/away from you until blur is first apparent on the left hand side of the target." A +1.00-DS trial lens was then placed before the right eye, and five individual measurements of blur discrimination were obtained and averaged. Instructions for this measurement were "Both sides of the target now appear equally blurred. Turn the dial away from you until the left-hand side of the target first appears more blurred than the right-hand side." The left eye of the subject was occluded throughout the process.

After baseline measurements, the subject was seated in an ophthalmic examination chair, 4 m from a logMAR chart and a 15-in. television screen. The subject wore a trial frame with optimum refractive correction, 4-mm artificial pupil, and a +1.00-DS blurring lens before the right eye and an occluder before the left eye. Visual acuity was then measured, followed by a period of 30 minutes blur adaptation, facilitated by observation of a broadcast television picture. After blur adaptation, visual acuity was remeasured using a different version of the logMAR chart. The subject was then repositioned at the Badal optometer, and blur sensitivity and blur discrimination thresholds were remeasured. The bipartite target remained at the position of best focus determined by the subject before blur adaptation. A further 10 autorefractor readings during distance fixation were recorded in addition to further near fixation readings to ensure cycloplegia remained at the required level.

Control trials (i.e., without blur adaptation) were also conducted as just outlined in a subset of the cohort, but these subjects were optimally corrected to the plane of the screen during the 30-minute television-viewing task. The order of trials (i.e., blur adaptation first or control trial first) was determined at random.

Data Analysis
Data were analyzed with commercial software (SPSS, ver. 13.0; SPSS, Inc., Chicago, IL). Pre- and postblur adaptation thresholds for visual acuity in defocused conditions, blur sensitivity, and blur discrimination were analyzed by one-way and repeated-measures ANOVA, with data grouped according to refractive error classification (i.e., EMM, EOM, and LOM).


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Addition of +1 DS over the best distance correcting lenses produced an equivalent reduction in visual acuity in all subjects, with no variation between refractive error groups (one-way ANOVA: F(2,21) = 0.881, P = 0.429).

Overall, visual acuity under refractive blur conditions (i.e., myopic defocus of 1 D, showed a significant mean improvement after 30 minutes of blur adaptation (–0.07 ± 0.07 logMAR, paired t-test P = 0.03). However, these changes in acuity did not differ significantly between refractive groups (one-way ANOVA F(2,21) = 0.167, P = 0.848), with all groups displaying improvements in acuity: EMMs (–0.06 ± 0.05 logMAR), EOMs (–0.07 ± 0.06 logMAR), and LOM (–0.07 ± 0.09 logMAR). Mean (±SD) pre- and postblur adaptation thresholds of visual acuity, proximal and distal blur sensitivity, total DOF, and blur discrimination are presented for EMMs, EOMs, and LOMs in Table 2 .


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  		TABLE 2. Group Mean ± SD Thresholds before and after a 30-Minute Period of Blur Adaptation

 
Blur adaptation had a significant effect on the proximal blur sensitivity threshold for all three refractive groups (two way ANOVA: F(1,40) = 4.786, P = 0.035), but refractive error failed to have a significant effect on proximal blur threshold (F(2,40) = 0.524, P = 0.60). Group mean increases in proximal blur sensitivity after blur adaptation of +0.13 ± 0.23, +0.23 ± 0.28, and +0.15 ± 0.21 D were found for EMMs, EOMs, and LOMs, respectively (Fig. 2) . One-way ANOVA showed that these changes were equivalent across refractive groups (F(2,20) = 0.38, P = 0.69).


Figure 2
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  		FIGURE 2. Mean changes in distal and proximal blur sensitivity threshold, total DOF, and blur discrimination threshold in EMMs, EOMs, and LOMs after 30 minutes of blur adaptation. Error bars, 1 SD.

 
Distal blur sensitivity thresholds were altered significantly by the period of blur adaptation (two-way ANOVA F(1,42) = 5.55, P = 0.02). Mean thresholds increased for all three refractive groups, with the largest mean increase observed in the EOMs. Mean changes in distal blur sensitivity thresholds were EMMs, +0.03 ± 0.14 D; EOMs, +0.30 ± 0.21 D; and LOMs, +0.08 ± 0.13 D (one-way ANOVA F(2,21) = 6.189, P = 0.01; Scheffé post hoc EMMs versus EOMs, P = 0.01; EMMs versus LOMs, P = 0.78; and EOMs versus LOMs, P = 0.05).

Total DOF increased significantly after blur adaptation (two-way ANOVA F(1,40) = 8.930, P = 0.005). The mean increases in DOF after blur adaptation in EMMs, EOMs, and LOMs were +0.15 ± 0.26, +0.53 ± 0.40, and +0.25 ± 0.26 D, respectively. These changes were equivalent across all three refractive groups (one-way ANOVA, F(2,20) = 3.06, P = 0.07).

Analysis of blur discrimination thresholds pre- and postblur adaptation using two-way ANOVA showed that blur adaptation had a significant effect on blur discrimination thresholds (F(1,42) = 7.723, P = 0.008), but when the factor of refractive error group was taken into account, no clear difference in the degree of change in blur discrimination threshold after blur adaptation could be found (F(2,42) = 3.17, P = 0.054).

Blur adaptation had no significant effect on the observers’ distance refractive error (two-way ANOVA; F(1,40) ≤ 0.001, P = 0.995).

Correlation coefficients (R) were determined for possible associations between changes in (1) VA and detection/discrimination thresholds and (2) discrimination and detection thresholds. A significant correlation was found only between the change in blur discrimination threshold and the change in the distal blur threshold (R(7), P < 0.01) for our EOM observers only. Figure 3A shows a plot of the change in total DOF against the change in visual acuity after blur adaptation. Similarly, Figure 3B is a plot of the change in blur discrimination threshold against the change in visual acuity.


Figure 3
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  		FIGURE 3. Change in (A) total DOFand (B) blur discrimination threshold against change in visual acuity after blur adaptation in EMMs, EOMs, and LOMs.

 
Preadaptation blur discrimination threshold values were found to be significantly less than preadaptation distal blur detection thresholds for all three refractive groups (paired t-test EMMs, P = 0.02; EOMs, P = 0.03, and LOMs, P = 0.01). This difference was also true of the corresponding postadaptation values, except in the case of the EMMs (paired t-test EMMs, P = 0.082; EOMs, P = 0.013; and LOMs, P = 0.01). The proximal and distal blur sensitivity thresholds were found to be equivalent in both the preadaptation and postadaptation conditions (three-way ANOVA; F(1,72) = 3.38, P = 0.07).

The positions of the proximal and distal edges of the DOF in free space were calculated, along with their midpoint for pre- and postblur adaptation. Blur adaptation was found to have no significant effect on the position of the midpoint of clear vision (two-way ANOVA; F(1,42) = 0.04, P = 0.84).

Control trials, where participants viewed broadcast television through optimal refractive correction, were conducted on two emmetropes, two EOMs, and two LOMs. Visual acuity with a +1-DS blurring lens placed briefly before the right eye was measured before and after 30 minutes of television viewing through optimal refractive correction. Visual acuity under these conditions did not differ significantly at the end of the control trial from the baseline value at the start of the trial (mean values, before the control trial 0.38 ± 0.11 logMAR; after the control trial 0.37 ± 0.05 logMAR; P = 0.911). Blur sensitivity was relatively unchanged at the end of the control trial in all refractive error groups (group mean changes of +0.01 ± 0.06, –0.01 ± 0.01, and +0.02 ± 0.04 D for EMMs, EOMs, and LOMs, respectively; pre- versus postadaptation detection, P = 0.306). Blur discrimination thresholds were also similar at the end of the control trial (group mean changes of –0.00 ± 0.02, –0.01 ± 0.05, and +0.02 ± 0.03 D for EMMs, EOMs, and LOMs, respectively; pre- versus postadaptation discrimination, P = 0.877).

To determine the repeatability of the measurements, we examined the intrasession differences in thresholds.2 The SD of the five readings that were recorded for each threshold measurement was calculated for each observer. Six threshold levels were determined for each participant (i.e., preadaptation proximal and distal blur sensitivity, postadaptation proximal and distal blur sensitivity, preadaptation blur discrimination, and postadaptation blur discrimination). Average standard deviations of thresholds for EMMs, EOMs, and LOMs are presented in Table 3 .


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  		TABLE 3. Repeatability of Proximal and Distal Blur Sensitivity and Blur Discrimination Measurements

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
The principal finding of this study is that the ability of EOMs to detect blur is degraded by a period of blur adaptation. The effect was manifest as an increase in the subjective DOF (i.e., proximal blur threshold plus distal blur threshold), either side of best focus. This deterioration in blur detection was also observed in LOMs and EMMs, but to a lesser extent. The shift in blur sensitivity threshold after blur adaptation is similar in EMMs and LOMs, indicating a degree of congruity between the neural adaptation processes of these two groups.

It is interesting to consider these results alongside what is understood about closed-loop accommodation control. Degradation of the ability to detect blur within a previously clear image, or an increase in blur of an already blurred image, is analogous to an increase in depth of focus. Elevation of the depth of focus component of the established models of accommodation control leads to a reduction in the blur-driven accommodation response. Jiang10 showed that adaptation of the accommodative sensory gain component of the accommodation control model could elevate the effective threshold of blur detection. The elevation of blur detection threshold was greater in myopes than emmetropes, although it is interesting to note that LOMs were studied. This elevation has potential implications for individuals undertaking near work. Low levels of myopic blur at distance can place an individual in a chronic state of blur adaptation if uncorrected and will lead to an increase in blur discrimination thresholds that may, in turn, reduce the accuracy of accommodation responses to subsequent near-vision tasks. These changes will lead to a state of hyperopic retinal defocus that, as seen in animal work, can cause structural recalibration of the axial length of the eye and consequently the development of myopia.25 26 In our experiment, all ametropic subjects habitually wore corrective lenses, and our protocol required the vision of our participants to be corrected in the period immediately before the experiment. It would be of interest to conduct a similar experiment on LOMs who choose not to wear corrective lenses for most tasks—for example, individuals who wear spectacles or contact lenses only for driving. From our existing data, we hypothesize that these individuals would have generally higher blur sensitivity thresholds at baseline (i.e., before a period of blur adaptation), and any increase in the blur sensitivity threshold after adaptation would be smaller than the changes seen in the current dataset.

Vera-Diaz et al.27 found that the accommodation responses of myopes increased after the introduction of blur during a near task with a diffusing filter. It would be of interest to examine the effect of longer-duration, lens-induced refractive blur adaptation on both static and dynamic accommodation responses and to evaluate any potential differences in progressing versus stable myopes. Also of note is the potential effect of blur adaptation on the magnitude of accommodative microfluctuations. If it is the case that accommodative microfluctuations play a role in accommodative accuracy, then an increase in the magnitude of these fluctuations would be expected after blur adaptation. It may be found that there is a differential shift in the magnitude of microfluctuations after blur adaptation between refractive error groups.28 This additional experimental work would create a link between the role of blur adaptation in the largely cognitive task undertaken in our experiment (i.e., the judgment of the clarity of a target) and its role in the autonomic mechanism of ocular accommodation.

Sensitivity to refractive blur was found to be reduced after a period of blur adaptation, with observers needing to place a greater amount of dioptric space between the two halves of the bipartite target before blur could be detected subjectively. This finding fits well with both the neural adaptation and variable gain spatial frequency channel models of blur adaptation proposed.14 29 Webster et al.30 examined the perception of blur after adaptation to sharpened or blurred images. It was found that the point of subjective best focus was shifted in the direction of the characteristics of the adapting image: After adaptation to an artificially sharpened image, the subject required subsequent images to be more sharp to be perceived as clear; after exposure to a blurred scene, the subject could accept as clear an image that was more blurred than the baseline image. Thus, the visual system appears to undergo recalibration of its blur perception mechanism, dependent on the visual diet. In relation to our data, after adaptation to a blurred scene, a subject will accept an increased amount of image blur as sharply focused, compared with the preadaptation baseline.

The mean blur sensitivity thresholds in the present study are higher than those reported by Rosenfield and Abraham-Cohen2 (0.11 and 0.18 D for emmetropes and myopes, respectively). This difference may be due in part to the methodological difference in the blur detection task in our study (i.e., detection of the point of first subjective blur of one target relative to a clear reference target), whereas Rosenfield and Abraham-Cohen used the point where the reference half of the target was clearer than the moving half of the target as the criterion for blur threshold.

Our findings are contrary to those of Wang et al.,18 who found an increase in blur sensitivity (i.e., a reduction in threshold) in myopes after a period of blur adaptation. Several procedural differences in the experiments may account for this disagreement. First, Wang et al.18 used a single target that was moved in dioptric space until blur was detected, thus introducing a temporal component to the blur detection task. In the experiment presented herein, the observer had a permanent reference target (the fixed half of the bipartite target) with which to compare the moving half in both the blur sensitivity and blur discrimination tasks. Second, a greater degree of optical defocus was imposed by Wang et al. during the blur adaptation phase (+2.50 D) than in the present experiment (+1.00 D). The imposition of higher levels of dioptric blur may cause adaptation in the gain of both low and high spatial frequencies, whereas lower levels of blur may affect only the higher spatial frequency channels. Finally, the target was repositioned at the point of subjective best focus for each trial, whereas in our study, the fixed reference target remained at a constant position within the Badal optometer.

The mean blur discrimination thresholds were less than the blur sensitivities across all our refractive groups and adaptation states. The ability to detect changes in defocus is known to vary with the initial level of defocus.19 Campbell and Westheimer20 varied baseline blur over a range of ±3 D from optimum focus. The maximum DOF was recorded at a target vergence of 0.7 D, whereas greatest blur sensitivity occurred symmetrically at between 1 and 1.8 D baseline blur for both myopic and hyperopic baseline defocus. Our blur discrimination measurements were in essence blur sensitivity measures undertaken at +1 DS from optimum focus. The relationship between our blur discrimination and blur detection thresholds concur with the results of Campbell and Westheimer.20 Remole21 proposed the concept of a near-focus plateau derived from spherical aberration, where defocus levels of up to ±0.50 DS fail to disrupt vision, although larger levels of defocus cause visibility to decrease at a greater rate. Direct comparisons of blur detection and discrimination thresholds also found lower discrimination thresholds than detection thresholds.22 When measuring blur discrimination thresholds before a period of blur adaptation, it is easier for a subject to detect a differential level of blur between two targets. After a period of blur adaptation, both halves of the target will be perceived as clearer due to the blur-adaptation process,30 and, as a result of this, the blur discrimination threshold will increase because of the now-extended range of clarity.

All refractive groups produced a significant improvement in high-contrast visual acuity under refractive blur conditions after a period of blur adaptation. However, there was no interrefractive difference in the magnitude of this effect, with EMMs, EOMs, and LOMs all producing equivalent acuity improvements. This follows the work of George and Rosenfield15 who failed to observe any significant difference in the magnitude of high-contrast VA improvement after blur adaptation between 13 emmetropes and 18 myopes. In both studies visual acuity was measured in conditions of major defocus. The variability of visual acuity measurements is known to increase in the presence of blur.31 32 This reduction in the repeatability of measurements could mask any population differences that may exist. An addition of +1 DS to the monocular distance refraction caused visual acuity to fall by equivalent amounts in our EMMs, EOMs, and LOMs. The visual acuity of myopes and nonmyopes has been observed to fall by equal amounts when positive lenses are added to distance refractions.33 This decrease will mean that all subjects will experience equivalent amounts of initial defocus, regardless of refractive error group. We failed to find a significant correlation between the change in visual acuity and change in total DOF or between the change in visual acuity and the change in blur discrimination threshold, after blur adaptation in any of the refractive groups studied. This lack of correlation may be due in part to the effect of myopic defocus imposed by the blurring lenses on the end point of visual acuity measurements. Carkeet et al.31 found that optical defocus extended the probit interval, and thus reduced the accuracy of the endpoint of high-contrast visual acuity measurements. Similarly, Rosser et al.32 found an increase in the test-retest variability of visual acuity measurement under conditions of induced myopic defocus.

Long-term exposure to myopic defocus has been implicated in increased myopia progression in children. Chung et al.34 showed in their study of the effect of undercorrection of myopia on myopia progression, that the undercorrected myopic children progressed at a greater rate than the fully corrected myopes over a 2-year period. The level of undercorrection was 0.75 DS binocularly. These undercorrected children would therefore be exposed to chronic defocus when observing objects at distance—a situation that is highly likely to induce neural adaptation to blur and that may influence the perception of blur at near. Consequently, accommodation responses of the undercorrected group may have been reduced, increasing the degree of hyperopic defocus to tasks at the close near-working distance adopted by children.35

We have shown that adaptation to a blurred scene modifies the blur detection mechanisms of the visual system in young adults. Blur sensitivity thresholds are elevated significantly in EOMs after blur adaptation. EMMs and LOMs do not appear to exhibit such a profound effect. The ability to discriminate between two differentially blurred images (i.e., blur discrimination) is impaired by a period of blur adaptation.


   Footnotes
 
Supported by a Clinical Demonstratorship from the Department of Optometry, University of Bradford (MPC).

Submitted for publication July 20, 2006; revised January 5, February 15, and March 1, 2007; accepted April 23, 2007.

Disclosure: M.P. Cufflin, None; A. Mankowska, None; E.A.H. Mallen, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Edward A. H. Mallen, Department of Optometry, University of Bradford, Richmond Road, Bradford, West Yorkshire BD7 1DP, UK; e.a.h.mallen{at}bradford.ac.uk.


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

  1. Rabbetts RB. Ocular aberrations. Clinical Visual Optics. 1998; 3rd ed. 288–289. Butterworth-Heinemann Oxford, UK.
  2. Rosenfield M, Abraham-Cohen JA. Blur sensitivity in myopes. Optom Vis Sci. 1999;76:303–307.[Web of Science][Medline][Order article via Infotrieve]
  3. Campbell FW. The depth of field of the human eye. Optica Acta. 1957;4:157–164.
  4. Atchison DA, Charman WN, Woods RL. Subjective depth of focus of the eye. Optom Vis Sci. 1997;74:511–520.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  5. Green DG, Powers MK, Banks MS. Depth of focus, eye size and visual acuity. Vision Res. 1980;20:827–835.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  6. Wang B, Ciuffreda KJ. Depth of focus of the human eye in the near retinal periphery. Vision Res. 2004;44:1115–1125.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  7. Miles PW. Depth of focus and amplitude of accommodation through trifocal glasses. Arch Ophthalmol. 1953;49:271–279.[Medline][Order article via Infotrieve]
  8. Layton A, Dickinson J, Plutznick M. Perception of blur in optometric tests. Am J Optom Physiol Opt. 1978;55:75–77.[Web of Science][Medline][Order article via Infotrieve]
  9. Wang B, Ciuffreda KJ. Foveal blur discrimination of the human eye. Ophthalmic Physiol Opt. 2005;25:45–51.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  10. Jiang BC. Integration of a sensory component into the accommodation model reveals differences between emmetropia and late-onset myopia. Invest Ophthalmol Vis Sci. 1997;38:1511–1516.[Abstract/Free Full Text]
  11. Bullimore MA, Gilmartin B, Royston JM. Steady-state accommodation and ocular biometry in late-onset myopia. Doc Ophthalmol. 1992;80:143–155.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  12. Jiang BC, Morse SE. Oculomotor Functions in Late Onset Myopia. Ophthalmic Physiol Opt. 1999;19:165–172.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  13. Schmid KL, Iskander DR, Li RW, Edwards MH, Lew JK. Blur detection thresholds in childhood myopia: single and dual target presentation. Vision Res. 2002;42:239–247.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  14. Mon-Williams M, Tesilian JR, Strang NC, Kochhar P, Wann JP. Improving vision: neural compensation for optical defocus. Proc Royal Soc Lond Biological Science. 1998;265:71–77.[Abstract/Free Full Text]
  15. George S, Rosenfield M. Blur adaptation and myopia. Optom Vis Sci. 2004;81:543–554.[Web of Science][Medline][Order article via Infotrieve]
  16. Portello J, Rosenfield M. Effect of intervening periods of clear vision on blur adaptation. Optom Vis Sci. 2002;79(suppl)24.
  17. Thorn F, Cameron L, Arnel J, Thorn S. Myopia adults see through defocus better than emmetropes. Tokoro T eds. Myopia Updates. Proceedings of the 6th International Conference on Myopia. 1998;368–374. Springer Tokyo.
  18. Wang B, Ciuffreda KJ, Vasudevan B. Effect of blur adaptation on blur sensitivity in myopes. Vision Res. 2006;46:3634–3641.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  19. Walsh G, Charman WN. Visual sensitivity to temporal change in focus and its relevance to the accommodation response. Vision Res. 1988;27:1207–1221.[Web of Science]
  20. Campbell FW, Westheimer G. Sensitivity of the eye to differences in focus. J Physiol. 1958;143:18P.
  21. Remole A. Spatial frequency thresholds vs border enhancement: sensitivity to retinal defocus. Am J Optom Physiol Opt. 1982;59:135–145.[Web of Science][Medline][Order article via Infotrieve]
  22. Jacobs RJ, Smith G, Chan CD. Effect of defocus on blur thresholds and on thresholds of perceived change in blur: comparison of source and observer methods. Optom Vis Sci. 1989;66:545–553.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  23. Elliott DB. Clinical Procedures in Primary Eye Care. 2003; 2nd ed. 244–246. Butterworth-Heinemann Oxford, UK.
  24. Atchison DA, Bradley A, Thibos LN, Smith G. Useful variations of the Badal optometer. Optom Vis Sci. 1995;72:279–284.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  25. Zhu X, Park TW, Winawer J, Wallman J. In a matter of minutes, the eye can know which way to grow. Invest Ophthalmol Vis Sci. 2005;46:2238–2241.[Abstract/Free Full Text]
  26. Zhing X, Ge J, Nie H, Smith EL, 3rd. Compensation for experimentally induced hyperopic anisometropia in adolescent monkeys. Invest Ophthalmol Vis Sci. 2004;45:3373–3379.[Abstract/Free Full Text]
  27. Vera-Diaz FA, Gwiazda J, Thorn F, Held R. Increased accommodation following adaptation to image blur in myopes. J Vision. 2004;4:1111–1119.[CrossRef]
  28. Day M, Strang NC, Seidel D, Gray LS, Mallen EAH. Refractive group differences in accommodation microfluctuations with changing accommodation stimulus. Ophthalmic Physiol Opt. 2006;26:88–96.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  29. Georgeson MA, Sullivan GD. Contrast constancy: deblurring in human vision by spatial frequency channels. J Physiol. 1975;252:627–656.[Abstract/Free Full Text]
  30. Webster MA, Georgeson MA, Webster SM. Neural adjustments to image blur. Nat Neurosci. 2002;5:893–840.
  31. Carkeet A, Lee L, Kerr JR, Keung MM. The slope of the psychometric function for Bailey-Lovie letter charts: defocus effects and implications for modelling letter-by-letter scores. Optom Vis Sci. 2001;78:113–121.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  32. Rosser DA, Murdoch IE, Cousens SN. The effect of optical defocus on the test-retest variability of visual acuity measurements. Invest Ophthalmol Vis Sci. 2004;45:1076–1079.[Abstract/Free Full Text]
  33. Radhakrishnan H, Pardhan S, Calver RJ, O’Leary DJ. Unequal reduction in visual acuity with positive and negative defocusing lenses in myopes. Optom Vis Sci. 2004;81:7–14.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  34. Chung K, Mohidin N, O’Leary DJ. Undercorrection of myopia enhances rather than inhibits myopia progression. Vision Res. 2002;42:2555–2559.[CrossRef][Web of Science][Medline][Order article via Infotrieve]
  35. Gwiazda JE, Hyman L, Norton TT, et al. Accommodation and related risk factors associated with myopia progression and their interaction with treatment in COMET children. Invest Ophthalmol Vis Sci. 2004;45:2143–2151.[Abstract/Free Full Text]




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