May 2007
Volume 48, Issue 5
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Visual Psychophysics and Physiological Optics  |   May 2007
The Effect of Myopia on Contrast Thresholds
Author Affiliations
  • Bistra D. Stoimenova
    From the Department of Physiology, Medical University Sofia, Sofia, Bulgaria.
Investigative Ophthalmology & Visual Science May 2007, Vol.48, 2371-2374. doi:10.1167/iovs.05-1377
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      Bistra D. Stoimenova; The Effect of Myopia on Contrast Thresholds. Invest. Ophthalmol. Vis. Sci. 2007;48(5):2371-2374. doi: 10.1167/iovs.05-1377.

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Abstract

purpose. To examine the relationship between the degree of myopia and foveally measured contrast thresholds.

methods. Monocular thresholds for positive and negative contrast were obtained from 60 myopes (−1.0 to −8.0 D) and 20 emmetropes of similar age. The contrast thresholds were measured at different background luminance levels, photopic and mesopic, by using a visual stimulus generator and a staircase method. Against a gray background, light (dark) letters were used to study positive (negative) contrast. ANOVA was used to identify factors influencing the contrast sensitivity. For the myopes, the contrast thresholds were regressed against the spherical equivalent refractive error, accounting for different intercepts and slopes between positive and negative contrasts.

results. Myopes yielded lower sensitivity to contrast than did emmetropes and, in contrast to emmetropes, exhibited higher contrast thresholds for negative than for positive contrast in both photopic and mesopic conditions. At all background luminance levels, contrast thresholds of myopic subjects increased systematically with a higher spherical equivalent refractive error, and the rate of increase was higher for negative than for positive contrast. The regression models yielded adjusted coefficients of determination (R 2 adj) of 0.735 and higher.

conclusions. Despite having corrected visual acuity, myopes exhibited reduced sensitivity to contrast in comparison to emmetropes. Furthermore, the contrast sensitivity decreased with an increasing degree of myopia, and the rate of decline was higher for negative than for positive contrast.

There is ample evidence, from both psychophysical and electrophysiological studies, that the differences between the myopic and emmetropic visual systems do not derive only from the biometrics of the eye. In particular, highly myopic subjects have been observed to have considerable impairment of the contrast sensitivity function. 1 2 3 Results obtained by means of automated static perimetry, show that subjects with high and moderate myopia have reduced threshold sensitivity. 4 Electrophysiological studies employing the multifocal electroretinogram (mfERG) reveal reduced N1 and P1 amplitudes and delayed latencies for subjects with high and moderate myopia. 5 A decrease in the amplitude of mfERG waves, most evident within the center of the retina (rings 1 and 2), has also been reported by other investigators. 6  
In view of the fact that the waves of human mfERG are generated mainly by the ON and OFF bipolar retinal cells, 7 the results for reduced amplitudes of waves of mfERG in myopic subjects 5 6 provide evidence of functional changes in the ON and OFF bipolar cells in the retinas of myopes. This fact implies changes in the entire ON and OFF pathways in the visual system of myopes. Furthermore, reduced sensitivity of the myopic visual system to positive and negative contrast is predicted, since the main function of the ON and OFF channels is to process separately the increment and decrement luminance inputs. 
The purpose of the present study was to test this hypothesis. For both positive and negative contrast, we examined the thresholds of myopic subjects with spherical equivalent refractive error (SER) ranging from −1.0 to −8.0 D, with no progression in myopia. To detect even small changes in contrast sensitivity, we used a sensitive psychophysical laboratory method and a wide range of background luminance levels, both photopic and mesopic. In addition, we studied differences between positive and negative contrast for emmetropic subjects, who served as a control group. 
Methods
Subjects
Sixty myopes, 18 to 31 years of age (mean age, 21.8), and 20 emmetropes, 18 to 33 years of age (mean age, 22.6), participated in the study. The spherical equivalent refractive error of the myopic subjects ranged from −1.0 to −8.0 D. Refraction was established subjectively after cycloplegia. All myopes had visual acuity corrected to normal (20/20 or better). Myopic subjects with SER from −6.0 to −8.0 D (11 subjects) were corrected with contact lenses and the rest (49 subjects) with spectacle lenses. The quality of refractive error correction was checked with a red-green test. Only right eyes were examined. All subjects gave their informed consent before participating. The studies were conducted in accordance with the principles embodied in the Declaration of Helsinki. 
Procedure
The monocular contrast thresholds of each subject were measured for positive and negative contrast at different luminance levels, with a visual stimulus generator (Cambridge Research Systems, Cambridge, UK). The luminance level and contrast polarity were chosen in random order. The subjects, who were untrained, viewed the stimuli foveally at a distance of 70 cm. A chin and forehead rest was used to minimize head movements. 
The stimuli were five Cyrillic letters of approximately equal legibility. A randomly chosen letter, with a size of 30 min arc (5 min arc for details), was displayed in the middle of a visible 3.5-deg arc circle, which was located in the center of a monitor screen. The circle was light or dark depending on the contrast studied. Light and dark letters against a gray background were used to study positive and negative contrast, respectively. Each appearance of a letter was preceded by an acoustic signal. The time that a letter was displayed (150 ms) exceeded that necessary for complete temporal summation. The contrast threshold, with a 67% probability of correct identification, was determined by applying a staircase method. The threshold was taken from one trial comprising 10 reversals of the staircase. 
The study was conducted at six background luminance levels: 0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2—three in the mesopic and three in the photopic region. Before each measurement, subjects took 5 minutes to adapt to the respective luminance. 
Results
To identify factors influencing the contrast sensitivity, contrast thresholds were first analyzed with a three-factor, repeated-measures ANOVA with one between-subject factor (refractive status) and two within-subject factors (contrast and background). The three factors were (1) refractive status with two levels (myopes and emmetropes), (2) contrast, with two levels (negative and positive), and (3) background luminance level with six levels (0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2). The ANOVA revealed significant main effects for all three factors: refractive status (F(2,78) = 1175.62; P < 0.0001), contrast (F(1,859) = 15.04; P < 0.0001), and background (F(5,859) = 1752.60; P < 0.0001) and significant interactions between refractive status and contrast (F(1,859) = 35.35; P < 0.0001) and refractive status and background (F(5,859) = 117.90; P < 0.0001). No interaction between contrast and background (F(5,859) = 0.41; P = 0.8441) and no three-way interaction (F(5,859) = 0.74; P = 0.5902) were found. 
The ANOVA results are depicted in Figure 1 , which shows mean contrast thresholds in myopic and emmetropic subjects, estimated separately for positive and negative contrast at the six background luminance levels. Emmetropes and myopes clearly differ in their average performance and, although the absolute differences become much less pronounced with an increasing background luminance level, these differences are all significant, according to Bonferroni-corrected two-sample t-tests conducted separately for each contrast and background luminance level. 
Myopic subjects conspicuously exhibit higher mean contrast thresholds for negative than for positive contrast, whereas in emmetropic subjects, the effect of contrast is reversed. To test the statistical significance of this phenomenon, we conducted Bonferroni-corrected paired t-tests for comparison of means, which were calculated separately for myopic and emmetropic subjects, at each of the six background luminance levels. Thus, the Bonferroni correction is given by the multiplication of each P-value by 6, which represents the number of tests conducted in each subject group. At all luminance levels, the tests revealed a significant difference (P < 0.0001) between the mean thresholds for negative and positive contrast, the difference being positive for myopes and negative for emmetropes. 
Concentrating on the subsample of myopic subjects, we analyzed the relationship between the degree of myopia and contrast thresholds (CT) by regressing the latter on the spherical equivalent refractive error (D). To account for different intercept and slope coefficients for positive and negative contrast, we ran the regression:  
\[CT_{i}{=}{\beta}_{0}{+}{\beta}_{1}I_{i}{+}{\beta}_{2}D_{i}{+}{\beta}_{3}D_{i}I_{i}{+}{\epsilon}_{i},\]
where the dummy variable I takes on the value 1 for positive and 0 for negative contrast. This coding of the variable enables β1 and β3 to be viewed as differences in the intercept and the slope, respectively, which are associated with a change from negative to positive contrast. The regression model, which is thus analogous to ANCOVA (analysis of covariance), was estimated separately for each of the six background luminance levels: 0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2
The six scatterplots are shown in Figure 2 . The solid and the dashed lines represent ordinary least squares (OLS) linear regressions for negative and positive contrast, respectively. In all subsamples, an increase in the refractive error led to an overall increase in the contrast threshold, although the slopes for positive contrast were lower than for negative contrast. The regression results are given in Table 1
The slope coefficient β2, measuring the influence of the degree of myopia on contrast thresholds for negative contrast, was positive and significant (P < 0.001) at all six luminance levels. The slope for positive contrast, given by β2 + β3, was smaller than that for negative contrast at all luminance levels. β3, representing the difference in the two slopes, was negative and except at 0.6 cd/m2 (P = 0.084) significant at the 1% level. Overall, the regression models yielded adjusted coefficients of determination (R 2 adj) of ≥0.735. As expected, the intercept terms for negative contrast, β0, were all positive. The regression lines for positive contrast exhibited even higher intercepts in all subsamples; all coefficients β1, measuring the difference between the intercepts, were positive, although in three cases not statistically significant. This latter result is consistent with the previously discussed, Bonferroni-corrected paired t-tests for comparison of means, according to which emmetropes display higher thresholds for positive than for negative contrast. 
Discussion
The results of the present study indicate that contrast thresholds are affected by myopia, the nature of contrast, and the background luminance level. In particular, despite having corrected refractive errors, myopic subjects show reduced sensitivity to contrast, relative to emmetropes, although the difference diminishes with a higher background luminance level. A decrease in contrast thresholds as a result of an increase in the background luminance level has also been reported in another psychophysical investigation, 8 and the phenomenon has been discussed in experiments on the molecular and cellular mechanisms of adaptation of the retina. 9 10  
We also find that emmetropes exhibit lower contrast thresholds for negative than for positive contrast in both photopic and mesopic conditions, the opposite pattern being exhibited by myopes. Other psychophysical 11 12 13 14 and electrophysiological studies 15 on emmetropes also confirm the existence of such functional asymmetry in the human visual system. This phenomenon can be explained with a morphologic asymmetry in the ON and OFF pathways 16 17 or with different mechanisms of contrast gain in these two channels of the human visual system. 15  
Furthermore, our study provides, for the first time, evidence that contrast sensitivity is negatively related to the degree of myopia. We find that contrast thresholds increase systematically with a higher spherical equivalent refractive error and that the rate of increase is higher for negative than for positive contrast. 
One might argue that this phenomenon results from the fact that myopia reduces the optical quality of the retinal image. Two factors leading to a deteriorated retinal image quality are optical defocus and astigmatism. For this reason, in our investigation, only myopic subjects without astigmatism and with good corrected visual acuity (20/20 or better) were examined. 
With respect to other optical imperfections, the so-called high-order aberrations, the empiric evidence is contradictory. Some investigators state that myopic eyes do not have significantly more aberrations than do emmetropic eyes. 18 19 20 21 22 Conversely, other studies suggest that myopes have higher aberrations than do emmetropes. However, the amplitude and type of aberrations are found to have little correlation with the amount of spherical equivalent refractive error. 23 24 According to a third group of researchers, high-order aberrations increase with a higher degree of myopia. 25 The last result is not consistent with our findings that the rate of increase in contrast thresholds with a higher degree of myopia differs between negative and positive contrast. However, this argument is based on the assumption that aberration-induced retinal image distortions do not introduce asymmetries related to the sign of stimulus contrast. Therefore, it is possible that the observed increase in contrast thresholds with a higher degree of myopia is caused by aberrations of the myopic eye, and/or aberrations added by corrective lenses. Alternatively, the causes could also lie in functional and/or morphologic changes in the retina of the myopic eye, especially because there is evidence that such changes exist. 4 5 6  
In conclusion, this study shows that, despite having good corrected visual acuity, myopes exhibit reduced sensitivity to contrast relative to emmetropes. The present study demonstrated, for the first time, that contrast sensitivity is negatively related to the degree of myopia. Contrast thresholds systematically increase with a higher spherical equivalent refractive error, and the rate of increase is higher for negative than for positive contrasts. These results warrant further investigation of the functional characteristics of the myopic visual system. 
 
Figure 1.
 
Mean contrast thresholds (%) of myopes (M) and emmetropes (E) for negative (−) and positive (+) contrast at six different background luminance levels: 0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2. The SEM is in brackets.
Figure 1.
 
Mean contrast thresholds (%) of myopes (M) and emmetropes (E) for negative (−) and positive (+) contrast at six different background luminance levels: 0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2. The SEM is in brackets.
Figure 2.
 
Scatterplots of negative and positive contrast thresholds (CT) as a function of the spherical equivalent refractive error (D) at the six background luminance levels. Solid and dashed lines: OLS linear regressions for negative and positive contrast, respectively. Note that the vertical axes differ in scale.
Figure 2.
 
Scatterplots of negative and positive contrast thresholds (CT) as a function of the spherical equivalent refractive error (D) at the six background luminance levels. Solid and dashed lines: OLS linear regressions for negative and positive contrast, respectively. Note that the vertical axes differ in scale.
Table 1.
 
Regression Results for the Relationship between the Contrast Thresholds and the Spherical Equivalent Refractive Error
Table 1.
 
Regression Results for the Relationship between the Contrast Thresholds and the Spherical Equivalent Refractive Error
cd/m2 n β0 P β1 P β2 P β3 P F R adj 2
0.6 120 16.396 0.000 0.817 0.330 4.099 0.000 −0.570 0.084 117.05* 0.745
1.6 120 12.996 0.000 0.942 0.157 2.550 0.000 −0.609 0.002 166.04* 0.806
2.6 120 10.741 0.000 1.761 0.009 2.176 0.000 −0.776 0.000 176.10* 0.815
3.6 120 8.915 0.000 1.508 0.007 1.952 0.000 −0.656 0.000 214.75* 0.843
100 120 5.797 0.000 0.261 0.169 0.655 0.000 −0.228 0.000 125.90* 0.759
125 120 5.510 0.000 0.624 0.007 0.606 0.000 −0.287 0.000 111.08* 0.735
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Figure 1.
 
Mean contrast thresholds (%) of myopes (M) and emmetropes (E) for negative (−) and positive (+) contrast at six different background luminance levels: 0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2. The SEM is in brackets.
Figure 1.
 
Mean contrast thresholds (%) of myopes (M) and emmetropes (E) for negative (−) and positive (+) contrast at six different background luminance levels: 0.6, 1.6, 2.6, 3.6, 100, and 125 cd/m2. The SEM is in brackets.
Figure 2.
 
Scatterplots of negative and positive contrast thresholds (CT) as a function of the spherical equivalent refractive error (D) at the six background luminance levels. Solid and dashed lines: OLS linear regressions for negative and positive contrast, respectively. Note that the vertical axes differ in scale.
Figure 2.
 
Scatterplots of negative and positive contrast thresholds (CT) as a function of the spherical equivalent refractive error (D) at the six background luminance levels. Solid and dashed lines: OLS linear regressions for negative and positive contrast, respectively. Note that the vertical axes differ in scale.
Table 1.
 
Regression Results for the Relationship between the Contrast Thresholds and the Spherical Equivalent Refractive Error
Table 1.
 
Regression Results for the Relationship between the Contrast Thresholds and the Spherical Equivalent Refractive Error
cd/m2 n β0 P β1 P β2 P β3 P F R adj 2
0.6 120 16.396 0.000 0.817 0.330 4.099 0.000 −0.570 0.084 117.05* 0.745
1.6 120 12.996 0.000 0.942 0.157 2.550 0.000 −0.609 0.002 166.04* 0.806
2.6 120 10.741 0.000 1.761 0.009 2.176 0.000 −0.776 0.000 176.10* 0.815
3.6 120 8.915 0.000 1.508 0.007 1.952 0.000 −0.656 0.000 214.75* 0.843
100 120 5.797 0.000 0.261 0.169 0.655 0.000 −0.228 0.000 125.90* 0.759
125 120 5.510 0.000 0.624 0.007 0.606 0.000 −0.287 0.000 111.08* 0.735
Copyright 2007 The Association for Research in Vision and Ophthalmology, Inc.
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