An Assessment of the Contrast Sensitivity in Patients with Ametropic and Anisometropic Amblyopia in Achieving the Corrected Visual Acuity of 1.0

Both visual acuity (VA) and contrast sensitivity (CS) are important parameters for measuring visual function. In this research, we investigated the CS of patients with ametropic or anisometropic amblyopia, whose corrected visual acuity (CVA) recovered to 1.0. Fifty-five cases with amblyopia and 22 control cases with a normal visual acuity of 1.0 were enrolled. The CS of the patients whose ametropic amblyopia had recovered to a CVA of 1.0 at 18 cpd spatial frequency was still lower than that of the normal control group under both photopic and scotopic conditions (P = 0.001, 0.025), but there were no significant differences at low- and middle-spatial frequencies. The CS of amblyopic eyes of the patients with anisometropic amblyopia was lower than that of the normal control group at the 18 cpd spatial frequency under photopic conditions (P = 0.005), and at the 6 cpd, 12 cpd, and 18 cpd spatial frequencies under scotopic conditions (P = 0.008, <0.001, 0.004, respectively). The CS between the amblyopic eyes and the sound eyes of patients with anisometropic amblyopia presented significant differences at the 6 cpd, 12 cpd, and 18 cpd spatial frequencies under scotopic conditions (P = 0.025, 0.045, 0.019, respectively). We suggest that amblyopia treatment should involve not only the correction of VA but also the improvement of CS.

Scientific RepoRts | 7:42043 | DOI: 10.1038/srep42043 cases (the spherical equivalent is 4.24 ± 2.130DS) with middle or high hyperopia of both eyes and 23 cases (the spherical equivalent of the amblyopic eye is 4.32 ± 2.175 DS, and the spherical equivalent of the non-amblyopic eye is 2.00 ± 1.663 DS) with anisometropia.
All patients were divided into three groups: the ametropic amblyopic group (MA), anisometropic amblyopic group (NA), and the normal control group (NC). The difference in spherical power and the astigmatic power between the two eyes was at least 1.5D and 1.0D, respectively, in the anisometropic amblyopic group. Measurements of visual acuity. The measurements of visual acuity employed the international standard vision acuity chart (E chart, Shengde Company, China), the background luminance of which was 300 cd/m 2 by a LCD monitor. The 1.0 lines of the chart were at the same height as the eyes. The chart was 5 meters away from the subject.

Ethical approval and informed consent.
Measurements of contrast sensitivity. The refractive error of all patients with amblyopia was corrected by wearing glasses. The sinusoidal gratings were used as the contrast sensitivity testing standards to conduct the Functional Acuity Contrast Test (OPTEC ® 6500 visual function tester (Stereo Company, U.S.)). The brightness values were 85 cd/m 2 (light) and 3 cd/m 2 (dark), respectively. The spatial frequencies of the sinusoidal grating were 1.5 cpd, 3.0 cpd, 6.0 cpd, 12.0 cpd, and 18.0 cpd. The right and left eyes were tested, separately. The test was carried out in the following steps: The remote switch was turned on to simulate the test distance of 6 m. Then, the Day button was pressed to test the bright mode using the 5 # to 9 # test pictures. Finally, the Night button was pressed for the dark mode test. If the answer was correct, the next test picture was presented; if not, the last correct answer was recorded 6 .
Statistical analyses. The data were analysed using a paired t-test by the Statistical Package for Social Sciences (SPSS) software (Version 17.0). P-values less than 0.05 were considered statistically significant. Table 1 and Fig. 1 show the contrast sensitivity of the three groups under photopic and scotopic conditions.

Results
The differences between the ametropic amblyopic group and the normal control group. Table 2 shows that the two groups exhibited significant differences at 18 cpd under both the photopic and the scotopic conditions (P = 0.001, 0.025).
The differences between the amblyopic eyes (AE) of the anisometropic amblyopic group and the normal control group. Table 3 shows significant differences at 18 cpd under the photopic conditions (P = 0.005) and 6 cpd, 12 cpd,and 18 cpd under the scotopic conditions (P = 0.008, < 0.001, 0.004).  The differences between the non-amblyopic eyes (nAE) of the anisometropic amblyopic group and the normal control group. Table 4 shows no differences between the groups under both photopic and scotopic conditions at all spatial frequencies.
The differences between the non-amblyopic eyes and the corresponding amblyopic eyes in the anisometropic amblyopic group. Table 5 shows no differences between the non-amblyopic eyes and the corresponding amblyopic eyes in the anisometropic amblyopic group under photopic conditions. Differences were shown at 6 cpd, 12 cpd, and 18 cpd under scotopic conditions (P = 0.025, 0.045, 0.019).

Discussion
CS is an important parameter in detecting visual dysfunction. There are two pathways from the retina to cortex: the parvocellular and the magnocellular pathways 7 . Both of them could transform and distribute information to the visual cortex (V1) 8 . The parvocellular system is selectively sensitive to middle to high spatial frequencies (low temporal frequencies); however, the magnocellular system is sensitive to a very broad temporal frequency range (low spatial frequencies) 9 . The contrast-sensitivity function (CSF) is controlled by the spatiotemporal characteristics of the visual pathway. The magnocellular system forms the basis of the achromatic CSF and dominates close-to-threshold detection. When the magnocellular system saturates, the parvocellular system dominates the higher contrast detection 10 . Amblyopia is a disease with visual abnormality, such as visual acuity, binocular function, and contrast sensitivity 11 . Amblyopia showed decreased contrast sensitivity under mesopic conditions and at intermediate spatial frequencies 12 . The visual acuity was significantly better correlated with improved CS during amblyopia treatment 13 . Despite the significant correlation between VA and CS, VA cannot predict CS 14 . VA is a measure of the resolution under high contrast. CS is a measure of the ability to distinguish the visual markers under different levels of contrast. The latter is more adaptively used in daily life. Unlike other studies that focused on the lower visual acuity of patients with ametropic amblyopia or anisometropic amblyopia, our study focused on the contrast sensitivity of those whose visual acuity had recovered to 1.0.
Our study demonstrated that different types of amblyopia still had contrast sensitivity dysfunction at different spatial frequencies, even when the CVA had recovered to 1.0. The CS dysfunction of the ametropic amblyopia emerged at a high spatial frequency. The CS dysfunction of the anisometropic amblyopia was observed at middle and high spatial frequencies, particularly under scotopic conditions. This result indicated that the recovery of the visual function was different from the low spatial frequency to the high spatial frequency during amblyopia treatment, and the parvocellular system remained uncured, even when the CVA recovered to 1.0.
Compared with photopic visual function mediated by the cone cells, mesopic visual function is more complex because it requires that the rod cells and the cone cells interact with each other properly 15 . This might be why the CS under scotopic conditions is more difficult to recover.     Although the visual acuity of patients with amblyopia had recovered to 1.0, their contrast sensitivity did not return to the normal range simultaneously. In other words, these patients had lower resolution capacities than did those with normal contrast sensitivity, especially under a dark environment. Therefore, we suggest that researchers focus on the improvement of visual acuity and the development of contrast sensitivity during the amblyopia treatment. Perhaps future studies should be directed toward evaluating the impact of reduced contrast sensitivity on the quality of life of patients with a history of amblyopia treatment in whom visual acuity was successfully restored to 1.0. Our findings have enriched the body of knowledge about amblyopia treatment in clinical work.