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Background: Dichoptic visual stimulation may be achieved using shutter goggles and mirror systems. These methods vary in their feasibility for use in children. This study aims to investigate the feasibility of use of a simple trial frame-based system to evaluate interactions in children.
Methods: Low contrast acuity, contrast sensitivity and alignment sensitivity were measured in the non-dominant eye of 10 normally-sighted children, 14 anisometropic children without amblyopia and 14 anisometropic amblyopic children (aged 5–11 years) using goggles and a trial frame apparatus (TFA). The dominant eye was either fully or partially occluded. The difference in visual functions in the non-dominant eye between the full and partial occlusion conditions was termed the ‘interaction index’. Agreement between the TFA and goggles in terms of visual functions and interactions was assessed in anisometropic children with and without amblyopia using the Bland-Altman method and t-test. Training sessions allowed subjects to become accustomed to the systems and tasks. The duration of training, the number of breaks requested by subjects and their willingness to attend further experiments were recorded in 10 subjects from each group and were compared between groups and between systems.
Results: Both Bland-Altman and t-test methods indicated acceptable agreement between the TFA and goggles in visual function and interaction measures (p > 0.05), except for contrast sensitivity measured in anisometropic children without amblyopia (p = 0.042). For all subject groups, contrast sensitivity training was significantly longer using goggles than using the TFA (p ≤ 0.001). Significantly more breaks were requested in acuity and contrast sensitivity testing, when goggles were used than when the TFA was used (p < 0.045). Anisometropic children without amblyopia showed a significantly greater willingness to attend more experiments using the TFA than using goggles (p = 0.025).
Conclusion: The TFA may be a useful tool in studies of interactions in amblyopes, particularly in studies of children's vision.
Dichoptic visual masking occurs when a target is presented to one eye and a mask is presented to the fellow eye. The stimulus and masking configurations may take various forms, including ‘masking light by light’, ‘masking a pattern by light’ and ‘masking a pattern by a pattern’.1 The mask may be presented either coincident with or adjacent to the stimulus.2–10 In the normal visual system, the target and the mask are processed binocularly and the mask can affect the perception of the target, resulting in an increment or a decrement in visual function thresholds of the tested eye.2,3,8–10 These masking effects are underpinned by interactions in the visual system that take place between signals from the two eyes, known as interocular interactions. In dichoptic masking studies, the impact of the masking stimulus on perception of the test stimulus is assessed. Comparison between visual functions measured under dichoptic and monocular viewing conditions allows the nature and the strength of interocular interactions to be investigated.
Mirror or prism devices and shutter goggles have been used in dichoptic masking studies;3,11–13 however, equipment of this kind may not be feasible for use in some populations.14 For this reason, a simple and portable viewing system, a ‘trial frame apparatus’ (TFA) was designed and validated in a previous study.14 The TFA consisted of a trial frame adjusted to the individual's interpupillary distance, a pinhole aperture (1.0 mm in diameter) placed in front of the tested eye and a partial occluder placed in front of the non-tested eye. The occluder was made using a +5.00 DS trial lens, on which four strips of 5.0 mm wide opaque black masking tape were placed to form a ‘star-shaped’ occlusion (Figure 1). Visual function and interocular interaction measurements made using this method were found to agree with those made using shutter goggles in children and adults with normal vision.14
Figure 1. The trial frame apparatus (TFA), 1.0 mm pinhole, opaque occluder and the star-shaped occluder used with the TFA. The star-shaped occluder was made using a +5.00 DS trial lens with four strips of 5.0 mm wide black masking tape. The opaque occlusion at the centre of the star-shaped occluder was 13 mm in diameter.
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Amblyopia is often known as ‘lazy eye’. It refers to a disorder of the visual system that is characterised by unilateral or bilateral loss of visual acuity without pathology.15 Anisometropia, which has a prevalence of two to six per cent in school children,16,17 is one of the major causes of amblyopia. Previous studies indicate that in humans, about one-third of cases of amblyopia are caused by anisometropia, one-third by strabismus and one-third by a combination of both.18,19 Deprivation due to conditions such as congenital cataracts can also lead to amblyopia.20
Numerous studies have demonstrated that interocular interactions differ between amblyopic and normal observers. For example, binocular summation is reduced or absent in amblyopes.21,22 Holopigian, Blake and Greenwald23 found that stereoacuity and binocular summation are absent at high spatial frequencies only, while Hood and Morrison24 found a higher level of binocular contrast summation in amblyopes with binocular single vision than in normal observers, for both low and high spatial frequencies. Baker and colleagues25 found that binocular contrast summation is normal in amblyopes when contrast sensitivity is normalised across the amblyopic and the fellow eyes. Studies by Sengpiel and Blakemore26, and Smith and colleagues27 have demonstrated inhibitory interaction in animals with amblyopia and Vedamurthy and colleagues2 found significant inhibitory interaction in anisometropic amblyopic adults.2 Most of these findings suggest that abnormal interactions occur in anisometropic amblyopia. Studying interactions in anisometropic amblyopes may provide information on visual processing in these subjects and the mechanisms underlying this disorder.
Comparison between interactions in anisometropes with and without amblyopia may be informative because both groups have significant differences in refractive error between eyes but only one of these groups has developed amblyopia. The aim of this study is to investigate the feasibility of use of the TFA as a means of studying interocular interactions in children with and without anisometropic amblyopia. Specifically, to investigate:
Agreement in visual function and interocular interaction measures between the TFA and shutter goggles in anisometropic children with and without amblyopia and
The feasibility of use of the TFA as a viewing system to evaluate interactions in children.
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Shutter goggles and the trial frame generally show acceptable to good agreement in visual function measures and in interaction index evaluations in anisometropes and amblyopes, with one exception. Contrast sensitivity in anisometropes showed poor agreement between the two viewing systems based on the paired t-test (but acceptable agreement was shown based on the Bland-Altman method). These findings agree with previous work in controls,14 and suggest that the trial frame may be applicable in the evaluation of interocular interaction in children and in investigation of these interactions in amblyopes, in populations where more complex devices cannot be used. It is important to note that the TFA does not have the flexibility of shutter goggles and mirror or prism devices and it may be used to study limited forms of dichoptic masking.
The poor agreement between shutter goggles and the trial frame in contrast sensitivity measurement in anisometropes is consistent with findings in controls.14 As the size of the central occlusion presented to the non-tested eye was 22° to 24° using the trial frame and 3.5° using goggles, it is possible that the size of the central occlusion and the amount of light transmitted from the periphery had a strong impact on contrast sensitivity but a less significant impact on acuity or alignment sensitivity. Perhaps better agreement in contrast sensitivity measurement may be found between these two systems if similar sizes of central occlusion were used.
As shown in Figure 3, it is interesting to note that the acuity of the non-dominant eye measured with partial occlusion did not differ significantly between amblyopes and anisometropes (one-way ANOVA, p > 0.05), which is consistent with previous findings using shutter goggles as a viewing system.41 This may be due to the use of the pinhole in front of the tested eye that resulted in the reduction in retinal illuminance.14,41 The present study found a significantly higher acuity interaction index in amblyopes than in anisometropes (evaluated using the trial frame). This suggests that the extent to which acuity of the non-dominant eye is influenced by stimulation of the dominant eye is greater in anisometropic children with amblyopia than in those without. This finding agrees with previous work using shutter goggles;41 however, the sample size in the present study is small and the statistical power in this sample is only 38 per cent (PS Power and Sample Size Program42). Further experiments with a larger sample may improve understanding of the relationship between interactions and anisometropic amblyopia.
Two amblyopic subjects had negative acuity interaction index (Figure 4), indicating better acuity with partial rather than full occlusion in these subjects. This suggests that the relationship between interocular interaction and anisometropic amblyopia may vary in amblyopic subjects. The reason for this inconsistency is unclear due to the lack of information about the history of the development of amblyopia and about the deficits at subcortical and cortical levels in amblyopic subjects. Similar studies on subjects with a clear history of development of amblyopia and accompanied by objective measures of amblyopia-related deficits would provide a better understanding of this relationship.
The pool of subjects tested here was recruited from a larger group of 106 children.43 While parents of all children gave consent for them to be tested using the TFA, consent for testing with the shutter goggles was received from parents of only 42 of these children. Some of the parents expressed concern that the use of unfamiliar equipment could be harmful to their children's vision. In addition, not all of the 42 children were able to complete the experimental processes using the goggles. Three children in the anisometropes group and one child in the amblyopes group did not complete testing with goggles due to discomfort when wearing the goggles and their parents were not able to persuade them to continue with the tests. Thus, only 38 of the 106 children underwent all tests with both viewing systems, due to difficulties encountered in using the shutter goggles.
In addition to discomfort and parental concerns about the shutter goggles, another complication with this viewing system was that the lenses were occasionally affected by condensation. This complication is related to the fact that the present study was conducted in a warm environment, in high humidity and might not arise in cooler, less humid conditions. This factor may in part explain the differences between the two viewing systems in terms of duration of training, number of breaks and willingness to participate in further test sessions.