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ISSN : 1229-6457(Print)
ISSN : 2466-040X(Online)
The Korean Journal of Vision Science Vol.18 No.2 pp.135-147
DOI : https://doi.org/10.17337/JMBI.2016.18.2.135

Visual Sensory and Motor Abnormalities in Amblyopia, and its Treatments

Ki-Seok Lee
Department of Optometry, Yeoju Institute of Technology

Address reprint requests to Ki-Seok Lee Dept. of Optometry, Yeoju Institute of Technology TEL: 02-737-0030, E-mail: skialee@hotmail.com

Abstract


Amblyopia affecting approximately three per cent of the population can be defined as a developmental loss of visual acuity with no detectable pathology of the visual pathway. Without adequate stimuli, the visual system continuously develops amblyopia during the period of neural plasticity. Visual sensory in amblyopia is abnormal in terms of low visual acuity, contrast sensitivity and spatial distortion, as well as oculomotor abnormalities such as saccadic and pursuit movements. Therefore, it should be considered that the most effective intervention is applied to amblyopic eye depending on factors influencing final outcome in amblyopia treatment.


amblyopia , neural plasticity , visual sensory , oculomotor abnormality , effective intervention

약시의 시감각과 시운동 이상 그리고 치료

이 기석
여주대학교, 안경광학과

    Amblyopia, commonly known as "lazy eye", is a developmental disorder in which best corrected visual acuity is decreased in one or(rarely) both eyes, and which is a consequence of abnormal visual experience during childhood,1,2) and its prevalence is approximately 1 to 5% of the normal population.3)

    Amblyopia is generally classified by it associations such as with strabismus, anisometropia, high astigmatism, stimulus deprivation, and ametropia,4)and presents levels of severity in terms of reduced visual acuity. Anisometropic amblyopia refers to a difference between two eyes of at least 1.0 dioptre in spherical equivalent(SE) refraction without the presence of a squint or other amblyogenic factor. Strabismic amblyopia refers to heterotropia and microtropia without the presence of anisometropia or high refractive errors. Mixed amblyopia is defined when both anisometropia and strabismus exist.5) Stimulus deprivation amblyopia is caused by an obstruction to vision such as ptosis or cataract.2)Clinical studies reported that of these types of amblyopia, a third by anisometropia, a third by strabismus and a third by mixed type are the main causes of human amblyopia.6) In addition, these types of amblyopia also show distinctive features in visual acuity and contrast sensitivity compared to those with normal vision(Fig 1).

    Abnormalities of visual function in amblyopia

    Much is known about visual deficiencies associated with amblyopia reflected in impairment of visual function in terms of visual sensory and motor systems. Sensory deficits include: decreased visual acuity8) decreased high contrast resolution acuity in one or both eyes of amblyopes9-11) decreased contrast sensitivity at high spatial frequencies7,12-18) low contrast resolution acuity19) grating acuity7,15,20) monocular spatial distortion21-23) suppression8) decreased alignment sensitivity7,15,24-27) and decreased stereoacuity.2,28) In addition, motor deficits include: unsteady fixation1,29,30) eccentric fixation1,31) defective saccadic and pursuit movement32-37) defective accommodation38-41) accommodative convergence8,39) and eye-hand coordination.42-44)

    Visual acuity

    Decreased visual acuity is a critical element in the diagnosis of amblyopia.3) Letter visual acuity is usually measured by a standard Snellen chart45) or logMAR letter chart.46) Inmeasurement of visual acuity in amblyopic vision, Morad et al.47) compared the full chart and single line of letters. They suggestedthat amblyopic vision had better visual acuity with a single line due to the crowding effect which is refer to impairment of target visibility due to the presence of adjacent factors such as surrounding stimuli. Evidence suggested that crowding influences visual activities of reading in daily life caused by letter identification.48)

    Contrast sensitivity

    Spatial luminance contrast sensitivity refers to the ability of the visual system to distinguish a difference between grey levels, and is usually measured at specific spatial frequencies. Reduction in contrast sensitivity is a typical characteristic of amblyopia and observed in individuals with amblyopic vision, particularly at high spatial frequencies.49-51)

    Abrahamsson and Sjostrand12) studied the loss of contrast sensitivity in different types of amblyopia and suggested that anisometropic amblyopia showed deficits in both visual acuity and contrast sensitivity whereas strabismic amblyopia had worse deficit in visual acuity than deficit in contrast sensitivity(Fig 1).

    Accommodation

    Following the first mention of abnormal accommodation in amblyopes by Abraham52 suggesting that slower and reduced amplitude of accommodation was observed in amblyopic eyes, a number of studies have found disorders of accommodation in amblyopic eyes such as inaccurate accommodative responses,40,41,53) particularly for chromatic targets,54) decrease in the accommodative controller gain(ACG),54,55)and increased depth of focus.39) This reduction of the accommodative response may be related to many factors such as abnormal fixational eye movements, defective contrast sensitivity and eccentric fixation,8,55)and may be a causative factor in anisometropic amblyopia.52,56) The non-amblyopic eye has been found to have normal accommodation.41,55)

    Stereoacuity(Binocularity) and suppression

    Cooper and Feldman28) found a decrease in stereoacuity in individuals with a high degree of strabismus with and without amblyopia, and suggested that strabismus may be the major element for the loss of binocularity. Consistent with this, Webber and Wood2)found that a lack of stereopsis was measured in most strabismic amblyopes despite recovery of visual acuity whereas some residual strereopsis was found in a number of anisometropic amblyopes.

    Membreno et al.57)found a disruption of binocular input in amblyopia during the critical period for the development of binocularity, which is associated with early childhood strabismus, anisometropia or both,58) and rarely with degradation of image such as congenital cataract or corneal opacity.

    Individuals with unilateral amblyopia were found to have a loss of binocular single vision59,60)and to have a greater reduction in stereoacuity than those with symmetric bilateral defocus. This is likely to be related to the interocular differences in visual quality.61,62)

    Regardless of the type of amblyopia, improvement in stereopsis has been foundlinearly as amblyopic visual acuity is improved during amblyopia therapy.2,63)

    Eccentric fixation

    The amblyopic eye frequently cannot keep steady or foveal fixation under monocular conditions. Eccentric fixation is a phenomenon in which, when the amblyopic eye is fixating on an object, the object is imaged on a non-foveal point.64) In one study, eccentric fixation was observed in 22% of strabismic amblyopia, 0% of anisometropic amblyopia and 58% of mixed amblyopia(both strabismic and anisometropic).31) The presence of eccentric or unsteady fixation results in a decrease in visual acuity1,65) and the amount of eccentric fixation is related to the severity of amblyopia.66) In particular, the decrease in visual acuity is likely to related to unsteady central fixation in anisometropic amblyopia, and to eccentric fixation in strabismic amblyopia.1) Eccentricity of fixation has been shown to decrease during amblyopia therapy.1,45)

    Eye movements

    Much is known about oculomotor disorders in amblyopia generally such as saccadic intrusion, and version and vergence movements manifest.1,36) For example, saccadic intrusions in amblyopia are observed typically in strabismus and infrequently anisometropia.1)

    Ciuffreda et al.67)stated that sensory processing and motor control of eyemovement might be affected by a consistently defocused or suppressed retinal image in one eye during an early developmental period, which contributed to functional amblyopia. Consistent with this, early studies have found abnormal fixation and movements in the amblyopic eye during fixation using electro-ophthalmography.29,30)

    Stark et al.68)found increased saccadic latencies in amblyopic eyes using a photoelectric method to measure horizontal eye position. Ciuffreda et al.69) also discussed oculomotor disorders in amblyopia(increased saccadic latencies, increased drift and abnormal pursuit reduced accommodative vergence) and strabismus(nystagmus, saccadic intrusions, abnormal pursuit, abnormal optokinesis, abnormal vestibular response and absence of disparity vergence).

    Schor70)found that eccentric fixation and decreased visual acuity in the amblyopic eye are associated with the amplitude and frequency of eye movements. In addition, a decrease in velocity sensitivity of tracking movements to nasal-ward retinal image motion of the amblyopic eyes was observed by infrared sensitive diodes. This could be related to the disorder of stabilization of an object image on the fovea for position and velocity, which influences the saccadic and pursuit movements in amblyopia. Ciuffreda et al.69)also suggested that visual acuity in amblyopia may be affected by increased drift but not by eye movement velocity.

    Eye-hand coordination

    A number of studies have demonstrated that depth perception deficits in amblyopia affect visually guided performances.42-44)Subjects with reduced or absent stereopsis for reasons other than amblyopia also show poor ability to complete visual-motor tasks such as bead-threading71) or grasping tasks72) compared with those with normal visual function.

    Lagreze and Sireteanu22)studied two-dimensional spatial distortions in strabismic amblyopes viewing with the amblyopic eye. They found greater distortions and spatial uncertainty in the amblyopes compared to controls and anisometropic amblyopes. They suggested that the localization errors in strabismus under monocular conditions may result from disorders between visual and the motor cortical maps, rather than simply sensory localization errors.

    Grant et al.42)reported deficts in movement execution in amblyopic adults and found that while initial reaching behaviour and grip shaping were almost normal, deficits in prolonged execution times and more errors in the final approach to the object were found compared to normals. Webber et al.44) explored deficits in fine motor skills in amblyopic children aged 8 to 10 years compared with visually normal children when using different types of task such as Visual Motor Control(VMC) andUpper Limb Speed and Dexterity(ULSD) items of the Bruininks-Oseretsky Test of Motor Proficiency. Using multiple regression, they found that fine motor skill performance on both tests was significantly influenced by a history of strabismus, but not by level of binocular function. More recent work by Suttle et al.43) studied eye-hand coordination performance(reaching and grasping) in children and adults with amblyopia using a 3D motion-capture system compared to those with normal vision, and found that children with amblyopia showed poorer performance under both monocular and binocular viewing conditions, and that performance depended on binocularity. They emphasized from the finding that binocular vision is critical for eye-hand coordination.

    As many studies suggested that as high-grade stereovision is required for optimal eye-hand coordination, the residualbinocularity in amblyopia may play an important role in supporting eye-hand coordination performance.44,71,73)

    Other symptoms

    In addition to decreased visual acuity and other visual abnormalities as mentioned earlier, a variety of deficits have been found in amblyopia such as separation difficulty,74) position uncertainty,75) and visual distortions of length, shape and direction.21-23) Strabismic amblyopia exhibits these disorders more than anisometropic amblyopia. For example, abnormalities linked to under-sampling at post-receptoral level,76-79) abnormalities in spatial localisation,26) and slower reading80) were found only in strabismic amblyopia. However, spatial distortions and temporal instability have been observed in both strabismic and anisometropic amblyopia.81,82)

    Although individuals with amblyopia do not show a specific visual field deficit compared to those with normal vision, both those with strabismic and anisometropic amblyopic vision showed a general loss of field at the central 30 degrees when compared to the sound eye as screened with a Humphrey automated visual field tester.83)

    Amblyopia treatments

    Unilateral amblyopia(occurring in one eye) is conventionally treated by refractive correction and occlusion(by patching or penalization(atropine)) of the non-amblyopic eye.31)

    The effective period for amblyopia treatment has been studied extensively. A number of studies reported no difference in effectiveness of occlusion treatment commenced at any time between 3 and 7 years of age,19,84-89)and treatment of this kind has generally been found to be effective in amblyopes aged younger than 5 years of age.90,91) However, Cobb et al.84) found an improvement in visual acuity in treated anisometropic amblyopes aged up to 12 years, and similarly PEDIG92) demonstrated visual acuity improvement during treatment in at least some amblyopes between the age of 7 and 17 years.

    Refractive correction

    Mitchell and Gingras93) noted the importance of refractive correction for amblyopia therapy. Cotter et al.94) demonstrated an improvement of 0.3 logMAR(three lines on a logMAR letter chart) in visual acuity in 83% of children with anisometropic amblyopia aged from 3 to 7 years with refractive correction only. Furthermore, other studies have found that visual acuity in strabismic amblyopic children aged from 3 to 7 years of age can be increased with only spectacles.95,96)

    Occlusion, penalization and near visual activities

    Occlusion(or patching) of the non-amblyopic eye was noted by de Buffon97) and is still the conventional treatment for unilateral amblyopia. Despite its availability over the centuries, shortcomings of occlusion treatment include disruption of schooling and social activities, lack of cooperation, limited improvement in binocularity and inconvenience of patching.2,98) For these reason, recently a number of randomised controlled studies have been conducted by the Pediatric Eye Disease Investigator Group(PEDIG) to find out the most effective protocol for occlusion therapy.

    PEDIG explored the effect of occlusion and penalization(e.g. atropine) treatments in children with anisometropic, strabismic or mixed amblyopia, and found similar improvement in visual acuity for children with moderate amblyopia aged 7 to 12 years of age between a regular single weekend atropine dose and two hours of daily patching of the non-amblyopic eye.99) PEDIG100)also suggested the benefit of atropine dose that visual acuity in children with severe amblyopia aged 3 to 12 years was much improved by the use of atropine only during weekend.

    A number of studies have reported that near activities such as puzzles, writing or computer use may be combined with occlusion therapy to improve its effectiveness. In a pilot study by the PEDIG group, PEDIG101)compared two hours of daily patching combining with one hour of near visual activities and only wearing refractive correction in children with moderated to severe amblyopia aged 3 to 7 years of age, and reported that visual acuity was slightly improved by the application of a combination of two hours of daily patching and one hour of near visual activities. However, a larger study by PEDIG102) reported no difference in improvement of visual acuity between near and distance activities in children during patching in children with strabismic, anisometropc and mixed amblyopia regardless of the severity of amblyopia.

    Although it was suggested by PEDIG102) that commonly near activities that stimulate accommodation did not improve the effect of occlusion, previous work by Renjie et al.103)suggested the possibility of the effectiveness of near activities with action video games in adults with amblyopia. This is an exciting treatment possibility since the treatment of amblyopia has been thought to be feasible only in children based on visual system plasticity during critical periods. They found that an improvement in the contrast sensitivity function(CSF) in the amblyopes after playing an action video game for 50 hours over 9 weeks. However, there are still limitations such as the proper gameselection for training and application of action games to children.

    Other treatments

    In addition to treatment of the amblyopic eye in isolation(with the non-amblyopic eye occluded or penalised), it has been suggested that binocular visual stimulation may be an important part of treatment, to ensure that binocularity(such as stereoacuity) is restored. For example, the Interactive Binocular Treatment(I-BiT) for amblyopia therapy98,104-106) aims to encourage the two eyes to work together by presenting some elements of a binocular stimulus to each eye, and some to both eyes. In this way, both eyes must be used in order to perceive the stimulus in full. This technique is computer based and has the potential to be more attractive to young children than conventional therapy, and higher compliance. They found significant improvement of 0.35 logMAR in both high and low contrast acuity in approximately half or more of subjects after this treatment, and also recent work found improvement of 0.125 logMAR units or more.105) However, the effect of I-BiT on the improvement in visual acuity is not strong as much as the effect of occlusion, and also it was limited due to small number of sample size.

    Perceptual learning is a visual task in which visual function is improved following practice. Polat et al.107)found an improvement in contrast sensitivity and letter acuity following perceptual learning in adults with amblyopia, and suggested that a combination of perceptual learning and occlusion was effective for amblyopia therapy in adults. Chen et al.108) investigated the effect of perceptual learning on anisometropic amblyopic adults and children by comparison of only patching. They found that letter acuity was improved to a greater extent by occlusion alone than by shorter periods of perceptual learning plus occlusion, but the improvement was achieved in a shorter period of time.

    Factors influencing final outcome with amblyopic treatment

    Stewart et al.31) suggested two main factors affecting final treatment outcomes such as type of amblyopia, initial age, initialseverity of amblyopia, fixation and binocular vision status as condition factors, and refractive adaptation and occlusion(total dose time and dose rate per day) as treatment factors. Webber and Wood2)mentioned that these factors are also helpful in determining the prognosis of maintaining normal vision in amblyopic eye. Stewart et al.31) also suggested that eccentric fixation contributes to poorer final visual outcome in amblyopic vision.

    Conclusion

    Amblyopia results in not only simply decreased visual acuity, but also a number of deficits of visual function in terms of visual sensory and motor. Even if the decreased visual acuity is one of most important factors to identify amblyopia, previous studies have reported a number of deficits affecting other visual function. Therefore, it is worthy of screening all aspects of amblyopic deficits relevant to visual sensory and motor function, not simply measuring visual acuity, in order for more accurate identification of amblyopia abnormalitiesand the best result in amblyopia treatment.

    Figure

    JMBI-18-2-135_F1.gif

    Mean locations of the 11 clinically defined categories in the two-factor space7)

    Table

    Reference

    1. Levi DM, Klein SA: Vernier acuity, crowding and amblyopia. Vision Res. 25(7), 979-991, 1985.
    2. Webber AL, Wood J: Amblyopia: prevalence, natural history, functional effects and treatment. Clin Exp Optom. 88(6), 365-375, 2005.
    3. Holmes JM, Clarke MP: Amblyopia. Lancet 367(9519), 1343-1351, 2006.
    4. Powell C, Hatt S: Vision screening for ambl yopiain childhood. Cochrane Database Syst. Rev. 8(3), 1465-1858, 2009.
    5. Robaei D, Rose KA, Ojaimi E et al.: Causes and associations of amblyopia in a population- based sample of 6-year-old Australian children. Arch Ophthalmol. 124(6), 878-884, 2006.
    6. PEDIG: A Randomized trial of atropine vs patching for treatment of moderate amblyopia in children. Arch Ophthalmol. 120(3), 268-278, 2002.
    7. McKee SP, Levi DM, Movshon JA: The pattern of visual deficits in amblyopia. J Vis. 3(5), 380-405, 2003.
    8. Griffin JR, Grisham JD: Binocular Anomalies: Diagnosis and Vision Therapy. 4th ed. Butterworth Heinemann, U S A., pp. 280, 2002.
    9. Caputo R, Frosini R, De Libero C et al.: Factors influencing severity of and recovery from anisometropic amblyopia. Strabismus Visual Sensory and Motor Abnormalities in Amblyopia, and its Treatments 15(4), 209-214, 2007.
    10. Donahue SP: The relationship between anisometropia, patient age, and the development of amblyopia. Trans Am Ophthalmol Soc. 103, 313-336, 2005.
    11. Weakley DR Jr: The association between nonstrabismic anisometropia, amblyopia, and subnormal binocularity. Ophthalmology 108(1), 163-171, 2001.
    12. Abrahamsson M, Sjostrand J: Contrast sensitivity and acuity relationship in strabismic and anisometropic amblyopia. Br J ophthalmol. 72(1), 44-49, 1988.
    13. Bradley A, Freeman RD: Contrast sensitivity in anisometropic amblyopia. Invest Ophthalmol Vis Sci. 21(3), 467-476, 1981.
    14. Freedman RD, Thibos LN: Contrast sensitivity in humans with abnormal visual experience. JPhysiol. 247(3), 687-710, 1975.
    15. Harvey EM, Dobson V, Miller JM et al.: Amblyopia in astigmatic children: patterns of deficits. Vision Res. 47(3), 315-326, 2006.
    16. Hess RF: Contrast sensitivity assessment of functional amblyopia in humans. Trans Ophthalmol Soc U K. 99(3), 391-397, 1979.
    17. Hess RF, Howell ER: Detection of low spatial frequencies: a single filter or multiple filters? Ophthalmic Physiol Opt. 8(4), 378-385, 1988.
    18. Thomas J: Normal and amblyopic contrast sensitivity function in central and peripheral retinas. Invest Ophthalmol Vis Sci. 17(8),746-753, 1978.
    19. Simmers Aj, Gray LS, McGraw PV et al.: Functional visual loss in amblyopia and the effect of occlusion therapy. Invest Ophthalmol Vis Sci. 40(12), 2859-2871, 1999.
    20. Harwerth RS, Smith EL, Okundaye OJ: Oblique effects, vertical effects and meridional amblyopia in monkeys. Exp Brain Res. 53(1), 142-150, 1983.
    21. Bedell HD, Flom MC: Monocular spatial distortion in strabismic amblyopia. Invest Ophthalmol Vis Sci. 20(2), 263-268, 1981.
    22. Lagreze WD, Sireteanu R: Two-dimensional spatial distortions in human strabismic amblyopia. Vision Res. 31(7-8), 1271-1288, 1991.
    23. Sireteanu R, Fronius M: Different patterns of retinal correspondence in the central and peripheral visual field of strabismics. Invest Ophthalmol Vis Sci. 30(9), 2023-2033, 1989.
    24. Birch EE, Swanson WH: Hyperacuity deficits in anisometropic and strabismic amblyopes with known ages of onset. Vision Res. 40(9), 1035-1040, 2000.
    25. Bradley A, Freeman RD: Is reduced vernier acuity in amblyopia due to position, contrast or fixation deficits? Vision Res. 25(1), 55-66, 1985.
    26. Levi DM, Klein S: Differences in vernier discrimination for grating between strabismic and anisometropic amblyopes. Invest Ophthalmol Vis Sci. 23(3), 398-407, 1982.
    27. Levi DM, Klein S: Hyperacuity and amblyopia. Nature 298(5871), 268-270, 1982.
    28. Cooper J, Feldman J: Random-dot-stereogram performance by strabismic, amblyopic, and ocular-pathology patients in an operant- discrimination task. Am J Optom Physiol Opt. 55(9), 599-609, 1978.
    29. Mackensen G: Fixation in amblyopia; electro- oculographic examination. Albrecht Von Graefes Arch Ophthalmol. 159(2), 200-211, 1957.
    30. Von Noorden GK, Burian HM: An electro- ophthalmographic study of the behavior of the fixation of amblyopic eyes in lightand dark-adapted state: a preliminary report. Am JOphthalmol. 46(1 Pt2), 68-77, 1958.
    31. Stewart CE, Fielder AR, Stephens DA et al.: Treatment of unilateral amblyopia: factors influencing visual outcome. Invest Ophthalmol Vis Sci. 46(9), 3152-3160, 2005.
    32. Ciuffreda KJ, Kenyon RV, Stark L: Increased saccadic latencies in amblyopic eyes. Invest Ophthalmol Vis Sci. 17(7), 697-702, 1978.
    33. Niechwiej-Szwedo E, Chandrakumar MC, Goltz HC et al.: Effects of strabismic amblyopia and strabismus without amblyopia on visuomotor behavior, I: saccadic eye movements. Invest Ophthalmol Vis Sci. 53(12), 7458-7468, 2012.
    34. Niechwiej-Szwedo E, Chandrakumar MC, Goltz HC et al.: Effects of strabismic amblyopia and strabismus without amblyopia on visuomotor behavior, Part II: Visually guided reaching. Invest Ophthalmol Vis Sci. 55(6), 3857-3865, 2014.
    35. Niechwiej-Szwedo E, Goltz HC, Chandrakumar MC et al.: Effects of strabismic amblyopia and strabismus without amblyopia on visuomotor behavior, Part III. Temporal eye-hand coordination during reaching. Invest Ophthalmol Vis Sci. 55(12), 7831-7838, 2014.
    36. Quéré MA: Abnormal ocular movements in amblyopia. Trans Ophthalmol Soc U K. 99(3), 401-406, 1979.
    37. Von Noorden GK, Mackensen G: Pursuit movements of normal and amblyopic eyes. An electro-ophthalmographic study. II. Pursuit movements in amblyopic patients. Am J Ophthalmol. 53, 477-487, 1962.
    38. Asper L, Crewther D, Crewther SG: Strabismic amblyopia. Part 1. Psychophysics. Clin Exp Optom. 83(2), 49-58, 2000.
    39. Ciuffreda K, Hokoda S, Hung GK et al.: Static aspects of accommodation in human amblyopia. Am J Optom Physiol Optics. 60(6), 436-449, 1983.
    40. Green DG: Visual Resolution When Light Enters Eye through Different Parts of Pupil. J Physiol. 190(3), 583-593, 1967.
    41. Kirschen DG, Kendall JH, Riesen KS: An evaluation of accommodation response in amblyopic eyes. Am J Optom Physiol Opt. 58(7), 597-602, 1981.
    42. Grant S, Melmoth DR, Morgan MJ et al.: Prehension deficits in amblyopia. Invest Ophthalmol Vis Sci. 48(3), 1139-1148, 2007.
    43. Suttle CM, Melmoth DR, Finlay AL et al.: Eye-hand coordination skills in children with and without amblyopia. Invest Ophthalmol Vis Sci. 52(3), 1851-1864, 2011.
    44. Webber AL, Wood JM, Gole GA et al.: The effect of amblyopia on fine motor skills inchildren. Invest Ophthalmol Vis Sci. 49(2), 594-603, 2008.
    45. Schor C, Gibson J, Hsu M et al.: The Use of Rotating Grating for the Treatment of Amblyopia: A Clinical Trial. Am J Optom Physiol Opt. 58(11), 930-938, 1981.
    46. Bailey I, Lovie-Kitchin JE: Visual acuity testing. From the laboratory to the clinic. Vision Res. 90, 2-9, 2013.
    47. Morad Y, Werker E, Nemet P: Visual acuity tests using chart, line, and single optotype in healthy and amblyopic children. JAAPOS. 3(2), 94-97, 1999.
    48. Pelli DG, Palomares M, Majaj NJ: Crowding is unlike ordinary masking: distinguishing feature integration from detection. J Vis. 4(12), 1136-1169, 2004.
    49. Hess RF, Bradley A, Piotrowski L: Contrast-coding in amblyopia. I. Differences in the neural basis of human amblyopia. Proc R Soc Lond B Biol Sci. 217(1208), 309-330, 1983.
    50. Hess RF, Howell ER: The threshold contrast sensitivity function in strabismic amblyopia: evidence for a two type classification. Vision Visual Sensory and Motor Abnormalities in Amblyopia, and its Treatments Res. 17(9), 1049-1055, 1977.
    51. Koskela PU: Contrast sensitivity in amblyopia. II. Changes during pleoptic treatment. Acta Ophthalmol. 64(5), 563-569, 1986.
    52. Abraham SV: Accommodation in the amblyopic eye. Am J Ophthalmol. 52: 197-200, 1961.
    53. Ciuffreda KJ, Hokoda SC: Spatial frequency dependence of accommodative responses in amblyopic eyes. Vision Res. 23(12), 1585-1594, 1983.
    54. Winn B, Heron G, Pugh JR et al.: Amblyopia, accommodation and colour. Ophthalmic Physiol Opt. 7(4), 365-372, 1987.
    55. Hung GK, Ciuffreda KJ, Semmlow JL et al.: Model of static accommodative behavior in human amblyopia. IEEE Trans Biomed Eng. 30(10), 665-672, 1983.
    56. Horwood AM, Riddell PM: Independent and reciprocal accommodation in anisometropic amblyopia. J AAPOS. 14(5), 447-449, 2010.
    57. Membreno JH, Brown MM, Brown GC et al.: A cost-utility analysis of therapy for amblyopia. Ophthalmology 109(12), 2265-2271, 2002.
    58. Hensch TK: Critical period plasticity in local cortical circuits. Nat Rev Neurosci.6(11), 877-888, 2005.
    59. Holopigian K, Blake R, Greenwald MJ: Selective losses in binocular vision in anisometropic amblyopes. Vision Res. 26(4), 621-630, 1986.
    60. Levi DM, Harwerth RS, Manny RE: Suprathreshold spatial frequency detection and binocular interaction in strabismic and anisometropic amblyopia. Invest Ophthalmol Vis Sci. 18(7), 714-725, 1979.
    61. Geib T, Baumann C: Effect of luminance and contrast on stereoscopic acuity. Graefes Arch Clin Exp Ophthalmol. 228(4), 310-315, 1990.
    62. Halpern D, Blake RR: How contrast affects stereoacuity. Perception 17(4), 483-495, 1988.
    63. Ciuffreda KJ, Kenyon RV, Stark L: Different rates of functional recovery of eye movements during orthoptics treatment in an adult amblyope. Invest Ophthalmol Vis Sci. 18(2), 213-219, 1979.
    64. Von Noorden GK: Binocular Vision and Ocular motility: Examination of Patient – III Sensory Signs Symptoms and Adaptation in Strabismus. St Louis: Mosby Co, 219-231, 1990.
    65. LEE KS, Chu BS: Correlation between eccentric fixation and visual acuity on different type of amblyopia. Korean J Vis Sci.16(3), 311-318, 2014.
    66. Flom MC, Weymouth FW, Kahneman D: Visual Resolution and Contour Interaction. J Opt Soc Am. 53, 1026-1032, 1963.
    67. Ciuffreda KJ, Levi DM, Selenow A: Amblyopia: Basic and Clinical Aspects. Butterworth-heinemann, pp. 165-189, 1991.
    68. Stark L, Vossius G, Young LR. Predictive control of eye tracking movements. IEEE Trans Biomed Eng. 52-57, 1962.
    69. Ciuffreda KJ, Kenyon RV, Stark: Increased drift in amblyopic eyes. Br J Ophthalmol. 64(1), 7-14, 1980.
    70. Schor C: A directional impairment of eye movement control in strabismus amblyopia. Invest Ophthalmol. 14(9), 692-697, 1975.
    71. O'Connor AR, Birch EE, Anderson S et al.: The functional significance of stereopsis. Invest Ophthalmol Vis Sci. 51(4), 2019-2023, 2010.
    72. Melmoth DR, Finlay AL, Morgan MJ et al.: Grasping Deficits and Adaptations in Adults with Stereo Vision Losses. Invest Ophthalmol Vis Sci 50(8), 3711-3720, 2009.
    73. Hrisos S, Clarke MP, Kelly T et al.: Unilateral visual impairment and neurodevelopmental performance in preschool children. Bri J Ophthalmol. 90(7), 836-838, 2006.
    74. Stuart JA, Burian HM: A study of separation difficulty. Its relationship to visual acuity in normal and amblyopic eyes. Am J Ophthalmol. 53, 471-477, 1962.
    75. Flom MC, Bedell HE: Identifying amblyopia using associated conditions, acuity, and nonacuity features. Am J Optom Physiol Opt. 62(3), 153-160, 1985.
    76. Hess RF, Wang YZ, Demanins Ret al.: A deficit in strabismic amblyopia for global shape detection. Vision Res. 39(5), 901-914, 1999.
    77. Levi DM, Klein SA: Limitations on position coding imposed by undersampling and univariance. Vision Res. 36(14), 2111-2120, 1996.
    78. Levi DM, Klein SA: Sampling in spatial vision. Nature 320(6060), 360-362, 1986.
    79. Levi DM, Klein SA: Vernier acuity, crowding and amblyopia. Vision Res. 25(7), 979-991, 1985.
    80. ZürcherB, Lang J: Reading capacity in cases of 'cured' strabismic amblyopia. Trans Ophthalmol Soc U K. 100(4), 501-503, 1980.
    81. Barrett BT, Pacey IE, Bradley A et al.: Nonveridical visual perception in human amblyopia. Invest Ophthalmol Vis Sci. 44(4), 1555-1567, 2003.
    82. Sireteanu R, Bäumer CC, Sarbu C et al.: Spatial and temporal misperceptions in amblyopic vision. Strabismus 15(1), 45-54, 2007.
    83. Donahue SP, Wall M, Stanek KE: Motion perimetry in anisometropic amblyopia: elevated size thresholds extend into the midperiphery. J AAPOS. 2(2), 94-101, 1998.
    84. Cobb CJ, Russell K, Cox A et al.: Factors influencing visual outcome in anisometropic amblyopes. Bri J Ophthalmol. 86(11), 1278-1281, 2002.
    85. Hardman Lea SJ, Loades J, Rubinstein MP: The sensitive period for anisometropic amblyopia. Eye (Lond). 3(Pt 6), 783-790, 1989.
    86. Hiscox F, Strong N, Thompson JR et al.: Occlusion for amblyopia: a comprehensive survey of outcome. Eye (Lond). 6(Pt 3), 300-304, 1992.
    87. PEDIG: The clinical profile of moderate amblyopia in children younger than 7 years. Arch Ophthalmol. 120(3), 281-287, 2002.
    88. Repka MX, Beck RW, Holmes JM et al.: A randomized trial of patching regimens for treatment of moderate amblyopia in children. Arch Ophthalmol. 121(5), 603-611, 2003
    89. Simmers AJ, Bex PJ: The representation of global spatial structure in amblyopia. Vision Res. 44(5), 523-533, 2004.
    90. Flynn JT, Woodruff G, Thompson JR et al.: The therapy of amblyopia: an analysis comparing the results of amblyopia therapy utilizing two pooled data sets. Trans Am Ophthalmol Soc. 97, 373-395, 1999.
    91. Holmes JM, Kraker RT, Beck RW et al.: A randomized trial of prescribed patching regimens for treatment of severe amblyopia in children. Ophthalmology 110(11), 2075-2087, 2003.
    92. Scheiman MM, Hertle RW, Beck RW et al.: Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Arch Ophthalmol. 123(4), 437-447, 2005.
    93. Mitchell DE, Gingras G: Visual recovery after monocular deprivation is driven by absolute, rather than relative, visually evoked activity levels. Curr Biol. 8(21), 1179-1182, 1998.
    94. Cotter SA, PEDIG, Edwards AR et al.: Visual Sensory and Motor Abnormalities in Amblyopia, and its Treatments Treatment of anisometropic amblyopia in children with refractive correction. Ophthalmology 113(6), 895-903, 2006.
    95. Cotter SA, Edwards AR, Arnold RW et al.: Treatment of strabismic amblyopia with refractive correction. Am J Ophthalmol 143(6), 1060-1063, 2007.
    96. Moseley MJ, Neufeld M, McCarry B et al.: Remediation of refractive amblyopia by optical correction alone. Ophthalmic Physiol Opt. 22(4), 296-299, 2002.
    97. de Buffon G: Dissertation sur la cause du strabisme ou des yeux louches. Académic Royale des Sciences. 231-248, 1743.
    98. Eastgate R, Griffiths G, Waddingham PE et al.: Modified virtual reality technology for treatment of amblyopia. Eye (Lond). 20(3), 370-374, 2006.
    99. Scheiman MM. Hertle RW, Kraker RT et al.: Patching vs atropine to treat amblyopia in children aged 7 to 12 years: a randomized trial. Arch Ophthalmol 126(12), 1634-1642, 2008.
    100. Repka MX, Kraker RT, Beck RW et al.: Treatment of severe amblyopia with weekend atropine:results from 2 randomized clinical trials. J AAPOS. 13(5), 258-263, 2009.
    101. Wallace DK, PEDIG, Edwards AR et al.: A randomized trial to evaluate 2 hours of daily patching for strabismic and anisometropic amblyopia in children. Ophthalmology 113(6), 904-912, 2006.
    102. PEDIG: A Randomized Trial of Near Versus Distance Activities While Patching for Amblyopia in Children Aged 3 to Less Than 7 Years. Ophthalmology 115(11), 2071-2078, 2008.
    103. Renjie L, Uri P, Walter M et al.: Enhancing the contrast sensitivity fucntion through action video game training. Nat Neurosci. 12(5), 549-551, 2009.
    104. Cleary M, Moody AD, Buchanan A et al.: Assessment of a computer-based treatment for older amblyopes: the Glasgow Pilot Study. Eye (Lond). 23(1), 124-131, 2009.
    105. Herbison N, Cobb S, Gregson R et al.: Interactive binocular treatment (I-BiT) for amblyopia: results of a pilot study of 3D shutter glasses system. Eye (Lond). 27(9), 1077-1083, 2013.
    106. Waddingham PE, Butler TK, Cobb SV et al.: Preliminary results from the use of the novel interactive Binocular Treatment (I-BiTTM) system, in the treatment of strabismic and anisometropic amblyopia. Eye (Lond). 20(3), 375-378, 2006.
    107. Polat U, Ma-Naim T, Belkin M et al.: Improving vision in adult amblyopia by perceptual learning. Proc Natl Acad Sci U S A. 101(17), 6692-6697, 2004.
    108. Chen PL, Chen JT, Fu JJ et al.: A pilot study of anisometropic amblyopia improved in adults and children by perceptual learning: an alternative treatment to patching. Ophthalmic Physiol. Opt. 28(5), 422-428, 2008.
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