On the relationship between strabismic amblyopia and eccentric fixation.

Landolt C and grating acuity are compared with that normally expected of the eccentric fixation region for 10 strabismic amblyopes. The findings suggest that 2 populations of amblyopes exist. For some amblyopes visual function is that predicted of the eccentric region used for fixation, whereas for other amblyopes there is a further pathological reduction in visual function. These findings may have an important bearing on the type of orthoptic treatment used for amblyopia.     

OCT对远视重度弱视患儿黄斑注视性质的观察

目的:通过OCT方法检查弱视患者的注视性质,思考和审视偏心注视的机制,指导弱视治疗.方法:对一组3~12岁儿童远视屈光性重度弱视患儿31例40眼治疗前后做黄斑光学相干断层扫描(OCT)检查,观察其注视性质的变化.结果:在31例40眼中,中心注视眼占63%,偏心注视眼占38%.单眼患者中心注视与偏心注视各占50%,双眼患者78%为中心注视,22%为偏心注视.治疗后15只偏心注视眼,分别在0.2~0.5视力之间全部转成中心注视,时间为1mo~2a.合并外斜和调节性内斜没有影响偏心注视转中心注视,且无论中心注视及偏心注视,其视力恢复对比观察,无明显差异.结论:采用OC T检查弱视眼注视性质客观、简便.在治疗过程中随着视力的提高,偏心注视转化为中心注视说明中心凹锥细胞功能被唤醒并且功能逐渐增强.     

Eccentric correction for off-axis vision in central visual field loss.

BACKGROUND: Subjects with absolute central visual field loss use eccentric fixation and magnifying devices to utilize their residual vision. This preliminary study investigated the importance of an accurate eccentric correction of off-axis refractive errors to optimize the residual visual function for these subjects. METHODS: Photorefraction using the PowerRefractor instrument was used to evaluate the ametropia in eccentric fixation angles. Methods were adapted for measuring visual acuity outside the macula using filtered optotypes from high-pass resolution perimetry. Optical corrections were implemented, and the visual function of subjects with central visual field loss was measured with and without eccentric correction. RESULTS: Of the seven cases reported, five experienced an improvement in visual function in their preferred retinal locus with eccentric refraction. CONCLUSIONS: The main result was that optical correction for better image quality on the peripheral retina is important for the vision of subjects with central visual field loss, objectively as well as subjectively.     

Is reduced vernier acuity in amblyopia due to position,contrast or fixation deficits?

Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.     

高度近视偏心注视优势位置的研究

目的 探讨高度近视眼因黄斑病理损害形成中心暗点以及丧失中心视力后偏心固视的形成规律,确定偏心注视点的优势位置.方法 应用微视野计(MP-1)对因黄斑病变形成偏心固视的40例高度近视患者的54只眼作固视检查.利用正常成人中心固视的90%置信椭圆,确定偏心固视相对于中心凹的位置.根据观察到的偏心固视的位置,将所有患眼分为preferred retinal locucs(PRL)优势组和PRL非优势组;并将两组视力作统计学比较.结果 54只高度近视眼中,中心视力丧失后偏心固视点形成在暗点下方视野者24只眼,占本组患眼的44.44%;左侧19只眼,占35.19%;上方6只眼,占11.11%;右侧5只眼,占9.26%.双眼均形成偏心固视者14例,其中13例双眼偏心固视模式一致,均为下方者7例,占双眼偏心固视者的50.00%;均为左侧者5例,占35.71%;均为上方者1例,占7.14%.偏心固视位于下方、左侧视野组与偏心位置位于右侧和上方的非优势组之间比较.其视力差异无统计学意义(F=0.144,P>0.05).结论高度近视患者偏心固视会形成在尽量靠近中心凹有功能的视网膜.下方视野是形成偏心固视的优势位置.     

Spatial and temporal crowding in amblyopia

Spatial crowding is a well-known deficit in amblyopia. We have previously reported evidence suggesting that the inability to isolate stimuli in space in crowded displays (spatial crowding) is a largely independent component of the amblyopic deficit in visual acuity, which is typically found in strabismic amblyopia [Bonneh, Y., Sagi, D., & Polat, U. (2004a). Local and non-local deficits in amblyopia: Acuity and spatial interactions. Vision Research, 44, 3009-3110]. Here, we extend this result to the temporal domain by measuring visual acuity (VA) for a single pattern in a rapid serial visual presentation (RSVP-VA, N=15) for fast ("crowded") and slow ("uncrowded") presentations. We found that strabismic amblyopes but not anisometropic amblyopes or normal controls exhibited a significant difference between VA under the fast and slow conditions. We further compared the "temporal crowding" measure to two measures of spatial crowding: (1) static Tumbling-E acuity in multi-pattern crowded displays (N=26) and (2) Gabor alignment with lateral flankers (N=20). We found that all three measures of crowding (one temporal and two spatial) were highly correlated across subjects while being largely independent of the visual acuity for a single isolated pattern, with both spatial and temporal crowding being high and correlated in strabismus and low in anisometropia. This suggests that time and space are related in crowding, at least in amblyopia.     

高度近视偏心注视优势位置的研究

目的 探讨高度近视眼因黄斑病理损害形成中心暗点以及丧失中心视力后偏心固视的形成规律,确定偏心注视点的优势位置.方法 应用微视野计(MP-1)对因黄斑病变形成偏心固视的40例高度近视患者的54只眼作固视检查.利用正常成人中心固视的90%置信椭圆,确定偏心固视相对于中心凹的位置.根据观察到的偏心固视的位置,将所有患眼分为preferred retinal locucs(PRL)优势组和PRL非优势组;并将两组视力作统计学比较.结果 54只高度近视眼中,中心视力丧失后偏心固视点形成在暗点下方视野者24只眼,占本组患眼的44.44%;左侧19只眼,占35.19%;上方6只眼,占11.11%;右侧5只眼,占9.26%.双眼均形成偏心固视者14例,其中13例双眼偏心固视模式一致,均为下方者7例,占双眼偏心固视者的50.00%;均为左侧者5例,占35.71%;均为上方者1例,占7.14%.偏心固视位于下方、左侧视野组与偏心位置位于右侧和上方的非优势组之间比较.其视力差异无统计学意义(F=0.144,P>0.05).结论高度近视患者偏心固视会形成在尽量靠近中心凹有功能的视网膜.下方视野是形成偏心固视的优势位置.     

Amblyopia in 4-year-old children treated with grating stimulation and full-time occlusion; a comparative study.

We have compared the effects on visual acuity and binocular functions of grating stimulation (CAM therapy) and full-time occlusion in 38 4-year-old, previously untreated amblyopic children. The patients were divided into subgroups with regard to amblyopia type and fixation pattern. We found that grating stimulation was slightly better than occlusion in improving visual acuity of anisometropic amblyopes with central fixation, but that both types of therapy were equally effective in strabismic amblyopia with central fixation and in amblyopia with eccentric fixation. However, maximal treatment effects were not reached with grating stimulation alone, as shown at follow-up after continued conventional therapy. Grating stimulation may be regarded as a valuable method at the initiation of treatment, particularly in anisometropic amblyopia, but it has to be supplemented with occlusion, which still must be regarded as the prime form of amblyopia therapy.     

Perifoveal function in patients with North Carolina macular dystrophy: the importance of accounting for fixation locus

PURPOSE: To quantify the extent of visual function losses in patients with North Carolina Macular Dystrophy (NCMD) and to demonstrate the importance of accounting for eccentric fixation when making comparisons with normal data. METHODS: Five patients with NCMD who were from a single family were examined. Multifocal electroretinograms (mfERGs) and psychophysical assessments of acuity and luminance visual field sensitivities were measured throughout the central retina. Comparisons of responses from equivalent retinal areas were accomplished by shifting normal templates to be centered at the locus of fixation for each patient. RESULTS: Losses of psychophysically measured visual function in patients with NCMD extend to areas adjacent to the locations of visible lesions. The multifocal ERG amplitude was reduced only within the area of visible lesion. Multifocal ERG implicit times were delayed throughout the entire central retinal area assessed. CONCLUSIONS: ERG timing is a sensitive assay of retinal function, and our results indicate that NCMD has a widespread effect at the level of the mid and outer retina. The findings also demonstrated that it is necessary to account for fixation locus and to ensure that equivalent retinal areas are compared when testing patients with macular disease who have eccentric fixation.     

Monocular fixation and acuity in amblyopic and normal eyes

We studied the relation between eccentricity of monocular fixation and the minimum angle of resolution invisually normal subjects and in both the amblyopic and dominant eyes of amblyopes. For the amblyopic and visually normal eyes, this relation was linear; resolution was poorer at the eccentrically fixating locus than at the corresponding region of the normal eye. The dominant eyes of the strabismic amblyopes had a nasal component of eccentric fixation that was less than that of the companion amblyopic eyes, but greater than the that found in the visually normal eyes. In terms of the eccentricity of monocular fixation, the domonant eyes of strabismic amblyopes were different from the normal eyes.     

Spatial resolution across the macaque retina

Grating acuity was measured as a function of eccentricity from the fovea in two macaques. A vertical-horizontal orientation discrimination was used to determine acuity, and the retinal locus of the test grating was controlled by training them to fixate a spot placed at various distances from the stimulus. Their head was fixed in place and fixation was monitored with a scleral search coil. The acuity of monkeys across the retina was similar to that previously measured in human subjects, reaching a peak of about 38 c/deg at the fovea, and decreasing about 10-fold by 30 deg eccentricity. Acuity was slightly higher in the temporal than in the nasal visual field. The shape of the acuity-eccentricity function suggested a dependence on cone density near the fovea, and on the density of P ganglion cells at eccentricities beyond 10 deg. Existing physiological data suggest the possibility that macaque acuity may also be limited in part by spatial averaging across the receptive field of retinal ganglion cells.     

Naso-temporal asymmetries in human amblyopia: Consequence of long-term interocular suppression

The resolution for vertical gratings was tested at different eccentricities in the visual field of 9 strabismic and 2 anisometric amblyopes.It has been found that, within the central 20° of the visual field of esotropic amblyopes, the acuity of the nasal retina was much more reduced than that of the temporal retina. Outside this region, acuity was practically identical to that of the non-amblyopic eye. This asymmetric loss of acuity was well correlated with the interocular suppression as determined by a method of interocular perimetry.For anisometropic amblyopes, the acuity loss was symmetric, diminishing gradually towards the periphery. The interocular suppression was also symmetric, uniformly affecting all tested positions in the visual field of the amblyopic eye.     

Contrast sensitivity and acuity relationship in strabismic and anisometropic amblyopia.

The contrast sensitivity function (CSF) and visual acuity were determined in children and adults with unilateral amblyopia due to strabismus or anisometropia with central fixation. The preschool children were examined repeatedly during occlusion treatment. All amblyopes had CSF deficits. The CSF was characterised by its peak value (the maximal sensitivity, Smax, and the spatial frequency at which Smax occurs, Frmax) calculated by a single peak least-square regression method. The two amblyopic groups showed discrepancies in relationship of both Smax and Frmax versus visual acuity both initially and during treatment. The strabismic cases had a more marked visual acuity deficit in relation to the contrast sensitivity losses, whereas these parameters are affected similarly in anisometropic amblyopes. The relationship between recovery of visual acuity and CSF during the initial month of occlusion treatment was of prognostic significance for the outcome of visual acuity improvement.     

Patients with AMD and a large absolute central scotoma can be trained successfully to use eccentric viewing,as demonstrated in a scanning laser ophthalmoscope

Twenty patients with age-related macular degeneration, an absolute central scotoma and a mean visual acuity of 0.04 (20/475) were studied. A scanning laser ophthalmoscope (SLO) was used for microperimetry and determination of preferred retinal locus, often located to the left of the retinal lesion (corresponding to a location to the left of the visual field scotoma), which is considered unfavorable for reading. All 20 patients were trained to use a new and more favorable retinal locus for reading, above (or occasionally below) the retinal lesion (corresponding to a location below or above the visual field scotoma), first by reading scrolled text under simultaneous fixation monitoring and instruction in the SLO and then by reading printed text, using high magnification (mean 14.3x). For the 18 patients who learned to use eccentric viewing, reading speed with adequate magnification prior to training was 9.0+/-5.8 words/min. With training (mean 5.2 hours), it increased significantly (p<0.001) to 68.3+/-19.4 words per min. Training of eccentric reading has thus proved to be very successful.     

视力表的标准化

回顾了我国多年来视力表在设计、制作和使用中存在的问题,着重分析了目前使用最普遍的两种视力表《国际标准视力表》和《标准对数视力表》存在的问题。认为视力表视力检查是评价人眼主觉视力简便而实用的方法,它只对视觉功能做粗略的评估,简便和实用是其最大的特点。因此今后在设计和使用视力表时应考虑:①设计合理、简便实用、兼顾习惯、国际接轨、全国统一;②除有通用表外,另设计出供3岁左右儿童使用的视力表。建议国家标准局在业务主管部门(卫生部及中华医学会)协助下,组织视力表问题研究专题组,广泛征求意见与建议,提出规范统一的方案。     

Binocular interaction in the peripheral visual field of humans with strabismic and anisometropic amblyopia

Residual binocular interaction was tested across the visual field of 9 strabismic and 2 anisometropic amblyopes in threshold and suprathreshold conditions.Binocular summation and interocular transfer of adaptation after-effects, both absent or very much reduced in the central region of the visual field of squint amblyopes, were highly significant in the periphery. The regions of preserved binocularity were well correlated with the areas for which there was no acuity loss and no interocular suppression. In anisometropic amblyopes, both binocular summation and interocular transfer of adaptation were lost at all tested eccentricities.Dynamic local stereopsis was also present in the periphery, but not in the central field of both squint and anisometropic amblyopes.     

Eccentric visual acuity in patients with macular disease

A series of cards each containing a two dimensional array of identical Snellen "E's" was used to determine best eccentric visual acuity in patients with macular disease having Snellen visual acuity of 20/70 or worse. Each "full field E" card simultaneously presents the same letter to foveal and parafoveal areas. This test can therefore determine quickly if potentially useful vision is present in any area of the central visual field. In our study of 37 eyes, 70% demonstrated potential visual acuity at least two times better than visual acuity measured by conventional methods, and 20% demonstrated at least a fourfold improvement. This suggests that most patients with macular disease do not spontaneously employ their best remaining area of retina for fixation.     

Positional uncertainty in peripheral and amblyopic vision

Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.     

Improving the reliability of visual acuity measures in young children.

Whilst the methodology of adult letter acuity measurement has been substantially refined over the last two decades, relatively little development has occurred in methods for quantifying letter acuity in young children. This study compares a recently developed visual acuity test (Glasgow Acuity Cards), which incorporates several key design features used in adult test charts to improve the sensitivity and reliability of visual acuity measurements. The equivalence of acuity measurements made with Glasgow Acuity Cards were compared with the Bailey-Lovie logMAR chart and Snellen chart in adults, and with traditional Single Letter Acuity and a modified Single Letter Acuity test in children. The test-retest reliability of acuity measurements made with Glasgow Acuity Cards and the Single Letter Acuity tests were also assessed in a large group of visually normal children. In addition, the ability of the pre-school letter acuity tests to detect differences in acuity between the two eyes, and to detect amblyopia were examined. Ninety-five percent of vision measurements made with the Bailey-Lovie chart and Glasgow Acuity Cards differ by less than 0.07 log unit. Furthermore, the sensitivity of Glasgow Acuity Cards to detecting changes in acuity longitudinally and inter-ocular differences in acuity is considerably greater as compared with traditional Single Letter Acuity tests. Improvements in paediatric acuity chart design are important for the effective detection and management of children with amblyopia.     

Acuity, crowding, reading and fixation stability

People with age-related macular disease frequently experience reading difficulty that could be attributed to poor acuity, elevated crowding or unstable fixation associated with peripheral visual field dependence. We examine how the size, location, spacing and instability of retinal images affect the visibility of letters and words at different eccentricities. Fixation instability was simulated in normally sighted observers by randomly jittering single or crowded letters or words along a circular arc of fixed eccentricity. Visual performance was assessed at different levels of instability with forced choice measurements of acuity, crowding and reading speed in a rapid serial visual presentation paradigm. In the periphery: (1) acuity declined; (2) crowding increased for acuity- and eccentricity-corrected targets; and (3), the rate of reading fell with acuity-, crowding- and eccentricity-corrected targets. Acuity and crowding were unaffected by even high levels of image instability. However, reading speed decreased with image instability, even though the visibility of the component letters was unaffected. The results show that reading performance cannot be standardised across the visual field by correcting the size, spacing and eccentricity of letters or words. The results suggest that unstable fixation may contribute to reading difficulties in people with low vision and therefore that rehabilitation may benefit from fixation training.