Can human amblyopia be treated in adulthood?

Amblyopia is a common visual disorder that results in a spatial acuity deficit in the affected eye. Orthodox treatment is to occlude the unaffected eye for lengthy periods, largely determined by the severity of the visual deficit at diagnosis. Although this treatment is not without its problems (poor compliance, potential to reduce binocular function, etc) it is effective in many children with moderate to severe amblyopia. Diagnosis and initiation of treatment early in life are thought to be critical to the success of this form of therapy. Occlusion is rarely undertaken in older children (more than 10 years old) as the visual benefits are considered to be marginal. Therefore, in subjects where occlusion is not effective or those missed by mass screening programs, there is no alternative therapy available later in life. More recently, burgeoning evidence has begun to reveal previously unrecognized levels of residual neural plasticity in the adult brain and scientists have developed new genetic, pharmacological, and behavioral interventions to activate these latent mechanisms in order to harness their potential for visual recovery. Prominent amongst these is the concept of perceptual learning—the fact that repeatedly practicing a challenging visual task leads to substantial and enduring improvements in visual performance over time. In the normal visual system the improvements are highly specific to the attributes of the trained stimulus. However, in the amblyopic visual system, learned improvements have been shown to generalize to novel tasks. In this paper we ask whether amblyopic deficits can be reduced in adulthood and explore the pattern of transfer of learned improvements. We also show that developing training protocols that target the deficit in stereo acuity allows the recovery of normal stereo function even in adulthood. This information will help guide further development of learning-based interventions in this clinical group

Field restriction and vignetting in contact lenses with opaque peripheries

Background: Some previous researchers have shown empirically that visual fields can be restricted by contact lenses with opaque and semi-opaque peripheral zones. However, there has been no formal statement of the optical theory behind such restriction. Methods: Calculations are made of fields of view with opaque periphery contact lenses, based on paraxial theory and meridional finite ray tracing in the Gullstrand-Emsley schematic eye. Results: This analysis shows that paraxial theory is adequate for predicting 'half fields of full illumination' for ocular pupil diameters smaller than six millimetres, although paraxial estimates of ‘half total field’ can substantially underestimate those obtained by finite ray tracing for a wide range of ocular pupil sizes. Therefore finite ray tracing may be a more appropriate method of calculating half total field. 'Total field' will be smaller with smaller ocular pupils and smaller contact lens apertures. A clinical method is suggested for assessing total field with such contact lenses in situ. The analysis applied to opaque periphery contact lenses is extended to the semi-opaque periphery contact lenses used for cosmetic alteration of eye appearance. Conclusion: Semi-opaque periphery contact lenses will decrease retinal illuminance most at regions outside those tested by previous empirical static perimetry studies

Spatio-temporal characteristics of vision in glaucomatous and normal subjects

This study investigated the ability of normal subjects and glaucoma patients to judge the temporal order of small, foveally presented stimuli. Two basic stimulus paradigms were used: Inter-Ocular Temporal Asynchrony (IOTA), and Temporal Order Discrimination (TOD). IOTA, a simple foveal dichoptic temporal order matching technique, was designed to investigate whether the visual system in glaucoma is characterised by interocular asymmetry of visual latency. Preliminary testing showed that IOTA was affected by monocular luminance attenuation using ND filters in normal subjects. Glaucoma subjects showed significant abnormalities on IOTA testing which correlated strongly with asymmetry of optic disc appearance, and less strongly with asymmetry of visual field indices. This is the first perceptual evidence of alterations in visual latency in glaucoma patients, and is in accord with previous findings that glaucoma causes increased latency in visually evoked potential responses. TOD was a monocular foveal test, measuring the smallest interstimulus interval required by a subject to distinguish the temporal order of two spatially proximal stimuli. As there had been little previous research in normal subjects on the effects of stimulus duration, contrast and temporal profile on TOD, these parameters were investigated for a small number of young, visually normal subjects. For constant contrast stimuli with rectangle temporal profiles, TOD thresholds depended on stimulus duration, performance being optimal for durations around 10 to 100 ms, and thresholds increasing for longer and briefer stimulus durations. For stimuli set at 4 times contrast threshold, TOD thresholds were constant for brief durations, but increased (i.e. became poorer) for longer durations. This duration-dependent increase in TOD thresholds was more marked for stimuli with Gaussian temporal profiles than for rectangular profile stimuli, suggesting that the high frequency information contained in the edges of rectangular pulses aided TOD judgments. TOD thresholds improved with increasing stimulus contrast, although the degree of improvement varied with stimulus duration, and between subjects. TOD performance was measured in glaucoma patients, age-matched old normal subjects, and young normal subjects. Stimuli with Gaussian temporal envelopes were used, and contrasts set at 4 times individual subject's contrast threshold so that all stimuli would be equally visible. Stimulus durations were varied over a large range (pulse standard deviations of 16, 64, and 256 ms). Older subjects showed increased TOD thresholds (i.e. poorer performance) compared to young normal subjects, and this was true for all stimulus durations. This result cannot be explained in terms of increased sluggishness of the aging visual system, but suggests that TOD processing of high and low temporal frequency information is impaired in older observers. The TOD thresholds of glaucoma patients did not differ significantly from· those of age-matched normals for all durations tested, despite the fact that glaucoma subjects had, on average, significantly poorer foveal contrast detection thresholds, greater optic disc cupping and a higher degree of visual field loss than for age-matched normals. For clinical testing of foveal function in glaucoma, contrast threshold measurement is of greater diagnostic value than TOD measurement. The subjects in each of the three groups were tested with MOTA (Monocular Temporal Asynchrony), a "method of adjustment" measure of temporal order matching ability. This technique was trialed because its speed of measurement offered an advantage over TOD in clinical testing. However MOTA showed poor potential as a substitute for TOD, correlating only weakly with TOD for the three groups, and being unable to distinguish glaucoma from age-matched normal subjects. In summary, glaucoma appears to cause interocular asymmetry of perceived latency, but does not affect precision of temporal order judgments, even in patients who show elevated foveal contrast thresholds

Modeling logMAR visual acuity scores: effects of termination rules and alternative forced-choice options

BACKGROUND: Logarithm of the minimum angle of resolution (logMAR) charts come in a variety of alternative forced-choice (AFC) formats and can be used with a variety of different rules to determine when to stop a subject reading down a chart (termination rules). METHODS: Exact calculation and Monte Carlo simulation techniques were used to compare logMAR scores for different termination rules and for infinite-, 26-, 10-, 8-, 4-, and 2-AFC logMAR chart formats. Slope-corrected standard deviation, an index of variability of the underlying ideal threshold, was used as a common metric for the different test conditions. RESULTS: The mean and standard deviation of logMAR scores can be significantly affected by termination rules and AFC format. For different AFC formats, different termination criteria were found to give optimal slope-corrected standard deviations. CONCLUSION: A number of clinically suitable termination rules are proposed for different AFC format logMAR charts. For letter-by-letter scoring of Bailey-Lovie and EDTRS charts, it is recommended that researchers and practitioners use a termination rule of stopping after four or more mistakes on a line

Confidence and coverage for Bland-Altman limits of agreement and their approximate confidence intervals

Bland and Altman described approximate methods in 1986 and 1999 for calculating confidence limits for their 95% limits of agreement, approximations which assume large subject numbers. In this paper, these approximations are compared with exact confidence intervals calculated using two-sided tolerance intervals for a normal distribution. The approximations are compared in terms of the tolerance factors themselves but also in terms of the exact confidence limits and the exact limits of agreement coverage corresponding to the approximate confidence interval methods. Using similar methods the 50th percentile of the tolerance interval are compared with the k values of 1.96 and 2, which Bland and Altman used to define limits of agreements (i.e. Formula+/− 1.96Sd and Formula+/− 2Sd). For limits of agreement outer confidence intervals, Bland and Altman’s approximations are too permissive for sample sizes <40 (1999 approximation) and <76 (1986 approximation). For inner confidence limits the approximations are poorer, being permissive for sample sizes of <490 (1986 approximation) and all practical sample sizes (1999 approximation). Exact confidence intervals for 95% limits of agreements, based on two-sided tolerance factors, can be calculated easily based on tables and should be used in preference to the approximate methods, especially for small sample sizes

Vision in competition pistol shooters: effects of distance defocus on performance

We examined the effects of adding positive spherical lenses to distance spectacle prescriptions on the pistol shooting performance of nine experienced shooters. Shot groupings were significantly smaller horizontally when shooters wore a +0.50D addition and significantly smaller vertically when shooters wore a +0.75D addition to their distance spectacle correction. Performance became significantly worse with spectacle additions of +2.00D and +4.00D. We discuss possible mechanisms for the improvement in performance obtained by using low plus additions for presbyopic and non-presbyopic pistol shooters