Sloan Letter Visual Acuity Charts and computer monitor pixilation

Purpose This research investigates how pixilation of computer monitors affects visual acuity measurement. Methods Stimuli were presented on a computer monitor as 8 lines of 5 Sloan letter optotypes in standard logarithmic progression format, ranging in size from -0.4 to 0.3 logMAR. Test distance was varied so that pixels on the monitor subtended different angles: 0.125, 0.200, 0.315, 0.50, 0.79, 1.25, 1.97 minutes of arc. Two pixel-sampling strategies were used: unfiltered sampling in which each pixel was rendered either black or white; or filtered sampling, in which pixel brightness was taken as the average letter brightness integrated across a pixel-sized aperture (i.e. grey-scale smoothing of letter edges). Binocular acuity was measured at each distance on 10 participants aged 19 to 38 years (mean 27.9 ± 7.0) using their best spectacle correction. Results Each observer’s logMAR acuity v log pixel size data was fitted with a broken line function, in which LogMAR acuity was described by a horizontal straight line below a critical pixel size (Pcrit) and by a linear relationship between acuity and pixel size above Pcrit. For small pixels sizes (below Pcrit), average asymptotic thresholds were a mean of -0.209 logMAR (SD 0.06) for the filtered letters, just slightly but significantly better than for the unfiltered letters with a mean -0.184 logMAR (SD 0.06) (t9 = 2.26, p = 0.050). Average Pcrit was significantly (p Conclusion For the purposes of visual acuity measurement, Sloan letters can be well rendered by relatively coarse sampling. Filtered letters (i.e. with grey-scale smoothed edges) can be rendered with much coarser (apparently sub-Nyquist) pixel sampling than unfiltered letters

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

Comment on: statistical methods for conducting agreement (comparison of clinical tests) and precision (repeatability or reproducibility) studies in optometry and ophthalmology

McAlinden et al.[1] have described various statistical tools for assessing repeatability of, and agreement between clinical measurements. One of the methods they described is that of Bland and Altman.[2] The method is well known and often used in health science, with the 1986[2] publication having been cited more than 24 000 times. As part of this method, differences d between each pair of measures are plotted against the mean of each pair of measures. This is illustrated in Figure 1, modified from figure 2 of McAlinden et al.,[1] for 10 pairs of measurements of spherical aberration using different instruments. The mean of the differences math formula is shown (in this case 0.04 μm). The standard deviation of the differences (sdiff) was 0.16 μm. For this 95% Limits of agreement (LoAs) were calculated as ..

Stand magnifiers for low vision : description, prescription, assessment

Stand magnifiers are still one of the most commonly prescribed classes of low vision devices. Their performance can be difficult to understand because stand magnifiers usually do not give an image at infinity. This review summarises the methods of describing image enlargement for stand magnifiers, emphasising their relationship to equivalent viewing distance (EVD). This is done in terms of the underlying optical equations, and measurement methods, and methods of prescribing. In the past, methods of determining EVD have been somewhat indirect, requiring accurate measurement of lens power, and image position. The use of digital photography provides an alternative, more direct, simpler method of determining EVD, which can be accomplished in-office. This method is described and it is demonstrated how it gives comparable results to older methods with small, clinically non-meaningful differences, that may be due to differences in image distance reference planes. Describing the performance of stand magnifiers in terms of their dioptric power, or in terms of ‘nominal magnification’ or ‘trade magnification’, is imprecise and misleading. It is better to use indices such as equivalent viewing power and EVD, which take into account the magnifier dioptric power, the image position of the magnifier and the distance a patient is from the magnifier. While EVD is a useful index for prescribing stand magnifiers, manufacturers do not always provide sufficient technical details to determine EVD for their stand magnifiers, and available tables of EVDs are more than a decade old and are likely to need updating. Photographic comparison provides a method for determining EVD, and this method can also be applied to other low vision devices.</p

Being conspicuous at night on a blurred road

Every year in the United States, more than 4500 pedestrians are killed and more than 75,000 pedestrians injured by motor vehicles. Worldwide, annually, one quarter of a million pedestrians are killed by motor vehicles. The vast majority of these deaths (70% or more) occur at night. Therefore, research into night-time pedestrian visibility is highly valuable..

Exact parametric confidence intervals for Bland-Altman limits of agreement

Purpose The previous literature on Bland-Altman analysis only describes approximate methods for calculating confidence intervals for 95% Limits of Agreement (LoAs). This paper describes exact methods for calculating such confidence intervals, based on the assumption that differences in measurement pairs are normally distributed. Methods Two basic situations are considered for calculating LoA confidence intervals: the first where LoAs are considered individually (i.e. using one-sided tolerance factors for a normal distribution); and the second, where LoAs are considered as a pair (i.e. using two-sided tolerance factors for a normal distribution). Equations underlying the calculation of exact confidence limits are briefly outlined. Results To assist in determining confidence intervals for LoAs (considered individually and as a pair) tables of coefficients have been included for degrees of freedom between 1 and 1000. Numerical examples, showing the use of the tables for calculating confidence limits for Bland-Altman LoAs, have been provided. Conclusions Exact confidence intervals for LoAs can differ considerably from Bland and Altman’s approximate method, especially for sample sizes that are not large. There are better, more precise methods for calculating confidence intervals for LoAs than Bland and Altman’s approximate method, although even an approximate calculation of confidence intervals for LoAs is likely to be better than none at all. Reporting confidence limits for LoAs considered as a pair is appropriate for most situations, however there may be circumstances where it is appropriate to report confidence limits for LoAs considered individually

Mask Barotrauma Leading to Sub-Conjunctival Haemorrhage in a Scuba Diver

This report describes the effects of mask barotrauma on a sports scuba diver. The patient presented with marked sub-conjunctival haemorrhages and peri-orbital haematomata; the result of not breathing into her diving mask during descent. Mask barotrauma is self-resolving, with little treatment necessary beyond patient reassurance. Patients should be instructed not to dive until the signs of the trauma resolve

Central Retinal Vein Occlusion in a Young Adult

This report describes a case of central retinal vein occlusion (CRVO) which was classified as papillophlebitis in a young female adult. In this age group, CRVO is relatively rare and tends to be mild in both its short-term and long-term visual consequences. The patient in this case showed a concurrent episode of bilateral intra-ocular pressure (IOP) elevation and the presence of cilio-retinal arteries in the affected eye. Despite extremely poor vision at presentation and the poor prognostic sign of cilio-retinal arteries, the patient made an excellent visual recovery with only mild paracentral field change

A Review of the Use of Confidence Intervals for Bland-Altman Limits of Agreement in Optometry and Vision Science

SIGNIFICANCE Confidence intervals are still seldom reported for Bland-Altman 95% limits of agreement. When they are reported, 50% of articles use approximate methods and 50% use exact methods. PURPOSE Bland-Altman limits of agreement can be unreliable estimates, especially for small sample sizes. However, authors seldom use confidence intervals for limits of agreement. This article reviews their use in Optometry and Vision Science. METHODS A keyword search for "Bland," "Altman," "Bland-Altman," "LoA," and "limits of agreement" was conducted on the Optometry and Vision Science website within a time range from January 2016 to December 2018. RESULTS Fifty articles were reported or were judged to use Bland-Altman analysis; sample sizes ranged from 3 to 2072. Eight of these article reported confidence limits for limits of agreement, four of which used exact methods and four used Bland and Altman's approximate method. CONCLUSIONS Use of confidence intervals for limits of agreement has increased in Optometry and Vision Science but is far from universal. To assist researchers in calculating exact confidence limits for Bland-Altman limits of agreement, spreadsheets are included for performing the calculations and generating Bland-Altman plots with the confidence intervals included.</p