Measurement of in vivo lens shapes using IOLMaster 700 B-scan images: Comparison with phakometry.

PURPOSE: This study compared in vivo crystalline lens shape measurements using B-scan images from the IOLMaster 700 with phakometry. METHODS: Twenty-four young adult participants underwent IOLMaster 700 and phakometry measurements under cycloplegia (1% cyclopentolate). The IOLMaster 700 generated B-scan images along six meridians in 30° increments, which were analysed using custom MATLAB software to determine lens surface radii of curvature. Phakometry measurements were obtained using Purkinje images reflected from the lens surfaces. RESULTS: The IOLMaster 700 image analysis method yielded a lower mean anterior lens surface spherical equivalent power (+6.20 D) than phakometry (+7.55 D); however, the two measurements were strongly correlated (R(21) = 0.97, p < 0.0001). The astigmatic power vectors (J0 and J45) for the anterior lens surface were significantly higher for the IOLMaster 700 measurements, with only J0 showing a significant moderate positive correlation (R(21) = 0.57, p = 0.005). For the posterior lens surface, the IOLMaster 700 measurements had a higher mean spherical power (+14.28 D) compared to phakometry (+13.70 D); however, a strong positive correlation (R(21) = 0.90, p < 0.0001) was observed. No significant correlations were noted for posterior lens surface astigmatic vectors (J0 and J45). The IOLMaster 700 estimates for the equivalent lens mean spherical power were slightly lower than those for phakometry, with a mean difference of -0.72 D, and both methods were positively correlated (R(21) = 0.94, p < 0.0001). CONCLUSIONS: The findings demonstrate that IOLMaster 700 B-scan image analysis technique provides similar estimates of lens surface powers to phakometry. These results highlight the potential of the IOLMaster 700 to provide measurements of lens shape, informing future research and clinical use.

Vision screening using the Acuidrive device.

CLINICAL RELEVANCE: Understanding devices used for vision screening, including their potential utilisation and validity, will facilitate proper utilisation of this technology. BACKGROUND: The Acuidrive is a self-illuminated, hand-held, visual acuity screening device intended for use in policing, with visual acuity assessed roadside to identify drivers who may not meet vision standards for driving. The target is presented binocularly at 24 cm, and +4.00 D lenses eliminate the accommodation requirement. This study investigates its validity and applicability relative to the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart. METHODS: There were 36 participants, half younger adults aged 18-30 years and half older adults aged 50-70 years. The subjects underwent binocular visual acuity testing using the Acuidrive device and an ETDRS chart displayed on a monitor. Eyes were corrected for distance using lenses in a trial frame, and additional trial lenses provided four blur levels: zero, +0.50 DS, +1.00 DS and +1.50 DS. Luminances for the devices were similar at approximately 100 cd/m2. ETDRS chart measurements were conducted both with room lighting on and off. RESULTS: The Acuidrive device underestimated the ETDRS visual acuity across all blur levels, with mean differences of 0.24 ± 0.07 logMAR and 0.18 ± 0.10 logMAR for room lights on and off, respectively. To predict ETDRS visual acuity of poorer than 0.34 logMAR (6/12=), a screening level of 0.50 logMAR (6/19) with the Acuidrive device provided high sensitivity and specificity (86% and 79% with lights-on, and 85% and 78% with lights-off). Visual acuity was better for the older group than the younger group by approximately 0.10 logMAR. CONCLUSION: There was an offset of 0.2 logMAR (two lines) between visual acuity measures for the Acuidrive device and an ETDRS chart. The Acuidrive device has the potential to be a viable screening tool with refinement to its construction.

Effects of induced aniseikonia on binocular visual acuity.

CLINICAL RELEVANCE: Binocular visual acuity is an important index of functional performance. Optometrists need to know how binocular visual acuity is affected by aniseikonia, and whether reduced binocular visual acuity is a marker for aniseikonia. BACKGROUND: Aniseikonia, the perception of unequal image sizes between the eyes, can occur spontaneously or can be induced after different types of eye surgery, or trauma. It is known to affect binocular vision, but there are no prior studies about how it affects visual acuity. METHODS: Visual acuity was measured for 10 healthy well-corrected participants aged 18-21 years of age. Aniseikonia of up to 20% was induced in one of two ways: (1) size lenses, which provided minification of field of view in one eye of each participant and (2) polaroid filters, which allowed vectographic viewing of optotypes on a 3D computer monitor. The best corrected acuity was measured on conventional logarithmic progression format vision charts and isolated optotypes, under both induced aniseikonia conditions. RESULTS: Induced aniseikonia caused binocular visual acuity thresholds to increase by small but statistically significant amounts, with the largest deficit being 0.06 logMAR for 20% size differences between the eyes. Binocular visual acuity was worse than monocular visual acuity for aniseikonia of 9% and greater. Acuity measured with the vectographic presentation gave slightly higher thresholds (by 0.01 logMAR) than for those viewed with size lenses. Acuity measured with charts gave slightly higher thresholds (by 0.02 logMAR) than with isolated letters. CONCLUSION: An acuity change of 0.06 logMAR is small and may be missed in a clinical examination. Therefore, visual acuity cannot be used as a marker of aniseikonia in clinical settings. Even with very marked induced aniseikonia, binocular visual acuity remained well within standards for licen*c*sing of drivers.

Changes in retinal and choroidal optical coherence tomography angiography indices among young adults and children over 1 year.

CLINICAL RELEVANCE: Optical coherence tomography angiography (OCT-A) indices are likely to change across time and optometrists should be aware of the variability expected during childhood development and in healthy adults. BACKGROUND: Cross-sectional studies have shown that OCT-A indices are associated with age in adults and children. The aim of this study is to investigate longitudinal changes in retinal and choroidal OCT-A indices over 1 year among healthy children and young adults. METHODS: This prospective longitudinal study captured macular OCT-A and OCT scans, and biometry measures at baseline and 1-year follow-up for 22 adults (18-30 years; -6.87 to +0.37 D) and 21 children (6-15 years; -5.75 to +0.25 D). Superficial and deep retinal layer, choriocapillaris and deep choroidal en face OCT-A images were analysed to extract magnification-corrected vascular indices in foveal, parafoveal and perifoveal regions. The retinal indices included foveal avascular zone metrics, perfusion, and vessel density. Flow deficit number, size, and density were extracted from choriocapillaris and perfusion density from deep choroid. Associations between annual changes in the OCT-A indices and axial length and baseline refraction were also studied. RESULTS: Among children, significant reductions were noted only in parafoveal superficial retinal and foveal and perifoveal deep retinal layer indices over 1 year (p < 0.05). Choroidal OCT-A indices in children and both retinal and choroidal OCT-A indices in adults did not show significant changes. Myopia was associated with a larger reduction in the perifoveal retinal OCT-A indices in children, and with increases in sub-foveal and sub-parafoveal choroidal indices in adults. There were associations between OCT-A indices and axial length changes but differently in adults and children. CONCLUSIONS: Significant changes were noted in retinal OCT-A indices over 1 year among children but not adults. In comparison, choroidal OCT-A indices in adults and children showed a stable morphology over this period of time.

Twenty-four hour diurnal variation in retinal oxygen saturation.

Retinal oxygen saturation is influenced by systemic and local vasculature, intraocular pressure (IOP), and individual cellular function. In numerous retinal pathologies, early changes take place at the level of the microvasculature, thereby affecting retinal oxygenation. The purpose of this study was to investigate diurnal variations in retinal oximetry measures and evaluate the relationship with other ocular and systemic physiological processes. Healthy adults (n = 18, mean age 27 ± 5.5 years) participated. Ocular and systemic measures were collected every four hours over 24 h and included retinal oximetry, IOP, optical coherence tomography (OCT), OCT-angiography (OCTA), biometry, blood pressure, and partial pressure of oxygen. Amplitude and acrophase for retinal oxygen saturation, axial length, retinal and choroidal thickness, OCTA parameters, and mean arterial and ocular perfusion pressure (MAP, MOPP) were determined were determined using cosine fits, and multiple regression analysis was performed to compare metrics. Retinal oxygenation saturation demonstrated a significant diurnal variation with an amplitude of 5.84 ± 3.86% and acrophase of 2.35 h. Other parameters that demonstrated significant diurnal variation included IOP, MOPP, axial length, choroidal thickness, superficial vessel density, heart rate, systolic blood pressure, and MAP. Diurnal variations in retinal oxygen saturation were in-phase with choroidal thickness, IOP, and density of the superficial vascular plexus and out-of-phase with axial length and MOPP. In conclusion, retinal oxygenation saturation undergoes diurnal variations over 24 h. These findings contribute to a better understanding of intrinsic and extrinsic factors influencing oxygenation of the area surrounding the fovea.

Diurnal changes in choroidal optical coherence tomography angiography indices over 24 hours in healthy young adults.

This prospective study investigated the magnitude and pattern of variation in choroidal optical coherence tomography angiography (OCT-A) indices every 4 h over 24 h in healthy young myopic (n = 24) and non-myopic (n = 20) adults. Choriocapillaris and deep choroid en-face images from macular OCT-A scans were analysed from each session to extract magnification-corrected vascular indices including choriocapillaris flow deficit number, size and density and deep choroid perfusion density in the sub-foveal, sub-parafoveal, and sub-perifoveal regions. Choroidal thickness was also obtained from structural OCT scans. Significant variations over 24 h (P < 0.05) were observed in most of the choroidal OCT-A indices excluding sub-perifoveal flow deficit number, with peaks observed between 2 to 6 AM. For myopes, peaks occurred significantly earlier (3-5 h), and the diurnal amplitude was significantly greater for sub-foveal flow deficit density (P = 0.02) and deep choroidal perfusion density (P = 0.03) compared with non-myopes. Choroidal thickness also showed significant diurnal changes (P < 0.05) with peaks between 2 to 4 AM. Significant correlations were found between diurnal amplitudes or acrophases of choroidal OCT-A indices and choroidal thickness, intraocular pressure, and systemic blood pressure. This provides the first comprehensive diurnal assessment of choroidal OCT-A indices over 24 h.

Changes in Retinal Optical Coherence Tomography Angiography Indexes Over 24 Hours.

Purpose: To evaluate changes in the retinal microvasculature of young adults over 24 hours using optical coherence tomography angiography (OCT-A). Methods: Participants (n = 44, mean age 23.2 ± 4.1 years, 24 myopes and 20 nonmyopes) with normal ophthalmological findings were recruited. Two macular OCT-A and OCT scans, systemic blood pressure, intraocular pressure (IOP), and biometry measurements were taken every four hours over 24 hours. Superficial and deep retinal layer en face images were analyzed to extract magnification-corrected vascular indexes using image analysis including foveal avascular zone metrics, vessel density, and perfusion density for the foveal, parafoveal, and perifoveal zones. Results: Significant diurnal variations (P < 0.001) were observed in the vessel and perfusion density in the three superficial retinal layer regions, with acrophase between 4:30 PM and 8:30 PM. Only foveal and parafoveal regions of the deep retinal layer exhibited significant diurnal variations with acrophase between 9 AM and 3 PM. Myopes and nonmyopes had different acrophases but not amplitudes in the parafoveal perfusion density of superficial retinal layer (P = 0.039). Significant correlations were observed between diurnal amplitudes or acrophases of superficial retinal layer indexes and systemic pulse pressure, IOP, axial length and retinal thickness. Conclusions: This study shows, for the first time, that significant diurnal variation exists in OCT-A indexes of macular superficial and deep retinal layer over 24 hours and were related to variations in various ocular and systemic measurements. Myopes and nonmyopes showed differences in the timing but not in amplitude of the superficial retinal layer parafoveal perfusion density variations but not in deep retinal layer.

Computer monitor pixellation and Landolt C visual acuity.

PURPOSE: To investigate the effects of computer monitor pixel density on Landolt C visual acuity measurements and to provide appropriate pixel density recommendations for Landolt C vision chart design. METHODS: Participants were 10 healthy observers aged 18-31 years (mean 21.7 ± 3.6). Logarithmic progression charts were used, consisting of eight rows of five Landolt C optotypes ranging from -0.40 logMAR to 0.30 logMAR. Monitor pixel angular subtense varied from 0.10 min arc to 1.97 min arc, achieved by changing the chart test distance. Testing took place with two filtering conditions: unfiltered optotypes, (pixels rendered either black or white) and anti-aliased optotypes with pixel luminance averaged for a pixel square and rendered as grey levels. RESULTS: Each participant's acuity versus pixel size data sets were fitted by a nonlinear relationship with acuity equal to an asymptotic threshold visual acuity (VAas ) for small pixel sizes below a critical pixel size (Pcrit ). For pixel sizes larger than Pcrit  there was a linear relationship between acuity thresholds and pixel size. For anti-aliased Landolt Cs, mean Pcrit  was 1.23 min, and for unfiltered Landolt Cs average Pcrit was 0.65 min. For anti-aliased LandoltCs, Pcrit  was 2.01xVAas , and for unfiltered Landolt Cs Pcrit  was 1.05xVAas . CONCLUSION: These results are quantitatively very similar to previous research on pixellation and Sloan optotype acuity. We have demonstrated that spatially filtering Landolt C optotypes acts as anti-aliasing, to make them more robust to the degradation effects of pixellation. Previous recommendations for maximum pixel size on Sloan letter vision charts can be applied safely to Landolt C charts.

Bearing fixing: A new computer algorithm method for subjective determination of astigmatism.

PURPOSE: This article aims to illustrate the principles of bearing fixing, a new computer-controlled procedure for subjective determination of astigmatism, and to compare it with conventional clinician-controlled Jackson crossed-cylinder refraction. METHODS: The principles and method for bearing fixing are described. Astigmatism was measured using bearing fixing and Jackson cross-cylinder twice on 20 visually normal participants aged between 18 and 34 years. After final sphere adjustment, acuity measurements were made with each refraction estimate. RESULTS: Bearing fixing results could be obtained for all participants. Mean bearing fixing cylinder magnitude was slightly higher than Jackson cross-cylinder measures of cylinder magnitude, by 0.05D. Using vector analysis to take into account cylinder power and axis, mean bearing fixing astigmatism was not significantly different from cross-cylinder astigmatism, but did have higher test-retest variability (p < 0.05). Acuity with bearing fixing and cross-cylinder corrections did not significantly differ in average value or repeatability. CONCLUSIONS: Bearing fixing is a feasible method for subjective determination of astigmatism, and there are a number of potential improvements to the method which could make it faster and more precise.

Induced Refractive Error Changes the Optical Coherence Tomography Angiography Transverse Magnification and Vascular Indices.

PURPOSE: To assess the effect of changing anterior eye refractive power with contact lenses on the transverse magnification of en face images and associated vascular indices from optical coherence tomographic angiography (OCT-A). DESIGN: Prospective crossover study. METHODS: Spherical soft contact lenses (-6 diopter [D] to +6 D in 2 D steps) were used to induce anterior eye refractive power changes in 11 healthy young adults and 3 × 3-mm macular scans were captured using OCT-A (Zeiss AngioPlex, software version 11.0; Cirrus HD-OCT 5000, Carl Zeiss Meditec Inc). Image transverse magnification was predicted based on refraction and biometry measurements and compared with empirical changes in the en face images measured with image analysis. Linear regression analysis was performed to assess the relationship between induced refractive ametropia and foveal avascular zone (FAZ) area, perimeter, circularity, and vessel density and perfusion density. RESULTS: The predicted transverse magnification was linearly related to induced refractive ametropia and to the empirical transverse magnification changes (average slope: 1.02, 95% CI: 0.90-1.34). All the OCT-A indices showed linear relationships with induced refractive ametropia (P < .05) with the 12 D tested range altering the indices by 7% to 12%. After correcting for transverse magnification, all OCT-A indices except FAZ area were linearly related to induced refractive ametropia (P < .05) and were reduced to 1% to 9%. CONCLUSIONS: This study is the first to show that induced refractive ametropia can affect OCT-A image magnification and indices. These changes are clinically important and need to be considered along with biometry effects when interpreting OCT-A indices. Transverse magnification changes can affect the ability of OCT-A to precisely measure linear dimensions of blood vessels.

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.

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.

Twenty-four hour ocular and systemic diurnal rhythms in children.

PURPOSE: Ocular diurnal rhythms have been implicated in myopia, glaucoma, diabetes, and other ocular pathologies. Ocular rhythms have been well described in adults; however, they have not yet been fully examined in children. The goal of this study was to investigate ocular and systemic diurnal rhythms over 24 h in children. METHODS: Subjects, ages 5 to 14 years (n = 18), wore a light, sleep, and activity monitor for one week to assess habitual sleep/wake patterns, then underwent diurnal measurements every 4 h for 24 h. Measurements included blood pressure, heart rate, body temperature, intraocular pressure (IOP), ocular biometry, and optical coherence tomography imaging. Saliva was collected for melatonin and cortisol analysis. Mean ocular perfusion pressure was calculated from IOP and blood pressure. Central corneal thickness, corneal power, anterior chamber depth, lens thickness, vitreous chamber depth, and axial length were determined from biometry. Total retinal thickness, retinal pigment epithelium (RPE) + photoreceptor outer segment thickness, photoreceptor inner segment thickness, and choroidal thickness were determined for a 1 mm diameter centred on the fovea. Subjects' amplitude and acrophase of diurnal variation for each parameter were determined using Fourier analysis, and mean acrophase was calculated using unit vector averaging. RESULTS: Repeated measures analysis of variance (ANOVA) showed that all parameters except anterior chamber depth exhibited significant variations over 24 h (p ≤ 0.005 for all). Axial length underwent diurnal variation of 45.25 ± 6.30 µm with an acrophase at 12.92 h, and choroidal thickness underwent diurnal variation of 26.25 ± 2.67 µm with an acrophase at 1.90 h. IOP was approximately in phase with axial length, with a diurnal variation of 4.19 ± 0.50 mmHg and acrophase at 11.37 h. Total retinal thickness underwent a significant diurnal variation of 4.09 ± 0.39 µm with an acrophase at 15.04 h. The RPE + outer segment layer was thickest at 3.25 h, while the inner segment layer was thickest at 14.95 h. Melatonin peaked during the dark period at 2.36 h, and cortisol peaked after light onset at 9.22 h. CONCLUSIONS: Ocular and systemic diurnal rhythms were robust in children and similar to those previously reported in adult populations. Axial length and IOP were approximately in phase with each other, and in antiphase to choroidal thickness. These findings may have important implications in myopia development in children.

Ocular and Systemic Diurnal Rhythms in Emmetropic and Myopic Adults.

Purpose: To investigate ocular and systemic diurnal rhythms in emmetropic and myopic adults and examine relationships with light exposure. Methods: Adult subjects (n = 42, 22-41 years) underwent measurements every 4 hours for 24 hours, including blood pressure, heart rate, body temperature, intraocular pressure (IOP), ocular biometry, and optical coherence tomography imaging. Mean ocular perfusion pressure (MOPP) was calculated. Saliva was collected for melatonin and cortisol analysis. Acrophase and amplitude for each parameter were compared between refractive error groups. Subjects wore a light, sleep, and activity monitor for 1 week before measurements. Results: All parameters exhibited significant diurnal rhythm (ANOVA, P < 0.05 for all). Choroidal thickness peaked at 2.42 hours, with a diurnal variation of 25.8 ± 13.44 µm. Axial length peaked at 12.96 hours, with a variation of 35.71 ± 6.6 µm. Melatonin peaked at 3.19 hours during the dark period, while cortisol peaked after light onset at 8.86 hours. IOP peaked at 11.24 hours, with a variation of 4.92 ± 1.57 mm Hg, in antiphase with MOPP, which peaked at 22.02 hours. Amplitudes of daily variations were not correlated with light exposure, and rhythms were not significantly different between emmetropes and myopes, except for body temperature and MOPP. Conclusions: Diurnal variations in ocular and systemic parameters were observed in young adults; however, these variations were not associated with habitual light exposure. Emmetropic and myopic refractive error groups showed small but significant differences in body temperature and MOPP, while other ocular and systemic patterns were similar.

Simulated image doubling and visual acuity: effects of doubling magnitude, orientation, and ghost image intensity.

PURPOSE: The effects of image doubling on logarithmic progression chart visual acuity were investigated by simulating diplopia on a computer monitor. METHODS: Ten participants (6M, 4F) aged 21-28 years (mean 22.4 ± 2.3) were assessed viewing with their left eye while wearing their best correction. Stimuli were eight rows of five Sloan letters, from 0.4 to -0.3 logMAR (6/15 to 6/3) arranged in logarithmic progression format, generated on an HD monitor. Stimuli were generated with different magnitudes of doubling, different directions of doubling, and different intensity ratios between the ghost image and main images. RESULTS: When the ghost image had the same intensity as the main image, there was a significant effect of doubling magnitude on visual acuity, with the mean acuity being -0.11 logMAR (6/4.7) for no doubling. Acuity thresholds (logMAR) increased when doubling exceeded 1 min of arc, reaching a level of 0.12 logMAR (6/7.9) for doubling of 16 min of arc. There were no significant effects of orientation on acuity, nor were there significant orientation-doubling magnitude interaction effects of visual acuity. Image doubling magnitude level affected visual acuity differently for different ghost image intensities, with the highest acuity elevation occurring when ghost and main images were equal or nearly equal. For faint ghost image intensities (10% and 20%) image doubling did not significantly affect visual acuity. CONCLUSIONS: Image doubling will degrade visual acuity if doubling is greater than 1 min of arc, and ghost images are sufficiently intense. However, even with very obvious visually-disturbing image doubling, visual acuity remains only slightly reduced.

Monocular Ghost Image Offset Thresholds: Dependent on Target Size and Ghost Image Relative Brightness.

SIGNIFICANCE: This is the first report of monocular ghost image offset thresholds measured using O optotypes. Monocular diplopia is a complaint of patients and is the result of a variety of etiologies. Furthermore, monocular image doubling also can be a confounding variable of vectographic stereoacuity tests, warranting an investigation of ghost image offset thresholds. PURPOSE: To measure ghost image offset thresholds of normal observers and how they are affected by offset orientation, target size, and ghost image relative luminance. METHODS: Participants were five individuals without ocular abnormalities aged 21 to 32 years. Stimuli were viewed monocularly and consisted of Sloan "O" optotypes generated on a computer monitor with varying levels of image doubling. Ghost image offset thresholds were determined using a spatial 2-alternative forced-choice paradigm and probit analysis of the frequency of seeing data. RESULTS: Under close-to-optimal conditions, monocular ghost image offset thresholds ranged between 14 and 22 arc seconds, a level that might be considered a hyperacuity. Ghost image offset detection thresholds demonstrated a U-shaped relationship with optotype size, with optimum thresholds occurring for optotypes sizes of approximately -0.15 logarithmic minimum angle of resolution. There was no measurable effect of offset orientation on ghost image offset detection thresholds. Monocular ghost image offset detection thresholds decreased as ghost image relative luminance increased. CONCLUSIONS: Ghost image offset detection thresholds can be quite low even when viewing under monocular conditions and relatively low ghost image relative luminance. This should be considered when designing and interpreting the results of vectographic stereoacuity tests.

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: see text]+/- 1.96Sd and [Formula: see text]+/- 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.

Computer monitor pixellation and Sloan letter visual acuity measurement.

PURPOSE: To assess the effects of changing computer monitor pixel density on visual acuity measurements made using Sloan optotypes. METHODS: Acuity was measured on 10 participants aged 19 to 38 years (mean 27.9 ± 7.0) measured binocularly wearing their best spectacle correction. Stimuli were eight lines of five Sloan letter optotypes in logarithmic progression, ranging in size from -0.4 to 0.3 logMAR. Test distance was varied so that pixels on the monitor ranged in size from 0.125 mins of arc to 1.97 mins of arc. Two sampling approaches were used: (1) unfiltered sampling, with each pixel rendered either black or white; and (2) filtered sampling with pixel luminance averaged across a pixel aperture, giving grey-scale smoothing of letter edges. RESULTS: A broken line fit was made to each data set, with acuity being stable at an asymptotic threshold VAas for small pixels sizes, with thresholds increasing linearly when pixel sizes exceeded a critical pixel size Pcrit . For unfiltered stimuli, Pcrit averaged 1.1 mins of arc and for unfiltered stimuli averaged 0.69 mins of arc. For filtered stimuli, Pcrit was 1.79xVAas , and for unfiltered Pcrit was 1.05xVAas . CONCLUSION: The results show that grey-scale filtering makes acuity measurement more resistant to the effects of pixellation. Based on a conservative interpretation of these findings, we make the recommendation that charts be constructed with, for filtered optotypes, a maximum pixel size of 0.6 x of the smallest MAR used and, for unfiltered optotypes, 0.35 x the smallest MAR used.

The Enright phenomenon. Stereoscopic distortion of perceived driving speed induced by monocular pupil dilation / Fenómeno Enright. Distorsión estereoscópica de la velocidad percibida al conducir, inducida mediante dilatación monocular de la pupila

Purpose: The Enright phenomenon describes the distortion in speed perception experienced by an observer looking sideways from a moving vehicle when viewing with interocular differences in retinal image brightness, usually induced by neutral density filters. We investigated whether the Enright phenomenon could be induced with monocular pupil dilation using tropicamide. Methods: We tested 17 visually normal young adults on a closed road driving circuit. Participants were asked to travel at Goal Speeds of 40km/h and 60km/h while looking sideways from the vehicle with: (I) both eyes with undilated pupils; (II) both eyes with dilated pupils; (III) with the leading eye only dilated; and (iv) the trailing eye only dilated. For each condition we recorded actual driving speed. Results: With the pupil of the leading eye dilated participants drove significantly faster (by an average of 3.8km/h) than with both eyes dilated (p=0.02); with the trailing eye dilated participants drove significantly slower (by an average of 3.2km/h) than with both eyes dilated (p<0.001). The speed, with the leading eye dilated, was faster by an average of 7km/h than with the trailing eye dilated (p<0.001). There was no significant difference between driving speeds when viewing with both eyes either dilated or undilated (p=0.322). Conclusions: Our results are the first to show a measurable change in driving behaviour following monocular pupil dilation and support predictions based on the Enright phenomenon (AU)

Objetivo: El fenómeno Enright describe la distorsión en la percepción de la velocidad experimentada por un observador que mira lateralmente desde un vehículo en movimiento, y la visión con diferencias interoculares en cuanto a luminosidad de la imagen en la retina, normalmente inducida por filtros de densidad neutra. Investigamos si el fenómeno Enright podría inducirse mediante dilatación monocular de la pupila utilizando tropicamida. Métodos: Realizamos pruebas a 17 adultos jóvenes con visión normal, en un circuito cerrado de conducción por carretera. Se solicitó a los participantes que viajaran a velocidades predefinidas de 40km/h y 60km/h mientras miraban a ambos lados del vehículo con: I) ambos ojos sin dilatación pupilar; II) ambos ojos con las pupilas dilatadas; III) dilatación únicamente en el ojo fijador; y IV) dilatación únicamente en el ojo no fijador. Registramos la velocidad real de conducción para cada situación. Resultados: Con la pupila del ojo fijador dilatada los participantes condujeron a una velocidad considerablemente superior (de 3,8km/h de media) que con ambos ojos dilatados (p=0,02); con la pupila del ojo no fijador dilatada los participantes condujeron a una velocidad considerablemente menor (de 3,2km/h de media) que con ambos ojos dilatados (p<0,001). Con el ojo fijador dilatado la velocidad fue superior, de 7km/h de media, a la velocidad con el ojo no fijador dilatado (p<0,001). No se produjo diferencia significativa entre las velocidades de conducción cuando miraban con ambos ojos, tanto estuvieran dilatadas las pupilas o no (p=0,322). Conclusiones: Nuestros resultados son los primeros que reflejan un cambio medible en el comportamiento conductor tras la dilatación monocular de la pupila, y respaldan las predicciones que se basan en el fenómeno Enright (AU)

Slope of psychometric functions and termination rule analysis for low contrast acuity charts.

PURPOSE: To assess whether the slopes of psychometric functions for measuring low contrast letter acuity were different from those for measuring high contrast letter acuity. METHODS: Ten participants, wearing their best spectacle correction, were assessed monocularly. Stimuli were logarithmic progression charts, generated on a computer monitor, with nine rows of five randomised Sloan letters generated in either high contrast format (Weber contrast 99.2%) or low contrast format (Weber contrast 18.7%). For each participant, psychometric functions were generated by probit analysis of the data on each of 16 attempts at a low contrast chart and 16 attempts at a high contrast chart. Each of these probit fits yielded an estimate of Probit Size which provided information about how steep or flat the psychometric function was, along with an estimate of Probit Acuity Threshold. RESULTS: Probit Size was significantly larger (p < 0.001) for low contrast charts than for high contrast charts, indicating that psychometric functions were flatter for low contrast charts. Mean Probit Sizes in logMAR were 0.099 (SEM 0.022) for low contrast charts and 0.071 (SEM 0.009) for high contrast charts if a guess rate of 1/26 was assumed, or were 0.086 (SEM 0.019) for low contrast charts and 0.064 for high contrast charts if a guess rate of 1/10 was assumed. Monte Carlo analysis showed that these means were likely to be biased estimates, with true Probit Size probably being larger (i.e. slightly flatter fits) by 0.016-0.019 logMAR. As expected, Probit Acuity Thresholds were poorer for low contrast charts than for high contrast charts (p < 0.001). CONCLUSION: Our Monte Carlo modelling showed that such differences in acuity psychometric functions would be expected to give greater intra-subject variability in low contrast letter-by-letter acuity thresholds than for high contrast letter-by-letter acuity thresholds, and that this difference would depend on the termination rule used when measuring acuity. Likewise the variation in letter-by- letter acuity thresholds with termination rule will be different for high and low contrast charts. For low contrast and high contrast Sloan letter charts in a standard logarithmic format, a termination rule of four mistakes on a row, will give close to optimum sensitivity-to-change.